“c - oil india · geophysics as a primary explorati on tool with change in technology, new...

“Changes are inevitable elements of existence and are escorted by perpetual growth all through.” The infallible achievements, unparalleled growth & development through decades has brought Geophysics Department to the pinnacle of exploration activity and placed Oil India Limited on the global picture with a distinct identity. ‘Insignia‘- particularly an emblem of specific distinction is depicting the decades of affirmative transformation and glorious journey that the department has gone through in quest of hydrocarbon resources. The cover design attempts to reflect the insignia of growth, accomplishments and the golden voyage right from the dawn of oil exploration through Gravity surveys to the most recently carried out multi-component seismic survey at OIL. With every step forward, the department is marching towards excellence in all facets of Geophysics & continually striving to conquer new horizons both in-country and overseas.

• Y. R. Singh

• D. S. Manral

• M. K. Banerjee

• P. K. Singh

• R. K. Srivastava

• M. Pereira

• P. Kumar

The views expressed in this souvenir are those of the individual contributors. Neither the Editorial Board nor Oil India Limited may endorse the views.

Published by Geophysics Department, Oil India Limited, Duliajan, AssamDesigned and produced by Exclusive Advertising Pvt. Ltd., Guwahati

INSIGNIA – is the special publication brought forth on the joint occasion of opening of the Duliajan chapter of Society of Petroleum Geophysicsts and the silver jubilee celebration of Geophysics Department. As the title and tagline proposes, this souvenir is intended to reflect the geophysical activities, capturing the illustrious, commendable work of OIL’s entire geoscience fraternity, in general, and geophysicists in particular.

Our main focus is to relive and witness the glorious eventful yesteryears which will ensure to beam our course to the future. The articles assembled herein are an endeavor towards this objective.

The abstracts section, which is a documentation of technical papers published in various journals or presented in the various conferences held in the past, is indeed a treasure of the technical expertise of the geophysical fraternity. The editorial board has delved through the various archives and put their level best to file them all into this edition in a very short timeframe.

The editorial team takes this opportunity to thank the authors for sparing their valuable time to pen down their thoughts. Our earnest gratefulness to the entire team of geoscientific community who have contributed to this publication.

We wish the readers a memorable experience.

From the

Desk

Reverence...To OIL’s geophysical unprecedented, unsurpassed, erudite!!!

To their infallible accomplishments & astounding determination

Their ability to reciprocate thoughts and inspiration through deeds...

Encouraging us to be what we are today,

capable of withstanding the exigent future ahead

The only INSIGNIA is YOU

A Humble Effort from us is dedicated all to YOU

We revere YOU

Apurba SahaPresident

SOCIETY OF PETROLEUM GEOPHYSICISTS, INDIA

It gives me indeed immense pleasure to learn that Geoscientists of Oil India Limited is opening “Duliajan Chapter” of Society of Petroleum Geophysicists (SPG), India. You are aware that SPG, India is affiliated with Society of Exploration Geophysicists (SEG), USA and European Association of Geoscientists and Engineers (EAGE), the Netherlands.

Energy security of the nation is the prime issue that needs to be addressed by all Geoscientists in order to maintain the double digit economic growth of India. The major part of the energy needs is being provided by the hydrocarbons which are non-renewable. As the growth of the nations ride on the continuous supply of energy, all efforts are being made to ensure continuous supply of hydrocarbons.

Over the years, breakthrough in the exploration of hydrocarbon is attributed to the advancements in petroleum geophysics. In the recent past, the geophysical methods have made immense contribution in the exploitation and management of hydrocarbon reserves in a cost effective manner. It is essential that our geoscientists get continuous exposure to these emerging technologies so that right technologies can be inducted and imbibed. Society of Petroleum Geophysicists, India could make the appropriate platform for showcasing technology by service provider through the biennial international conference and expositions which helped E&P industries in India to adopt such technology.

The objectives of SPG are distinct and thrusts are on helping to bridge the gap between Academic Institute, Professional Institute and Organization, creating awareness for Earth Science among School Students and their parents, promoting excellence & technological advancements in Geoscientists and to create opportunity for pursuing research on Geophysical Problems/Challenges being faced by E&P Industry.

I hope “Duliajan Chapter” will fulfill the need of geoscientists over north Assam and neighbouring places as well as to contribute in the economic growth of our country and wish all success for future endeavors.

(Apurba Saha)

B. N. TalukdarDirector (Exploration & Development)

This is a proud occasion for all the geoscientists in OIL INDIA LIMITED that a Chapter of SOCIETY OF PETROLEUM GEOPHYSICISTS (SPG) is being opened at Duliajan on 28th June, 2009.

SPG has made its significant mark in the development of petroleum geophysics in India through its association with national / international geoscientific societies, organizing training programme, bi-ennial conferences and talks by international experts. I sincerely believe that our geoscientists operating in the N-E will derive benefits of SPG through its Duliajan Chapter. Keeping track of latest developments in geoscientific field is the need of the hour for the explorations in the E&P Sector. Duliajan Chapter of SPG is definitely going to be a common and strong platform for the entire geoscientific community of OIL to develop new trend of technology, new concept/innovation towards achieving the exploration success in the present challenging and competitive environment.

I hope this chapter will flourish and become a matter of pride for both OIL and SPG

(B N Talukdar)

U. BorahResident Chief Executive

I am extremely happy that a chapter of Society of Petroleum Geophysicists (SPG) is going to be opened at Duliajan. The importance of Geophysics in the entire value chain of the E&P industry can never be understated.

In Oil India Limited, Geophysics played a very important role in all the exploratory and field development efforts. SPG has significant contribution in the development of petroleum geophysics in the country.

I sincerely hope that Duliajan chapter of SPG will provide a platform for development of our geoscientists towards facing the future challenges in exploration and field development.

(U. Borah)

9

Preamble

E&P sector of the petroleum industries globally recognizes geophysics as the primary hydrocarbon explorati on tool. OIL as a second largest NOC in the upstream sector of India, has its own geophysical history. In N-E primarily, OIL discovered many producing fi elds based on geophysical explorati on. Discovery of OIL’s old fi elds like Naharkati ya, Moran, Jorajan, Kusijan etc., was the result of old geophysical applicati ons based on geological requirements. However, discovery of oldest Digboi fi eld was an excepti on. Digboi fi eld was discovered in the year 1889 by inquisiti ve eyes who found slick of oil on an elephant’s feet.

With ti me OIL has been intensifying explorati on acti viti es in its diff erent operati onal areas using geophysics as a primary explorati on tool with change in technology, new concept/ideas towards identi fying hydrocarbon prospects. While old fi eld

reservoirs deplete new discovery is essenti al and therefore, explorati on conti nues. Hence, for any E&P Company, explorati on is a cyclic process and geophysics is the integral part of it. In today’s arena, fi nding a new fi eld is a

great challenge for the explorati onists in respect of having best possible accuracy in every geophysical signature to properly understand the sub-surface geological complexiti es. Geophysics plays a key role to achieve explorati on success. A focus on geophysical evoluti on of any E&P Company by itself is a focus on organizati onal growth/progress with ti me in respect of geoscienti fi c advancement, increase in explorati on acreage and discovery of new fi elds. Proper & systemati c geophysical applicati on and the eff ecti ve integrati on of its results with other relevant geoscienti fi c data defi nitely reduce explorati on risks.

To be more generalized, geophysical applicati on

Geophysics - Hydrocarbon Exploration by Oil India Limited (OIL)

in the upstream sector primarily includes gravity survey, magneti c survey, seismic survey and magneto-telluric survey at ti mes. Gravity survey with Torsion Balance was the fi rst pioneering step in the globe for hydrocarbon explorati on and so was the case in the Upper Assam basin. Subsequently, all other geophysical survey techniques/equipment were developed as the necessity arises from ti me to ti me for hydrocarbon explorati onists. In seismic, 2D/3D has become a common term now-a-days. E&P Companies and Geophysical Service Providers started adopti ng latest seismic technology and OIL has already stepped into it. OIL takes pride in having its vast experience in geophysical operati ons parti cularly in N-E part of India.

Early Age (ti ll 1960)

Discovery of oldest Digboi oilfi eld opened a new vista for hydrocarbon explorati on in diff erent sedimentary basins. As days progressed, the geoscienti sts all over the world visualized that below the near-surface cover of alluvium and diff erent layers of rocks, there is a possibility of oil and gas reserves. In order to identi fy such reservoirs below surface, a new science “explorati on geophysics” emerged in early 1900s.

Though today, geophysics, parti cularly petroleum

For any E&P Company, exploration is a cyclic process and Geophysics is the integral part of it.

K.K. Nath

10

geophysics, is dominated by seismic, it was not the case during early days of oil explorati on. In Hungary, during 1915, Lorand van Eotvos pioneered gravity survey with torsion balance. Though originally developed for geodesy-related studies, the instrument was successfully used for oil explorati on in USA thereaft er from 1922. A few years later, seismic refl ecti on survey came into existence and then onwards it slowly became the prime technique for prospecti ng of oil and natural gas.

With respect to Assam, the concerns were similar. The geoscienti sts like others in diff erent parts of the world were naturally of the opinion that the formati ons buried below the alluvium in the plains of Upper Assam might be hydrocarbon bearing. It was then in 1925, the fi rst gravity survey using torsion balance was taken up in the plains of Upper Assam. The torsion balance survey was fi rst carried out near Bordubi and nearby areas by Eotvos Geophysical Insti tute, Budapest. The team comprised of two geophysicists of Eotvos Geophysical Insti tute and three Indian assistants. The picture of sub-surface emerged from the survey led to some interpretati on problems and the idea of drilling in the area based on torsion balance gravity data did not materialize.

In the meanti me, during early thirti es, there was some progress in other geophysical techniques for explorati on parti cularly seismic. During this period, refl ecti on seismic was getti ng fast established as the main tool for explorati on of hydrocarbons. In 1937, Burmah Oil Company jointly with Briti sh Petroleum (then Anglo Iranian Oil Co.) and Shell proposed to the Government of India for carrying out geophysical surveys in the plains of Upper Assam. Considering the requirement of extensive geophysical surveys in the area, a new license for geophysical surveys was then introduced. The license was valid for fi ve years and was meant for geophysical surveys over an area of 5320 sq.miles.

The aim of the gravity survey was to identi fy and defi ne the prospecti ve areas so as to plan for seismic survey only in the areas of interest selecti vely. Thereaft er, from December, 1937, the then single fold seismic refl ecti on survey was carried out. The survey was done by M/s Pett y Geophysical Engineering Company, USA. This was possibly the

fi rst refl ecti on seismic survey in India. The survey indicated the presence of a structural high near Naharkati ya which was not far from the gravity high identi fi ed by torsion balance gravimeter survey in 1925-26. In view of outbreak of Second World War, the gravity and seismic survey programmes were disconti nued then. Later these geophysical survey programmes led to the discovery of Naharkati ya oil fi eld in 1954, the fi rst oil fi eld of independent India.

The discovery of oil in Naharkati ya brought in excitement and vigour in explorati on acti viti es. Geophysical prospecti ng were again started with renewed energy. The enti re basin was quickly covered by magneti c and gravity surveys. These surveys could not identi fy the prospects with confi dence. Single fold refl ecti on seismic survey was again

carried out in detail in diff erent parts of the basin and the same conti nued upto 1957.

Prior to the second world war, as the surface geophysical techniques were gathering momentum and were extensively deployed in Upper Assam, M/s Schlumberger set-up its wireline logging base in 1933. This was the fi rst wireline logging acti vity

These geophysical survey programmes led to the discovery of Naharkatiya oil fi eld in 1954, the fi rst oil fi eld of independent India.

The Perpetual Growth...

11

in India.

Period Between 1960-1980

This period witnessed unprecedented growth in all areas of geophysics. In such a scenario, OIL could not be far behind. There were series of geophysical campaigns in Upper Assam and Arunachal Pradesh. Aft er a pause, seismic survey resumed again in 1964-65. The geophysical service provider, M/s Seismograph Services Ltd. in this case, brought in new equipment which could record data using analogue magneti c tapes instead of paper records of the past. The seismic surveys during this phase went to remote corners of the concessions in Arunachal Pradesh, which had never been seismically explored before.

It may be noted that the data acquired either in paper secti ons or in analogue magneti c recorders

were digiti zed and thereaft er processed in the data processing centers of ONGCL. This helped in interpreti ng large areas of the basin.

The seismic campaigns conti nued therein. Every year between 1969-1974, seismic data was acquired

in the basin using the services of ONGC. The CDP technique provided bett er quality data and all these surveys led to the discovery of fi elds in Jorajan and other areas.

Apart from that, in the winter seasons of 1972-74, parti es from ONGC carried out extensive seismic survey in diff erent parts of the basin. Additi onally, a gravity party of ONGC carried out gravity survey in the hills of Manabum and neighbouring Lohit district of Arunachal Pradesh.

As the days progressed, it was necessary for the Company to keep up with the level of producti on. This, in turn, required new discoveries. On the other hand, the discoveries were also becoming smaller in size. The Company then decided to have its own seismic party and on 07th February, 1977, with DFS V equipment, OIL’s own fi rst seismic crew recorded the initi al seismic data. This was the fi rst DFS V system in India. This 24 channel system was later upgraded for higher channel count.

Additi onally, in 1978 aero-magneti c survey was carried out in some parts of OIL’s operati on areas in Upper Assam and Arunachal Pradesh by NGRI.

In the meanti me, OIL got Petroleum Explorati on License (PEL) in Mahanadi Basin in 1977. The geophysical programmes were then initi ated in this basin. For the fi rst ti me aeromagneti c surveys were introduced in OIL. NGRI carried out extensive aero-magneti c survey in the basin in 1978. The fi rst off shore seismic data acquisiti on programme by the Company was carried out in 1978. This was done by M/s Delta Explorati on Incorporated of USA.

The acquired digital seismic data required immediate processing. So far, seismic data from all spheres of the Company were mainly processed at ONGC’s center at Dehradun. The off shore seismic data from Mahanadi Off shore was processed at Digicon’s processing Center at Singapore. OIL then felt the requirement of becoming self-suffi cient in this regard. Consequently, OIL acquired a highly sophisti cated computer system, the Megaseis and successfully commissioned it in early 1980 at Duliajan. With this powerful tool, self-reliance is assured in all fi elds of seismic prospecti ng.

Period Between 1980 – 2000

During this period, OIL started venturing out to diff erent areas outside N-E. It acquired new PELs in Rajasthan, Saurashtra and Ganga Valley. Diff erent geophysical acti viti es were carried out in these projects. In Rajasthan and Ganga Valley vibroseis surveys were carried. In Rajasthan, M/s CGG carried out the survey between 1984-87. A

The Company then decided to have its own seismic party and on 07th February, 1977, with DFS V equipment, OIL’s own fi rst seismic crew recorded the initial seismic data.

12

computer center was also established there to process the data therein immediately. In Saurashtra off shore seismic data acquisiti on took place. In Bay Explorati on Project, covering Mahanadi basin and N-E Coast, seismic data was acquired both on land and off shore. In Mahanadi Onshore, Geophysical Services Inc. carried out extensive seismic survey in 1981-82.

During this period, there was a thrust in Assam and Arunachal Pradesh to cover the enti re basin by seismic survey. Oil India Limited acquired three (3) more DFS V systems and one (1) SN 358 system between 1983 and 1987. With all the fi ve (5) in-house seismic crews, every nook and corner of the basin was seismically covered in the next decade and half. This extensive seismic coverage led to the discovery of a new play in the basin – the Eocenes.

OIL always adopted the cutti ng edge technology. In the 80s, 3D seismic survey started getti ng popular. In 1987-88, 3D seismic was carried out in off shore north-east coast by M/s Western Geophysical.

This was the fi rst 3D off shore survey of OIL. Thereaft er, in 1991-92, 3D seismic survey was carried in Jaisalmer Basin of Rajasthan. This was the fi rst land 3D

seismic survey of OIL.

Thereaft er, 3D seismic was introduced in Assam in 1992-93. Aft er initi al problems, 3D seismic survey became a part and parcel of the explorati on programmes in Upper Assam. In-house experti se in this matt er was developed and diff erent areas were covered by 3D seismic survey using SN 368 line telemetry system. During the mid 90s, 3D seismic started getti ng prominence and the requirement of 2D seismic survey were getti ng reduced gradually.

As the quantum of seismic increased, new processing systems were installed. In 1992, GeoMASTER processing system based on Cyber 830 was commissioned. Thereaft er, in order to process 3D seismic data through in-house eff orts, PROMAX processing system based on SGI Hardware was installed and commissioned in 1996. Apart from PROMAX, the system had Ray tracing package – GXII, Inversion and AVO package – Hampson Russell etc. All these systems opened up new vista in the

fi eld of processing of seismic data.

With the increased volume of acquired data, its interpretati on required to be modernized. In late eighti es and early nineti es, fi ve (5) stand-alone interpretati on work stati on were procured. This enabled quick interpretati on and analysis of the enormous volume of seismic data.

The conti nuous upgradati on of the processing system and introducti on of interpretati on work stati ons enabled OIL to interpret seismic data independently not only from Assam and Arunachal Pradesh but also from other spheres of the Company.

Seismic was not the only technique used during this period. Aeromagneti c surveys were carried out in diff erent phases in many areas like Assam and Arunachal Pradesh and Ganga Valley. Second phase of aero-magneti c survey was carried out by NGRI in 1989. During the end-nineti es, the problem of explorati on in logisti cally diffi cult areas was in the forefront of explorati on challenges therein. Before planning for seismic survey, it was decided to carry out ground gravity-magneti c survey in Manabum and adjoining areas of Arunachal Pradesh.

... in 1991-92, 3D seismic survey was carried in Jaisalmer Basin of Rajasthan. This was the fi rst land 3D seismic survey of OIL.


13

OIL had a feather in its cap in 2000, whence it eff ort for acquiring seismic data connecti ng two wells located at north and south banks of mighty River Brahmaputra was recognized and the group was awarded a special menti on for creati vity and innovati on by NPMP.

This period has witnessed exciti ng developments in Geophysics throughout the world. OIL, in its own, way became a part of this development process and incorporated new techniques and technology in its exploratory and development ventures

Period Beyond 2000

This is an era of NELP and the paradigm of explorati on changed in the enti re country. OIL took acti ve part in this new era of liberalizati on. It not

only bagged blocks within the country but also in other countries like Libya and Gabon, where OIL is the operator.

The requirement of geophysics slowly changed with the increase of computi ng power. The hardware was upgraded in 2004. Additi onally new soft ware for processing and depth imaging – FOCUS and GEODEPTH were introduced in 2005. In the meanti me, other soft ware packages from M/s Hampson Russell were procured to have a full suite for fruitf ul reservoir characterizati on. The interpretati on system was augmented in 2000 with the introducti on of GeoFrame system.

In 2001 & 2002, three (3) new seismic data acquisiti on equipment were procured with an objecti ve to have higher channel counts and for deployment in logisti cally diffi cult and environmentally sensiti ve areas. The radio telemetry RSR system – the fi rst

of its kind in India was installed in 2001. This equipment was found to be extremely suitable for logisti cally diffi cult and environmentally sensiti ve areas like Brahmaputra River bed etc. The two other equipment were line telemetry systems – SN 388 of Sercel make were commissioned in 2001 and 2002 respecti vely.

In 2009, two (2) new equipment viz. SN 428 were again purchased. These equipment has high channel counts and now the in-house seismic crews are in a positi on to acquire seismic data with even high channel count. This will also enable acquisiti on of seismic data which may enable bett er illuminati on of sub-surface. The procurement of these systems brought in new technology in OIL. The multi -component data acquisiti on is now initi ated.

This decade also saw introducti on of reservoir characterizati on tools like inversion, pore pressure predicti on, spectral decompositi on. Att ribute analysis which are becoming a routi ne aff air now.

OIL hired the services of well reputed geophysical service providers to carry out large 3Ds in its normal areas only, 2D surveys in logisti cally diffi cult areas. The logisti cally diffi cult areas were imaged using modern imaging tools like CRS technique and an integrated interpretati on of gravity-magneti c, Magneto-telluric and seismic data provided some picture of the complex sub-surface of areas like Manabum. Incidentally, the enti re non-seismic geophysical programme (gravity-magneti c, Magneto-telluric) was acquired between the years 2000-2004 by NGRI.

NELP bidding rounds and internati onal bidding processes brought in new acreages. The logisti cs, environment etc varied from place to place. With

Seismic Data Acquisiti on in Libya

MC meeti ng in progress at OIL offi ce, Libya

14

full considerati on of the above, detailed geophysical surveys viz. 2D and 3D seismic, aero-magneti c were carried out.

So far as collaborati on with academic insti tuti ons is concerned, OIL has also left its mark in the fi eld of Geophysics. Through a collaborati ve study with Rice University, USA, a new technique, Unifi ed Imaging, is developed for imaging the sub-surface in geologically complex areas. This is a new fronti er in the quest for excellence in Geophysics.

Conclusion

A. PAST – Major landmark

(i) ‘Bordubi high’ was the result of Torsion Balance gravity survey which was fi rst in India.

(ii) Discovery of Nahakati ya fi eld was from geophysical contributi on and this discovery was fi rst in Independent India.

(iii) First digital seismic data acquisiti on system of 16-bit technology DFS-V was procured by OIL in India.

(iv) North Bank and South Bank across the mighty river Bhahmaputra was connected seismically with in-house eff ort for the fi rst ti me in OIL and for which OIL got a nati onal award (NPMP award for innovati on & creati vity).

B. PRESENT - Major resources & competence

(i) OIL has experienced and competent geophysical staff to carry out explorati on acti viti es in N-E India.

(ii) OIL has its own seismic data acquisiti on systems

of latest technology having Multi -Component Survey facility.

(iii) OIL has its own seismic data processing systems with latest soft ware.

(iv) OIL has its own seismic data interpretati on system with latest soft ware.

C. FUTURE – Synergeti c approach

(i) More emphasis to be on Reservoir Geophysics.

(ii) Att empt to be made to intensify Multi -component survey and Time- Lapse 3D survey parti cularly as a fi eld development process.

(iii) Pilot project on Passive Seismic to be taken up.

(iv) Processing Hardware System to be upgraded.

(v) OIL – Academia Collaborati on Projects to be taken up/renewed for explorati on success in thrust belt areas.

(vi) More emphasis to be on Seismic Strati graphy and Reservoir Characterisati on.

OIL needs changes in conventi onal geophysical approach to contribute more in old fi eld development as well as in explorati on process in the Fronti er area and in the new acreages. Enti re Geophysical fraternity of OIL to be committ ed to achieve its geoscienti fi c goal through excellence and contribute towards organizati onal growth by discovering new fi elds. OIL’s MANTRA is:

“BRIGHT PAST IS THE MIRROR FOR PROGRESSSIVE PRESENT AND BRIGHT PRESENT IS THE MIRROR

FOR PROGRESSIVE FUTURE.”

Multi componenet Seismic Data Acquisiti on


Introducti on

In the ancient ti mes, oil was mainly used for medicinal purposes and collected mainly from the surface seepages or spills. When the demand for oil increased, the traditi onal methods of fi nding oil proved to be ineffi cient. It was at this ti me, that mankind started depending on geophysics to trace out the locati on of oil beneath the earth’s surface. While some preliminary geophysical studies were made in the 19th century and even earlier, explorati on geophysics fi rst became important about 1922-23 with the mapping of salt domes in the United States and Mexico by torsion balance and seismic refracti on methods.

In the mankind’s search for natural resources geophysics has served as a potent tool. In the last century, various geophysical techniques came into existence and became the most important contributor for explorati on and exploitati on of diff erent natural resources. Geophysical methods

and seismic prospecti ng in parti cular have played an important role in unraveling the secrets of the subsurface in the quest for hydrocarbons. Seismic methods have provided unparalleled high resoluti on defi niti on of the subsurface on a regional scale compared to any other method.

With the passage of the century, there were development in physical & biological sciences, technology also evolved in leaps and bounds, which resulted in the development of the geophysical techniques. The professionals in the fi eld of geophysics imbibed all these and with innovati ons at every stage tried to provide soluti ons to diff erent problems regarding explorati on and exploitati on of diff erent natural resources. The role of geophysics in the upstream division of oil and gas industry has been expanding. Geophysics is now used not

Geophysics

only for explorati on as in early days but also for exploitati on, development, producti on monitoring and reservoir management.

In this arti cle, we delve into development of the discipline of geophysics, especially in the last century with special reference to seismic prospecti ng.

Explorati on Geophysics

Explorati on geophysics, in general, and seismic refl ecti on techniques, in parti cular has evolved in twenti eth century. This applied science always lived upto its reputati on of rising to every occasion and provided soluti ons to many of the problems and challenges faced by the petroleum industry from ti me to ti me. The geophysical industry shaped itself through innovati on and improvisati on. The development of geophysics has not taken place in isolati on – its innovati ve ability has merged with developments of science and technology in other fi elds as well. It is worthwhile menti oning that the industry has throughout imbibed the inventi ons and advancements of other disciplines, varying from physical to biological sciences, from communicati on theory to computers. The industry has always recognised the potenti al of these inventi ons and technological developments at the very early stage and almost immediately such developments came into knowledge. In this paper, we will elaborate how the industry throughout the checkered history has showed, on one hand, its resilience to innovate and on the other hand, displayed its capability to imbibe the best of other disciplines and incorporated them into its routi ne analysis and operati ons.

Development of Geophysics in petroleum industry

The evoluti on of geophysics dates back to the early part of the twenti eth century. Till date, geophysics, in general, and petroleum geophysics, in parti cular, is dominated by seismic but it was not the case in the early days of oil explorati on. The theoreti cal developments in seismic wave transmission dates back to as early as 1907 when Wiechert

D.S.Manral & R.Dasgupta

in Quest of

- A Historical PerspectiveHydrocarbonResources

Geophysical methods and seismic prospecting in particular have played an important role in unraveling the secrets of the subsurface in the quest for hydrocarbons.

15

and Zoeppritz worked out the theory for seismic wave transmission and gave theoreti cal soluti ons to the problem of seismic wave propagati on viz. refracti on and refl ecti on. However, these path breaking theoreti cal soluti ons were then far ahead of their experimental counterpart and it took many years since then to be practi ced for search of hydrocarbons.

Meanwhile, in Hungary, during 1915, Lorand van Eotvos pioneered gravity survey with torsion balance. During 1915 and 1916 the Torsion balance was employed for the fi rst ti me to detail the structure of what was then a one well oilfi eld at Egbell, now in Czechoslovakia. A few years later, seismic refl ecti on survey came into existence and then

onwards it slowly became the prime techniques for prospecti ng of oil and natural gas.

The fundamental work in the development of explorati on seismology and seismic prospecti ng techniques and instruments was pioneered by the works of Reginald Fessenden, Dr. Ludger Mintrop, Everett e Lee DeGolyer, and J. Clarence Karcher. Refl ecti on seismic prospecti ng stemmed principally from the pioneering works of Reginald Fessenden about 1913, a work which was directed towards measuring water depths and detecti ng icebergs using sound waves. Although the abstract idea of refl ecti ons of sound energy from underground formati ons preceded a few seasons to J.C.Karcher’s work, however, it was not unti l J.C.Karcher devoted himself to the challenge that the idea was implemented with equipment and procedures. The pioneering eff ort rapidly transformed an idea into the most important technique available to geophysical prospecti ng ti ll date.

The earliest experiments with seismic refl ecti on technique were carried out by J.C.Karcher from 1919 to 1921 wherein the potenti al of seismic refl ecti on methods in mapping the shallow subsurface horizons of interest in hydrocarbon explorati on were demonstrated in central Oklahoma.

This experiment was a signifi cant revoluti on in the history of Geophysics in Oil & Gas explorati on.

Further, in 1924, the discovery of an oil fi eld beneath the Nash salt dome in Brazoria County, Texas, was the fi rst to be based on single-fold seismic data. However it was not unti l 1927, that seismic refl ecti on method was put to work for routi ne explorati on in search of hydrocarbon resources.

The exigencies of world war II led to the adaptati on of many geophysical methods/techniques for their use in detecti ng mines, submarines and enemy fi re positi ons. This encouraged basic research, which led to tremendous advances in electronics, signal processing concepts, and computer methods. Consequently, post-world war era witnessed rapid industrialisati on on one hand, and advances in diff erent fi elds of science and technology on the other. These resulted in quantum leap in demand of fossil fuels which necessitated increase in the

J.C.Karcher at the 1971 ceremony dedicati ng the monument which commemorated the inventi on of refl ecti on seismology (Courtsey, Geophysics, November 1987)

Lorand van Eotovos

16

Photostat of record obtained by J.C.Karcher in Oklahoma’s Arbuck-le Mountains in 1921 (Courtsey: Geophysics, November,1987)

The GloriousExpedition ...

explorati on & exploitati on of fossil fuels. This scenario enabled seismic refl ecti on technique to fl ourish and acted as a catalyst to the development of the seismic refl ecti on technique.

During the 1940s improvements in refl ecti on instruments resulted in the development of more sensiti ve & sturdier geophones and recording equipments which had greater uniformity, and were smaller and lighter in weight. This enabled having more accurate and portable equipment, which were bett er adapted to extremes of temperature, humidity and atmospheric pressure. Improved refl ecti on fi eld techniques including multi ple patt erns of both shot holes and geophones were also introduced in an eff ort to improve directi vity and increase the refl ecti on amplitude relati ve to noise. Another notable development in refl ecti on fi eld technique introduced during 1940 has been off shore shooti ng.

During the 1950s, developments in seismic refl ecti on techniques were initi ated with the primary objecti ve of reducing the noise that interferes with the refl ecti on events. A number of innovati ons were introduced to achieve greater safety, economy and fl exibility in fi eld operati ons as well. During the 1950s the vibroseis technique was also introduced. Another advancement during the mid-1950s was the recording of seismic signals in variably magneti zed tracks along the length of a magneti c tape. The adopti on of magneti c media for recording seismic data enabled frequency analysis for opti mizati on of fi lter setti ngs among others. It was a signifi cant development at the ti me as the structures could now be seen directly from the seismic wiggles. Moreover, changing from paper to tape records paved the way to machine processing, development of the analog processor, and a total change in the way seismic data were collected and processed.

The 60s witnessed a rapid progress in many fronts. The advent of digital recording heralded a new era in seismic data acquisiti on. Mass storage media like magneti c tapes started getti ng used extensively in seismic data acquisiti on. The industry shift ed from analogue to digital form of recording. M/s Geophysical Services Inc. introduced the fi rst

digital fi eld system and computer for seismic data processing in the year 1961, in a joint eff ort with Texas instruments and several other oil companies. On the other hand, digital signal theory which was used in electronics and radio communicati ons were imbibed in geophysics. This increase in computi ng power during the decade led to the development of digital seismic signal processing technique which heralded a new era in seismic data processing. This created a revoluti onary change in seismic industry – it started the use of digital fi lters of diff erent kinds. One of the most signifi cant developments of the decade was the introducti on of Common Depth Point (CDP) recording technique invented by William Harry Mayne of M/s Pett y Geophysical Company and is the heart of seismic data acquisiti on even today. New concepts and development in acquisiti on methodology and acquisiti on techniques started developing at pace during this period.

Though the concepts of 3D seismic surveys existed since the early days of geophysics, the ability to implement the concepts were restricted by the effi ciency & accuracy of data acquisiti on and the cost and computi ng power necessary to condense, process, display and interpret data. Despite all these, the fi rst 3D seismic survey was shot by Exxon over the Friendswood fi eld near Houston as early as 1967. The era also witnessed development of theoreti cal concepts for analysis of seismic datasets for direct detecti on of hydrocarbons resources which was pioneered and demonstrated by Carl H. Savit in the 60s.

The 70s was an era of consolidati on and upgradati on of existi ng methods and technology. This decade witnessed boom in oil prices. This led to increase in seismic acti vity through out the world. In seismic data acquisiti on, there was gradual progress in the amplifi cati on part of the recording system IFP amplifi ers came into existence in this decade. The channels per shot increased signifi cantly. Apart from that, sophisti cated marine vessels with long streamers became a typical part of marine seismic. With the progress in computi ng power and digital technology, by the early 1970s seismic data processing became a new branch of seismic prospecti ng. In this area, new techniques like diff erent types of deconvoluti on, velocity analysis, processors for single and multi channel processing, residual stati cs computati on, noise reducti on etc. were developed for obtaining bett er imaging & prospect identi fi cati on. Apart from this, computer intensive data processing tools like migrati on

17

William Harry Mayne

became well recognized in the industry. Another signifi cant development during the decade was the identi fi cati on of bright spot phenomenon on seismic datasets for direct indicati on of gas saturated reservoirs. By mid 1970s seismic att ributes became an important part of data analyis and interpretati on. Apart from this inversion of poststack seismic amplitudes into acousti c impedance was another important development to aid in understanding the subsurface in terms of lateral changes in lithology & porosity.

The 80s saw signifi cant development in seismic techniques. During the mid-80s, 3D seismic changed the course of seismic data acquisiti on forever. 3D seismic enabled the industry to acquire high resoluti on, high fi delity seismic data, which gave a new dimension to sub-surface imaging through cost eff ecti ve means. Petroleum industry reaped the benefi t of 3D in several ways. In order to have larger channel capacity for catering to the demands of 3D seismic, lighter telemetry acquisiti on systems came into existence. Transiti on zone surveys could be taken up in right earnest with the introducti on of radio-telemetry data acquisiti on systems. The impact of 3D was such that slowly it almost replaced conventi onal 2D seismic survey throughout the world.

The decade also saw a breakthrough in identi fi cati on of hydrocarbon reservoirs by means of att ribute analysis of seismic waves. AVO analysis, alongwith new processes in seismic inversion, opened up a new vista in seismic explorati on. Seismic techniques were mainly tools for explorati on so far. But, with the introducti on of 3D seismic, alongwith att ribute related analysis (AVO, Inversion, etc.), seismic techniques slowly developed into a tool for fi eld development as well. Meanwhile, computi ng power increased exponenti ally and workstati ons with improved graphics package became available off the shelf. The advent of 3D brought about signifi cant increase in data volume. Hence, interpretati on moved from paper secti ons to workstati ons. So far as seismic data processing is concerned, DMO became an essenti al part of routi ne processing and pre-stack migrati on of data became aff ordable with the available computi ng power. This decade also witnessed yet another revoluti on in the fi eld of surveying and navigati on with the introducti on of Global Positi oning System (GPS).

During the 90s, the industry expectati on from 3D seismic, coupled with att ribute studies, towards development of oil and gas fi elds soared. Geophysics

came to be recognised also as a tool for reservoir development & monitoring and a new fi eld of geophysics, namely reservoir geophysics, came into being wherein eff orts were largely focussed on aspects of fi eld development. Existi ng IFP based data acquisiti on systems seemed inadequate to meet such requirements. Thus, IFP based systems made way for data acquisiti on systems based on delta-sigma technology, which could bett er preserve the seismic amplitudes. Keeping pace with the changing paradigm, survey technology & practi ces also evolved simultaneously. Navigati onal and positi oning aids saw signifi cant improvement in precision. The modern version of DGPS provide sub-centi meter accuracy in positi oning. In the process, newer concepts were conti nuously embraced leading to improved practi ces besides an overall enhancement of on-line quality control & effi cacy.

Seismic data acquisiti on became characterised by large channel counts – land 3D seismic surveys with higher acti ve channels counts for acquiring data with bett er geophysical att ributes became a standard in the industry. Marine seismic surveys, comprising of large ships with multi ple streamers and multi ple sources became the order of the day. Combinati on telemetry systems (line and radio) were increasingly deployed in logisti cally diffi cult areas. Apart from this as a corollary to and a natural out growth of 3D seismic survey, the decade saw introducti on of 4D seismic or ti me-lapse 3D seismic. 4D seismic (repeated 3D seismic surveys) during the life span of an oil fi eld enabled effi cient management of hydrocarbons. In this decade, multi -component seismic (3C), seen as a major tool for oil fi eld development and reservoir management, was introduced. Computer graphics based applicati on also became more widespread and interacti ve. Throughout the 90s, the industry witnessed introducti on of modern interacti ve and visualisati on systems in all fi elds of seismic prospecti ng.

With passage of ti me increased computi ng power allowed pre-stack depth migrati on and velocity model building of 3D seismic data for bett er imaging. During this decade, tools from biological sciences were introduced. The techniques like neural network, geneti c algorithm were used routi nely. Another important achievement in the decade was integrati on of geological, seismic, well log, well data and petrophysical data to provide a complete image of reservoirs.

The present decade witnessed integrati on of

18

The GloriousExpedition ...

diff erent oil fi eld with seismic data. Unlike the past wherein geophysicist were focused primarily on identi fi cati on of new explorati on targets, technological advances in various facets of seismic opened up exciti ng opportuniti es and lead to signifi cant value additi on to the development and producti on stage of the oilfi eld cycle. During this era the transiti ons from ti me to depth imaging & att ribute processing, from line-based to 3D volume-based interpretati on, att ribute analyis & volume visualizati on among several others played key role in bett er understanding and characterizati on of the subsurface.

Road ForwardDuring the 21st century the industry is expected to play more decisive role in reservoir monitoring and management. This will require ti me-lapse measurements, whether they are in the form of 3C-3D or 4C-4D seismic or in the form of conti nuos measurements of physical parameters within the reservoir. Thus the industry is expected to witness an increase in 3C and 4D surveys. On the other hand, smart oilfi elds will merge with intelligent downhole measurements. The permanent downhole sensors in the intelligent wells will help in proacti ve management of the reservoirs. These intelligent measurements will involve improvements in geophysical measurements both in surface and down hole. The recently introduced digital geophones will be extensively used to obtain high fi delity data. Seismic att ribute analysis will play a key role in the near future to bett er understand and characterizing the reservoir properti es. Additi onally, passive seismic is expected to play a major role in explorati on and reservoir monitoring.

The major area, which will need a larger investment in research, is the delineati on of thin-beds i.e. the beds whose thickness is less than one-eighth of the wavelength. The conventi onal seismic measurements will need more precision in this regard. Additi onally, this might need technological improvements in acquisiti on and analysis of cross-well seismic. Apart from this, research will be directed mainly towards reservoir property mapping in the near future. The signifi cant areas of interest in this regard are expected to be anisotropy and att enuati on (Quality factor).

In ulti mate analysis, the emphasis of the industry in the near future will be on integrati on of various disciplines. Geophysicists will play the nodal role in integrati ng geophysical data with well and other

geoscienti fi c informati on. Integrati on of seismic and non-seismic measurements to map, image and characterize the subsurface with higher fi delity, feed the reservoir simulati on model, drilling & reservoir producti on management models through fi t for purpose technology.

The technological focus would be on acquiring and extracti ng every bit of informati on from the data for bett er defi niti on of the subsurface both at the reservoir level and across the overburden. These would aid in developing the reserve replacement opportuniti es, identi fi cati on of by-passed reserves in existi ng oil fi elds & eff ecti ve recovery of the remaining reserves. These requirements would conti nue to place demands on the seismic data quality, processing turnaround, integrated interpretati on workfl ows, dynamic visualizati on, and more then ever, the people from various discilplines. Quanti tati ve seismic interpretati on on full wave fi eld data along with seismic att ribute studies, geostati sti cal tools/technicques, 3D geo-cellular modeling, high performance cluster computati on, fracture modeling for enhancing explorati on & producti on in fractured reservoirs, S-wave birefringence & tomographic studies among others would play key role in future.

ConclusionIn this arti cle the developments of explorati on geophysics with respect to petroleum industry have been presented. In its course of development and in future, the discipline has and is expected to take innovati ve approaches to serve the requirements of the petroleum industry. Geophysical industry shaped itself in accordance with the requirement of ti me. It is hereby concluded that the key to the progress of geophysics is its ability to adopt the developments of other disciplines, break new fronti ers using such developments and thereby, adapt to the changing scenario. The eff ecti veness of geophysical methods has been greatly increased by improvements in instrumentati on, operati onal procedures, new processing algorithm and interpretati on. Extensive research and development, supported mainly by the oil industry, has greatly strengthened the fundamental scienti fi c base on which rests the future of geophysics as a potent tool in hydrocarbon explorati on.

ReferencesJ. C. Karcher, 1987: The refl ecti on seismograph: its inventi on and use in the discovery of oil and gas, Geophysics.

19

20

Geology and Hydrocarbon potential of sedimentary sequence in North Bank of River Brahmaputra of Upper Assam Basin

Deb S.S, Barua I, Baruah P.N

Abstract

The sedimentary sequence of the northern part of Upper Assam Basin was deposited on a Pre-Cambrian Basement during Cretaceous and early Cenozoic to Recent ti me. It forms the northern part of the petroliferous Upper Assam Basin. Structurally and strati graphically it resembles the southern part of this petroliferous basin. However, ti ll date, no commercial success has been established even though indicati ons of presence of hydrocarbon have been observed in a few wells. A fair amount of subsurface geoscienti fi c informati on has been observed from seismic survey and drilled wells, which will help determine the future course of

explorati on. More than 5000m of thick sediments had been deposited in shallow marine – deltaic to fl uvial conditi on through ti me having Type II & III organic matt er. Rock sample

analysis results also suggest the source rocks to be predominantly gas prone and generally immature in the drilled wells.

Introducti on

The elongated basinal part from the foothills of the

Himalaya to the northern bank of the Brahmaputra river covers an area of over 5500 Sq.Km and is located in the northeast part of India along the Assam – Arunachal border. (fi g-1).

The mighty Brahmaputra river topographically divides the Upper Assam Basin into two parts. The southern part of the Basin has been established as a prolifi c petroliferous basin and as a result this part is highly explored. On the other hand the northern part of basin is less explored and has not yet been established as a potenti al area from the hydrocarbon point of view. A total of around 6200 GLK seismic data of diff erent foldage have been acquired by both ONGCL and OIL since 1965 which has led to the identi fi cati on of 9 structures on the basis of seismic data interpretati on. A total of fi ve wells have since been drilled in fi ve diff erent structures of this area.

Though no major hydrocarbon fi nds has been encountered ti ll date, recent discovery of hydrocarbon in Baghjan, Barekuri and Mechaki structures in the eastern fl ank of the Basement High in the south bank area which have similar strati graphic framework as the north bank area has increased the prospecti vity of the area. The paper presents signifi cant informati on on this relati vely unknown part of the Upper Assam Basin.

Geologic Setti ng and History

The Upper Assam Basin is a part of the Assam-Arakan geological province. It is bounded in the north by the Eastern Himalayas, in the east by the Misimi Massif, in the south by the Naga-Patkai Hills and in the west by the Mikir Hills and Shillong Plateau. The thickness of sedimentary fi ll varies from 3.5 km in central part to more than 7 Km in peripheral part. A depth contour map on top of Basement presented on fi g.-2, shows the tectonic framework of the study area.

A fair amount of subsurface geoscientifi c information has been observed from seismic survey and drilled wells, which will help determine the future course of exploration.

Fig-1: The study area showing identi fi ed and drilled structures

Through thebigger canvass...

21

The Sylhet Limestone Formati on lying on Pre-Cambrian graniti c/gnessic Basement consists of sandstone, shale and fossiliferous limestone bands inferred to be inner shelf to nearshore deposits.

The overlying Kopili Formati on consists mainly of splintery and carbonaceous shale with calcareous sandstone seems to be deposited in inner shelf to near shore deltaic setti ng.

The Barail Formati on overlying the Kopili Formati on, comprising of sandstone with shale bands with minor mudstone and coal is developed in a limited area only.

The Neogenes are represented by a clasti c sequence mainly made up of sandstone beds deposited by braided channels with palaeocurrent directi on mainly towards northwest.

Structural Elements

The available seismic data reveals a strong refl ector

Fig-2: Depth contour map on a refl ector close to Basement

Strati graphy and Depositi onal Environment

The generalized strati graphic succession of the Basin established ti ll date in the North Bank areas based on drilled well evidence is presented below.

Age Group Formati on LithologyThickness (Meters)

Recent to Pleistocene

Alluvium and high level terraces

Unconsolidated to semi consolidated coarse grained sandstone with minor siltstone bands.

800-1100

Plio-Pleistocene Dihing Dhekiajuli BedsCoarse to medium grained sandstone with siltstone bands.

750-1350

Mio-Pliocene Dupiti la Namsang BedsCoarse to fi ne grained sandstone with interbeds of clay and occasional carbonaceous matt er.

400-1000

Miocene Tipam

Girujan Clay Mott led clay with sandstone 0-20

Tipam sandstone

Medium to fi ne grained sandstone with interbeds of silty clay, shale and occasional presence of coal in the lower part.

900-1100

Oligocene BarailBarail (Argillaceous & Arenaceous)

Bluish grey mudstone , alternati on of shale and coal bands with subordinate sandstone underlain by mainly sandstone with minor shale bands.

0-350

Eocene-Upper Palaeocene

Jainti a

Kopili Alternati onSplintery shale, siltstone, claystone, carbonaceous shale with fi ne grained calcareous sandstone.

200-300

Sylhet LimestoneThin Fossiliferous limestone beds with fi ne grained sandstone , shale, clay stone and siltstone

100-300

Pre Cambrian BasementPinkish and grayish granite/granite gneiss.

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

22

corresponding to the top of Sylhet Formati on level present in the enti re area. A 2D seismic line shot across the river Brahmaputra linking the north and south banks of the river is presented as Fig-3, depicts the formati ons to be monoclinally dipping to the north-west. The faulti ng in the area appears to be associated with the Himalayan Orogeny and hence much younger in origin. The general trend of the identi fi ed faults is E-W to ENE-WSW with dip towards N to N-W. Another set of faults are seen to be dipping towards the S-E and make up the horst and graben feature of the area (fi g 2). The fault trends have conformity with the fault trend/tectonic alignment observed in the southern part of the basin.

Fig-3: Seismic secti on (ti me) across river Brahmaputra

Geological Informati on from Drilled Wells

Based on the drilling evidence and seismic data, isopach maps were prepared for the Sylhet Limestone Formati on, Kopili Formati on, Barail and post Barail Formati on .The thickness in these formati ons are 100-300m for the Sylhet, 200-300m for Kopili and 0-350m for the Barail Formati on. A seismogeological secti on connecti ng the two parts of basin across the river depicti ng the strata connecti vity between the two parts of the basin (fi g-4).

Fig-4: Seismo-geological secti on across the river Brahmaputra

The combined thickness of the Palaeogene sequence (Sylhet + Kopili + Barail) is around 300-900m and the rest comprises of the Neogene sequence (Tipams + Namsangs) and younger sediments. It has been observed that the thickness of Palaeogene sequence increases towards south-east while the Neogene and younger sequences show an opposite trend (fi g-4,5 & 6).

Figure-5: Isopach map of Neogene+Younger Sequence

Figure-6: Isopach map of Palaeogene Sequence

The results of rock eval pyrolysis analyses of selected shale samples recovered from drilled wells ranging from Dhekiajuli to Sylhet Formati on indicate that the Kopili and Sylhet Formati ons have fair to good source rock potenti al. The cross plot of HI and T-max values indicate that this area contain Type II and Type III organic matt er (fi g 7). The study indicates that the organic matt er in these formati ons have been derived from both marine and terrestrial source. The analyses results also suggest that the source rocks are predominantly gas prone and generally immature in the drilled wells.

Through thebigger canvass...

23

Temperature recorded in all fi ve drilled wells show a temperature gradient ranging from 1.44 - 1.51 degC/100m as against the temperature gradient of 2.46 - 2.55 degC/100m in the South Bank. The formati on pressure in Sylhet Formati on reservoirs is hydrostati c in western part of the study area whereas in the deeper basinal side (towards eastern side) it is 600 psi above hydrostati c which is similar to the South Bank Eocene wells.

Figure-7: Type of organic matt er in Kopili Formati on

Conclusion

The study area can be broadly divided into two sub areas based on lithology & tectonic considerati on viz; Area I: located to the west of Dhakuakhana area with dominantly sandy sequence characterized by horst-graben and step faulti ng type of tectonics; Area II: located east of Dhakuakhana area with alterati on of sand shale sequence characterized by horst and graben type of tectonics

The maximum Terti ary sequence encountered in the

study area is around 5000 m of which 3800-4200 m comprises the Neogene and younger sediments dominated mainly by sandstones deposited under fl uvial conditi ons. The lower part of the Terti ary sequence (300-900m) comprise dominantly of shales, splintery shales, thin limestone bands, siltstone, calcareous sandstones etc. deposited under shallow marine environment.

The thickness of Neogene + younger sediments increases towards northwest while the Paleogene sequence shows an opposite trend. The Lakadong + Therria member of Sylhet Formati on which acts as the main hydrocarbon bearing reservoir in the South Bank does not appear to have developed in the western part of study area(Area I). However, there is a thin to fair development of the same in the eastern part of the area (Area II).

The Girujan Clay formati on which acts as regional cap rock in the South Bank area (except in northeastern part) of the Basin shows a sporadic appearance in the North Bank. Barail Formati on which is considered as the main reservoir as well as source rock for majority of oil/gas fi eld discovered in the southern part of the Basin is found to be absent in the western part, but is found to be developed in the eastern part of the study area.

Source rock studies suggest that Kopili and Sylhet Formati ons have fair to good source potenti al. Studies also indicate that the source rocks are predominantly gas prone and generally immature in the drilled wells. Low temperature gradient in the North Bank area could be the contributi ng factor for source rock immaturity in the structurally higher parts of the area. Therefore, mature source rock could be available in the deeper part of the basin (eastern part).

Recent discovery of hydrocarbon in Baghjan, Barekuri and Mechaki structures in the eastern part of the South Bank area which have similar strati graphic framework as North Bank area has increase the prospecti vity of the area.

24

Reservoir Architecture in central part of Nahorkatiya Field – A Prolific Producer Of Its Time

M.K. Sharma

Abstract

The Barail Group of rocks of Oligocene age represents a transiti onal phase of evoluti on of the Assam-Arakan Basin, located in Northeast India, during which depositi on of sediments took place in a fl uvio-deltaic environment. The Barail Group conformably overlies Eocene sediments, deposited under shallow/open marine environments and is in turn overlain un-conformably by the Tipam Group fl uvial sediments of Miocene Age. The Barail Group comprises some of the prolifi c hydrocarbon reservoirs of this Basin. A major oilfi eld with primary reservoirs

in the Barail Group is the Nahorkati ya Field. Discovered in 1953, by Oil India Limited (OIL), the fi eld has been producing prolifi cally over the years. However in order to assess the remaining potenti al

of this mature fi eld a multi disciplinary review was undertaken which commenced with studies for bett er understanding of the reservoir architecture. The Nahorkati ya Field is compartmentalized into a number of discrete blocks by normal faults. This study refers to a new approach adopted for deciphering the reservoir architecture of the Barail 13+95 Block in the central part of the Nahorkati ya Field. Available data was studied for detailed delineati on of the depositi onal setti ng on the reservoir scale. The study helped in identi fi cati on and mapping of correlatable horizons with the available wireline logs which helped in building of a robust correlati on scheme. The correlati on scheme made it possible to decipher the reservoir architecture which has in turn proved to be a major input in understanding the geologic control on hydrocarbon distributi on in the reservoirs. With minimal wireline log data available on the porosity and permeability of this reservoir, the geological approach has aided in the delineati on

of fl ow units which is envisaged to help in improving the oil recovery sti ll further by draining the unswept oil and to improve overall sweep effi ciency in the area by implementi ng additi onal and/or enhancing pressure maintenance and drilling infi ll locati ons.

Introducti on

The Nahorkati ya Structure, in the petroliferous Assam-Arakan basin, is located in northeastern corner of India (fi gure-1). The structure is a faulted anti cline, 36 sq. km in size at the Oligocene level, with the major axis of the structure trending in a NE-SW directi on. Nahorkati ya Field has produced a signifi cant percentage of the original hydrocarbons-in-place. Presently, remaining potenti al in this mature fi eld is being realized by current producers, new completi ons and by implementati on of pressure maintenance schemes.

The primary hydrocarbon reservoirs in the

Nahorkati ya area are located in the Barail Group which was deposited in the Oligocene period subsequent to the fi rst phase of Himalayan Orogeny (HOM-1). The Barail Group was later eroded during HOM-2 (end Oligocene) which resulted in the truncated secti on and major unconformity seen today.

The Barail Group consists of multi stacked reservoirs which are in turn compartmentalized into discrete blocks by normal faults. One such major block in the

to assess the remaining potential of this mature fi eld a multi disciplinary review was undertaken which commenced with studies for better understanding of the reservoir architecture.

Figure-1: Locati on Map

Unconventional Approach- Unconventional Solution

25

central part of the Nahorkati ya structure is the NHK 13+95 Block. This block is one of the most prolifi c producers from the Nahorkati ya Structure and has yielded a major share of the producti on from the Nahorkati ya Field.

The Barail Group in Nahorkati ya area comprises of multi -stacked sand channels. This has resulted in correlati ng the sand bodies encountered in diff erent wells a highly challenging job. An inappropriate sandstone-to-sandstone correlati on could lead to an incorrect correlati on of reservoir units which could have direct bearing on reservoir connecti vity and consequently on eff ecti ve reservoir management. Furthermore, in spite of a large well density, a major geological study of the area is hampered by the absence of a complete dataset

It may also be menti oned that Nahorkati ya being one of the oldest oilfi eld in the Assam Arakan Basin, the available wireline logs are of wide ranging vintages. The oldest wireline log recorded in the Block was of 1955 and the most recent one is of 2006. The main drilling acti vity in the Block took place from 1955 ti ll 1993 following which there was a drilling pause of about 13 years from 1993 ti ll 2006. Most of the wells were without complete suite of porosity logs. Therefore, the study of the area had to be conducted by using a limited suite of logs which made the process considerably diffi cult. During the course of correlati on relati vely newer generati on wireline data, wherever available, were taken into considerati on for refi nement of the correlati on. Furthermore, detailed review of the geology of the conventi onal cores was hampered largely by the vintage of the cores.

A study conducted recently to identi fy the unswept oil from this Block required building of a fi ne scale geological model of the reservoir. Therefore a geological approach was adopted to decipher the reservoir architecture of Nahorkati ya fi eld and the Block in parti cular, based on the limited data available. This paper presents the fi nding of the study which is envisaged to play a major role in deciphering the reservoir architecture of similar fi elds within OIL’s operati onal areas in the Assam Arakan Basin.

Depositi onal Setti ng

The fi rst step in building the geological model consisted of understanding the depositi onal

setti ng of the area. This was necessary in order to construct an appropriate framework for correlati ng and extrapolati ng sandstone reservoirs in the area of study. Therefore a detailed study of the overall paleo-depositi onal environment of the Barail rocks was conducted for the enti re Nahorkati ya Field based on the available conventi onal cores.

The sandstones seen in conventi onal cores are products of high and low energy depositi on. The high-energy sandstones are typically fi ne to medium grained and occasionally coarse grained. They are unbioturbated with sub-angular to rounded red mudstone rip-up clasts that are up to pebble size in disti nct lags. The sandstones are occasionally carbonaceous with internal bedding surfaces picked out by carbonaceous material. Carbonaceous materials are also present as discrete grains.

These sandstones are oft en massive and featureless, oft en cross-bedded on a centi metre to decametre scale, with tabular cross bedding predominant. There is common evidence of ‘sand on sand’ erosion in core.

The lower energy sandstones are typically thin (few centi metre to decametres) fi ne grained, are unbioturbated, have more aggradati onal bedfroms (e.g. climbing ripples) and have laminar fl ow bedforms on a millimeter to centi meter scale. Carbonaceous material in these beds tends to be found only as disti nct laminae. The available biostrati graphic informati on indicates these sandstones to be barren.

The mudstones and shales observed in the cores have numerous immature palaeosols consisti ng mostly of light grey to green waxy mudstones, rootlets and early diageneti c mineralisati oin. In additi on, there are abundant reddened horizons, which are not controlled by bedding. Some of the reddened horizons are ti ghtly cemented, siliceous mature caliche horizons of upto approximately 30 cm thick. Sub-angular clasts of these facies are a common component of the sandstone deposits.

Highly sulphurous coal beds are also present in the core occurring within sandstone beds, with thicknesses of upto 20 cm, but more commonly within the argillaceous deposits where they are upto 50 cm thick.

The evidence from the cores indicates that the observed sandstone reservoirs consist of a series of

26

small fl uvial channels usually amalgamated where seen in core and logs (multi -storey and multi -lateral). These sandstones are associated with coastal plain overbank sediments with some minor evidence of marine infl uence. Earlier sandstone depositi on is in relati vely wide, linear channel belts passing up into a system of smaller, meandering channels with occasional crevasse splays. The common evidence of ‘sand on sand’ erosion in the core makes deducti ons about original channel thickness diffi cult. The lower energy, thinner sands are interpreted as overbank sand depositi on, probably crevasses plays, given the interpretati on of the thicker sandstones.

Corroborati on with seismic and well evidence suggests the above depositi on occurred within well-defi ned channel belts of up-to a few kilometers width. Corroborati on with log data shows that the degree of channel amalgamati on decreases up the strati graphic secti on, passing from what is essenti ally an amalgamated sheet sand, formed by coalescing of numerous channels braided together into more isolated sand bodies. This coalescence has resulted in fl uid communicati on between the individual lobes leading to a virtual layer cake geology. In places where these sand bodies are more isolated, parti cularly in the upper part of the Barail sequence, the SP log response is that of a fi ning up sequence. The ‘fi ning-upwards’ response of the sandstones represents waning current fl ow and eventual abandonment of the channel.

Abundant reddened secti ons occurring both in overbank and channel facies indicate oxidati on, therefore exposure. The ubiquitous red shale clasts found in channel sandstones are clear evidence of reworking of the exposed interfl uves. These clasts commonly occur in channel lag deposits; they would have become entrained due to channel bank erosion. Their shape and internal structure indicates that they were semi-lithifi ed at the ti me of depositi on, and have been derived locally. This, together with the extensive reddening seen in overbank deposits, shows that there were signifi cant periods of non-depositi on/exposure in the locality.

Soil generati on in a overbank setti ng requires a signifi cant hiatus in depositi on. Laterally persistent

palaeosols may have formed during falling base level, at ti mes when the rate of relati ve sea level fall exceeded the rate of basin subsidence. River incision accompanying relati ve sea level fall would have resulted in sediment bypass of interfl uves, allowing extensive pedogenesis to take place. Coal and peat accumulati on may have occurred in response to a rising relati ve sea level, caused by a rise in the water table in the coastal plain. The extent and thickness of these deposits will have been infl uenced by variati ons in topography.

The development of barren channel sandstones together with brackish water mudstones/siltstones, immature to mature palaeosols and immature brown coals in an aggrading tabular system is typical of an upper coastal plain environment.

Reservoir Correlati on

Following completi on of a detailed review of the sedimentology of the sand bodies a broad

geological picture of reservoir emerged. Using the understanding of the depositi onal setti ng, an exercise was conducted to build a robust correlati on scheme using geological responses of the wireline logs which was then used to determine the reservoir strati graphy. The latest geological concepts were incorporated along with the seismic interpretati on of the area.

In the correlati on scheme, due cognizance has been accorded to depositi onal patt ern of fl uvial bodies, depositi onal dips of fl uvial regimes as well as to the traditi onal nomenclature scheme of the area. Sand geometry and depositi onal environment are having a mutual relati onship which has been used extensively for well to well correlati on. Furthermore, due to the mechanism of formati on, the observed palaeosols and coal deposits within the sequence are relati vely conti nuous in comparison to the more ephemeral sandstone deposits. Consequently, as

Rootlets penetrati ng leached overbank shale

Channel sandstone with rounded pebbles of reddened overbank shale


27

they have been recognized in wireline logs, they have off ered a way of dividing the sedimentary sequence into a number of pene-contemporaneous units.

The Barail Group is characterized by a number of log signatures within the shale secti ons that can be correlated over large areas of the fi elds. These features are usually sharp troughs in the AM64 inch resisti vity log and usually on density curves and slow events of sonic. The AM64 log response seen is usually that expected for a high resisti vity bed below the tool resoluti on. The response is therefore not a true response but a tool arti fact, but a useful means of identi fying thin, highly resisti ve beds (Schlumberger 1972). These log signatures are related to the immature soils and thin coal beds recognized in the cores. Majority of the wells have either one or more of the curves suitable for the recogniti on of these pedofi ed horizons. The methodology adopted in the correlati on procedure therefore was to correlate all the wells with these curves fi rst and the remaining wells are fi tt ed into the correlati on (fi g.-2). This parti cular approach was uti lized successfully in identi fying isochronous correlatable markers to diff erenti ate the deltaic/coastal plain sediments of North Sea. Based on the same, the sandstone reservoirs have been fi tt ed into isochronous strati graphic frame work defi ned by the pedofi ed horizon markers.

Conclusions

The primary goal of the correlati on was to obtain a robust descripti on of the hydrodynamic fl ow units within the sand zones. In order for the sands to be defi ned as a single hydrodynamic fl ow unit the assumpti on was that the sandstones of neighbouring wells had to have some strati graphic overlap constrained within the correlatable pedofi ed horizons.

In the area under study seventeen pedofi ed horizons have been identi fi ed and correlated across the fi eld based on this approach.

The sandstones in upper part of the Barail Group are diachronous in nature and thus play a dominant role in reservoir connecti vity.

In the Block under study the lowermost reservoir termed as Barail Fift h Sand is an amalgamated sheet sand body formed by coalescing of numerous channels braided together and discerning of individual lobes is not possible. This coalescence has resulted in fl uid communicati on between the individual lobes leading to a virtual layer cake reservoir. In places within the Block the individual channels of the overlying reservoir termed as Barail Fourth Sand has incised into the Barail Fift h Sand establishing reasonable to parti al communicati on between the sand bodies. The thickness of the shale layer, in areas where it separates the Barail Fourth and Fift h Sands, varies across the fault block.

The Barail Fourth Sand reservoir has been identi fi ed to be a combinati on of two diachronous channels in the area under study (Channel-A and Channel-B).Of these, the lower channel (Channel-A) of Barail Fourth Sand is found to be confi ned to the northern part of the Block. The upper channel in Barail Fourth Sand (Channel-B) is more extensively developed compared to the lower channel. The development and orientati on of individual channels are seen from their isopach maps presented in Figure-3.

On the contrary the overlying reservoirs in the upper

Fig.-2: Correlati on between some of the wells in the area under study

28

part of the Barail Group in the Block under study have been identi fi ed to be isochronous in nature. The isopachs of one of the isochronous reservoir (comprises of Channels-1, 2 and 3) is shown in fi g.-4. The isopach of the channels in the fi gure unambiguously depicts the isochronous nature of the sand channels.

The electro-log evaluati on, core lithology and correlati on in the light of the total facies variati on of the Barail litho-strati graphic column has aided in synthesizing the working model. The need for such a conceptual model stems from the fact that the realm of clasti c sedimentati on is characterized by diff erent depositi onal environments, sedimentary process, sequence and patt erns (Le Blanc, 1972).


29

Reservoir Geophysics- Future in the Making

Y.R. Singh

Introducti on

The concept of petroleum reservoir geophysics is relati vely new. In the past, the role of geophysics was largely confi ned to explorati on and, to a lesser degree, the development of discoveries. As cost-effi ciency has taken over as a driving force in the economics of the oil and gas industry and as major assets near abandonment, geophysics (in parti cular reservoir geophysics) has increasingly been recognized as a tool for improving the bott om line closer to the wellhead. The reliability of geophysical surveys, parti cularly seismic, has greatly reduced

the risk associated with drilling wells in existi ng fi elds, and the ability to add geophysical constraints to stati sti cal models has provided a mechanism for directly delivering geophysical results to the reservoir engineer.

What is Reservoir Geophysics?

Reservoir geophysics can be defi ned as the applicati on of geophysical techniques within a known hydrocarbon reservoir. It is an integrated, systemati c study of the various physical properti es of the reservoir (atleast one drilled well informati on within that reservoir), with the available geophysical tools (mainly seismic) in order to manage, monitor and also design the explorati on and development of an asset. Seismic att ributes, AVO analysis, seismic inversion, multi -component seismic, VSP surveys, Passive seismic, Reservoir characterizati on etc., are just a few examples of an integrated approach of comprehending the reservoir. We can further subdivide “reservoir geophysics” into development” and “producti on” geophysics, depending on the immediate applicati on: Development geophysics is applied to the initi al effi cient development of a fi eld, whereas producti on geophysics is applied to the understanding of the fi eld as it evolves during producti on.

Bird’s view of Reservoir Geophysics

Several techniques are in place to facilitate the

study of the reservoir. Following are few of the tools which aid a geophysicist to explore the reservoir very precisely.

Multi -Component Seismology

Improving reservoir performance and enhancing hydrocarbon recovery are criti cal to the future of the petroleum industry -- and to do this, it must be possible to characterize reservoir parameters, including fl uid properti es, their movement and pressure changes with ti me. Multi -component seismology has great potenti al for monitoring fl uid movements in reservoirs. The main reason is simply the presence of fl uid-fi lled fractures.

Shear waves (S-waves) are much more sensiti ve than compressional waves (P-waves) to the presence of fractures or microfractures and the fl uid content within the fracture network. Seismic shear wave anisotropy in the reservoir causes two shear modes to form (S1 and S2) and to propagate with diff erent velociti es -- a phenomenon called S-wave splitti ng, or birefringence. Seismic shear wave anisotropy is key to monitoring fl uid property changes in fractured media.

Shear wave analysis allows for visualizati on of inter-well distributi on of secondary porosity, permeability, and fracture zones. Due to rigidity changes associated with fl uid replacement in the reservoir, dynamic monitoring with shear wave data provides a means to follow acti vely the

... reservoir geophysics has increasingly been recognized as a tool for improving the bottom line closer to the wellhead.

30

displacement of reservoir fl uids with CO2 and other injecti on techniques. This dynamic reservoir characterizati on will provide the industry with the ability to be more proacti ve, rather than reacti ve, in the management of reservoirs.

4D-Seismic

3D seismic data acquired at diff erent ti mes over the same area (generally termed as 4D seismic) are becoming useful as a reservoir management tool to potenti ally detect dynamic reservoir fl uid movement and try to predict the hydrocarbon fl ow capacity. By using 4D seismic data, reservoir engineers and geoscienti sts can locate bypassed oil to opti mize reservoir management. Additi onally, allows asset

teams to observe the dynamic processes in and around the reservoir, making the development and producti on stages more successful by placing wells more eff ecti vely.

Seismic Att ributes

From the processed seismic data, a host of additi onal features can be derived, and used in interpretati on. Collecti vely, these features are referred to as seismic att ributes. The simplest att ribute, and the one most widely used, is seismic amplitude, and it is usually reported as the maximum (positi ve or negati ve) amplitude value at each common midpoint (CMP) along a horizon picked from a 3D volume. The use of seismic att ributes extends well beyond simple amplitudes. Most of the “original” seismic att ributes were based on the Hilbert transform and consisted of the instantaneous amplitude (or amplitude of the wave envelope), the instantaneous phase (most useful for accurate ti me picking), and the instantaneous frequency. Spati al variati ons in

lithology and fl uid content are among the primary goals of reservoir geophysics, and these are typically established through calibrated seismic att ributes.

Seismic Inversion

Seismic inversion is the process of transforming seismic refl ecti on data into a quanti tati ve rock property descripti on of a reservoir. Seismic inversion based on the seismic data actually used and a priori informati on available (reservoir measurements such as well logs and cores), seeks to determine compressional wave velocity, Poisson's rati o and acousti c impedance, by minimizing the diff erences between observed and syntheti c seismic data. Seismic waveform inversion can be made approximately linear so that soluti on by iterati ve linear approaches is possible.

Inversion of geophysical data to obtain subsurface material properti es is criti cal to increasing our quanti tati ve understanding of geologic structures and lithology and has direct applicati on in hydrocarbon explorati on and exploitati on.

Geostati sti cs

Geostati sti cs is very powerful and standard tool in geological modelling of hydrocarbon reservoirs in the last two decades. It could be used in interpolati on or simulati on improving as well as mathemati cal tool for linking of two dependant variables, e.g. porosity and seismic. Geostati sti cs deals with spati al data, i.e. data for which each value is associated with a locati on in space. In such analysis it is assumed that there is some connecti on between locati on and data value. From known values at limited sampled points, geostati sti cal analysis, more specifi cally kriging and Co-kriging techniques can be used to predict spati al distributi ons of properti es over large areas or volumes.

RedefiningProspect

31

Spectral Decompositi on

In recent years, a couple of techniques in parti cular have been developed that appear to help the interpreter identi fy properti es of extremely thin beds, well below what has traditi onally been considered the quarter-wavelength resoluti on of seismic data. These techniques make use of the various frequency components within a band-limited seismic wavelet; one operates in the frequency domain, and the other in the ti me domain. The frequency-domain approach (see, for example, Partyka et al., 1999) called spectral decompositi on, looks for notches in the frequency band representi ng a sort of ghost signal from the interference of the refl ecti ons from the top and bott om of the thin bed. The frequency at which that ghost, or spectral notch, occurs corresponds to twice the (two-way) ti me thickness of the bed. Because the seismic wavelet contains frequencies well above the predominant frequency, spectral notches can be indicati ve of extremely thin beds. The thinning out of a channel or shoreline, for example, can be observed by mapping the locati ons of successively higher-frequency notches in the spectrum. Such analysis facilitates the proper planning and managing of further well-locati ons.

Verti cal Seismic Profi ling

Getti ng either the receiver or the source closer to the imaging target results in a much higher-resoluti on image. Placement of a string of seismic receivers in the borehole (verti cal seismic profi ling or VSP) or a source in the borehole (reverse VSP) accomplishes this, and allows for 3D imaging if the surface components (sources for VSP and receivers for reverse VSP) occupy appropriate large swaths of the surface. Development of extremely high-quality multi channel receiver strings has made the service aff ordable by minimizing acquisiti on ti me, which oft en requires loss of producti on.

Cross-Well Seismic Imaging

The deployment of a string of receivers in one well, and a source in another well, allows the imaging of the plane between the two wells. The ti ming of the fi rst arrivals allows a 2D image of interval velociti es to be obtained as a velocity tomogram,

and the refl ected events can then be migrated into proper positi ons for a crosswell refl ecti on image. The primary advantage comes from a tremendous increase in resoluti on, oft en exceeding a full order of magnitude improvement over the surface data in the same area.

Passive Seismic

The technique uses naturally occurring micro-seismicity (1-10 Hz) of the earth or the tremors induced due to fractures created by movement of water-front during EOR/IOR injecti on techniques in the reservoir. Deployment of specially designed sensors in boreholes and also use of permanent stati ons on surface, allows detecti on of these events. When the seismic events that accompany sti mulati on for hydraulic fracturing are located, the result is a temporally changing map of the fracture during its creati on. The mapping of events from other reservoir practi ces (usually, although not always, injecti on) can also be accomplished. This technique is increasingly being used for reservoir monitoring, characterizati on and management.

In a Nutshell

A reservoir geophysicist by the use of technologies listed above, analyzes the target of the study, calibrates legacy seismic data to wells, and investi gates the seismic petrophysical responses of the various scenarios anti cipated in the reservoir. These provide a good opportunity to collect that data, map and design an explorati on survey and also strategically plan the opti mal well locati ons for improved producti on standard.

Reservoir Geophysics in OIL

In OIL, innovati on and adopti on of newer technology has always been of paramount importance, furthermore geophysics being the core integral part, constantly requires implementati on of advanced experti se. Typically, unti ll now geophysics has been used as an explorati on tool, with the adopti on of newer techniques such as ti me lapse 3D (4D Seismic), multi -component surveys, seismic att ribute studies, AVO, inversion studies, Curvature analysis, geo-hazard analysis etc., exploitati on of reservoir geophysics in OIL is at a very promising stage.

32

Geophysical Data ManagementK. L. Mandal, D. S. Manral & A. K. Khanna

Introducti on

Minimizati on of explorati on risk is the quest of geo-scienti sts in E&P industries. The geoscienti sts are engaged in relentless research to bring up new technology/idea in the fi eld of data acquisiti on, processing and interpretati on towards minimizati on of explorati on risk. The advent of high speed computers and cutti ng edge technologies as a result of such eff ort has made the data processing & interpretati on faster and cheaper. Additi onally, new techniques are increasingly being used on the old vintages of dataset to extract new informati on which could not been inferred before. Fresh data acquisiti on campaigns are also carried out when the requirement arises. Thus, in the course of ti me, geo-scienti sts need to store and maintain diff erent types of geo-scienti fi c data of diff erent vintages. The storage and management of geophysical data is one of the major challenges to the industry nowadays. Thus there is a franti c need to implement safe preservati on measures of data media and effi cient maintenance of the database.

Seismic Data Library/Archives

Geophysical data in an E&P industry consti tute a major volume of seismic data. Hence, such geophysical data library is generally termed as seismic data library. Seismic data library contains raw, processed and migrated datasets along with the related informati on like observer logs (datasheets), near surface velocity models, processing & interpretati on reports, various maps etc. The seismic data fi les are purely digital data and can be stored in digital media only, whereas, the supporti ve informati on such as observer logs, processing & interpretati on reports and various maps are in both digital and hard copy formats.

Unti l early nineti es, seismic datasets were generally recorded in digital magneti c tapes. Thereaft er, with the advent of high volume cartridges, data storage got shift ed to digital cartridges in which larger amount of data could have been stored whereas

the cartridges themselves required much smaller storage space in tape libraries. Presently, data are recorded in digital form in CDs/DVDs and portable hard disks also. The hard disks (magneti c disks) and CDs/DVDs may be easier to use but are less reliable than the digital cartridges/tapes. The DVDs/CDs and the portable hard disks can go bad at any point of ti me. Hence, digital cartridges are always preferred for long term storage of seismic data.

Depending on usages, data are stored in two types of media viz. working and backup. The aim of having Working Media is to provide data to the users with the minimum loss of ti me. Nowadays, seismic data processing involves a huge amount of data churning and, therefore, entails working media to be of high

capacity so that it can cater huge volume of data in a short period of ti me. In case tapes/cartridges are used as working media, they need to be rugged enough for frequent usages. IBM-3595 cartridges are an example of such high capacity & robust magneti c cartridges. Additi onally, high capacity

magneti c disks like NAS (Network Area Storage) and SAN (Storage Area Network) are also being used for the purpose. Working media are generally stored within or close to the premises of data processing & interpretati on centers.

Backup media are meant for long term storage, for about 10 years. Normally, media selected for this purpose are cartridges, because of their stability & reliability. IBM-3592 and LTO-3 cartridges are designed for backup purpose and have been proved to be reliable in the industry.

Considering the rapid change in technology, selecti on of backup media may be done for a period of next 10-15 years only. Otherwise, the media and media drives may become obsolete before the planned ti me. As for example, the gold plated DVDs are very stable for the ti me being and one may think that they will remain be reliable for next 100 years, but the technology may change aft er few years and they may become obsolete.

... there is a frantic need to implement safe preservation measures of data media and effi cient maintenance of the database.

The MagicBox

33

Additi onally, following points should be considered for the storage of data:

i) A special storage environment is required for increasing the longevity of the storage media where following things should be considered:

a) Tape storage areas should be cool and dry - temperature should be maintained between 20-24 degree Celsius and humidity should never exceed 55% RH.

b) The tapes should be kept away from strong sunlight and contact with water should be avoided.

c) Contaminati on of the tapes by dirt, dust, fi ngerprints, food, cigarett e smoke and ash, and airborne pollutants should be avoided.

d) The tape should be kept away from magneti c fi elds and stacking of the same on top of the equipments producing such fi elds should be prohibited.

ii) Periodic transcrip-ti on from old to new media is necessary – it saves data in me-dia which are not obsolete.

iii) Storage of multi ple copies at diff erent locati ons can help in saving data in case of disasters like fl ood, fi re, earthquake etc.

Database Management

E&P industries store a huge amount of geo-scienti fi c data in their Data Tape Libraries/Archives generated by conti nuous acquisiti on, processing and interpretati on. These invaluable datasets are used for various purposes like planning & designing of fresh seismic survey campaigns, chalking out the future explorati on strategy (analyzing the locati ons of producing & dry wells, Basin model and available seismic data) and minimizing the downti me in data processing (starti ng processing work from any intermediate stage by picking suitable datasets). Catering the required datasets for these acti viti es from the huge data library in a short period of ti me is a quite challenging job. Hence, various types of

effi cient database management systems are being used nowadays.

Since explorati on acti viti es are carried out at diff erent geographical locati ons, the Database & its management system is docked on a Geo-Spati al platf orm called Geographical Informati on Systems (GIS). It helps to pick up the correct dataset without any error and has now become industry standard.

In GIS, images/objects have coordinates as per the geographical positi ons/extents. The informati on of any image/object is defi ned as its att ributes and is stored in the database. GIS displays images/features as informati on layer in geospati al space and selecti on of any image/feature displays the pertaining att ributes. Various queries can also be run to generate reports on the basis of the

att ributes of the images/features. Some of the examples of the features are 2D seismic lines, 3D seismic blocks, shot points, receiver points, roads, river courses, forests etc. The att ributes comprise of several classes of informati on viz. co-ordinates of boundary points of the 3D-seismic block, acquisiti on &

processing parameters, the tapes pertaining to the fi eld data etc. As an example, a 2D-seismic line can be displayed as per its geographic positi on on the satellite imagery or SOI (Survey of India) map and its att ributes like acquisiti on parameters etc. can be seen when the line is highlighted.

Nowadays, maintaining a cluster of very high capacity NAS/SAN disks has become well within an aff ordable cost. So, for further minimizati on of data rendering ti me in data acquisiti on (planning & designing), processing and interpretati on, some E&P companies are using comprehensive online data library comprising of NAS/SAN cluster. This arrangement enables users at workstati ons to navigate the details of individual 2D seismic line, 3D seismic block etc. through GIS applicati on and to locate/access/view the pertaining seismic data set residing in the data library/archive.

The following data are stored in digital form in the data library:

34

a) Seismic data

- Field data (Geom. loaded), Processed data (Stack & Migrated);

- Observer log, Navigati on data and Velocity fi les in ASCII;

- Acquisiti on parameters & detail reports, Processing parameters & detail reports and Interpretati on reports;

b) Well log data, G-M data, Geological maps & reports, Explorati on and producti on reports of lease areas.

c) GPS survey points (surveying control points/benchmarks) & reports, satellite imageries & SOI maps.

All the data menti oned above are stored in the database in a structured way with proper path names.

The most criti cal part of making such database system is the collecti on of geophysical data and other relevant informati on as inputs for GIS applicati ons. The following data are loaded into GIS database in the forms of maps and features with att ributes and geographic informati on:

a) 2D seismic lines: 2D seismic lines are

represented as features under ‘line’ class and have the following att ributes:

- Path names of Acquisiti on & processing parameter fi les;

- Path names of raw data (geometry applied), Observer logs Navigati on & Processed data;

- Path names of Acquisiti on, processing, Interpretati on and geological reports etc;

- Image fi les of interpreted migrated secti ons;

- Other informati on like fold, year of recording, name of the area, length of the line in terms of GLKM (Ground Line Kilo Meter) etc. required for frequent queries & various report generati on;

b) 3D seismic blocks: 3D blocks are represented under ‘area’ class and have similar types of att ributes as menti oned above for 2D seismic lines.

c) Shot & Receiver points of 2D lines: Shot and receiver point coordinates are defi ned as ‘point’ class. The shot point has four att ributes viz. line name, hole-depth, up-hole ti me and elevati on apart from geographical coordinates (Easti ng & Northing). These pieces of informati on are required for calculati on of the volume

The MagicBox

35

of 2D data acquired in a parti cular lease area (Explorati on or Mining) as well as to know near-surface logisti cs in regards of shot hole drilling for future surveys. A Receiver point has only one att ribute and that is elevati on.

d) Shot & Receiver points of 3D blocks: The defi niti on and att ributes of shot and receiver points are the same as in the case of 2D lines menti oned above.

e) Surveying Bench marks/Control points: These bench marks/control points are defi ned as ‘point’ class. Each point has ten att ributes viz. three WGS-84 coordinates (Longitude, Lati tude and Ellipsoidal height), three coordinates (Lati tude, Longitude and Elevati on) based on Indian datum (Everest 1830 PAK, 1962 editi on), three coordinates (Easti ng, Northing and Elevati on) based on local projecti on system, and the path name of processing & modeling parameter fi le.

f) Satellite imageries: These imageries are geo-referenced and loaded into GIS database as ‘raster image’ class for providing much required informati on about the surface like latest river courses & river beds, deep and shallow water bodies, abandoned river channels, existence of highly moisturized land, forest cover, tea-

gardens, roads and the extent of a town/ urban development etc..

g) Wells: Wells are defi ned as ‘point’ class. The att ributes are the name of the well, status of the well (producing or abandoned), name of the prospect, name of the fi eld and the path name of Log data fi les.

In a similar way menti oned above, other relevant data like terrain, logisti cs, PEL / ML and other maps can be incorporated in GIS to obtain an integrated image in the form of layers of diff erent properti es in the study area. Thus a user can navigate & access the data in a visual geospati al environment and can pick the correct dataset without any error.

Conclusion

As the volume of geophysical data is increasing day-by-day, their management in a systemati c way is of paramount importance for an E&P industry. Apart from increasing the longevity of the data, one can decrease the retrieval ti me also by managing them well. The need for the quest is to have a proper planning for future, storing the data in proper conditi ons and to be updated in terms of technology. Proper storage media as well as effi cient retrieval method can ensure eff ecti ve use for present users as well as a safe storage for future users.

36

An Alternative Processing Approach for Noise Attenuation in Sirte Basin, Libya: A Case Study

A. K. Khanna, D. S. Manral & D. N. Murty

Summary The prime objecti ve of seismic data processing is to improve S/N rati o so that the resultant seismic secti on can be used for meaningful interpretati on of the subsurface geological features of interest in hydrocarbon explorati on. Though various algorithms are in practi ce to segregate the signal from the noise for achieving bett er S/N rati o, conventi onal processing techniques are not successful in eff ecti ve noise suppression in geologically complex setti ngs. In this arti cle the results of eff ecti ve noise att enuati on achieved by using an advanced processing technique called Leading Intelligent Filtering Technique (LIFT).

Introducti onOil India Limited (OIL) and Indian Oil Corporati on Limited (IOCL) consorti um was awarded the license for Area 86 on 29th January, 2005 in the Sirte Basin of Libya. Explorati on & Producti on Sharing Agreement (EPSA) signed on 20th March 2005 between NOC, Libya and OIL-IOCL consorti um with the

Explorati on Phase of 5 years. OIL is the operator for the Area. The area of around 7087 SqKm is located in West Sirte Basin. In the meanti me on 2nd October, 2005 the consorti um was awarded another Block 102/4 in the Sirte Basin of Libya (Fig. 1). OIL is the operator for this block. EPSA was signed on 3rd December, 2005 between NOC, Libya and OIL-IOCL consorti um. The Block is located over the eastern Hon Graben and the western part of the Zella Trough in West Sirte Basin with an area of around 2,710 SqKm.

Basinal Informati onThe Sirte Basin with an area of 492,000 SqKm is a Mesozoic-Terti ary rift system formed on the northern edge of the conti nental plate margin of Africa. The Sirte Basin can be divided in to three divisions viz. Eastern Sirte Basin, Central Sirte Basin & Western Sirte Basin.The main structural elements present viz. Uaddan uplift , western Graben, Dor EI Abd Graben, Zellah Trough and Dahra Platf orm (Fig. 1).The

basin is characterized by multi ple source rocks, multi ple reservoirs, good seals and variety of traps and a series of platf orms, horsts and grabens. The average elevati on of the area is 240m and covered with scrub desert. The Sirte Basin province contains one dominant & principal petroleum system, the Sirte Zelten system. The Upper Cretaceous Sirte Shale is the eff ecti ve source rock in the area. The reservoirs range in rock type & age from fractured Precambrian basement, clasti c reservoirs in Cambrian-Ordovician Gargaf sandstones and Lower Cretaceous Nubian (Sarir) Sandstone to Paleocene Zelten formati on and Eocene carbonates. Hydrocarbon resources are approx. equally divided between carbonate & clasti c reservoirs (Pre-Terti ary dominantly clasti c , Terti ary dominantly carbonate reservoirs).

Although carbonates & shales dominate much of the Terti ary sequence, the evaporites & salt deposits of the Eocene Gir formati on (Hon Evaporites member) are of utmost importance to petroleum accumulati on in the Sirte Basin province because they form essenti al seals for the hydrocarbons migrati ng out of the Upper Cretaceous into reservoirs of many ages along the faulted horsts and grabens. The dominant trap style is structural (around 84 percent) with remainder considered strati graphic or a combinati on of the two.

Seismic Data Acquisiti onArea 86, covering 7087 SqKm, is situated on the western rim of the Sirte Basin within which several major structural elements are present viz. Uaddan Uplift , Western Graben, Dor El Abd Graben, Zellah Trough and Dahra Platf orm (Fig. 1). Block 102/4 (Area102 Block 4), conti guous to the north with Area-86, with total surface area of 2710 SqKm is located in the south-eastern porti on of the Uaddan Platf orm, the northern fringe of the Abetar High and the intersecti on of the Hon Graben with the Zella Trough in Sirte Basin (Fig. 1). The 2D-seismic lines proposed and acquired in the Area 86 & the Block 102/4 are shown in the seismic coverage map (Fig.

37

2).The Lines taken up for processing are the hilighted ones on the Base map . The 2D seismic data acquired with 80 fold in this region (Acquisiti on parameters shown in the table-1) characterized by strong coherent (Ground roll and linear noise of diff erent type with diff erent velociti es that can not be completely suppressed by using conventi onal processing tools) and random noise shown (Fig. 3). The complex subsurface geology along with the shallower high velocity anhydrate layer masks the energy penetrati on, resulti ng in poor S/N rati o. Also the refl ecti ons are shallow and confi ned to 1.0 second and obscured by very strong coherent noise.

Acquisiti on Parameters: (Table-1)Recording instrument SN 428XLGeometry Symmetrical-Split SpreadSource VIBROSEISSweep Length 16 secondsNo. of sweeps 12 (4 x 3)Sweep –Frequency 8-80 HZSource spacing 40 m Receiver spacing 20 mNear 30 mFar off set 3210mSampling interval 2 ms

Seismic Data ProcessingMany conventi onal processing techniques are in place to segergrate signal from undisered noise, which have their own limitati ons, such as Band pass fi lter alone and/or F-K fi lters alone. For instance band pass fi lter can not eliminate the enti re spectrum of ground roll and the data will be sti ll left with some undesired noise. Similarly using F-K fi lter alone, Transforming the data from T-X domain to F-K domain in order to eliminate the low frequency noise, some ti mes the original signal will be lost in this eff ort. However, the present study required a special fi ltering procedure know as

LIFT processing technique which provides a bett er approach to improve S/N rati o by att enuati ng undesired coherent noise and there by retrieving the masked signal.

In this Arti cle we are demonstrati ng how eff ecti vely the linear coherent noise and ground roll are suppressed and hidden refl ectors are brought out using an advanced processing technique called LIFT to get confi dence for further velocity analysis and to bett er image the subsurface. In principle splitti ng the original data in to two separate frequencies and working separately on diff erent frequency windows LIFT can address the noise eff ecti vely. In endeavor of removing these undesired noises the original data was processed in two separate parts. Initi ally the low frequency data that is of below 30Hz (where the data is mostly obscured by strong ground roll and linear noise of diff erent kind) was separated from the original input data to have outputs of both low frequency component data as well as high frequency component data. Now the low frequency data is subjected to fi lters such as Low Frequency array fi ltering based on velocity in order to remove the strong noise. Then the output of fi ltered low frequency data is added to the high frequency component separated in the previous step to achieve improved S/N rati o. High frequency noise att enuati on is done by using TFCLEAN

(noise suppression via Time-Frequency transform). Fig. 4 showing the results of eff ecti ve noise att enuati on achieved by this approach. Noise att enuati on has been achieved eff ecti vely by using an advanced processing approach LIFT. The valuable signal obscured by the strong coherent noise of diff erent type and ground roll is retrieved by the technique which can not be achieved by conventi onal processing. Also Strong coherent noise att enuati on using the technique leads to the improved quality of the secti on. Aft er noise att enuati on using the technique the fi nal stack secti ons obtained with LIFT (improved) and with out LIFT is shown in Fig. 5&6 respecti vely.

ConclusionConventi onal seismic processing techniques fail to deliver the desired outputs especially when the imaging challenge is to address logisti cally diffi cult & geologically complex areas. Such areas are generally characterized by poor signal to noise rati o. Therefore, proper preconditi oning of the raw shot gathers is a must. Advanced processing approaches are the best alternati ve soluti ons for imaging the subsurface in such areas.

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Potent tool- Promising future

Introduction of Multicomponent Acquisition technology in OIL–A quantum jump

A.S.R.K.Shastri, V.Kara, K.Dasgupta P. K. Singh, S Maji, A.Sonowal, M. Gupta, R.P.C.H. Ganesh Karri & P.K.Paul

Introducti on and basic Principle

The introducti on of multi -component seismic survey has brought a new era in E&P industry. In comparison to only verti cal component of ground vibrati on in normal seismic surveys, all the three components of ground vibrati on is recorded in multi -component seismic survey. This measurement of the three components of the ground vibrati on enables measurement of two components of S-wave informati on (viz. SV & SH) alongwith the traditi onal P-wave data. In land seismic, it is termed as three-component or 3C survey whereas in marine, the additi onal hydrophones adds one additi onal component and therefore, it is known as four-component or 4C.

Multi -component seismic is based on the principle of energy parti ti oning and conversion of seismic data (from P to S). At any interface, seismic waves are refl ected, transmitt ed or refracted. The incident P or S waves on refl ecti on and transmission gets converted to S or P waves whatever the case. This principle of conversion of seismic wave at any boundary forms the basic premise of multi -component seismic method, which is also called converted wave seismic technique. Thus, in this method, instead of Common Depth Point (CDP) in conventi onal seismic, Common Conversion Point (CCP) is the keystone.

The measurement of both P and S waves makes

multi -component seismic a potent tool for characterizati on of reservoirs. It essenti ally is a fl uid discriminator tool especially for identi fi cati on of the extension of gas. Multi -component seismic is also a tool for identi fi cati on of measurement of fracture density and orientati on.

Multi -Component Seismic in Oil India Limited (OIL)

With the procurement of new SN 428 XL seismic data acquisiti on equipment, OIL acquired the

capability of acquiring 3C seismic data. Already 3C data is acquired in and around Moran fi eld area. The data is acquired along 2D profi les. A step-by-step approach is taken in carrying out this 2D-3C programme. An experimental programme of recording data both with 3C digital geophones and

with normal geophones laid along parallel profi les was initi ated. A comparison of P-wave data from 3C digital geophones and that from normal geophones showed recording of higher fi delity data by 3C digital geophones. Thereaft er, routi ne 2D-3C data acquisiti on is being carried out in this area. This 2D-3C programme will be further extended to other areas like Nahorkati ya, Makum, Deohal etc.

Future Road Ahead

This initi al pilot programme of 2D-3C seismic data acquisiti on and analysis of the acquired data will help in identi fi cati on of diff erent att ributes towards reservoir characterizati on, direct indicati on of hydrocarbons etc. It is, thereaft er, planned to enhance the capability and acquire 3D-3C data for accurate mapping of diff erent att ributes and anomalies.

Multi-component seismic is based on the principle of energy partitioning and conversion of seismic data (from P to S).

Dip consideration in 3D survey planning in North East coast offshore area- A case study

K. K. Nath & S. N. Singh

Today, in oil industry it is envisaged that 3D seismic survey may eliminate the 2D seismic survey in future developmental survey programme. 2D survey could, however be used in semi-developmental and exploratory survey programmes as a fi ner substi tute to the in-vogue reconnaissance survey. The expectati on from the 3D data and the resources development in 3D survey operati on creates more concern to the geophysicist while planning and designing the survey parameters so that more and more meaningful data could be acquired in most cost eff ecti ve manner.

In this paper, the considerati on of formati on dip in planning the 3D survey data acquisiti on programme has discussed with reference to the marine 3D survey carried out by Oil India Limited in north east coast off shore area, off the coast of Orissa, India. Finding no signifi cant success in striking commercial hydrocarbon in the area, even aft er carrying out varieti es of geoscienti fi c study, the 3D survey was planned with great expectati on to pinpoint reservoirs within the Paleogene shelf–slope carbonate sequence. The possible subtle structural/Strati graphic traps in overlying Neocene sequence were assessed to be secondary target of explorati on for hydrocarbon.

In above situati on, all-out eff orts were mandatory for a meti culous planning of the survey parameter considering all the available data in the area, mainly the target depth, target size and the formati on dip at the zone of interest. With knowledge on target depth and size as in the instant case, it could be seen that a slight negligence of the dip of the target horizons would have caused a signifi cant loss to the 3D data volume and would have led only to a parti al fulfi lment of the expectati ons which could be avoided through adjustment of the shooti ng programme with least technical compromise.

Published in AEG-1994

Land 3D survey design and quality control – A case study from OIL’s Oil field in Upper Assam

S.N Singh & P.V. Reddy

To facilitate the development of it’s newly discovered fi elds of Hapjan, Dikom, Shalmari in Upper Assam valley; OIL India limited recently acquired about 340 sq.km. of 24 (8 x 3) fold, seismic data. The subsurface bin size was 25m

X 50 m. SN-368 cable telemetry system was used for data acquisiti on. Presurvey planning, simulati ons and other data quality checks were performed through Geomicro-II soft ware on Microvax computer. The present paper deals with the planning and executi on of the 3D seismic survey carried out to achieve the desired geological objecti ve.


Mixing geometries in 3D seismic data acquisition – A case study

R. Dasgupta, A. Das, A. Kumar, T. Bhatt acharya,

D. S. Manral & B. M. Sinha

Oil India Limited has been carrying out its 3D seismic data acquisiti on with its own crew and has, so far, successfully completed 6 blocks in its PEL/ML areas in Upper Assam. In all the initi al fi ve blocks and partly in the sixth block, 3D seismic data acquisiti on was carried using an acquisiti onal geometry comprising of 6 lines, each line having 119 channels. The acquisiti onal parameters for the sixth block (to be referred as Block 6 henceforth in this paper) were designed keeping in view the geological objecti ve of targeti ng the formati ons of Eocene age, expected to in the depth range of 4000-4500 m.

During the course of operati on in the sixth block, it was found that it may not be possible to conti nue six-line recording due to various operati onal constraints. The quantum of fi eld electronics handled during 6-line operati on was found to be quite large and it was decided to restrict the same to and opti mum level. The enti re 3D acquisiti on parameters were then reviewed and a 5-line seismic operati onal technique (each line comprising of 91 channels) was proposed to complete the sixth block. This study was carried out keeping in mind that the data to be acquired by the new geometry matches with the data already acquired using six-line operati on in the block under questi on (in context to the bin size, off sets and azimuth-distributi on), and the economic viability of the method is not sacrifi ced. In this paper, we demonstrate how the recently acquired 5-line seismic data is matched with the already acquired 6-lines.


Hydrocarbon exploration in the North Bank of river Brahmaputra by Oil India Limited – A retrospective

G. R. Saini and B. M. Sinha

The Brahmaputra valley in Assam and Arunachal Pradesh is covered by alluvial plains on the two sides of the mighty

40

Di id ti i 3D

Abstracts

Seismic Data Acquisition“Covering uncommon Ground - Probing Miles Down”

river Brahmaputra. The northern fl ank of the valley is long strip of plain land with an average width of around 25 Kms in the east to around 60 km in the west. This area is bounded by the Himalayan foot hills in the North and the river Brahmaputra in the South.

Morphologically the area is highly torn-off by a large number of tributaries of the river Brahmaputra. This has left the area totally devoid of major infrastructural faciliti es like roads and railways. Only one meter gauge railway and one nati onal highway run West to East all along the area. Lack of basic infrastructural faciliti es has rendered it as one of the most inaccessible terrain in the country or probably in the world and as a result the area is poorly explored from hydrocarbon point of view. Though ONGCL had started geophysical surveys in the area long back in 1965-66, it did not reach to the stage of drilling.

Aft er nati onalizati on in 1981, OIL expanded its explorati on acti viti es in other areas beyond South Bank of the river Brahmaputra. One of the areas of its new venture was the North Bank of the river Brahmaputra. OIL acquired its fi rst PEL in the eastern parts of North Bank comprising of Murkongselek and Pasighat area in Assam and Arunachal Pradesh and started its in-house seismic data acquisiti on in this area in 1986-87 fi eld session. Since then OIL has acquired around 3300 SKLM of 2D seismic data with varying CDP foldage, mostly in the eastern parts comprising of Pasighat, Jonai, Shilapathar, Dhemaji and to some extent Dhakuakhana and Lakhimpur areas.

ONGCL conti nued to work in the adjacent north bank areas ti ll 1988-89 fi eld season. With OIL’s entry, it withdrew from this area and under an agreement all the seismic data acquired by ONGC in these areas were taken over by OIL for further explorati on work. OIL has since then intensifi ed its explorati on acti viti es in North Bank and based on the interpretati on of data acquired by ONGCL and by OIL, an exploratory drilling programme has been worked out. Overcoming the various logisti c diffi culti es, OIL was successful in spudding the fi rst well “Bihpuri-1” on Bupuria structure on June 16, 1997.

The area lying west of Bihpuria – Narayanpur has so for not been explored at all. OIL intends to cover seismically these areas during current fi ve year plan. The area is expected to witness intense explorati on acti viti es including drilling of few more wells on already identi fi ed structures based on the interpretati on of the existi ng seismic data. In this paper a brief descripti on of logisti cs and the informati on deduced from the acquired seismic data is presented.


3D seismic survey design in a logistically difficult area - A case study

R.Dasgupta, D.S.Manral, A.Kumar,

T.Bhatt acharya & P.K.Paul

Oil India Limited has been successfully carrying out 3D seismic survey in its PEL/ML areas in Upper Assam. Recently OIL carried out 3D seismic survey in Nahorkati ya area, in order to develop and enhance producti on from the aging Nahorkati ya Oil Field, discovered way back in 1953. The oil & gas producti on over here is mainly confi ned to the sands pertaining to Barail formati ons. The planned seismic survey is required to cover the Nahorkati ya Oil Field, primarily to delineate the Barail reservoir sands, which are in the depth range of 2600-3200m. The other purpose is to provide a structural confi gurati on of the Eocene formati ons, which are in the depth range of 4000-4400m. The area considered for the survey is logisti cally quite diffi cult, as a signifi cant porti on of the block is comprised of marshy lowland, covered with this vegetati on and paddy fi elds. Apart from this Burhi Dihing River meanders through the enti re block and virtually fl ows over the central porti on of the Nahorkati ya Oil Field. This river has changed its path quiet frequently and has left oxbow lakes and river channels. The area has a very high populati on density which needs to be negoti ated during the survey planning. There are a large number of wells in the area, which also need to be taken care during the planning phase of seismic survey.

There are a few important factors that conventi onally characterize the design of 3D acquisiti on parameters; these are depth to the target zone, off set and azimuthal distributi ons, bin size, foldage and logisti cs of the area. In this block the main problem was the scale of logisti cs, therefore, it was required to design a seismic survey which meets our scienti fi c and technical requirements overcoming the logisti cs of the area. Keeping all the above into considerati on and analyzing the results obtained from simulati on studies, it was planned to cover the area using four line (brick patt ern) operati onal technique, having 120 channels laid out per line. This four line acquisiti on geometry, having a symmetrical split spread defi niti on provides an adequate coverage of 28 fold data (7 inline × 4 crossline), with a fair amount of bin to bin consistency. In this paper we present in detail the methodology employed in the planning and designing of the survey.


41

The analysis reveals a defi nite trend in stati c correcti ons with respect to elevati on. For extracti ng the informati on of interdependencies between variables i.e. to interrelate the data in a stati sti cal sense a computer algorithm has been writt en for fi tti ng a curve, to the obtained dataset, using Least Square technique. In the enti re area, at the surface there is a thin weathered layer of variable thickness. The thickness of this layer is generally less than 4m and has velocity values ranging from 300 to 350 m/sec. This layer has a signifi cant contributi on to stati cs. In this paper, we present the problem in stati cs computati on, analysis of the stati cs data and its trend in Upper Assam. The result of our stati cs and its eff ect on seismic refl ecti on data will also be presented.


Seismic data acquisition in sandy areas of river bed

P.K. Sharma, V.P. Singh, Anup Kumar, F. Siddiquee & D.N. Lahkar

Oil India Limited (OIL) has been carrying out seismic survey operati ons for last several decades in upper Assam basin. River Brahmaputra has divided this basin into two separate regions, the region lying in the south of the river Brahmaputra has been widely explored and the availability of various geophysical data has helped in understanding the geology of the area. However, the region lying in the north of the river Brahmaputra has been sparsely covered by the geophysical methods. No seismic data could be acquired in the river bed area lying between South Bank and North Bank due to diffi cult logisti cs of the area. An att empt was made by an in-house crew to cover the unexplored river bed area by seismic and to connect the South Bank to that of the North Bank of the basin by using a Radio Telemetry equipment of Nati onal Geophysical Research Insti tute (NGRI) Hyderabad.

The data gap over the river Brahmaputra because of its wide course and logisti cally diffi cult approach has posed a major hurdle in correlati ng the geological sequence of the two banks. The objecti ve of the seismic survey was to get the conti nuous of subsurface from North Bank & South bank formati on. A regional line was completed to connect the North and South Bank, which crosses the water channels ti mes and its tributes several ti mes. Due to varying width of water neither hydrophones nor shots points were possible inside the water channels because Oil India does not have hydrophones and seismic sources for water covered areas. Prior to shooti ng, a simulati on study was carried out in Geomicro VAX-II. The objecti ve of simulati on study was to fi ll up the data gap due to various water channels and accordingly recovery shots were planned. Aft er completi on of shooti ng it was found in the water channels that near off set channels could

A practical approach to seismic modeling for field parameter design for exploration of Hydrocarbon in Belt of Schuppen

G.R.Saini, B.J.Reddy & C.V.G. Krishna

The south eastern part of the OIL’s PEL area is aff ected both at surface and subsurface levels by a series of imbricate thrust faults. This area covering surface and subsurface positi on of these thrust is termed as Belt of Schuppen. Due to complex surface and subsurface geology the Belt of Schuppen has been a challenge for explorati on. Through hydrocarbons had been discovered in Digboi fi eld (located in the BOS) more than hundred years back, no major breakthrough could be made in the adjoining areas covered by surface folded hills of Naga-Kumchai-Disang thrusts. Scanty seismic coverage of the areas like Kumchai, Ledo-Lekhapani and Pengree-Bordumsa where thrusts are not exposed at surface, shows that mapability of refl ectors over the thrust zones is very poor. Apart from the surface and near surface logisti c constraints, other cause of poor refl ecti on quality may be due to improper acquisiti on parameters. An att empt has been made in this study to analyze the reasons for poor seismic refl ector quality and to suggest the alternati ve approach for improvement therein by the way of modeling of refl ector dips and formati on velociti es and their lateral variati on over the area.


Trail analysis of statics – A case study

R.Dasgupta, D.S.Manral, A.Kumar & T. Bhatt acharya

Applicati on of proper stati c correcti on has been a big problem in explorati on seismology. Stati c correcti ons are required in refl ecti on seismology to compensate for velocity and /or thickness variati on within the “weathered zone”. Even a minor variati on in the weathered layer thickness can distort the spectrum of the signal and cause aberrati on on fi nal stacked data. In seismic survey, using explosive source, stati c correcti on has two components, the shot and receiver components. The shot hole ti me is generally used to compute the shot component of stati cs, whereas the low-velocity layer (LVL) survey or the Uphole survey (UH) are carried out to calculate the receiver component. The shot hole ti me records are generally not very accurate, whereas the accuracy of LVL and uphole survey is far bett er. In order to circumvent these inherent errors in stati c computati on, an analysis of the trend of “stati cs” has been carried out. Around 500 LVL/UH values covering OIL’s PEL area have been used for the analysis, as there are higher confi dence level of on stati cs computed from UH/LVL surveys.

42


Each trace line comprised of 120 channels and the lines were 300/350m apart. In the later blocks, the availability of adequate numbers of fi eld was the major additi onal constraint. Innovati ve design, using lesser volume of ground electronics, enabled us to generate 28 fold data (7 inline* 4 crossline) with four line swath geometry. This was achieved without sacrifi cing the geophysical att ributes and geological objecti ves of the surveys.

In this paper, we will present the various aspects of the diff erent 3D surveys carried out in Upper Assan by Oil India Limited (OIL). We also demonstrate that with innovati ve design of 3D geometries, by opti mally deploying the resources and by taking care of logisti cs and socio-economic conditi ons, how seismic data of good quality is acquired in Upper Assam.


3D seismic survey design – A cost – quality perspective

Y.R. Singh, D.S. Manral, T.Bhatt acharya & R.Dasgupta

3D seismic surveys have become the most widely accepted geophysical tool for explorati on and development of hydrocarbon resources worldwide. Designs of 3D seismic surveys require balancing of two confl icti ng concerns – achievement of geological objecti ves and minimizati on of survey cost. In this era of relentless pursuit by geophysical companies for effi ciency and cost advantage, the cost of seismic surveys vis-à-vis survey objecti ves has become the buzzword. Though cost-quality analysis of 3D survey design has been widely practi ced in internati onal geophysical industry, the same is scarcely reported in the Indian context.

The 3D acquisiti on geometry is generally characterized by key parameters viz. target depth, bin size, foldage, off set & azimuth distributi on, aspect rati o, etc. Innumerable geometries can be conceived to achieve the same set of geological objecti ves. Out of this large set of possible acquisiti on geometries, the choice of the opti mum geometry is generally made on the basis of the cost criteria without compromising the desired geophysical att ributes.

In this paper various 3D acquisiti on geometries have been simulated and each of them has been weighed vis-à-vis the achievement of geological objecti ves and its cost implicati on. The role of receiver deployment overlap patt ern, bin size, foldage, receiver & shot density, etc. in determining the cost of the survey has been analyzed to discern any percepti ble patt ern in their relati onship aimed at defi ning a probable cost-quality trade-off for selecti on of 3D seismic acquisiti on parameters, which is perti nent in the Indian context.


not be recorded, due which informati on from younger formati ons were not available however, we were able to get the informati on from Eocene formati on which is our zone of interest in the near by hydrocarbon producing South Bank area.


Optimization of 3D seismic survey design in Upper Assam basin

R. Dasgupta, D.S. Manral and T. Bhatt acharji

Three dimensional seismic surveys have become accepted in the geophysical industry as an important tool for providing a clearer subsurface defi niti on of geological features of interest in hydrocarbon explorati on. Yet, the cost of 3D seismic data acquisiti ons and will always be considerable, for which it is important to opti mize the survey design.

Design of 3D seismic survey is dependent upon many factors, which needs criti cal analysis for the choice of opti mum acquisiti on parameters. The designer needs to achieve the geological objecti ve of the survey, keeping in view of the logisti cs, socio-economic conditi ons of the survey area, the resources (fi eld electronics) at the disposal of the seismic party, acquisiti on system available for data recording and last but not the least, the cost of the enti re 3D project. There are few important factors that conventi onally characterize the design process. However, stress should be laid out on key geophysical data att ributes such as off sets, azimuths and inline/crossline multi plicity. The survey needs to focus on the geological targets (primary and secondary), thereby providing vital leads to the depositi onal characteristi cs of the producing formati ons and accurate imaging of the same.

In Upper Assam Basin, the focuses of the 3D seismic survey are the sand units pertaining to the two mains producing i.e. Barail formati on of Miocene/Oligocene age and Lakadong –Therria of Eocene age. In this part of the basin, the producing Barail sands are in depth range of 3500-4500 m, depending on its locati on in the basin.

The SN-368 distributed line equipment of Sercel make is used in all the surveys. The bin size of 3D surveys carried out in this basin has been fi xed as 25m (inline) and 50m (crossline). Keeping in crew the aforesaid constraints, 3D acquisiti on geometries that can provide proper distributi on of off sets, azimuths and multi plicity are designed. Only such geometries can help in delineati ng and mapping the producing sands pertaining to the target formati ons.

In the initi al few blocks, six-line swath geometry having a symmetrical split spread defi niti on was used for data acquisiti on. This generated 24 fold (8 inline * 3 crossline).

43

replaced by wide azimuth 3D seismic survey which required deployment of large number of channels (>1200 channels per shot). The wide azimuth 3D surveys are generally characterized by aspect rati os more than 0.5 (rati o of width to length of acti ve swath). Wide azimuth 3D seismic surveys, when managed eff ecti vely, have both technical and economical merits. One of the important aspects towards meeti ng both technical & economic objecti ves of survey concerns the deployment of large number of channels and managing the same in the fi eld. Usage of wide angle 3D surveys drasti cally reduces the source eff ort which signifi cantly reduces the cost of the survey.

The present paper deals with 3D seismic surveys in the logisti cally as well as culturally diffi cult areas in the parts of Upper Assam basin. The areas covered are of varied logisti cs and are covered by large swamps, water bodies and forests, which constrain the achievement of desired results. The paper covers a detailed techno-economic analysis of various orthogonal geometries for future surveys in such a areas considering operati on constrains, channel deployment, block dimension etc.

The study reveals that higher channel count yields a more economic soluti on, though there exists a limit to which the economics is favourable with respect to increasing channel count. There is a criti cal point beyond which increase in channel becomes uneconomic.


2D seismic survey design considerations in logistically difficult areas in parts of Arunachal Pradesh – A case study

K.L. Mandal, B.J. Reddy, T. Bhatt acharya & R.S. Ram

The belt of Schupen in the fronti er part of Upper Assam basin has been considered highly prospecti ve from hydrocarbon point of view. However, these areas have remained largely unexplored due to inaccessible terrain and extreme logisti cs. As part of its intensifi ed explorati on acti viti es in the fronti ers areas, Oil India Limited (OIL) had undertaken extensive seismic surveys in the foothill areas surrounding Kumchai and Namchik in Arunachal Pradesh. The topography of the area is rugged and is covered with thick forests, numerous river channels, commercial plantati ons, habitati ons etc. The near surface is covered very thin alluvium, along with thick near surface boulders and riverine sands. This rendered shot-hole drilling up to the desired depth considerably diffi cult. In order to overcome the diffi culti es posed by terrain and near surface conditi ons, best available portable mechanized shot-hole drilling rigs were deployed for the survey. Acquisiti on geometry deployed was end-on with 120

Seismic survey in logistically difficult hilly terrain with boulder beds underneath in the proximity of the foothills of Himalayas using state- of-the-art shot hole drilling techniques

Akshaya Kumar & Rajiv Nigam

Oil India Limited (OIL)’s explorati on for hydrocarbons today has moved into challenging and new fronti er areas. The days of easy oil fi elds are behind us. Present day seismic surveys are being carried out in logisti cally diffi cult and hosti le terrain conditi ons. OIL is conducti ng seismic surveys in rugged terrain of the Himalayan foothills with thick forest cover in Kashmir PEL areas in the state of Utt aranchal. The environment is extremely testi ng. It is seen that conventi onal shot hole drilling techniques are no longer applicable in such complex areas with boulder beds, loose gravels, wet and loose sands for good quality data acquisiti on.

For obtaining meaningful data/informati on of the subsurface, it is extremely important to minor the quality at the acquisiti on stage. For achieving good data quality which mainly dependent upon the opti mum shot hole depth, planning has been made for data acquisiti on in the Himalayan foothills. There is a need to deploy the mechanized shot hole drilling rigs which have the capabiliti es to drill in boulder bed areas of the logisti cally diffi cult terrain covered with dense forest.

Any compromise at the “shot-hole drilling depth” will directly aff ect the quality of the data acquired, which cannot be improved by any processing technique at a later stage if missed at the stage of acquisiti on.

The present paper discusses case history of the acquisiti on of good quality seismic data by OIL in the logisti cally hosti le, hilly and rugged terrain of Himalayan foothills through suitable shot-hole drilling rigs(hydraulic, pneumati c and mechanical) for achieving opti mal shot-hole depth even in areas where the only approach is through step climbing. These miniature marvels have made shot hole drilling possible even in the toughest and steepest terrains, which otherwise would have been diffi cult even with heli-portable operati ons.


Wide angle land 3D seismic survey, optimizing to find the limit

F. Siddiquee & R. Dasgupta

In recent years, approach towards land 3D seismic surveys has undergone signifi cant changes. The narrow azimuth 3D seismic surveys are being progressively

44


channels acti ve channels per shot. However, diffi culti es in shot hole drilling in boulder beds, poor shot-hole drilling in boulder beds, poor shot-hole coupling due to thick sand column/boulder, and other logisti c, ecological & environmental considerati ons lead to large data gaps in seismic profi les, translati ng into disconti nuous mapping of the subsurface. Alternate geometry with large number of channel deployment was discounted in view of logisti c, operati onal & survey cost considerati ons, for proper imaging of the subsurface. Therefore, an att empt was made to image the subsurface using an unconventi onal scheme of recovery shots, without escalati ng the cost of survey.

To start with, the patches in a profi le that generated good seismic signals were identi fi ed. Thereaft er, the seismic profi le was simulated considering such good shots in order to identi fy the fold & off set patt ern generated for each Common Mid Point. Possible recovery shots in such good patches were identi fi ed that could compensate for the required fold gaps & missing off sets. Recovery shots with reverse spread were planned through simulati on in a way that made the geological objecti ves of the survey successful. The paper highlights the methodology adopted to overcome the diffi culti es in undertaking 2D seismic surveys under logisti cally diffi cult terrain conditi ons.


Problems and challenges in conducting seismic survey operations in logistically difficult hilly and rugged terrain in the foothills of Uttaranchal- A case study

Akshaya Kumar & Bedanta Pd. Sarama

Oil India Limited (OIL)’s explorati on for hydrocarbons today has moved in to challenging and new fronti er areas. The days of easy oil fi nds are behind us. Present day seismic surveys are being carried out in logisti cally diffi cult and hosti le terrain conditi ons. OIL recently for the fi rst ti me conducted seismic survey (dynamite) in rugged terrain of the Himalayan foothills having dense forest cover in Kashipur PEL areas in the state of Utt aranchal. The environment is extremely challenging and logisti cally diffi cult, beside the surface logisti cs, it includes the diffi cult subsurface conditi ons as well. Past experience has shown that conventi onal shot-hole drilling techniques are no longer applicable in such complex areas which are underlain by boulder beds, loose gravels mixed with wet and loose sands, fl owing water at shallow depths for acquiring good quality seismic data.

It is extremely important to monitor the “Quality” at

the acquisiti on stage for obtaining meaningful data/informati on of the subsurface. For achieving good data quality, which is mainly dependent upon the opti mum shot hole depth and shooti ng medium, proper advance planning for data acquisiti on in the forest infested areas in Utt aranchal foothills has played a pivotal role. Based on earlier experiences need is felt for deploying mechanized shot-hole drilling rigs having demonstrated proven capabiliti es for drilling in boulder bed areas in logisti cally diffi cult terrain.

The present paper discussed the case history of the acquisiti on of good quality seismic data by OIL in the logisti cally hosti le, hilly and rugged terrain of Utt ranchal foothills, deploying suitable state-of-the-art shot-hole drilling rigs (hydraulic and pneumati c) for achieving opti mal shot-hole depth even in areas where the only approach is through steep climbing. These portable rigs, which are really miniature marvels has made shot-hole drilling possible even in the toughest and steepest terrains, which otherwise would have been diffi cult even with heli-portable operati ons. During seismic survey operati ons various challenges have been faced in view of several factor like use of dynamite as energy source, reserve forest (ANR region), presence of wildlife, highly undulati ng terrain conditi ons etc. which called for practi cing of stringent safety aspects and at the same ti me ensuring the quality of data acquired within the laid down ti me frame in the most cost-eff ecti ve manner has also been dwelt upon.


Near surface modelling for statics computation in Upper Assam, India - A case study

R. Dasgupta, T. Bhatt acharya, D.S. Manral & K.K. Nath

Wide range of near surface anomalies can be caused by the presence of weathered layer. The velocity and the thickness of the weathered layer vary signifi cantly. Such near surface anomalies are required to be compensated during processing of seismic data, otherwise not only it degrades the stacks but it might also produce arti facts in the seismic data. The shallow refracti on survey (LVL) measurements provided the near surface interval velocity and layer thickness based on which the initi al model was constructed. The model was fi ne-tuned using the velocity measurements as derived from Uphole surveys.

Prior to 3D seismic coverage in Upper Assam, India, the near surface model (of layer thickness and velocity) was esti mated for every 2D seismic line from which the stati c correcti ons to the datum were computed. With extensive 3D coverage and wealth of Uphole and LVL observati on available, development of a conti nuous 3D near surface

45

parti cular and Ganga basin, in general has so far proved futi le through oil/gas shows have been reported from many parts of the basin. Limited exploratory drilling, so far, has not encountered the much expected Eocene source rock i.e. Subathu Formati on, which are reportedly exposed in several thrust sheets in the area. Oil India Limited (OIL), for the fi rst ti me conducted seismic surveys (dynamite) in geologically complex and extremely rugged terrain of the Utt aranchal foothills. Processing and interpretati on of the date, parti cularly applicati on of advanced techniques elucidated the hydrocarbon prospecti vity of the study area from the explorati on point of view. Identi fi cati on of Triangular zones (triangle zones having thrust-stack prospects with apparent closure as well as new plays, etc.) which appear analogues to many other oil/gas bearing triangular zones around the world, have renewed interest in the area. Triangular zones in the study area are made-up of stacked south-traveling thrust sheets and their north-vergent back thrust, build on a detachment at top of Vindhyan where the target is Dharamsala (reservoir) and Subathu (source and reservoir).

The area merits a sustained explorati on programme with techno-economic considerati on. The present paper highlights focus on the explorati on in triangular zones for deep as well as shallow gas/oil reservoirs, which is expected to result in the discovery of hydrocarbon in Utt aranchal foothills.

Published in GEO-INDIA-2008

3D seismic survey in a highly populated area - A case study

R. Dasgupta, Y.R. Singh, D.S. Manral, T. Bhatt acharya,

K.L. Mandal & D.N. Lahkar

Designing a 3D seismic survey is a criti cal and elaborate exercise if it is to be carried out in a thickly populated area of geological interest from the hydrocarbon prospect point of view. In such an area, apart from considering various geophysical data att ributes like off set distributi on, azimuthal distributi on, opti mum bin size and foldage, other two important factors such as populati on distributi on and near sub-soil conditi on also play a vital role which need a careful considerati on while designing a 3D geometry. In 1998-99, Oil India Limited (OIL) carried out 3D seismic survey over the ageing Moral oil fi eld in its PEL/ML areas in Upper Assam basin. The primary geological objecti ve was to extract all possible structural informati on from the deeper Eocene prospects as well as to extract lithological/structural informati on from the relati vely shallow horizons viz. Barail and Tipam of Oligocene-Miocene age so that future development of the old fi eld can be eff ecti vely planned. The area is highly populated- the Moran town and quite a few villages fall

model was att empted. In Upper Assam the seismic data acquisiti on is generally done in dry winter season and this results in very litt le fl uctuati on in upper layer. Hence, it is envisaged that a near surface velocity-depth model would allow for accurate computati on of stati cs at any point in Upper Assam.

Published in EAGE-1999

Solution for imaging in logistically constraint areas: A case study from Upper Assam

K. Dasgupta, M. K. Banerjee, S. Maji & R. Dasgupta

The study area falls within the operati onal areas of Oil India Limited at Upper Assam. A discovered oil and gas fi eld is present in the study area. Area known as Baghjan is a promising fi eld with good coverage of 3D data having medium size combinati onal Traps. However, a major part of the fi eld could not be covered by previous 3D due to the presence of North-South elongated water fi lled marshy swamp having thick volume of recent sediments mostly loose fi ner parti cles of clayey nature. Average length of the swamp is 7.0 Km and the width is greater than 1.8 Km at places. The objecti ve of the survey is to compensate the data gap of previous 3D survey, provide conti nuous imaging at prospect level to facilitate integrated interpretati on and DHI responses detecti on.

It is diffi cult to execute Geodeti c survey prior to seismic and to plant sensors during seismic over the Bagjhan swamp. Shot hole drilling is also diffi cult within the swamp because of high sand pressure. A Close-Grid 2D Survey of 200m line spacing and 50 m Shot spacing was designed with high channel count split-spread geometry having adequate shots and receivers placed on both side of the swamp. Moreover, to provide bett er coupling of receivers to the ground, the geophone bunches were planted on soil mounts which provided bett er coupling to the ground instead of planti ng directly on the very lose swampy soil. To image the subsurface below the swamp, where neither shots nor receivers could be placed, and reverse shots were taken at one side of the swamp with 25 m shot interval and higher no of receivers on the other side of the swamp to provide high foldage. The innovati ve CDP coverage-Off set compensati on technique adopted in acquisiti on parameter design was successful to obtain desired result aft er processing of the Data.

Published in GEOINDIA-2008

Re-look from the prospectivity of Uttaranchal foothills in OIL's Kashipur PEL area

Akshaya Kumar, P. Barua & B. P. Sarma

The hydrocarbon explorati on in Himalayan foothills, in

46


inside the block. The area also covers the residenti al colony of the Oil India. Apart from this, a river fl ows through the block.

Considering the above, the 3D seismic survey was to designed not only to meet our geological objecti ves but also to overcome the problems posed by the surface logisti cs and urbanizati on. Taking all the above into account and analyzing the results obtained from simulati on studies, it was planned to cover the area with four line swath shooti ng geometry, having 120 channels per line, with 112 acti ve channels laid out in each line. This four line acquisiti on geometry had a symmetrical split spread defi niti on. It provided focusing of the higher off sets in the zones defi ning the producing sands of Barail formati on.

In this survey, seismic gelati ns in couplable tubes were used as energy sources. The logisti cs of the area were such that the shot points had to be carefully planned so that the structures were not damaged during the survey, whereas the objecti ve of the survey could be met. In this paper we will also present the methodology adopted in placement of the shots using the sub-soil informati on from uphole survey data in such semi-urban/thickly populated area.

Published in IGU-1999

Utilization of Radio Telemetry Acquisition System in Brahmaputra river bed area - A case study

P.K. Sharma, F. Siddiquee, Y.P. Singh,

Anup Kumar & D.N. Lahkar

The Upper Assam basin has been divided into two separate regions by mighty river Brahmaputra which is fl owing more or less in middle of the basin. Most of the areas lying towards the south of river Brahmaputra (South Bank) have been adequately covered by seismic survey and it is proved as a main bowl of hydrocarbon of the upper Assam basin. Its counter part on the north of Brahmaputra has been separately covered by the seismic survey and ti ll date no signifi cant discovery of hydrocarbon has been made in this region. The area between two regions is occupied by the river Brahmaputra and its tributaries which forms a logisti cally hosti le terrain of about 20 km width. The region is widely covered by swamps, river deltas, river channels of varying depth and width, loose sand patches, wild grasses etc. where the possibility of acquiring seismic data by uti lizati on of conventi onal system with OIL’s existi ng infrastructural faciliti es is next to impossible. This is because of the limitati on that the conventi onal CDP shooti ng requires conti nuous cable layer all along the seismic profi le and

also it requires the shift ing of instrument positi on as the shot point progresses along the profi le. In additi on to the deployment and transportati on of instrument and ground electronics there are other limitati on with the CDP cable such as leakages, resistances and crossfed among channels which get very prominent as CDP cables are laid out in the swampy, marshy and water logged areas.

Thus to acquire the data in this wide gap the one opti on that looked feasible was the uti lizati on of Radio Telemetry system. With the above objecti ve in mind Oil India Limited for the fi rst ti me in its operati onal area deployed the RF (Radio Frequency) Telemetry system (Eagle SN388) which has been hired from Nati onal Geophysical Research Insti tute (NGRI). Thereby a courage has been demonstrated by Oil India Limited by shooti ng a regional line of about 30km connecti ng south to north of river Brahmaputra which was made possible only by uti lizati on of Radio Telemetry system.

It is observed that recepti on and transmission of RF signals improves a lot as the ground electronics (Remote Data Acquisiti on Unit, RDAU) are moved inside the river bed. In such a challenging and environmentally sensiti ve area this instrument could be eff ecti vely uti lized to gather data within a range of 18 to 20km from the central recording unit without much of signal distorti on and clear recepti on of RF signal. The drawback that has been observed with this instrument is that the antenna which transmit the data from RDAU to the Central Recording unit is a unidirecti onal one and as a result when the shot point moved further inside the river it was diffi cult to align the RDAU antenna directi on towards the line of sight of antenna that was fi xed with the recording unit. This short coming can be overcome by the uti lizati on of multi directi onal antenna. Further the antenna height in the RDAU has to be increased for getti ng the good RF signals. The Radio Telemetry system was also uti lized in the normal terrain of South Bank of river Brahmaputra. In this area, it was observed that in case of thick vegetati on, forest covered area, the recepti on of signal from RDAU to central recording unit was not god. Based on the limited experience/use of equipment (eagle SN388) of about one and half month. It is practi cally felt that the range of instrument deteriorates considerably in the normal terrain (3-4 km only) than that of river bed area.

The present paper illustrates the advantages and disadvantages of Radio Telemetry system over the conventi onal CDP cable system in acquisiti on of seismic data in logisti cally diffi cult terrain.


47

Challenges in Thrust-Belt exploration - A case study from Upper Assam basin, India

K.L. Mandal, D.S. Manral & A.K. Khanna

The thrust-belt areas lying in East and South-East part of the Upper Assam Basin, India are a geologically prospecti ve province in terms of hydrocarbon resources. Over the years few oil fi elds have been discovered in these areas. The conti nuous endeavour to unravel the complex subsurface geology in and around these thrust- belts has led to a series of seismic survey campaigns in the quest for hydrocarbon resources. There are a few success stories; however these are sti ll limited, primarily because of poor imaging of subsurface geology. Severe constraints in terms of diffi cult logisti cs & complex subsurface geology make seismic data acquisiti on and processing programs challenging for imaging of subsurface geology.

We present a case study over a part of these thrust belt areas, where a fresh seismic campaign had been carried out recently based on the geological leads of previous seismic survey to obtain improved subsurface images. Ingenuity in planning, eff ecti ve implementati on of the survey design, stringent quality control measures, fi t-for-purpose operati onal methodology & seismic data processing has enabled in achieving the primary objecti ves of imaging the Girujan and Tipam formati on of Oligocene age in the area. The structural geometry of the subsurface features of interest in hydrocarbon explorati on could be delineated and understood in bett er details, during the course of interpretati on and therefore yielded in validati on and release of locati ons for exploratory and development with an increase degree of confi dence.

Published in ISM-2007

Cost effective and optimised 3D seismic surveys – Examples from Upper Assam

R. Dasgupta, D.S. Manral & T. Bhatt acharjee

In modern E&P business, 3D seismic survey has become an integral tool for oil fi eld development. The implementati on of 3D seismic surveys needs a well thought designing of the acquisiti on parameters that will enable the geophysicists to achieve the geological objecti ves in cost eff ecti ve manner. Thus, the objecti ve of the geophysicist is to design geometry, the geophysical att ributes like bin-size, fold, off set and azimuth distributi on etc. will meet the geological objecti ves of the survey (target depth, geological features of the reservoir etc.) and at the same ti me it should be cost-eff ecti ve. The 3D seismic survey design therefore leads to a process of

opti mizing the geophysical att ributes of the survey vis-a-vis its cost implicati ons.

The opti mizati on process has broadly three steps:

i) Choice of few appropriate geometries from a basket of geometries depending on the geological objecti ve of the survey, logisti cs of the area where survey is planned and the operati onal constraints;

ii) Parameterizati on of the geometries for getti ng proper geophysical att ributes so that the geological objecti ves of the survey is achieved. This process involves fi nding out appropriate bin-size, in-line and cross-line folds, maximum off set range, off set and azimuth distributi on. Well-data, oil-fi eld informati on like oil-water contact, previous seismic data and structural contour maps are used extensively in this process. Operati onal constraints (like maximum fi eld electronics that can be handled), logisti cs and size of the block are some of the determining factors in this exercise.

iii) Aft er parameterizati on of the geometries, diff erent opti ons are generated and the cost-implicati on of each opti on is deduced. The cost is generally derived from historical cost data of the area of operati ons. The geometries are then ranked with respect to the cost.

The opti mizati on technique for 3D seismic survey design is applied for obtaining the opti mized geometries for diff erent blocks in Upper Assam. The blocks were of diff erent sizes and the geological objecti ves of the surveys were at ti mes diff erent. The areas had diff erent degrees of logisti cs diffi culti es. The opti mizati on method described above enabled us to design geometries that can provide a balance between the source eff ort and receiver eff ort. In the actual paper, some of the examples of opti mizati on exercises of 3D seismic survey geometries will be presented.

Published in PETROTECH-2003

Acquisition challenges in logistically difficult areas and its solution - A case study from Upper Assam

Dasgupta Koustav, Dasgupta Rahul,

Banerjee Mayank & Maji Subhadeep

The study area falls within the operati onal areas of oil India limited at Upper Assam. A discovered oil and gas fi eld is present in the area. The area is characterized by presence of a Nati onal Park at its north where seismic acti vity is prohibited. Additi onally, the area, at its west and south, is bounded by a swamp. With the use of GIS diff erent logisti c problems were analyzed and a three

48


phase explorati on programme was designed. The project helped to cover the seismic data gap essenti al to image below the swamp.


Effects of seismic blasting in thickly populated areas – An experimental study in Upper Assam, India

R. Dasgupta, P. K. Paul, Akshaya. Kumar,

P. Pal Roy, C. Sowmliana & R.P. Singh

The avoidance of damage to structures during land seismic survey in thickly populated areas is a challenge. A novel experimental trial was conducted at diff erent sites in the thickly populated area of Upper Assam, India. The methodology of the experiments was adopted from the mining industry. The experiments included measurements of vibrati ons (mainly peak parti cle velocity) recorded at diff erent distances caused due to blasti ng of seismic explosives of variable sizes at diff erent depths. An empirical equati on relati ng parameters like peak parti cle velocity, charge size, distance of recording was determined using stati sti cal regression of the recorded data, keeping in view the state’s standards. In this paper, the principle of the method, details of experimentati on and results are presented.

Published in SEG-2008

3D seismic survey in parts of Upper Assam Basin - A Critical review

S. Rath, M. Chatt erjee & R. Dasgupta

3D seismic survey is being carried out in operati onal areas of Oil India Limited in upper Assam Basin from the start of this decade. The 3D seismic surveys are conducted in these areas to image accurately structural features, complex fault patt erns, develop newly discovered fi elds, explorati on of subtle traps, revitalizati on of oil matured fi elds and to explore the untapped deeper Eocene prospects below established Oligocene-Miocene producers. In the design of the 3D surveys, various factors including geology depth of reservoirs, logisti cs, etc. were considered. Such surveys were successfully completed over thickly populated semi-urban areas and townships apart from lands with agriculture and forest cover. The 3D surveys have enabled to generate new prospects, bett er imaging of faults and structures of various magnitudes.

Published in SPG-2000

Optimizing land 3D seismic survey design - Methodology and examples

R. Dasgupta, D.S. Manral, T. Bhatt acharya & Anup Kumar

Three-dimensional seismic survey is a major tool in explorati on for hydrocarbons. In order to achieve the

objecti ves of the survey, the acquisiti on needs to have the proper geophysical att ributes and should be cost eff ecti ve. In this paper, we present a methodology for opti mizati on of land 3D seismic survey geometries. The process involves study of appropriate geometries with respect to their geophysical att ributes and their relati ve cost. The relati ve cost is determined by a relati ve cost factor equati on which takes care of diff erent factor which are appropriate in the context of our country. Two examples are also presented to show the effi cacy of the method.


An unconventional approach to cover the data gaps in 2D seismic survey – A case study

P.K. Paul, A. Kumar & R. Dasgupta

Accessibility to the area of operati on is one of the key factors governing the ease of seismic operati on. However, data gaps due to inaccessibility in the form of restricted area, bad logisti cs and environmental restricti ons poses the biggest hurdle which results in low fold for CMP and also leading to poor signal to noise rati o which in turn lead to poor stack, insuffi cient number of diff erent off sets for velocity analysis and inaccurate residuals stati cs.

In such cases the data gaps, which could not be covered seismically due to logisti cs and also because of conventi onal recording methodology, can now be covered with the use of non-conventi onal acquisiti on geometry in amalgamati on with the new technology like radio telemetry system or combinati on of radio and line telemetry system.

Oil India Limited (OIL), a Govt of India Enterprise, took one of such areas up from its operati onal areas of Upper Assam. These areas could not be covered seismically earlier because of the logisti cs and the other being the restricted area. Radio Telemetry system hired from NGRI, Hyderabad was used for seismic data acquisiti on in these areas along with a non-conventi onal methodology as adopted to cover these areas depending on the logisti cs.

In this paper we present a case study to demonstrate the use of unconventi onal data acquisiti on in order to cover such seismic data gaps. The data gaps existed in the vicinity of the major oil and natural gas producing province of Upper Assam basin. In view of the diffi cult logisti cs and proximity to a nati onal park, the seismic acquisiti on could not be carried out in this area. By using unconventi onal data acquisiti on parameters along with applicati on of proper technology, the area could be covered and an interpretable seismic data could be acquired. A signifi cant prospect was identi fi ed on the interpretati on of this acquired data for development in

49

future. The use of unconventi onal technique not only helped in this regard, but also was cost eff ecti ve. This type of unconventi onal acquisiti on will be further used to cover other areas where such data gaps exist.


Environment friendly seismic surveys in logistically difficult areas within Brahmaputra River-Bed – A case study

K.L. Mandal, T. Bhatt acharjee & R. Dasgupta

Recent hydrocarbon discoveries in Upper Assam have raised the hydrocarbon prospects of the Brahmaputra riverbed. However, diffi cult logisti cs and ecological fragility of the area makes conventi onal seismic surveys across the river-bed challenging. Oil India Limited deployed an in-house 2D seismic survey crew recently to undertake a seismic survey across parts of the river-bed. Using a mix of innovati ve survey design and operati onal methodology, the primary objecti ves of imaging the Upper Paleocene-Eocene sediments beneath the Brahmaputra river-bed was achieved.


Problems and challenges in conducting seismic survey operations in logistically difficult hilly and rugged terrain of the foothills of Uttaranchal – A case study

Akshaya Kumar, Pramitabha Barua & Bedanta Pd. Sarma

Oil India Limited (OIL)’s explorati on for hydrocarbons has moved today into challenging and new fronti er areas. The days of easy oil fi nds are behind us. Present day seismic surveys are being carried out in logisti cally diffi cult and hosti le terrain conditi ons. OIL, recently, conducted seismic surveys (dynamite) in rugged terrain of the Himalayan foothills having dense forest cover in Kashipur PEL areas in the state of Utt aranchal. The environment is extremely challenging and logisti cally diffi cult, beside the surface logisti cs, it includes the diffi cult subsurface conditi ons as well. Past experience has shown for acquiring good quality seismic data that conventi onal shot-hole drilling techniques are no longer applicable in such complex areas which are underlain by boulder beds, loose gravels mixed with wet and loose sands, fl owing water at shallow depths.

It is extremely important to monitor the “Quality” at the acquisiti on stage for obtaining meaningful data/informati on of the subsurface. For achieving good data quality, which is mainly dependent upon the opti mum

shot hole depth and shooti ng medium, proper advance planning for data acquisiti on in the forest infested areas in Utt aranchal foothills has played a pivotal role. Based on earlier experiences need was felt for deploying mechanized shot-hole drilling rigs having demonstrated proven capabiliti es for drilling in boulder bed areas in logisti cally diffi cult terrain.

The present paper discusses the case history of the acquisiti on of good quality seismic data by OIL in the logisti cally hosti le, hilly and rugged terrain of Utt ranchal foothills deploying suitable state-of-the-art shot-hole drilling rigs (hydraulic and pneumati c) for achieving opti mal shot-hole depth even in areas where the only approach is through steep climbing. These portable rigs, which are really miniature marvels has made shot hole drilling possible even in the toughest and steepest terrains, which otherwise would have been diffi cult even with heli-portable operati ons. During seismic survey operati ons various challenges have been faced in view of several factors like use of dynamite as energy source, reserve forest (ANR region), presence of wildlife, highly undulati ng terrain conditi ons etc. which called for practi cing of stringent safety aspects and at the same ti me ensuring the quality of data acquired within the laid down ti me frame in the most cost-eff ecti ve manner has also been dwelt upon.


Optimization of seismic survey in logistically difficult area: A case study from the upper Assam basin

Surender Singh Rana, V. Kara,

A.S.R.K. Sastry, P.K. Paul & A. Kumar

Ingenuity in planning, designing and simulati on of 3D seismic surveys plays a pivotal role in realizing the geological objecti ves of the survey. However, stringent implementati on of the plan & acquisiti on design in the fi eld during the course of seismic data acquisiti on assumes much more signifi cance when the area is compounded by diffi cult logisti cs besides being environmentally sensiti ve. OIL recently acquired 3D seismic data in Majuli Island (world largest river island) in River Brahmaputra. An effi cient methodology covering all the aspects of the said seismic survey was chalked out, which other than technical aspects included mentoring of OIL staff , appraisal of the challenges in the area of operati on to the enti re crew, awareness of the project to local authoriti es & people etc. for effi cient implementati on of the survey. OIL was successful in carrying out the proposed 3D seismic survey in an environment friendly manner.


50


Spectral Coherency Technique for interpolation on seismic traces

V. K. Sibal & R. K. Pathak

Although the development in Geophysical world has

been multi -fold in the last two decades in terms of

precision delineati on technique for very high temporal

and spati al resoluti on, the need for extracti on of more

and more informati on from the existi ng data has grown

simultaneously at an equal pace.

Interpolati on of spati ally aliased seismic data is oft en

used to improve the quality of multi -trace processing,

viz. migrati on. At ti mes, interpolati on of stacked seismic

traces is also desired during interpretati on to generate

traces of uniform interval all over the study area.

A technique for interpolati on of seismic traces, based on

the magnitude of the spectral coherency at the dominant

frequency is ascribed in the paper. A program has

been developed on SIDIS* workstati on using FORTRAN

77 programming language for interpolati on of SIDIS

formatt ed seismic data. The technique has been applied

and tested on a set of seismic traces from one of oil fi elds

in upper Assam area and the results are presented in the

paper. The technique will go a long way in enhancing

the spati al resoluti on of seismic data parti cularly from

the stable zone of off shore region and would contribute

tremendously for the off shore 3D seismic data acquired

by adopti ng the single streamer methodology.


Delineation of subthrust formation through application of Dip Move Out (DMO) – A case Study from Upper Assam valley

A. K. Khanna & S. N. Singh

With the rapid growth of technology in the fi eld of

hydrocarbon explorati on, it has been quite challenging

for the explorati on geophysicists to steer the present

day search for oil more towards the high risk areas with

seemingly insurmountable surface, near surface and

subsurface complexiti es. This has already established

the need for high resoluti on seismic data acquisiti on to

facilitate objecti ve oriented data processing with a view

to achieve bett er subsurface defi niti on. On the other

hand, the conti nuing search for hydrocarbon has resulted

in accumulati on of a huge volume and wide variety of

data of diff erent vintages. Eff orts are, therefore, felt

mandatory to conti nuously view, review and integrate

these data, not only to decide on an appropriate

explorati on strategy but also for sustaining a cost eff ecti ve

explorati on programme.

In OIL’s (Oil India Limited) PEL area around the Naga-

Thrust area, OIL acquired 2D seismic data in late eighti es.

The diffi cult terrain conditi on prevented in uniform

shooti ng patt ern, oft en the data acquisiti on as per

planned parameter was not plausible. The conventi onally

processed seismic secti on though depicted the subsurface

features but confi dent mapping, especially around the

sub-thrust formati ons, was not possible.

An att empt has been made in this paper to enumerate

the applicati on of “Dip Move Out (DMO)” on the above

acquired data and its possible success in providing

conclusive leads for explorati on around the geologically

complex Naga-Thrust region around the Upper Assam

oil fi elds. Processed seismic secti ons from this parts of

Assam Arakan basin, individual seismic lines and line

joining data of diff erent vintage, have been presented

for bett er appreciati on of the complex tectonics and the

related explorati on problems in the region.


Seismic velocities for effective planning of exploratory well in virgin area – A case study from North Bank areas of Upper Assam

C. S. Singh, A. K. Khanna & S. N. Singh

The conti nuing explorati on venture of Oil India Limited

(OIL) in the South Bank of river Brahmaputra provided

possible clue of hydrocarbon accumulati on in North-Bank

of the river. In conti nuance of its extensive explorati on

programme, OIL acquired the PEL of about 4200 Sq. Km

covering the north bank of the river and subsequently

collected about 1700 GLKM of 2D seismic data of diff erent

vintages from the erstwhile ONGC to assess the various

explorati on objecti ves in the area.

Based on the interpretati on of these seismic data and

other available geological informati on’s, OIL planned to

drill a few exploratory wells in the area. Simultaneous to

the above geoscienti fi c work and planning for drilling, OIL

deployed one of its 2D seismic crew to acquire data in

the area during the year 1995-96. It was felt prudent to

process these data on a high priority basis and review the

various explorati on targets prior to spudding the fi rst well

in the area.

The acquired data was processed using OIL’s in-house

Landmark Seismic Data Processing system with ProMax

52

Seismic Data Processing“No Magic - Just Solid Science”

6.1 processing soft ware and the results were made

available as expected. This paper discusses the result of

this processing with specifi c emphasis on the advantage

of interacti ve velocity analysis which provided reliable

lead in guiding the well planning. It has been observed

that the seismic velocity macro model esti mated from the

available drilling informati on in adjacent South Bank wells

and the geological correlati on from the South Bank area,

needed considerable validati on to meet the objecti ves.

The generated seismic secti ons were converted to depth

secti on using the products of interacti ve velocity analysis/

manipulati on and was a major input to the esti mated

depth model.


Indentification of possible Gas Hydrate prospects in offshore, Saurashtra, India

V.K.Sibal & S.N.Singh

Conti nuing explorati on eff orts by Oil India Limited in

Off shore, areas of its Saurashtra Explorati on Project, off

the western coast of India has led to accumulati on of

a good volume of 2D seismic data, in additi on to other

geological and drilling leads. In view of the growing global

and nati onal interest in tapping deep marine natural Gas

Hydrate resources it was felt prudent to initi ate a pilot

study to look for the occurrence of any such possible

resources in Saurashtra Coast.

It is envisaged that that the off shore areas of Saurashtra

posses a good potenti al of housing sediments rich in

Gas Hydrate due to high sediment infl ux from Indus Fan

complex and Narmada-Tapti river system coupled with a

high pressure-low temperature environment conducive

to generate Natural Gas Hydrates.

The present study uses the available Multi Channel

seismic Data (MCD) from the parts of a few seismic

lines picked. The present day conti nental shelf and used

model based seismic modeling data processing with

the objecti ve of making the BSR’s (Bott om Simulati ng

refl ectors) discernible. The paper discusses the various

aspects of Data preparati on, Data processing used for

enacti ng the BSR in the middle of loose, unconsolidated

sediments close to the sea bott om and the subsequent

interpretati on of possible Gas Hydrate Prospects.

The results have been extremely encouraging and have

postulated a tremendous scope for further special studies

for objecti ve oriented planning, acquisiti on, processing

and special processing of seismic data to arrive at

convincing and conclusive inferences on Gas Hydrate

prospects in the area.


Some aspects of Time to Depth conversion from seismic velocities: A case study

A.K. Malkani, G.R. Saini & K.K. Nath

One of the most common, yet diffi cult task in seismic data

processing is to determine the correct spati al positi on

of seismic refl ectors. The refl ectors as seen on the ti me

secti on may deviate (due to velocity variati ons at the

interfaces) from the actual subsurface positi ons of the

geologic features. Depth secti ons generated by conversion

of arrival ti mes into depth are used to esti mate correct

positi on of these subsurface features. Time to depth

conversion is a crucial step; aft er all, a well is drilled in

depth not in seismic ti me. Conversion of ti me secti on into

depth is performed using velociti es derived from seismic

data, which are further modifi ed by correcti on with well

velociti es. Seismic velociti es have poor verti cal resoluti on

and good spati al control in comparison with well data. A

combinati on of seismic and well log data is, therefore,

preferable for accurate depth conversion of seismic data.

Interval/average velocity in depth derived using Dix’

equati on and RMS velociti es converted from horizon

velocity analysis were used to produce the depth

secti on for comparison. RMS velociti es produced noisy

results; therefore, a smooth velocity functi on was used.

Smoothing may cause suppressions of subtle features if

smoothening operator is not selected carefully. In the

present study, an att empt has been made for selecti on

of the opti mum velocity fi eld for depth conversion/depth

migrati on through appropriate velocity modeling which

produces reliable depth informati on prior to drilling a

well.


Determining the proper Post-Stack Migration technique for 2D seismic data from Upper Assam

C.V.G. Krishna, Y.P. Singh & B.M. Sinha

Oil India Ltd. (OIL), a premier nati onal oil company, has

acquired large amount of 2D seismic data in its operati onal

areas especially in Upper Assam, in OIL’s operati onal

areas substanti al oil producti on comes from the Eocene

reservoirs which are mostly associated with host and

graben features at around 3000-4000m depth. Most of

53

these strictures were identi fi ed based on 2D seismic data.

During the processing of the 2D seismic data, utmost

care is required to build a proper velocity model for input

to the stacking and post stack migrati on method to be

used. Diff erent migrati on methods like Kirchhoff , fi nite

diff erence, frequent domain migrati on etc. were tested

for the proper imaging of these hydrocarbon bearing

structures. It may be menti oned here that velocity is

the most important parameter for any proper migrati on

technique. To have a good velocity model normal

moveout analysis were carried out at diff erent stages of

processing using a sequence which involves picking of

initi al velociti es, computati on of residual stati cs and then

using the fi rst pass residual stati cs in a second pass velocity

analysis. This is followed by the computati on of second

pass residual stati cs. Velociti es were picked interacti vely,

in the fi rst pass using constant velocity stacks and in the

second pass using 15 velocity functi ons centered on

initi al velocity picks, with a minimum and maximum limit

of 10 % either side. With the two pass velocity analysis

the velocity model was considerably improved.

In this paper the results of the testi ng of the diff erent

migrati on methods have been discussed which have

shown that Kirchhoff migrati on has shown bett er results

compared to the other methods in properly imaging of

the deeper structures in our operati onal areas.


Residual statics application in seismic data processing for thick boulder covered areas of Arunachal Pradesh

C.V.G. Krishna, T.Bhatt acharya, Anup Kumar,

G.V.J. Rao & B.M.Sinha

Seismic refl ecti on ti mes are oft en aff ected by irregulariti es

at the near surface. Field stati c correcti ons remove a

signifi cant part of these travel ti me distorti ons from the

data caused by these irregulariti es. Nevertheless, these

correcti ons usually do not account for rapid changes

in elevati on, the base of weathering layer and the

weathering velocity. Oil India Limited (OIL) has recently

embarked upon an accelerated geophysics explorati on

programme in the logisti cally diffi cult and geologically

complex fronti er areas in North-East India. The parti cular

area under study- Manabhum in Arunachal Pradesh is

located at the juncti on of two most signifi cant folding

movements in the region namely the East-West folding

parallel to the Mishmi Hills. The area is geologically

complex with the presence of thrusts and fold structures.

Recently, OIL has initi ated acquisiti on of 2D seismic

data in this area by hiring services. Data acquired so far

has been collected over & across an anti cline structure

which was partly exposed on the surface. Nearer to the

anti clinal structure the surface is mostly hilly with surfaces

elevati ons varying between 100 to 500 meters, with the

top few hundred meters mostly of loose sediments and

boulders/pebbles.

The data has been processed using elevati on stati cs, with

the latest version of ProMAX seismic data processing

soft ware. The processed seismic line does not show any

coherent seismic events over the anti clinal structure

where the variati on of surface elevati on is very high. The

proper calculati on & applicati on of the residual stati cs,

with multi ple pass velocity analysis is found to improve

the quality of the processed seismic secti on considerably

over the anti clinal features in the form of improved

conti nuity and bett er alignment of refl ected events. A

case study from the Manabhum area is presented in this

paper highlighti ng the methodology used for calculati on

of the residual stati cs and its applicati on in processing the

seismic profi le.


Problems in the thrust-belt imaging – A case study from Upper Assam

K.L.Mandal, D.S.Manral & B.J.Reddy

In any thrust belt, seismic data quality is generally poor due to complex geological setti ngs associated with such areas. The imaging becomes more complicated due to scatt ering of seismic energy because of complex geological setti ngs. This is further aggravated; as such areas are logisti cally diffi cult and oft en covered by boulder beds. This makes the seismic acquisiti on more diffi cult and seismic data very noisy in thrust belt areas. In the paper we present the imaging problems of seismic data along the ‘Naga Thrust’ in Upper Assam Basin. In this area, so far, the success in explorati on is limited primarily due to inaccurate and poor imaging of the thrust.

Across this thrust, lateral velocity various in shallow as well as in deeper formati ons. At places rapid changes in elevati ons also been observed in this area. All these in combinati on lead to poor and inaccurate images. In order to obtain bett er image of this thrust a diff erent approach is taken at each step in processing. In our study, the meaningful velocity analysis/modeling and bett er stati cs soluti on applied along with the pre-stack depth migrati on has helped immensely to improve the subsurface image close to real geological setup.


54


Suppression of multiples using differential NMO on Saurashtra offshore data – A case study

C.S. Singh, B.N. Singh & A.K. Malkani

Raw seismic data generally associated with coherent

and incoherent noise obscures some relevant primary

events of geological interest. It therefore, becomes

primary objecti ve during processing to remove them

in order to draw the meaningful conclusions from the

data. Random noise generated by external sources and

having no defi nite property can be handled eff ecti vely

by stacking process but the coherent noise generated by

seismic source and geological features exhibit a defi nite

property. Based on their properti es many fi lter operators

are designed to esti mate them from primary events.

Multi ples termed as coherent noise and generated as a

result of multi ple refl ecti ons of the down going seismic

waves by two ore more geologic interfaces exhibit two

primary properti es viz. the periodic behavior and the

diff erenti al NMO from primary refl ecti ons.

Based on the periodic behavior of multi ples, inverse

fi lters are designed to suppress them. These inverse

fi lters designed on the periodic behavior of multi ples

in the beginning of data do not prove always to be an

eff ecti ve tool for the suppression of multi ples as the

multi ples change someti mes their periodic behavior

due to surface and sub-surface inhomogeneti es. It then

becomes essenti al to apply the other tools to eliminate

them from seismic data.

Oil India Ltd. started seismic data acquisiti on in Saurashtra

off shore region during the year 1991 and acquired about

8800 ground line kilometer of seismic data. As the data

was greatly interfered by low and high velocity multi ples,

the identi fi cati on of primary refl ecti on on it was quite

diffi cult. It became necessary for the processors to

remove them from the data for bett er interpretati on

specially in its deeper part. As the inverse fi lters were not

very eff ecti ve to suppress them, the authors adopted to

do the same based on the diff erenti al NMO exhibited by

the multi ples as they have velocity always diff erent from

the primary refl ecti ons. In the present work, att empts

have been made to convert the multi ples which generally

follow the patt ern to primary refl ecti ons into dipping

events and then fi lters were designed to remove the

dipping events and thus the multi ples from the primary

refl ecti on data.


Improvement in signal to noise ratio by Spatial Deconvolution (F-XY Decon) in frequency domain on 3D seismic data: A case study from Upper Assam basin

A. K. Malkani, G. R. Saini & K. K. Nath

It has been a common problem all over the world to

delineate the deep seated subtle structures parti cularly

in the areas where strong marker refl ectors obscure the

events down below. In the parts of Upper Assam basin

in OIL’s operati onal areas, mapping of Paleocene-Lower

Eocene refl ectors in 3D seismic data becomes very

diffi cult due to poor refl ector conti nuity marred by noise.

Even the DMO processing does not improve the signal

to noise rati o in these areas. Therefore to improve the

mapability of such events it is essenti al to increase the

S/N rati o while processing the data.

Since pre-stack applicati on of various S/N enhancement

techniques was unable to produce the desired results

therefore post stack applicati ons were adopted for

generati ng the required output to map the deeper

subtle structures. There are various methods available

to increase the post stack S/N rati o while processing

the 3D Seismic data viz. Eigen Vector Stack, F-K fi ltering,

F-XY Decon etc. An att empt has been made to improve

the quality of deeper refl ecti ons using F-XY Decon fi lter

on a 3D data set. The FXY Deconvoluti on process uses a

complex rectangular predicti on fi lter to predict the signal

at the center point of the spati al fi lter. Any diff erence

between the predicted complex value and the actual one

can be classifi ed as noise, and removed.

In the present study the input data for F-XY Decon was

prepared using the sequence – Trace edit, AGC applicati on,

Band Pass fi lter, Spiking (minimum phase) Deconvoluti on,

Elevati on stati cs, NMO applicati on, 3D DMO stack. The

out put DMO stack as well as DMO Stack with F-XY Decon

was migrated.

A comparison of the four secti ons (3D DMO Stack,

Migrati on on DMO Stack, 3D DMO Stack with F-XY

Decon and Migrati on on 3D DMO Stack with F-XY Decon)

clearly shows the increase in the Signal to Noise rati o

improving the conti nuity of deeper horizons in both 3D

DMO Stack with F-XY Decon and Migrati on on 3D DMO

Stack with F-XY Decon. The conventi onal 3D DMO Stack

secti ons appear bit noisy and diffi cult to map the events

below 3.0 seconds, the zone of interest. The secti ons

aft er the applicati on of F-XY Decon appear noise free

55

and conti nuous, parti cularly in the deeper horizons. It

is evident that in more geological complex areas, where

data quality is very poor to see and map any thing from

conventi onally processed seismic secti on, applicati on of

F-XY Decon can help in imaging the subsurface.


Imaging issues in Himalayan foothills - A case history from oil’s acreages

Akshaya Kumar, A. K. Khanna, V. K. Kulshresta,

R. K. Shrivastava & J. P. Singh

Oil India Limited embarked upon an ambiguous seismic

programme, targeti ng the Assam Shelf trend below

the Naga Thrust, geologically complex and boulder rich

formati on in the foot hills of Himalaya in Utt aranchal and

Arunachal Pradesh. Explorati ons of this trend has been

hampered by the poor quality of seismic data due in part

to the rugged topography, steep dips, hanging walls, and

diffi culti es in modeling seismic velociti es.

The objecti ve of this work is to platf orm an alternati ve

and reliable approach that could be incorporated in

the conventi onal seismic data processing for the high

resoluti on imaging of complex subsurface structures

through a zero-off set simulati on from seismic

multi coverage refl ecti on data. The CRS method is based

on a multi -parameter travelti me approximati on, called

the hyperbolic travelti me which relates the travelti me

of the reference or central ray (NIP) chosen as Zero-

off set ray and a hypotheti cal ray called the Normal wave

(N-wave) generated by an exploding refl ector. Unlike

the customary refl ecti on imaging methods, such as the

NMO/DMO Moveout/Stack or Prestack, which asks a

suffi ciently true velocity model to yield appropriate

result, the Common Refl ecti on Surface (CRS) Processing

methods off ers excellent signal-to-noise rati o and renders

bett er refl ector conti nuity supplemented by an enhanced

imaging of structural details and dipping elements and

does not depends on macro velocity model.

The CRS method assumes local refl ector segment in the

subsurface with curved interface which are characterized

by their locati ons, local orientati on/dip and local

curvatures. These structural subsurface properti es in

CRS technique are related to three kinemati c wavefi eld

att ributes, which can be described in terms of so called

eigen waves of the seismic data. In multi coverage seismic

data a refl ector wave may be characterized by the

following imaging parameters;

• Dip of events,

• Curvature of events in mid point directi on (Zero-

off set)

• Curvature of events in the off set directi on (central

CMP gathers).

A case study was carried out in order to illustrate the

possible improvement in imaging by CRS method in the

explorati on blocks located in the logisti cally diffi cult

terrain with signifi cant subsurface velocity variati on and

steep dips along with major faults in the study area.

Special att enti on was paid in the processing phase to

accurately image the thrust and the boulder affl uent

formati ons. In this respect, Common Refl ecti on Surface

(CRS) approach lead to a signifi cant increase in image

quality. Additi onally, velocity informati on was also derived

from the inversion of CRS att ributes that lead to similar

even bett er results as PSDM. Uti lizing the CRS inversion

velocity fi eld to PSDM the CRS stacks signifi cantly

improves the turnaround without compromising the

imaging quality.


Pore pressure predictions prior to drilling – A case study from Moran-Shalmari area in upper Assam basin

A.K. Khanna, K.L. Mandal, T. Bhatt acharya & D.S. Manral

Upper Assam Basin is prolifi c in terms of its hydrocarbon potenti al. Prospects have been identi fi ed within the enti re sedimentary column lying within formati ons from Eocene to late Miocene age. At places the drillable prospects within sand shale alterati ons are quite deep (4000 to 6000 meters) and oft en underlie thick formati on of shale. Down-hole problems are oft en encountered during drilling, both in the Foreland & in the Thrust-Belt (Belt of Schuppen) areas of the basin, leading to unsuccessful completi on, damage to prospecti ve zones and cost escalati on. Specifi c problems encountered include diff erenti al pipe stuck up, hole caving, mud loss and kicks etc. These also prevent recording of good quality logs for formati on & prospect evaluati on and exploitati on of hydrocarbon at its full potenti al.

To overcome these challenges, drilling & well log data have been analyzed and it has been observed that these problems usually occur due to the presence of abnormal formati on pressure that needs to be addressed using proper mud weight & an eff ecti ve casing policy in order to complete these wells successfully. It is also observed that the abnormal formati on pressure is encountered predominantly in the thick shale formati ons.

56


In view of above, esti mati on of Pore Pressure Profi le

accurately prior to drilling the wells is criti cal in

hydrocarbon explorati on. It helps in designing an

eff ecti ve casing policy & appropriate mud weight profi le

that can minimize the down-hole problems & risk of

blow outs and also prevent damage to the formati on in

prospecti ve zones. In pore pressure predicti on studies

undertaken in any area prior to drilling, the compacti on

dependant geophysical properti es of formati ons like

seismic velociti es, densiti es & resisti viti es are normally

integrated with other geo-scienti fi c data. A case study is

presented here on a successful pore pressure predicti on

study carried out in the operati onal area of M/s Oil India

Limited (OIL) in Upper Assam Basin.

For successful completi on of wells in such diffi cult areas,

eff orts have been made to esti mate the pore pressure

profi le prior to drilling for planning of eff ecti ve casing

policy and mud weights. The study suggests that in the

area the abnormal formati on pressure within the shale

is primarily due to under-compacti on & diagenesis / fl uid

expulsion. Seismic velocity yields reliable responses in

these conditi ons and can be extensively used for pore

pressure studies, integrated with other available geo-

scienti fi c data.


Improvement of deeper eocene reflections from Upper Assam basin using iterative processing sequence

C.V.G Krishna, Y.P. Singh & K.K. Nath

The objecti ve of seismic data processing is to improve the

S/N rati o so that a good seismic secti on can be provided

to the interpreter for a meaningful interpretati on of the

subsurface geological features such as structural highs,

lows, faults, pinchouts etc., which are of paramount

importance in Oil explorati on. The accuracy of mapping

in general decreases with increasing depth due to

inelasti c att enuati on, wavefront spreading, etc., because

of which, generally decrease seismic amplitudes and high

frequencies, with increasing ti me. It is observed that the

normal conventi onal processing may not always be able

to bring out subtle features such as pay sands of 3-10 m

thickness at large depths for a proper mapping. In this

paper the results of the normal processing sequence and

the iterati ve sequence will be presented. The iterati ve

processing sequence shows the improvement in refl ecti on

quality of the deeper Eocene refl ecti ons.


Automated stacking velocity computation in thrust belt areas – A case study from Upper Assam

P. Jaiswal, S. Choudhuri, T. Bhatt acharjee,

T. Borgohain & R. Dasgupta

Esti mati on of velocity during processing of seismic data

in thrust belt areas are diffi cult as the CMP gathers

may not show consistent coherency functi on from one

gather to another. The lack of coherency is mainly due

to steeply dipping refl ectors, strong lateral variati on in

velocity and rough topography. In such cases, either the

velocity picking should be made at fi ner intervals or an

independent guide for velocity picking should be made

available. In this paper, eff ort is made to esti mate velocity-

depth model using travel-ti me inversion to be used for

processing of seismic data from thrust belts. In this study,

the seismic fi rst arrivals are picked in shot domain at a

regular interval aft er making correcti ons for polarity of

the data. The elevati on and other co-ordinate details

were also the input. With all these and a broad velocity

functi on/model as input, travel ti me inversion of the fi rst

arrivals were carried out using the algorithm of Zelt and

Smith (1992 hereaft er referred as ZS 92) for obtaining a

velocity-depth model along the seismic profi le. Repeated

runs of travel-ti me inversion were performed unti l the

results were within the permissible limits.


Improvement of Signal/Noise ratio and fault imaging with full 3D Prestack Kirchhoff Time Migration: A case study from Upper Assam

C.V.G.Krishna, B.M.Sinha & K.K.Nath

The importance of seismic data in hydrocarbon

explorati on is realized by the extent to which the

subsurface informati on can be derived during the data

interpretati on. To achieve this objecti ve seismic data

processing should aim at improving the S/N rati o so that

a good seismic secti on can be produced for a meaningful

interpretati on of the subsurface geological features such

as structural highs and lows, faults, pinch-outs etc. which

are of paramount importance in hydrocarbon explorati on.

It is oft en seen that conventi onal processing is not always

suffi cient to bring out clearly certain subtle features of

interest in oil explorati on. In this paper, the results of

Kirchhoff Prestack Time Migrati on process which was

carried out over a 3D data is presented. The results have

shown a very good improvement (when compared with

57

the conventi onal post stack migrati on process) in the fault

imaging as well as mapping the horst/graben features

and the associated structures for the Eocene reservoirs in

OIL’s operati onal areas in Upper Assam.


The study of static correction for seismic data acquired in boulder bed areas of Pasighat (Arunachal Pradesh)

B. Singh, V.K. Kulshreshth & F. Siddiquee

Stati c correcti on has been a major challenge for seismic

explorati on parti cularly in geologically complex areas

having boulder beds and mountains. Such areas are

characterized by lateral velocity variati on in the weathered

zone. It has been observed from the fi rst breaks of seismic

monitor records showing disti nct features of bending

events of near off sets. Therefore, assuming equivalent

medium with quadrati c velocity variati on in depth, an

att empt has been made to derive proper stati c correcti on

in order to apply on the seismic data acquired in Pasighat

areas of Arunachal Pradesh. The technique for quadrati c

velocity model for solving the raypaths bending at near

off set and linear events (refracti on) for the far off set

arrivals has been used world wide. But the technique has

been adopted for the fi rst ti me in Pasighat area, where

acquiring seismic data in loose boulder beds has been

58


increasingly diffi cult due to shot-hole drilling problems.

Thereby the shot and receiver stati c has been calculated

and used them in seismic data processing. Considerable

improvement has been observed in seismic secti ons

aft er the applicati on of the method in the shallow events

where as there is only marginal improvement in the

deeper events.


Processing of 2D seismic data from Western Sirte Basin, Libya: a case study

C.V.G.Krishna, V.V.Bhadrappa & S.Rath

The importance of seismic data in hydrocarbon

explorati on is realized by the extent to which the

subsurface informati on can be derived during the data

interpretati on. To achieve this objecti ve seismic data

processing should aim at improving the S/N rati o so that

a good seismic secti on can be produced for a meaningful

interpretati on of the subsurface geological features such

as structural highs and lows, faults, pinch outs etc. which

are of paramount importance in hydrocarbon explorati on.

In this paper, the results of Kirchhoff Pre-stack Time

Migrati on process together with pre-stack FK fi ltering in

shot and receiver domain and pre-stack random noise

att enuati on which is carried over 2D data from an area in

the Western Sirte Basin, Libya is presented.


Resolution of structural and stratigraphic complexity by 3D seismic in Dandewala area of Jaisalmer Basin

T.K. Banerjee, G.R. Saini, R. Hura & R. Devi

Two wells in Dandewala area of Jaisalmer basin were

drilled in 1990 based on 2D seismic data. Natural gas was

encountered in multi ple reservoirs in these wells.

Initi ally, this area was interpreted as a horst bound by

two North-South trending faults, merging towards south.

Within horst, two independent structures were identi fi ed

and drilled. The structural and strati graphic confi gurati on

of the area however was found to be more complicated

than initi ally anti cipated aft er drilling of these two wells.

In order to understand the geological confi gurati on of the

area in detail and to delineate the prospect properly a 3D

seismic survey was conducted in 1991. It is observed from

3D interpretati on that the Dandewala horst block consists

of more or less one single structure instead of two, as

previously interpreted by 2D data. This has enhanced the

hydrocarbon prospects of the area and more locati on

are identi fi ed to ascertain and exploit these hydrocarbon

reserves. Detailed discussion of the results of 3D survey

and a comparison of 2D & 3D interpretati on results is

presented in the paper.


3D view from closely spaced 2D seismic data

R. N. Pandey & Rupendra Singh

The combinati on of 3D data along with the power of

the workstati on has basically been responsible for the

development of Geophysics. Modern Geophysics has

got its applicati on in areas requiring precision geology,

with parti cular reference to the small and subtle

geological variati ons. In this paper, one case history has

been presented wherein the precision Geophysics has

to be deployed before deciding about the exploratory

programme.

The advantages of 3D seismic data and its applicati on

are well established and the petroleum industry has

been reaping the benefi ts for more than 15 years by

designing a cost eff ecti ve exploratory or development

drilling programme. However, in the absence of 3D data,

att empts have been made to generate ti me slices along

with other 3D specifi c uti liti es to obtain 3D perspecti ve

from the conventi onal 2D seismic data.

With the discovery of oil from the Eocene secti ons in Upper Assam basin, with in the OIL’s PEL area, it has become desirable to obtain 3D picture of the structure and the adjoining areas, for designing a most cost eff ecti ve drilling programme.

Due to certain limitati ons, conventi onal swath shooti ng data could not be obtained and hence the closely spaced 2D lines were loaded as a 3D prospect on the SIDIS (Seismic Interacti ve Data Interpretati on system) workstati on. All the tools and soft ware available on the workstati on were eff ecti vely used to generate diff erent 3D outputs like Time-slices, Horizon-Slices, cube, etc.

Based on this unique exercise enti re explorati on/development programme had to be reviewed and changed and the wells drilled so far have proven to be hydrocarbon bearing apart from identi fi cati on more drillable exploratory prospects.

This paper covers the basis and the methodology of obtaining a 3D picture from closely spaced 2D seismic data. The limitati ons and advantages have also been discussed briefl y.


Seismic attenuation as a hydrocarbon indicator - A case study

Rahul Dasgupta & G.R.Saini

The search of hydrocarbons in various sedimentary basins has become a need of humanity at least since the beginning of 20th century. We have reached to a stage where more hydrocarbon discoveries are required but the methods used conventi onally are not capable of giving desired results. It is therefore necessary to fi nd out improvements or modifi cati ons on the conventi onal methods or re-look new ideas to extact maximum informati on from the dataset. In the present study an att empt has been made to correlate the presence of hydrocarbons in a formati on with the variati on in att enuati on coeffi cient in these formati ons. Att enuati on represented by Q factor is calculated by two methods using VSP and Sonic data from two wells for the devoirs formati ons and it has been inferred that presence of hydrocarbon gas in a formati on has drasti c eff ect on the Q factor. This signifi cant observati on can be used as an indicator for the presence of hydrocarbons in a formati on. Further if the results can be synergized for surface seismic data then the property can be used to establish the presence of hydrocarbons in the areas prior to drilling as well.


60

Seismic data interpretation-Hitting the Bull's Eye...

Seismic attenuation estimation and use in Hydrocarbon Exploration

Rahul Dasgupta

At present, lot of work is being carried out on esti mati on

of seismic att enuati on of quality factor (Q) from diff erent

types of seismic data like VSPs, check-shots and surface

seismic data. There are quite a few methods of measuring

Q from VSPs and check-shots like spectral rati o method;

spectral, wavelet, instantaneous phase, instantaneous

frequency modeling techniques; rise-ti me method. These

methods have their usefulness depending on various

conditi ons and its esti mati on is aff ected by the presence

of noise (both coherent and incoherent) and multi ples

(both long and short path multi ples). The measured

att enuati on (termed apparent att enuati on) in the sum

total of the att enuati on inherent to the rock mass (intrinsic

att enuati on) and that due to tuning eff ects of multi ples

in thin beds (strati graphic att enuati on). In general, from

seismic data the apparent att enuati on is measured. Its

esti mati on from surface seismic data is more diffi cult

due to multi plicity of ray-paths which not only aff ects the

lengths of ray-paths but also in given CMP the angle of

incidence of ray and consequently, the refl ecti vity diff ers

from trace to trace. Therefore, standard techniques of Q

esti mati on which are used for VSPs cannot be used for

surface seismic data. With a view to this, Dasgupta and

Clark (1994) devised a method of accurate esti mati on

of Q from surface seismic data. A modifi ed version of

this method is discussed in the paper. The role which

att enuati on can play as a lithology discriminator and how

the presence of hydrocarbons in a formati on can aff ect

att enuati on and whether att enuati on can serve as a

“Direct Hydrocarbon Indicator (DHI)” is explained.


Horizontal seismic section- A powerful tool for stratigraphic interpertation

B.M.Sinha, G.R.Saini & C.V.G.Krishna

The recent advances in Seismic data acquisiti on,

processing and interpretati on technology have given

some additi onal powerful tools for revealing the

depositi onal patt erns and basinal history. 3D data volume

interpretati on on workstati on generates very useful

products like horizontal seismic secti ons which have

capability to detect subtle features including lithological

variati ons and strati graphic traps.

Verti cal secti ons are excellent tools for delineati on of

structural features like faults and folds but they prove

to be inadequate to delineate the subtle depositi onal

features. Creati on of Horizontal secti on at a given ti me/

depth provides the interpreter an opportunity to look at

the earth surface in geological past, thus enabling him to

visualize the depositi onal history of the sediments in the

basin.

As major structural features already having been

discovered worldwide, the future thrust in hydrocarbon

explorati on is intended towards the search for strati graphic

or subtle depositi onal traps. Use of Horizontal Secti ons in

delineati on of strati graphic depositi onal features in one

of the oilfi elds of Oil is presented as a case study in this

paper.


Stratigraphic approach by 3D seismic: A future thrust for exploration and development of oil fields in Upper Assam basin

P.V.Reddy & K.K.Nath

The eff ecti ve discovery and development of a hydrocarbon

reservoir primarily require the basic understanding of its

depositi on system and the various dynamic processes

responsible for its depositi on. Also, the concept behind

the methodology being used for reservoir delineati on

essenti ally determines the eff ecti veness of its

development plan and economic viability of the fi eld.

In upper Assam basin, the structural aspect is so far the

main considerati on for the discovery and development

of oil fi elds. Apart from good discoveries, the subsurface

reservoir models and the mechanism of hydrocarbon

trapping and distributi on, especially in Oligocene and

Eocene formati ons are yet to be fully understood. The

reason could be due to the disconti nuous nature and

orientati onally deposited reservoir sands which do not

have any direct relati on with the structure. The structure

however, appears to have infl uenced on the hydrocarbon

trapping and distributi on patt ern in the reservoir sands.

Thus, the prospects are basically the resultant of both

strati graphic and structural phenomena. Hence, the

integrati on of both structure and strati graphic informati on

is essenti al for eff ecti ve identi fi cati on and delineati on of

reservoirs in upper Assam basin.


61

Seismic attenuation as a hydrocarbon indicator - A case study

R.Dasgupta & G.R.Saini

With the passage of ti me, as the quality of decision

making process is being expected/demanded to be as

high as possible, analysis of various att ributes related

to seismic has turned into an essenti ality. One such

att ribute, seismic quality, Q could play important role in

the hydrocarbon explorati on as it can provide signifi cant

informati on if put to proper test in this regard. Recently,

considerable amount of work has been carried out on

esti mati on of seismic att enuati on or quality factor, Q. In

this paper, the result of Q esti mati on from VSPs and sonic

log data of two wells from a gas fi eld located within OIL’s

PEL area in the state of Rajasthan, using spectral rati o and

seismic-sonic drift methods are presented. The analysis

from two wells adjacent to each other reveals that the Q

esti mates for all the formati ons encountered in both the

wells are similar except for Khuiala formati on (of Eocene

age), the Q values being 34±4 and 39±5 for well A and

21±2 and 24±3 for well B respecti vely. This signifi cant

diff erence in Q is due to the fact that the formati on in

well B is hydrocarbon bearing whereas, in well A, it is

not. For future exploratory and development work this

signifi cant observati on can be used as an indicator for the

presence of hydrocarbon in formati on.


Validation of anomaly from the seismic data –A case study from Shaurashtra offshore area

B.N. Singh & S.N. Singh

Non uniqueness of geophysical anomalies have always

been a stumbling block in geophysical interpretati ons.

The observed seismic responses oft en lead to diff erent

explanati ons whereas the real expected soluti on is

unique. In the context of oil explorati on, it has been

diffi cult, at ti me, to disti nguish the seismic anomalies

caused by major structural features e.g. salt dome, salt

diaper, igneous intrusions, basement horst etc., owing

to the similariti es of the response manifestati ons on

the seismic secti ons. This has led to pitf alls in seismic

data interpretati on and have necessitated studies in

convincingly inferring the nature of causati ve bodies, as

well as have made the integrated studies imperati ve in

the industry.

During the explorati on venture pursed by Oil India Limited

(OIL) in Saurastra off shore area, a few similar cases were

encountered where the uniqueness in interpretati on

and thus an unanimous conclusion on the nature of the

causati ve body could not be ascertained. In an att empt

to explain the anomalies, a study was carried by the

authors with preclusion that these seismic anomalies are

worth investi gati ng from hydrocarbon explorati on point

of view. The authors have tried to explain the individual

possibiliti es, based on their characteristi c depicti on on

seismic secti on to att ain the best possible soluti on. Special

and advanced processing such as Wavelet Processing,

PSDM (Pre Stack Depth Migrati on) etc. have been carried

out to enact the anomalies and the environment bett er.

The results of gravity and magneti c observati ons have

been interpreted and integrated to validate the diff erent

possibiliti es.

This paper deals with the validati on of one of such seismic

anomalies which resembled the response which could be

due a various type of causati ve bodies. An att empt has

been made in this paper to give a best possible soluti on

to assert the nature of the causati ve body through the

process of eliminati on of diff erent possibiliti es.


Scanning through the seismic anomalies in search of oil traps – A case study from Upper Assam oilfields

S. N. Singh & V.K. Sibal

Detecti on of seismic anomalies and subsequent

exploitati on of these anomalies have always provided

front end leads in oil explorati on. The fact that the seismic

signatures are a form of analyti cal signal, the seismic

characteristi cs depicted by subsurface features allows

the dissecti on of the overall responses into consti tuent

contributors. This has pursued the geophysicists to

unravel the embedded informati on from the seismic

signals through a wide variety of att ribute studies. The

att ribute analysis have been quite revealing when used

systemati cally with cauti on.

This paper discusses a case study on systemati c scanning

through a seismic anomaly at a depth of about 4000 mtrs

(2850 ms) within an identi fi ed drilling prospecti ng Upper

Assam oilfi eld of Oil India Limited. A meek “contact like

appearance” at Oligocene level (Barails) on conventi onally

processed seismic secti on, prompted the study to isolate

the anomaly by reprocessing the data with advanced

processors and scan the anomaly for various att ributors

e.g. Instantaneous Amplitude, Instantaneous Phase,

62


Cosine of Phase, Instant frequency and apparent polarity,

in and around the anomalous zone. The consistency

observed over the scanning of the anomaly further

glorifi ed the prospect. The fi nal drilling results when

ti ed with the seismic anomaly revealed it to be a contact

within the structural oil trap.

The study emphasizes on capturing each and every

aspects of the seismic anomaly, preserve and scans

the anomalies for various att ributes. This approach, in

additi on to the conventi onal tracking for major structural

and strati graphic features could help avoiding pitf alls;

oft en encountered in seismic explorati on due to masking

of the anomalies or meek representati ons. Occasionally, it

has been noti ced that certain perti nent aspects of seismic

signatures are either not preserved during acquisiti on or

are frequently lost in the gimmick of rigorous seismic

data processing algorithms.

Recommendati ons have been made on the need for

re-looking at the already accumulated seismic data of

diff erent vintages in order to prepare the data for enacti ng

the anomalies and redressing the potenti al hydrocarbon

prospects from the att ribute response of the anomalies

would lead to intra-structural mapping to guide the

explorati on plan and assess the techno-economic viability

of the prospects.


A first look at seismic anisotropy from long offset data – A case study from Upper Assam

F. Siddiquee, A. Kumar, & R. Dasgupta

Certain amount of unconventi onal seismic refl ecti on

data has been acquired in an area which forms a part of

petroliferous basin of Upper Assam. The seismic profi le,

which has been laid out of in the fi eld to acquire the data, is

of almost 10km length. In the area of study, the producing

formati ons are expected to be in the depth range of 3500-

3800 m. Hence the seismic data has off sets much larger

than those acquired in conventi onal refl ecti on seismic

data. The recorded data, showed two clear-cut trends

of seismic events viz. refl ected and the refracted events.

These two trends were studied. The refracti on data was

interpreted in order to develop a broad geological model

of the area of study, which compared well with the one

generated from seismic refl ecti on data.

The interpretati on of refracti on data clearly images the

major geological sequence of the area which can be well

corroborated with those of refl ecti on data. The velocity

along the main interfaces obtained from refracti on data,

when compared with the interval velocity from refecti on

data, gives a hint of anisotropy. The interval velociti es of

refracted events were found to be higher than those from

the refl ected events. This higher refracted wave velocity

can be att ributed to the presence of shale streaks within

sand beds, which have got higher velocity along the

bedding plane, whereas the refl ected wave, characterized

by a wave moti on perpendicular to the bedding plane,

has its contributi on proporti onal to each member of the

bedding sequence encountered during its propagati on

through the medium.

These interesti ng results can be studied further with the

help of AVO modeling and seismic refracti on tomography

and, accordingly, step has already been taken in this

directi on to substanti ate the results. This might also

give us insight of the depositi onal patt ern of diff erent

formati ons in the area.


Use of Principle Component Analysis and stepwise regression for estimation of porosity from 3D sesimic and well data- A case study from Upper Assam, India

Y.P.Singh, R.K.Pathak & K.K.Nath

A scheme for esti mati on of porosity from 3D seismic

att ributes and well data is described. The methodology

aims to fi nd stati sti cal relati onship from available well

data and seismic att ributes to predict porosity accurately

away from well bores. From 3D seismic volume a set of

horizon-based att ributes (seismic att ributes) is extracted.

This is followed by Principle Component Analysis (PCA)

of extracted seismic att ributes that creates a new

series of independent att ributes (PC att ributes). Larger

sets of att ributes are esti mated by linear/nonlinear

transformati on of both seismic and PC att ribute sets.

Forward stepwise regression derives opti mum set of

att ributes from larger sets based on minimum error

criteria. The generated relati onship is used to predict

porosity away from the well bores. In this paper stepwise

regression analysis used to judge the effi cacy of both

seismic and PC att ribute sets.

To esti mate the reliability of driver relati onship,

cross validati on is used. In this process, each well is

systemati cally removed from the training sets, and

the relati onship is rederived for remaining wells. The

63

predicti on for the hidden well is then calculated. The

validati on error, which is the average error for all the

hidden wells, is used as a measure of the likely predicti on

error when the relati onship is applied over the area away

from wells.

The scheme has been tested on the real data set from

one of the oil fi elds in upper Assam, India. The predicti on/

validati on error observed with PC att ributes is lower than

that of the same derived from seismic att ributes. For

non linear transformed sets of PC and seismic att ributes,

predicti on/validati on error reduces signifi cantly from one

observed with the linear sets of PC and seismic att ributes.

The predicti on/validati on error was lowest in case of non

linear transform set of PC att ributes. This establishes

that the porosity map for enti re study area calculated

by using non linear att ributes would be most accurate.

The data reducti on and other computati onal advantages

associated with PC att ributes make it a favorable soluti on

for accurate porosity esti mati on.


Mapping of sub-thrust structures for hydrocarbon exploration: An integrated approach

S.K. Basha, N.R.Hazarika, S.Rath & N.M.Borah

Explorati on for hydrocarbon in the Thrust belt lying along

the southern margin of the shelf region of Upper Assam

basin started in the last part of the nineteenth century.

Discovery of oil & gas from sub-thrust Barail formati on of

Oligocene age associated with the Naga thrust in Kusijan

fi eld located towards west of Digboi in early sixti es had

establish the hydrocarbon potenti al of sub-thrust play.

Explorati on in thrust belt is associated with both surface

and subsurface challenges. Till date major part of this belt

is relati vely poorly explored due to complex geological

setti ng, diffi cult logisti cs, and lack of infrastructure

faciliti es for carrying out survey in such rugged terrains

and poor seismic response below thrust. Moreover,

available seismic data quality in the area is also poor,

especially adjacent to thrust making imaging of structure

below the thrust diffi cult.

Drilling evidence in Kusijan structure, which is a well-

defi ned sub-thrust ti me high associated with Naga thrust,

has shown that in depth, the positi on of the high has

shift ed in the up-dip directi on. To overcome the pitf all in

interpretati on, two seismic lines passing through a ti me

high in nearby Digboi area have been reprocessed and

depth secti ons have been generated by applying Velocity

modeling and Post Stack Depth Migrati on(PSDM). The

ti me and depth migrated secti ons have been interpreted

and the shift of the structural high has been observed.

This paper highlights on the fi nding and observati on of

the study carried out for the structure identi fi ed below

the Naga thrust in the frontal thrust zone.


Accurate estimation of velocity and density relationship in hapjan field of Upper Assam basin, India

Y.P.Singh, R.K.Pathak & K.K.Nath

Identi fi cati on of stratagraphic traps is a challenging task

even today and it requires integrati on of various types

of data such as geological informati on, well log, core,

seismic and producti on data. However, the integrati on of

well log and seismic data is mostly used and is the basis

for Amplitude variati on with off sets (AVO), Inversion and

various other applicati ons. In the study area the seismic

data is available on regular grid, however, the well log

data is available only for very few drilled locati ons. Even

at few well locati ons the well log data does not have the

complete suite of well logs, which are mandatory for

integrati on with seismic data. Wherever, the desired well

logs are available, the stati sti cal relati onship between

logs can be established and the same can be used for

generati on of syntheti c logs at the places where the

parti cular log is not recorded.

The rock velocity and density depend upon the mineral

compositi on, granular nature of the rock matrix,

cementati on, porosity, fl uid content, environmental

pressure, depth of burial and geologic age. In this paper

we have established the relati onship between sonic

(velocity) and density logs for Barail reservoir in Hapjan

fi eld, Upper Assam basin in OIL’s operati onal area. Barail

reservoir sands are of upper Eocene age deposited in

deltaic environment and lie at a depth of about 2850-

3050 m. Gardner equati on was used for establishing

the relati onship between velocity and density. We have

selected the three well locati ons where both sonic and

density logs were recorded. The data in Barail reservoir

show the systemati c relati onship between velocity and

density. The Gardner’s coeffi cients calculated for Barail

reservoir are 0.18 and 0.27 in place of their standard value

0.23 and 0.25 respecti vely. The syntheti c logs computed

using new values and standard values are compared and

these syntheti c logs were further used for generati on of

syntheti c seismograms. The syntheti cs calculated using

new esti mated Gardner coeffi cients for the study area

provide more accurate match with real seismic traces at

64


well locati ons. In this way well logs and seismic can be

integrated more accurately for various applicati ons such

as ti me-depth conversion, source wavelet generati on,

AVO and Inversion.


Seismic study of possible Gas Hydrate – A case study from Indian Offshore

R. Dasgupta, P. K. Singh, B. Singh & P. Jaiswal

Gas hydrates and associated free gas aff ect the elasti c

properti es of the host sediment in ways that are seismically

detectable. Seismically, gas hydrates are inferred on the

basis of the presence of high amplitude reserved polarity

events on seismic refl ecti on records that mark the Base

of Hydrate Stability Zones (BHSZ) known as Bott om

Simulati ng Refl ectors (BSRs). Temperature, pressure, pore

fl uid salinity and methane concentrati on are important

factors that aff ect the formati on or dissociati on of

Hydrates, Geophysical att ributes viz. velocity of hydrate

bearing formati ons, AVO signatures etc. play a major role

in identi fi cati on of gas hydrates. In this paper, we present

a case study pertaining to Saurashtra off shore area of

western India shelf, where possible presence of hydrates

are inferred from detailed study of diff erent geophysical

att ributes and geological setti ngs of the area, although

explicit BSRs could not be identi fi ed.


A case study on Pore Pressure Prediction of a well in Upper Assam basin

K.L.Mandal, A.K.Verma, B.Singh & C.V.G.Krishna

For more than a decade Oil India Limited has been

exploring and exploiti ng hydrocarbons from the deeper

horizons of lower Eocene/Paleocene formati ons which

occurs at depths of around 3500-4500m depth. At ti mes

severe down-hole pressure problems are encountered in

the wells, which lead to problems during well completi on.

Pore pressure, which is the pressure acti ng upon the

fl uids (gas, oil, water and highly water saturated clay) in

the pores of the formati on, becomes a very important

factor to know before the well is drilled, for a proper

planning of the casing and mud policy for exploratory

and development wells in the areas having down-hole

pressure problems.

In this paper a study on pore pressure predicti on of

an Eocene well, which has been done using Presgraf

soft ware from M/s Landmark is presented. The study

was carried out before drilling, using seismic data and

other available well informati on from the nearby areas

and is presented along with the actual mud weights used

during the drilling of the well. The comparison of the mud

weights computed from the pore pressure predicti on

using mostly seismic data has shown a very good fi t with

the actual mud weights used during the drilling of the

well. Also, an exercise was conducted later using the log

informati on of the well which gave a very similar result to

that of earlier study.


Facies distribution based on Seismic Attribute & Petrophysical data for deeper reservoirs- A case history from Upper Assam basin

S.Rath, P.K.Sharma & Mrs. Reba Devi

The seismic att ribute studies for drawing meaningful

interpretati on for thin sand reservoirs with limited well

control at deeper depths have always been a tough

challenge for geoscienti fi c community worldwide. This

is mainly due to the inherent limitati on of the high

frequency content of seismic data. But the composite

responses of a group of reservoirs at deeper depth, with

depositi onal and reservoir distributi on model based on

well data, log moti fs etc. along with some new tools for

seismic att ribute analysis may help at ti mes in making

some broad assessment about the reservoir facies

distributi on. The present paper deals with such a case

history where recently Oil India Limited has discovered

a number of good Paleocene-Eocene reservoirs with

relati vely thick sands (about 10m) against the traditi onal

thin sand reservoirs (about 2-5m) at a depth of around

3900m. A basinwide geological model of broad reservoir

distributi on patt ern at diff erent strati graphic levels based

on mapping of maximum fl ooding surfaces, sequence

boundaries etc. have been prepared. This model has

been coupled with geo-stati sti cal analysis of both

surface & volume based seismic att ributes, well data

etc. to delineate the reservoir distributi on patt ern. This

has helped in discriminati ng diff erent reservoir facies

in a spati al domain and guided in the placement of the

future exploratory/extension locati ons to delineate the

Palaeocene-Eocene prospects with a minimal risk.

Published in APG-2004

65

Estimation of seismic attenuation from CMP gathers

R. Dasgupta & R .A. Clark

The esti mati on of seismic att enuati on is of prime

importance in hydrocarbon explorati on, but it has met

with litt le success except to a certain extent in VSP studies.

Diff erent methods have been developed for measuring Q

and in recent years Q has been successfully computed

from VSPs (Tonn, 1991, Stainsby and Worthington, 1985,

Jannsen et a1, 1985) and the strengths and weaknesses

of these methods have been documented. Esti mati on

of Q from surface seismic data has not met with much

success, maybe because of the multi plicity of ray-paths

aff ecti ng both the elasti c (refl ecti on/transmission,

geometric spreading etc.) and inelasti c components of

energy losses with off set. As well as this, surface seismic

data is aff ected by NMO-stretch along with the usual

problems of the presence of noise and multi ples. In this

presentati on, a new scheme for computi ng Q (which is

assumed to be constant within the frequency range of

interest in hydrocarbon explorati on) from CMP gathers is

proposed. The method has provided accurate Q values

when applied to models.


Successful estimation of Q from surface seismic data - A case study

R. Dasgupta & R.A. Clark

Att empts have been made to esti mate seismic quality

factor, Q of diff erent rocks in the laboratory as well as from

diff erent forms of seismic data and the eff orts have met

with varied degree of success. The computati on of Q from

VSPs and check-shot surveys is quite successful (Ganley

and Kanesewich, 1980. Jannsen et al. 1985; Stainsby and

Worthington, 1986; Tonn, 1991) whereas there exists

no reported evidence of its successful esti mati on from

surface seismic data. The main reason this is att ributed to

the multi plicity of the ray-paths associated with surface

seismic data, where every sample represents a diff erent

ray-path. Apart from this, even for a CMP, path lengths

and the refl ecti vity/transmissivity vary from one trace to

another. Dasgupta and Clark (1993) proposed a method

of computi ng Q from surface seismic data which solves

the problems arising due to multi plicity of the ray-paths,

and tested the technique using model data. In this paper,

the method is reviewed and results of computi ng Q from

surface seismic data from the central part of the southern

UK North Sea are discussed. The method is further

validated by comparison of the surface seismic results to

esti mates from VSPs both adjacent to the seismic line and

in similar lithologies elsewhere in the North Sea.


Estimation of Q from CMP gathers and case histories

Roger A. Clark & Rahul Dasgupta'

There have been many att empts to measure seismic the

quality factor, Q, from explorati on seismic data. These

have met with a diff ering degree of success depending on

various factors including the type of data from which the

esti mati on has been att empted. Q has been successfully

computed From VSPs and check-shots (Ganley. and

Kanesewich, 1980; Jannsen et al, 1985; Stainsby and

Worthington, 1985; Tonn, 199I), but to our knowledge

there exists no reported evidence of its successful

esti mati on from surface seismic data. The main reason

for this is that, within a CMP, path length and refl ecti on/

transmission vary from one trace to another and those

wavelets as a stacked trace - though corrected for travel

ti mes do not have correct spectra. Dasgupta and Clark

(1993) proposed a method of esti mati ng Q from surface

seismic data which solves the problems that arise due

to the multi plicity of the ray- paths, and tested the

technique using model data. In this paper, the method

is reviewed and further validated by comparison the

results of Q esti mati on from surface seismic data from

the central part of the southern UK North Sea (Dataset l),

to esti mates from VSPs both adjacent to the seismic line

and in similar lithologies elsewhere in the North Sea. Two

case histories are also discussed Q has been esti mated

from surface seismic data from off shore South-East Asia

(Dataset II), to test whether Q can be used as a lithology

discriminator in the fronti er area with no well control.

Secondly, with a dataset (Dataset III) from the central

part of UK North Sea, we have studied the eff ects of

hydrocarbons on att enuati on.


Velocity modeling of Naga Thrust, North-East India

P. Jaiswal, C.A. Zelt, A.W. Bally, & R. Dasgupta

A seismic line is shot over a relati vely fl at area in North-East

India that has a thrust fault in the subsurface. The quality

of the seismic data does not allow adequate imaging of

the thrust. Also, the surface expression of the thrust is

too subtle to be used as a key for interpretati on. To aid

the interpretati on, fi rst arrival ti mes have been inverted

using a well- established regularized inversion algorithm

66


and a shallow (~1.5 km deep) velocity-depth model has

been developed. The thrust has been interpreted on

the velocity-depth model based on the variati ons of the

velocity contours. This interpretati on can be converted in

ti me and used to interpret deeper parts of the thrust in

the seismic data.


Travel-time and full-waveform inversion for improved seismic imaging in geologically complex areas

P. Jaiswal, C.A. Zelt & R. Dasgupta

A complex geology of the thrust belt and acquisiti on

related noise limit the ability of conventi onal processing

to yield a reliable velocity model which in turn leads to

a poor subsurface image. We demonstrate the uti lity of

2D travelti me and 2D waveform inversion as supplements

to conventi onal imaging and interpretati on processes

in geologically complex areas such as the thrust belt.

First, a smooth velocity model with long wavelength

characteristi cs of the subsurface is esti mated through

fi rst arrival travelti me inversion. Next, the velocity

model is used for depth-migrati on. The migrated image

allows unambiguous interpretati on of the thrust fault.

Interpretati on of the thrust fault in parts of the migrated

image that are not adequately resolved due to pre-

processing constrains, is achieved on high-resoluti on

velocity model from waveform inversion developed using

results from travelti me inversion.


Near-surface imaging with traveltime and waveform inversion

Priyank Jaiswal, Colin A. Zelt & Rahul Dasgupta

Due to acquisiti on related noise and ground roll only

a limited ti me window of seismic data is available for

depth imaging through conventi onal processing. This

ti me window excludes coda near the direct arrivals and

far off set refracti ons as they cannot be handled by the

conventi onal processing methods. Moreover due to NMO

stretch and mute, shallow refl ecti on events have limited

off sets and do not stack as well as deeper refl ecti on

events. As a result, in terrains with rough topography

and rapidly varying near surface conditi ons, near surface

images from conventi onal processing are oft en not

appropriate. We present a case study of near-surface

(< 1.5 km from topography) imaging using multi scale

waveform inversion. We also show that multi scale

waveform inversion not only yields an interpretable near

surface image but also a velocity model that can be used

in conventi onal processing for improved depth imaging.

However, due to a high degree of nonlinearity inherent in

waveform inversion, its success calls for a robust starti ng

model and careful applicati on. In this paper we esti mate

a suitable starti ng model for waveform inversion using

regularized inversion of direct and refl ecti on travelti mes.

The seismic data used in this paper are from the Naga

Thrust and Fold belt in Northeast India.


Estimation of Q from surface seismic reflection data

Rahul Dasgupta & Roger A. Clark

Reliable esti mates of the anelasti c att enuati on factor, Q, are desirable for improved resoluti on through inverse Q deconvoluti on and to facilitate amplitude analysis. Q is a useful petrophysical parameter itself, yet Q is rarely measured. Esti mates must currently be made from borehole seismology. This paper presents a simple technique for determining Q from conventi onal surface seismic common midpoint (CMP) gathers. It is essenti ally the classic spectral rati o method applied on a trace-by-trace basis to a designatured and NMO stretch-corrected CMP gather. The variati on of apparent Q versus off set (QVO) is extrapolated to give a zero-off set Q esti mate. Studies on syntheti cs suggest that, for reasonable data quality (S/N rati os bett er than 3:1, shallow <5°) dips, and stacking velocity accuracy <5%), source-to-refl ector average Q is recoverable to within some 3% and Q for a specifi c interval (depending on its two-way ti me thickness and depth) is recoverable to 15–20%. Three case studies are reported. First, Q versus off set and verti cal seismic profi ling (VSP) Q esti mates for a southern North Sea line was in close agreement, validati ng the method. For Chalk, Mushelkalk-Keuper, and Bunter-Zechstein, Q was esti mated as 130 ± 15, 47 ± 8, and 156 ± 18, respecti vely. Next, two alternati ve lithological interpretati ons of a structure seen in a fronti er area were discriminated between when Q esti mates of 680 to 820 were obtained (compared to some 130–170 in the overlying units), favoring a metamorphic/crystalline lithology rather than (prospecti ve) sediments. This was later confi rmed by drilling. Third, a profi le of Q esti mates along a 200-ms-thick interval, known to include a gas reservoir, showed a clear and systemati c reducti on in Q to a low of 50 ± 11, coincident with the maximum reservoir thickness, from some 90–105 outside the reservoir. Q for the reservoir interval itself was esti mated at 17 ± 7.

Published in GEOPHYSICS-1998

67

Dip considerations: An effective tool for fault mapping on seismic data

C.V.G. Ksishna, S.N. Singh & V.K. Sibal

Role of “Geological Faults” in “Geophysical Explorati on” has always been vital in explorati on and development of oil fi elds. The geophysical data interpretati on and presentati on involves considerable eff orts in identi fi cati ons and esti mati on of fault characteristi cs prior to drawing informati on on tectonics and geology of subsurface strata. Dip considerati on; both regional and local not only play important roles in deciphering geological structures and mapping of structural closures of determining oil accumulati ons in a trap but also provides defi niti ons on migrati on conduits, entrapment conditi ons well as volumetric of hydrocarbon in a reservoir.

OIL’s operati on areas in Upper Assam, being a part of the Assam-Arakan basin, has undergone a series of extensive tectonism from the ti me of rift ing & drift ing phase in cretaceous period ti ll the major tectonic event of evoluti on of Basement Ridge and Himalayas in Miocene. This various phases (broadly diff erenti ated to four major phases), has led to severe faulti ng and thrusti ng of the basin where the major oil fi elds are located. Most of the identi fi ed oil traps are found to be fault related closures.

In this paper, an established method, based on dip/azimuth related algorithms on fault identi fi cati on and correlati on has been discussed with validati on by comparison with the methods generally adopted by the interpreters. The method discussed, uses the creati on of local dip and azimuth att ributes (only dip for 2D and both dip & azimuth for 3D data) over relati vely horizontal strata and across fairly steeply dipping faults. The local dips and azimuth are computed using the Dip/Azimuth algorithm along user selected CDP range for 2D data interpretati on and across a user designed spati ally extended window for 3D data to generate the initi al fault patt ern model. The ti me att ribute mapped from the geological horizons along with the inputs from the generated fault model could be used eff ecti vely in achieving faster results.

It is recommended that this procedure when used as a routi ne in interpretati on of seismic data to generate dependable preliminary interpretati on in a very short ti me (2-3 min.) to guide the enti re interpretati on. The authors have presented a few case studies and have discussed the data preparati on with the parameter design aspects, required for using the algorithm to yield meaningful results. This has been found to expedite the turn around ti me in completi on of 3D data interpretati on projects by leaving behind the job of the interpreter to augment the preliminary results with fi ne tuning and through adjustments for faults.


Seismic inversion in hydrocarbon exploration - A case study

C.V.G. Krishna, R.K. Pathak, S.K. Basha & K.K. Nath

Seismic refl ecti on method, the most widely used method for hydrocarbon explorati on, provides subsurface informati on in terms of refl ecti on coeffi cient series of amplitudes that is related with acousti c impedance (product of velocity and density) contrast between diff erent subsurface layers. The att ributes mapped in seismic method relates with the rati o of impedance contrast between diff erent subsurface layers instead of att ributes directly related with individual layers.

In hydrocarbon explorati on, it is envisaged to get subsurface informati on in terms of att ributes directly related with lithology, porosity and fl uid contents. Velocity and density are two such att ributes that directly relates with the subsurface formati on. Their variati on and magnitude provide valuable subsurface informati on in terms of reservoir characteristi cs. Seismic inversion links between seismic and such geological att ributes. Inversion process converts seismic informati on (impedance contrast) in to acousti c impedance log directly related with the geological units. The variati on of these and their magnitude within each unit when identi fi ed and interpreted gives informati on in terms of lithology and reservoir properti es viz. porosity, fl uid contents etc.

The value of seismic data parti cularly in hydrocarbon explorati on is enhanced by the extent to which the geological and petrophysical informati on can be incorporated into the data interpretati on. A common approach to integrate these various types of informati on is through modeling studies that att empt to approximate the earth’s acousti c response. Two modeling approaches are generally used, which are : (1) Direct modeling and (2) Inverse modeling.

The direct modeling approach involves use of mathemati cal models to determine the seismic response of an acousti c impedance sequence. One dimensional modeling converts velocity and density logs into band limited refl ecti vely ti me series known as syntheti c seismograms.

Inverse modeling is a mathemati cal approach to convert seismic data into an Acousti c Impedance sequence. This approach assumes that the stacked seismic data (which is deconvolved, preserved amplitude processed and migrated) closely approximates the earth’s refl ecti vely functi on, basically, the inverse approach starts with the observed data and proceeds to calculate the model or physical situati on, namely, the acousti c impedance sequence. The inverse approach converts each seismic trace into an acousti c impedance log. In this paper

68


various aspects of seismic inversion techniques are briefl y discussed.

A study was carried out to outline the prospecti ve sands at a depth a about 2000 mtrs from the inversion of seismic data over a producing fi eld in Upper Assam basin. The prospecti ve sand shows lower acousti c impedances (lower velocity) than the overlying and underlying shale formati on. An inverted seismic line has shown the prospecti ve sand from the area study.


Evaluation of reservoir potential in a developing oil field from 3D seismic survey - A case study

P. V. Reddy & B. M. Sinha

Introducing fi rst ti me, Oil India Limited (OIL) recorded 3D seismic survey in Hapjan oil fi eld located in Upper Assam basin, India in 1993. The fi eld was discovered in early 1980's. Prior to 3D survey, 24 wells were drilled mostly based on structure. But, the producti on and sand distributi on patt ern observed varied within the structure.

The reservoir under the study exists in Barail Formati on of Oligocene/Upper Eocene age at a depth of around 2900m and occur as distributory channels and barrier bars deposited in deltaic environment. The gross reservoir zone consists of two main sand units, Barail 4th and 5th, separated by a shale band.

The 3D horizon amplitude maps of reservoir sands showed good correspondence with the producti on results and sand development patt ern at drilled wells. Bett er sand prospects were observed in those wells, which are located in the high amplitude zones. The amplitude maps disti nctly brought out the lateral disconti nuiti es, which are likely to act as fl ow/permeability barriers within the reservoir sands.

A linear relati onship observed between 3D refl ecti on amplitudes and resisti vity from the wells reaffi rms that the amplitudes can be directly used for identi fying prospecti ve sand in the study area. Using the above relati onship and producti on results of the wells, amplitude maps have been transformed into resisti vity maps. The output maps indicated the distributi on patt ern of prospecti ve sand and also identi fi ed additi onal prospects within the study area.


Analysis of blowout effect through high resolution 3D survey - A case study

V. K. Sibal, S. N. Singh & P. V. Reddy

This case study is from one of the major oilfi elds in Upper Assam basin, India. The fi eld is located on an elongated

anti clinal structure along upthrown side of a major normal fault. In 1992, while drilling the second well on the structure, a blowout occurred which conti nued for about 20 hours and got subsided on its own. Later, the development drilling conti nued in the other parts of the structure leaving the safe distance from the blowout well. It was a general apprehension that the blowout might have resulted in surcharged zones in the shallow unconsolidated formati ons in the uncased well secti on due to accumulati on of high pressured hydrocarbons, which were released from the deeper reservoirs.

Oil India Limited planned to carry out a high resoluti on 3D survey around the blowout well to analyze the blowout eff ect and its areal extent before going for drilling a few more wells for fi eld development in the vicinity of the blowout well. Prior to blowout, the structure was covered by a close grid 2D survey that enabled to compare with the 3D data and observe the blowout eff ect on seismic data. Post blowout seismic data showed signifi cant loss of refl ecti on amplitudes in the vicinity of blowout well at lower Eocene level where the producti on reservoirs are established. Another major eff ect of blowout on 3D data has been observed at the depth of isolati on casing shoe. This paper presents detailed analysis of post blowout 3D data for identi fi cati on and delineati on of blowout aff ected area in the verti cal secti on around the well in order to demarcate the area of possible drilling risk.


Amplitude versus offset (AVO) study in a gas sand - A case study from Upper Assam, India

K. K. Nath, R. K. Pathak & C. B. Reddy

Of late, Amplitude variati on with off set (AVO) has been

found to be one of the successful geophysical tools to

delineate gas bearing sands parti cularly within shallow

prospects. With the signifi cant advances in AVO technique,

it has become a reliable tool to study the details of

reservoirs in terms of lithology and rock properti es.

In the Upper Assam basin, the presence of commercial

quanti ty of hydrocarbon has been established in the sands

within the Barail secti on (from upper Eocene to Oligocene

age) comprising of fi ne to medium grained sandstone with

shale bands. One of these sands in the area of study has

been found to be gas bearing having thickness of about

15 mtrs (with thin shale band in between) at a depth

of about 2440 mtrs. In the area, study has been carried

out to understand the seismic characteristi cs and AVO

response of the thin gas bearing sand. The AVO response

of the gas bearing sands and corresponding AVO anomaly

has been presented in the paper.

69

The syntheti c AVO response for the gas sand matched well

with real seismic data at well locati on. The AVO curves

show variati on of amplitudes with off set from the top

and bott om of the gas sands from the real and syntheti c

AVO traces. The scatt ering of some points from model

AVO curves are primarily due to shale bands present

in the gas-bearing zone. The identi fi ed AVO anomaly is

being used to map the extent of gas bearing sand in the

area of study.


Reservoir Characterization from Principal Component Analysis of 3D seismic attributes - A case study from Upper Assam, India

Y. P. Singh, R. K. Pathak & K. K. Nath

Seismic att ributes are specifi c measurement of geometric,

kinemati c, dynamic or stati sti cal features derived

from seismic data. They are very useful in predicti ng,

characterizati on and monitoring hydrocarbon reservoir

as they are the manifestati on of reservoir characteristi cs

and their variati ons are representati ve of variati ons

in reservoir parameters. With the advancement in

processing and imaging technology, 3D seismic att ributes

(at regular grid) integrated with well att ributes provide

detail descripti on of reservoir properti es. However, out of

number of att ributes, only few can be used for reservoir

characterizati on and some of them are stati sti cally

dependent. Principal Component Analysis (PCA) is

used to determine the correlati on among the seismic

att ributes and provide a set of new att ributes that are

stati sti cally independent and orthogonal to one another.

It is observed that the att ributes along few principal

components are insignifi cant and may not contain

informati on about reservoir properti es. The signifi cant

principal components retain maximum informati on

contained in the seismic att ributes. The integrati on of

various seismic att ributes for reservoir characterizati on

in principal component domain is robust primarily as the

numbers of signifi cant principal components are less than

the original seismic att ributes. These signifi cant principal

att ributes when correlated with well att ributes provide

meaningful informati on of reservoir properti es.

In this paper a case study from upper Assam is

presented wherein PCA of seismic att ributes has been

used in reservoir characterizati on. Seismic amplitude,

instantaneous amplitude, instantaneous frequency,

instantaneous phase etc. have been extracted from

3D seismic data against reservoir interval aft er proper

calibrati on with the sonic logs available within the study

area. PCA has been carried out using above att ributes

to derive a set of att ributes orthogonal to one another.

These PCA derived att ributes and their correlati on with

well-derived reservoir properti es such as porosity and

sand distributi on are presented in this paper.


Application of seismic attribute analysis and Post-stack inversion in reviewing hydrocarbon prospect within deep lower Eocene–Paleocene sediments – A case study from Upper Assam basin

S. Rath & S.N. Singh

Conti nuing oil producti on from Oligocene/Miocene

prospects in one of the major oilfi eld of Oil India

Limited (OIL) in Upper Assam and the increasing success

in producing from Lower Eocene-Paleocenes in its

neighboring oilfi elds inspired OIL to drill on a structure

in the oilfi eld to probe hydrocarbon prospects in deeper

Lower Eocene-Paleocene secti ons. However, a few

discouraging results of exploratory drilling for similar

Lower Eocene-Paleocenes in nearby structures, compelled

OIL to hold up the drilling ti ll more supplements from

drilling of bett er rated prospects in close by structures

and outcome of a proposed 3D survey on the structure,

are made available. Parallely, it was thought prudent to

undertake a focused reprocessing of relevant seismic data

with latest technology and study the reservoir through

seismic att ribute analysis and post-stack inversion. The

present paper deals with this reprocessing and inferred

results that helped OIL in reviewing its decision on drilling

of the locati on.


Identification and validation of possible Paleo-sediment incursion from 3D Time Slices - A case study from Upper Assam

N.M. Bora, K.K. Nath & S.N. Singh

The paper deals with processing and scanning of 3D data

volume for a series of ti me slice that revealed a possible

relati on of the seismic refl ecti on anomaly to the paleo-

sediment depositi on during early period of basin fi ll in

parts of Upper Assam shelf. The anomaly that appeared

at the southeastern edge of the 3D block, at a level close

to graniti c basement, was observed to gradually encroach

and fi nally swarming the enti re 3D Block within a ti me span

70


of about 300 ms (TWT). The observed anomaly showed a

progressive landward marine incursion, which appears

to have played a crucial role in sediment depositi on and

distributi on system in the region. The observed anomaly

has been validated from the available geological and

drilling evidences. Considering the aspects of proximity

of source to accumulati on, paleo-geomorphology and

observed basement features around the zone marked by

the anomaly, a new explorati on target has been postulated.


Seismic guided nonlinear extrapolation of Porosity using Probabilistic Neural Network – A case history from Upper Assam, India

Y.P. Singh, R.K. Pathak, K.K. Nath & Prof. K. D. Gogoi

Accurate esti mati on of reservoir properti es such as

porosity, permeability, sand/shale rati o and pay zone

thickness etc. is necessary for effi cient management of

the reservoir. All these properti es on well locati ons can

be esti mated very accurately with available well logs.

The interpolati on/extrapolati on of these properti es away

from the well bores will not be accurate due to limitati on

of well distributi on. 3D seismic data available in the

study area can enhance the accuracy of the esti mati on of

reservoir properti es away from well bores. Probabilisti c

Neural Network (PNN) based data driven method has

been applied over one of the oilfi elds of OIL at upper

Assam for interpolati on/extrapolati on of porosity.


Successful estimation of Q from surface seismic data: Methodology and case studies

R. Dasgupta & R.A. Clark

The seismic quality factor, Q, can provide important

informati on for hydrocarbon explorati on, but it is rarely

used. Though Q has been successfully from VSPs, use of

surface seismic data has failed to deliver reliable results.

In this paper, the method proposed by Dasgupta and

Clark (1993) for esti mati ng Q from surface seismic data is

reviewed and further validated by comparing the results of

Q esti mati on using this method from surface seismic data

of southern UK North Sea to esti mates from VSPs close to

the seismic line and in similar lithologies elsewhere in the North Sea. There is a good agreement of interval Q values between those esti mated from surface seismic data and those derived from VSP’s, the Q esti mates being in the

range of 50- 150 for units from Chalk to Bunter-Zechstein. Furthermore, Q has been computed from surface seismic data from off shore South-East Asia to discriminate between sediments and basement, in absence of any well control. Here, the interval in questi on has Q values of 680-800 suggesti ng metamorphosed sediments or crystalline

rocks with some amount of jointi ng or porosity.


Near-surface imaging with Traveltime and Waveform Inversion

Priyank Jaiswal & Colin A. Zelt & Rahul Dasgupta

Due to acquisiti on related noise and groundroll only a limited ti me window of seismic data is available for depth imaging through conventi onal processing. This ti me window excludes coda near the direct arrivals and far off set refracti ons as they cannot be handled by the conventi onal processing methods. Moreover due to NMO stretch and mute, shallow refl ecti on events have limited off sets and do not stack as well as deeper refl ecti on events. As a result, in terrains with rough topography and rapidly varying near surface conditi ons, near surface images from conventi onal processing are oft en not appropriate. We present a case study of near-surface (< 1.5 km from topography) imaging using multi -scale waveform inversion. We also show that multi -scale waveform inversion not only yields an interpretable near surface image but also a velocity model that can be used in conventi onal processing for improved depth imaging. However, due to a high degree of nonlinearity inherent in waveform inversion, its success calls for a robust starti ng model and careful applicati on. In this paper we esti mate a suitable starti ng model for waveform inversion using regularized inversion of direct and refl ecti on travel ti mes. The seismic data used in this paper are from the Naga Thrust and Fold belt in Northeast India.


Well log and synthetic seismogram analysis of an oilfield in Assam, India: A 3C seismic development feasibility study

Robert R. Stewart & Maria F. Quijada

K. L. Mandal & Romen Borgohain

Well logs from several fi elds in Assam, India were analyzed

to determine the feasibility of a 3C seismic development

study. One of the challenges in the area is to identi fy

sand-rich regions within a structural geologic setti ng.

We fi nd that representati ve reservoir sands (e.g., Barail)

have an identi fi able response; in parti cular, higher S-wave

velociti es compared to their surrounding sediments.

Syntheti c seismograms generated from the logs suggest

71

that these reservoir sands may be detectable with PP and

PS seismic techniques.


Estimating seismic attenuation from surface seismic data - Proposed method and case histories

R. Dasgupta & Roger Clark

Recently, lots of works are being carried out on

esti mati on of seismic att enuati on or Quality Factor (Q)

and how it relates to diff erent variables like rock-matrix,

pore-space/pore-shape/pore orientati on and above all

the fl uid present in the pores. The att empts to esti mate

seismic att enuati on from VSPs. Check-shots, sonic

waveforms have met with reasonable success (Ganley

and Kanesewich, 1980; Jannsen et al, 1981; Stainsby

and Worthington, 1985; Tonn, 1991). Its esti mati on from

surface seismic data was not successful mainly due to the

multi plicity of ray-paths associated with surface seismic

data. Dasgupta and Clark (1993) presented a method of

successful esti mati on of seismic att enuati on from surface

seismic data which accounted for multi plicity of ray-paths

and NMO-stretch associated with surface seismic data. In

this paper, a modifi ed version of the method, its validati on

through model studies and actual fi eld data is presented.

The use of Q as a tool for gross lithology discriminati on

and the eff ect of hydrocarbon on it is also discussed. All

the three datasets used in the work are marine datasets.


Porosity estimation from multi-attribute analysis from 3D seismic and well data- A case study from Upper Assam

R.K. Pathak & K.K. Nath

Esti mati on of porosity of a hydrocarbon reservoir is

essenti al for reserve esti mati on and planning for producti on

operati on. In most cases, however, spati al porosity

variati on can not be delineated from the measurements

made at sparsely located well. The integrati on of seismic

att ributes along with well measurements signifi cantly

improves spati al descripti on of porosity. Esti mati on of

porosity is made using deterministi c approach based

on stati sti cal relati onship between porosity and seismic

att ributes (Schultz et. Al. 1994, Russell et. al. 1997).

The area under study is a part of Upper Assam basin

where commercial quanti ty of hydrocarbon has been

established in the sands of Barail formati ons of upper

Eocene to Oligocene age. The area has been covered by

3D seismic survey during the year 1993-94 with 25m x

50m bin size with the objecti ve to delineate prospecti ve

reservoir sands for opti mal fi eld developments.

A study has been carried out to map porosity from multi ple

regressions of seismic att ributes and well derived porosity

esti mates in the reservoir interval and the results have

been presented in the paper. In the study, porosity has

been considered as weighted sum of seismic amplitude,

instantaneous amplitude and instantaneous frequency.


Q from surface seismic land data-methodology

R. Dasgupta & Y.P. Singh

The Seismic inelasti c att enti on, which is measured in

terms of quality factors, Q, is an important att ribute, and

is required in various amplitude analyses like AVO. It is

generally measured from VSPs and partly from marine

seismic data. In this paper we present a modifi ed version

of AVO technique, suitable for measuring Q from land data.

The amplitude spectrum is compensated for NMO stretch.

The log of rati os of amplitude spectrum for diff erent

refl ecti ons are used to esti mate Q. Slope esti mati on in

spectral rati o method is very crucial. Therefore, it was

esti mated using M-norm robust esti mati on. We also

present a case study from Upper Assam to demonstrate

its practi cal applicati on.


Understanding of seismic pitfalls through Seismo-geological Modeling: Some examples from Upper Assam basin

B.N. Singh, A. K. Malkani & K.K. Nath

Oil India Limited (OIL) has been operati ng in Upper Assam basin for many years for the hydrocarbon explorati on. During the interpretati on of seismic data the possible basement was mapped at around 3 seconds near the Brahmaputra arch. A well defi ned refl ector below the possible basement was noti ced near the arch, which was thought to be either due to the presence of pre-rift sediments or due to the seismic arti facts. The questi onable refl ector was noti ced both on stacked and migrated ti me secti ons of 2D seismic data. A well drilled in the area on a similar feature encountered the basement before reaching the questi onable refl ector.

The depth encountered in many other wells drilled on the basis of seismic ti me interpretati on did not match with the esti mated depth. The saddles of the structures on

72


the seismic ti me secti ons were encountered at relati vely shallower depth. Many structural features associated with extensional and compression tectonics mapped on the ti me secti ons proved to be dry while drilling. By comparing the two secti ons i.e. seismic secti on in ti me domain and seismic secti on in depth domain, it is evident that in the ti me domain secti on it appears a structural high whereas in the depth domain the events are relati vely fl at.

In the thrust fold areas of Upper Assam basin, the interpretati on of seismic data was found very tedious due to the complexity of the geology and the misleading features present on the seismic ti me secti ons. The diffi culti es have been faced in proper understanding of the thrust planes and the structural features presents in the sub-thrust. Many structures mapped in the sub-thrust were found to be dry aft er drilling.

In order to understood the above problems encountered during the interpretati on of seismic ti me secti ons geological models were prepared and seismic ti me secti ons were generated using a two dimensional ray tracing. Several models were prepared for each problem and the unique soluti on was derived with the comparison of modeled and real data.


Porosity estimation from multiple regression analysis of principal component seismic attributes - A case study from Upper Assam, India

Y.P. Singh, R.K. Pathak & K.K. Nath

In this paper a method for esti mati on of porosity from multi ple regression analysis of principal component seismic and well att ributes is presented. Principal component analysis of extracted seismic att ributes creates a new set of independent att ributes (principal component seismic att ributes). The principal component seismic att ributes along with well data are used for multi ple regression analysis. The weights so generated are used to predict porosity. The method has been validated by uti lizing 3D seismic and well att ributes from one of the oil fi elds in upper Assam. The predicted porosity has shown substanti al improvement over the porosity predicted from well att ribute and marginal improvement over the porosity predicted from multi ple regressions of seismic and well att ributes. The data reducti on and other computati onal advantages associated with this technique make it a favourable alternati ve for accurate porosity esti mati on. The area under study is part of upper Assam basin where the presence of commercial quanti ty of hydrocarbon has been established in the sands of Barail Formati on of upper Eocene age. The reservoir sands are

deposited in deltaic environment and lie at a depth range of 2850-3050 m having approximate thickness of 30-50m. The reservoir sand show lower seismic velocity in comparison to overlying and underlying shale formati on and can be characterized as class III type (Rutherford and Williams, 1989).


Fluid type distribution based on seismic attributes in Makum field, Upper Assam basin, India

N.M. Borah, S. Rath, B.N. Singh, P.K.Kakoty & Anurag Grover

More the well data the more is the accuracy in approximati on of rock properti es and thereby, the interpretati on. To obtain bett er well data for petroleum explorati on means allotti ng a high budget, which may not be cost eff ecti ve always. For any explorati on eff ort is made to strike a balance between a cost eff ecti ve budget and obtaining adequate well informati on. In these scenario 3D seismic att ributes comes as an aid. It has been revelati ons that simultaneous analysis of 3D seismic att ributes and well log properti es lead to bett er esti mates of spati al distributi on of reservoir or rock properti es as compared to esti mates generated only from well data.

However, the relati onships of seismic att ributes to log properti es are usually not obvious, and furthermore, they vary from one region to another, and even from one layer to another. This study is an att empt to classify the att ribute space based on fl uid type using seismic att ributes constrained with log properti es. In this method advance techniques for seismic guided property esti mati on are used that combines seismic att ributes and log properti es in a quanti tati ve way to predict porosity distributi on in the reservoir.

Makum oilfi eld is situated in Upper Assam basin in northeastern India. The fi eld is a faulted anti clinal structure. The main pay interval is within the Barail formati on of Oligocene age. The reservoirs under study have been deposited in lower delta plain setti ng of a prograding delta system. The depth of these sandstone reservoirs is of the order fo 2550 to 2650 meters below Mean Sea Level. The pay thickness is of the order of 20 to 50 meters.

The fi eld is in acti ve development stage with 18 wells drilled ti ll date. Large areas of the fi eld are yet to be developed and the present recovery from the fi eld is around 5% of the original oil-in-place. The fi eld is covered by 3D seismic data.

This fi eld has been taken up for study to predict the spati al distributi on of gross porositi es in order to guide an opti mum development plan.


73

New imperatives in the exploration of oil & gas

V. K. Sibal

Oil and gas are the prime sources of energy on which the

human race depends. Both these energy sources play a

very signifi cant role in the economic, social and politi cal

lives of the nati ons of the world. However, the reserves

of oil and gas being fi nite, there is an ever-widening

gap between demand and supply. No alternati ve

economically viable source of energy has been found.

Thus, oil and gas have become scarce, and thereby,

precious commoditi es.

It is imperati ve that alternati ve explorati on programme.

This is a challenging task, involving extensive and

strenuous explorati on, and most importantly, a global

consciousness and appreciati on of the problem. Innovati ve

thoughts and ideas, based on intensive research, need

to be encouraged in order to achieve a breakthrough in

discovering alternati ve energy sources.

Till such ti mes alternati ve energy sources are found, our

explorati on programmes will have to discover/produce

oil and gas to meet the increasing demand, so as to

prevent the inevitable use of these precious commoditi es

as politi cal weapons. The very survival of the human

race depends on the explorati on of oil and gas and other

alternati ve energy sources. It is for this very reason that

our present percepti on of hydrocarbon explorati on need

to be redefi ned. A global data base needs to be created

for a complete understanding and evaluati on of world’s

hydrocarbon prospect. For this, it is imperati ve that a

global explorati on programme is chalked out, whereby

no nati on works in isolati on; all nati ons must integrate

their studies to create a data base that will cover complex

geological boundaries in all the conti nents. Integrati on

of these data may lead us to the discovery of the much

needed alternati ve energy sources.


Advances in High Performance E&P Workstations for oil industry

V.K Sibal & A. Subramanian

High performance of E&P workstati ons are powerful

desktop workstati ons with enhanced graphic capabiliti es

and effi cient network communicati ons aimed to provide

cost-eff ecti ve soluti ons for explorati on and producti on

problems of oil industry. Integrati on of geology (organic

geochemical and hydrodynamic data), petrophysics

(borehole and well log data), geophysics (2D and 3D seismic

data), reservoir engineering along with 3D visualizati on

of the subsurface conditi ons is prime objecti ves of E&P

workstati ons. To full fi ll this objecti ve, the various existi ng

stand-alone E&P applicati ons needs to be integrated

in a network environment. This integrati on of existi ng

multi ple applicati ons can help the E&P problems to be

addressed in a somewhat integrated fashion, but data

handling will be an enormous task. This integrati on in

compliance to evolving industry standard will be of great

help in future which will mainly solve the data handling

problems that presently exist. One such a major standard

evolving in E&P sector of oil industry is POSC standard.

Year 1994-95 is set to be a starti ng for implementati on

of POSC specifi cati ons. The future requirements in E&P

workstati ons as perceived by the authors are soft ware

available adhering to some industry standard, eliminati on

of specialized, non-standard graphic processors which

at present creati ng maintenance problems, high speed

network link between systems, 3D visualizati on of the

prospect or reservoir and virtual reality in oil fi led drilling

operati ons.


Impact of innovation in Petroleum Geophysics

R. Dasgupta & K.K Nath

In mankind’s search for natural resources geophysics has

served as an important explorati on tool. In the last century,

various geophysical techniques came into existence

and emerged as the most important contributor for

explorati on and explorati on of various natural resources

all over the globe. With the passage of centuary, physical

& biological sciences and technologies witnessed rapid

advancement, which gave impetus to the development

of geophysical techniques. The professional in the fi eld

of geophysics conti nuously imbibed & embraced this

advancement towards providing innovati ve soluti on to

a wide spectrum of problem concerning explorati on and

exploitati on of hydrocarbon resources. In this paper, we

delve into the development of geophysics as a discipline

over the last century, specifi cally in regards to seismic

prospecti ng. The impact of innovati ons, at every stage of

development of this discipline is also highlighted.


75

Spectral Analysis of Gravity data-An integrated approach towards hydrocarbon exploration in geologically complex and logistically difficult area of Manabum

G. K. Ghosh & F. Siddiquee

Imaging of subsurface in the perspecti ve of hydrocarbon

explorati on in the geologically complex and logisti cally

hosti le terrain of the Assam-Arakan basin near fore

deep of Himalaya foot hills is one of the daunti ng and

challenging job for the geoscienti st. In such a complex

mountainous and thrust terrain, the seismic has its own

limitati on and it is always recommended to uti lize some

passive geophysical methods to supplement and add

value to the seismic data. In similar such eff orts, OIL

in its endeavor towards the accelerated hydrocarbon

explorati on approach ventured into the Manabum area.

The area of study primarily falls close to the foot hills of

Himalayas i.e. fore deep region in the North-East and

partly in Belt of Schuppen (thrust belt) to the south-

east, facing the Upper Assam foreland shelf. In order to

obtain some meaningful geophysical data, OIL decided

to acquire ground gravity data on the experimental basis

along few profi les which falls in the periphery of seismic

lines. These data was collected with a great diffi culty as

in the study area the ground elevati on varies from 250

meters to 800 meters. The Gravity data was collected

with the joint eff ort of NGRI and OIL using Lacoste and

Romberg Gravimeter (Model G) with an accuracy of 0.01

mGal covering total 2000 gravity observati ons at spacing

of 0.05 km to 1.0km interval as per availability of tracks

and approach. While carrying out interpretati on studies

of Gravity data, an att empt has been made to esti mate

the average depth of the sedimentary strata and the

basement from the Bouguer anomaly map, using spectral

analysis method. The necessary program uti lized for

the spectral study was developed by the authors. This

spectral analysis approach gives the result which maps

the variati on of Namsang depth formati on from 1.96 km

to 2.28 km where as the average depth of the basement

from the study varies from 6.5 km to 7.6 km. the result

so obtained from these study substanti ate and correlate

very well with that of seismic.

The present paper illustrates the details of the spectral

analysis approach in the depth esti mati on and the result

so obtained encourages uti lizing similar techniques to

obtain some meaningful results in such complex terrain. Published in AEG-2005

Magnetotelluric field investigations in the logistically difficult Manabum of Arunachal Pradesh

C.V.G. Krishna, B.M.Sinha, K.K.Nath & T.Harinarayana

Oil India Limited (OIL), a Government of India Enterprise having its E&P as core acti viti es in the upstream sector of petroleum industries, has embarked on an accelerated geophysical explorati on programme parti cularly in the logisti cally diffi cult & & geologically complex fronti er areas in Assam & Arunanchal Pradesh. As a part of this programme, there has been renewed thrust on explorati on acti viti es in & around Manabum prospect in Arunanchal Pradesh. Manabum area is located at the juncti on of two of the most signifi cant folding movements in the region namely the east west folding parallel to the Naga Hills and the north west-south east folding parallel to the Mishmi Hills. The folding movements are the result of massive tectonic acti vity witnessed during the Miocene period. The area is geologically complex with the presence of thrusts and folds structures and comprised mostly of hilly terrain with surface elevati ons varying between 100 to 300 meters. Top few hundred meters in the area is covered mostly by loose sediments and boulders/pebbles leading to extreme diffi culti es in the executi on of the seismic survey plan and hence causing diffi culti es in mapping of diff erent sub-surface horizons of geological interest with high degree of confi dence based only on the seismic data. In view of the complex nature of the said area, both logisti cally and geologically, an integrated geophysical explorati on programme was initi ated comprising of 2D seismic, ground Gravity-Magneti c (GM) and Magnetotelluric (MT) surveys.

In the present paper, the result of the MT modeling exercise and that of the subsequent MT Field investi gati ons conducted jointly by Nati onal Geophysical Research Insti tute, Hyderabad and OIL are being discussed. The objecti ve of the modeling exercise is to see how the diff erent geological formati ons respond to MT signal. The log data, litho-strati graphic formati on encountered in the well and borehole resisti vity has been used for the purpose of the modeling. The MT responses for both 1D and 2D schemes have been modeled assuming 40 ohm-m for the Namsang/alluvium sediments having thickness of about 2-3 Km, overlying the conducti ve sediments of the Girujan/Tipam/Barail formati ons varying the thickness up to 6 Km at places. The resisti vity of the sediments below Namsangs is varied and MT response for various models has been studied and it has been observed that irrespecti ve of sedimentary thickness of diff erent

76

Beyond Seismic ...Integration is the key

formati ons, both Girujan and basement tops can be map with reasonably high level of confi dence.

With the encouraging results obtained from the modeling exercise MT fi eld investi gati ons were undertaken along three profi les in the area with a stati on interval of about 2-3 Kilometers. The frequency of the recorded signals ranges from about 0.001 second (1 KHz) to 500 seconds (.002 Hz) which facilitates to probe the subsurface from shallow depths of a few meters to few tens of kilometers. The processed results of the fi eld data and subsequent 1D and 2D modeling of the same have clearly brought out the interfaces between Dhekiajulis/Namsangs, Namsangs/Girujans and the overlying sedimentary formati ons and the basement top thus validati ng the results of the modeling undertaken prior to the fi eld investi gati ons.


Re-look on the prospectivity of Uttaranchal foot hills in OIL’s acreages in Ganga Basin

Akshaya Kumar, Pramitabha Barua & Bedanta Pd. Sharma

The hydrocarbon explorati on in the Himalayan foothills, in parti cular the Ganga basin, in general has so far proved futi le though oil/gas source have been reported from many parts of the basin. Limited exploratory drilling, so far, has not encountered the much expected Eocene source rock i.e. Subathu Formati on, which are reportedly exposed in several thrust sheets in the area. Oil India Limited, for the fi rst ti me conducted seismic surveys (dynamite) in geologically complex and extremely rugged terrain of the Utt aranchal foothills. Processing and interpretati on of the data, parti cularly applicati on of advanced techniques elucidated the hydrocarbon prospecti vely of the study area from the explorati on point of view. Identi fi cati on of Triangular zones (Triangle zones having thrust-strike prospects with apparent closure as well as new plays, etc) which appear analogues to many other oil/gas bearing triangular zones around the world, have renewed interest in the area. Triangular zones in the study area are made up of stacked south-travelling thrust sheets and their north-vergent back thrusts, build on a detachment at top of Vindhyan where the target is Dharmsala (reservoir) and Subathu (source and reservoir).

The area merits a sustained explorati on programme with techno-economic considerati on. The present paper highlights focus on the explorati on in triangular zones for deep as well as shallow gas/oil reservoirs, which is expected to result in the discovery of hydrocarbons in Utt aranchal foothills.


Weiner Deconvolution of Magnetic data for Automatic Depth Estimation in a geologically complex and logistically difficult area of Assam Arakan Basin

G.K. Ghosh, S. K. Basha, Md. Salim & V.K.Kulshreshth

Hydrocarbon explorati on in unexplored virgin areas like foot hills of Himalayas in the North-Eastern part of India is a greater challenge for explorati on geoscienti sts. The area is logisti cally diffi cult and geologically complex with thick reserve forests and rugged topography. Near surface boulder beds, dipping strata, coupled with severe thrusti ng made the acquisiti on of good quality seismic data a diffi cult task. Proper esti mati on of basement depth and delineati on of basement confi gurati on is essenti al for an integrated interpretati on of geophysical data pertaining to these areas. In this study, Wenner Deconvoluti on Technique (Wenner 1953) has been used to confi gure and esti mate the basement depth using total magneti c fi eld data. This technique assumes that the basement is supposed to be a series of thin semi infi nite verti cal dykes lengths and it can give direct depth informati on to the magneti c causati ves bodies without prior knowledge of strike, inclinati on and suscepti bility of the magneti c bodies. This technique has been applied on some of the selected profi les and corroborated the results with the 2D seismic data. The results thus obtained can be used along with other data in an integrated interpretati on to constrain basement depth.


Imperatives of Knowledge Management in upstream oil & gas Industry

A.K. Khanna, K.L. Mandal, T. Bhatt acharya & D.S. Manral

In the organizati onal context, Knowledge Management (KM) helps in building upon what we learn from others and reduces the response ti me to adapt to changes in the business environment. In a competi ti ve business scenario where every moment is precious & every decision is strategic, it is a necessity to capture & create a repository of both explicit & tacit knowledge. Such a knowledge repository can aid in faster & improved decision making within the organizati on.

KM has been leveraged by the oil & gas industry for more than a decade, during which the industry has witnessed radical changes. Notable among them have been the spate of mergers & acquisiti ons, expansion into new geographies, rigid environmental norms, advances in

77

technology, deep-sea explorati on, etc. Through these changes, KM initi ati ves have played an important role in making the operati ons more eff ecti ve & effi cient. Although KM initi ati ves in upstream oil & gas industry in vogue for quite some ti me, an eff ecti ve KM System can be of help in addressing some of the emerging issues facing the industry today.

Availability of skilled professional across various disciplines is a key area of concern, compounded further by aging of the workforce & rising level of att riti on. Putti ng in place an eff ecti ve KM System can help upstream oil & gas companies retain valuable knowledge in such a scenario. As oil & gas companies expand into new geographies and operati ons move into geologically complex environments, the challenge will be to disseminate the best practi ces across the organizati ons. Experience of various oil & gas companies in implementi ng KM suggest that Content Management Systems & Communiti es of Practi ce are eff ecti ve towards realizing this objecti ve.

Content Management Systems provide ti mely & useful informati on to users by creati ng processes that identi fy, collect, categorize and update contents using a common standard across the organizati on. Contents can be in the form of databases, presentati ons, reports, publicati ons, e-mails, etc. Communiti es of Practi ce create a channel for knowledge to cross boundaries created by functi ons, geographies, ti me, etc. It promotes the development & standardizati on of practi ces across operati ons & regions.

Looking at the business case rati onale, the business impact of KM initi ati ves can be enormous in terms of quality improvements, reducti on in downti me, fewer lost opportuniti es, improved performance, imparti ng skills & enhancing capabiliti es of the workforce, etc. which more than justi fi es the investment made on KM initi ati ves. However, criti cal to the success of such initi ati ves is the unsti nted commitment of the senior management towards management of the change and enlist support of all stakeholders towards such initi ati ves.


Numeric integration of Infinite and Singular Integrals using Tanh transform

Y.P. Singh

The applicati on of Tanh transform is considered in the effi cient and accurate evaluati on of infi nite integrals generated in electrical and electromagneti c modeling of geophysical system. Numerical results are given and compared with existi ng techniques like digital fi ltering and

Fast Fourier Transform (FFT). The tanh transform method converges for very small and very large values of distances between source and receiver points. The accuracy of Tanh transform is more than the other existi ng techniques. At the same ti me it takes less CPU ti me and memory, and to achieve the desired accuracy tanh transform requires less number of sample points in comparison to digital fi ltering and FFT technique.


Gravity and Magnetic surveys in Assam-Arakan Basin

A.K. Verma, Y.R. Singh, Anup Kumar, R. Dasgupta & K.K. Nath

The explorati on of oil and gas in the petroliferous basin of Assam-Arakan is conti nuing for several decades. Potenti al fi eld methods (Gravity and Magneti c) played an important role in the explorati on planning and integrated geophysical data interpretati on. Oil India Limited (OIL) carried out airborne magneti c survey in two phases. The aero-magneti c provided the basement confi gurati on of the basin based on which subsequent seismic explorati on programme was planned and executed.

The ground gravity and magneti c surveys in the thrust belt areas of Assam and Arunachal Pradesh not only helped in planning the seismic survey programme but also served as an input for integrated geophysical explorati on in the area.


Seismic Database and Management System for oil & gas exploration: A GIS based application

K.L. Mandal, D.S.Manral & A.K.Khanna

A huge amount of geoscienti fi c data is conti nuously acquired, processed, interpreted & stored in the E&P industry. A proper & catalogued preservati on of the dataset generated during the various phases of the E&P cycle is of paramount importance. These datasets need to be stored at a single source in a systemati c and structured manner for carrying out planning & designing of fresh seismic survey campaigns. Since explorati on acti viti es are carried out at geographically diff erent locati ons, the Database & Management system needs to be platf ormed on a Geo-Spati al applicati on. In view of above at Oil India Limited (OIL) an in-house eff ort has been made to build up a comprehensive Geo-Spati al Database & Management system to cater to various facets of data/informati on required for Seismic Data Acquisiti on, Processing & Interpretati on in context of OIL’s explorati on acti viti es.


78

Beyond Seismic ...Integration is the key

HSE management in seismic operations in OIL’s acreages in North-East india- A case study

Akshaya Kumar, K. K. Nath, R. K. Shrivastava & J. P. Singh

Oil India Limited provides a broad range of Oil and Gas Explorati on, Producti on and Transportati on services essenti al for a rapidly evolving global economy. OIL, however, fi rmly believe that growth should be well harmonized with "people fi rst" policies, environmental conservati on and social well-being. In order to be a responsible corporate citi zen and a partner of choice across the globe, the company upholds the highest standards of health, safety and environment management - an intrinsic value system which forms the core of our corporate governance. Our constant and collecti ve eff orts for ensuring incident-free operati ons, fail proof risk management and a cleaner, safer environment have paid rich dividends over the decades, leading to bett er growth opportuniti es and enhanced trust.

Oil India Limited (OIL’s) seismic explorati on for hydrocarbon has moved into challenging and new fronti er areas. Presently OIL’s Seismic Operati on (dynamite) are being carried out in logisti cally diffi cult, hilly and hosti le, rugged terrain with dense forest, geologically complex, culturally sensiti ve and highly populated areas like Upper Assam, India. The environment is extremely testi ng and challenging.

As seismic operati ons are highly technology driven, knowledge based, scienti fi c and integrated operati ons involving a large number of vehicle, manpower, sophisti cated instrument and explosive materials lead to safety hazards, health risk and environmental issues.

The present paper discussed the awareness of safe operati ng practi ces and implementati on of sound procedure essenti al for achievement of high standard of health, safety and environment during acquisiti on of good quality of seismic data by OIL. Also addressing the salient HSE issues and correcti ve measures/Dos & Don’ts in general to be followed by concerned seismic party. During seismic survey operati ons various challenges have been faced in view of several factors like use of dynamite as energy source, reserve forests underlain by boulder beds, loose gravels mixed with wet and loose sands, fl owing water at shallow depths, presence of wild life, highly undulati ng terrain etc. which are called for practi cing of stringent safety aspects and at the same ti me ensuring the quality of seismic data acquired within laid down ti me frame in most cost eff ecti ve manner has also been dwelt upon.


An innovative approach to safety during seismic survey operations in Upper Assam – A case study

R. Dasgupta, C.S. Singh & T. Bhatt acharjee

Oil India Limited (OIL), a Govt. of India enterprise, has been engaged in explorati on, exploitati on and transportati on of hydrocarbons in Assam and Arunachal Pradesh for last several years. The seismic survey operati ons of the company dates back to early 60s. OIL has its own in-house seismic crews which have been operati ng in diff erent areas of Assam/Arunachal Pradesh since 1977. In all these seismic surveys, explosives (special gelati ns/seismic gelati n) have been used as energy source. The impact of the explosions may create damage to buildings and properti es. The demography of the operati onal areas has changed drasti cally in the last decade– the existi ng township/villages have expanded and in many places new dwellings have come up. The seismic industry needs to adopt to this changing scenario, so that while realizing the objecti ves of seismic survey, it does not cause harm and damage to life, environment and buildings and properti es. In this paper, we present before you a case study which describes how a problem that arose from seismic explorati ons in recently surveyed Moran 3D block was solved with existi ng resources and without jeopardizing the objecti ves of the 3D seismic survey in Moran area.

IV OIL Industry Safety on Upstream Operati on-1999

79

“c - oil india · geophysics as a primary explorati on tool with change in technology, new...

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