2888105 prospecting for subdeccan trap mesozoicformationhosted hydrocarbons

8/6/2019 2888105 Prospecting for SubDeccan Trap Mesozoicformationhosted Hydrocarbons

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Abstract

Thick Mesozoic sediments under the basalt cover of the

Deccan traps along the northwest coast of India are consid-

ered to be potential targets for hydrocarbon exploration

together with the Indus basin of Pakistan. Sub-basalt seismic

imaging is difficult in these formations, as the Deccan traps

are composed of multilayered lava flows. Seismic imaging

issues associated with high velocity basalt include:

1. Multiples generated in interbedded units of basalt and

between the top of the basalt and the sea floor.

2. Energy scattering from and a bsorption by hetero-

geneities.3. Wave mode conversion at the top of the basalt.

Rado n-based an alysis can solve some of the d ifficulties relat-

ed to multiples. The use of low frequency sources and multi-

component technology are other methods commonly sug-

gested to improve sub-basalt imaging. The Deccan basalt

area is very heterogeneous and has not been mapped effec-

tively by high resolution seismic. At present, only standard

streamer data are available in this area. Model-based imag-

ing together with other methods can improve the sub-basalt

image and add confidence in the reservoir modelling.

Imaging challenges in this setting are discussed using exam-ples from the Kutch basin on the northwest coast of India,

where the primary reservoir (Bhuj formation) is overlain by

thick basalt.

IntroductionThe Gulf of Kutch on th e northern part of the west coast of

India is a unique basin where the basaltic flow separates the

Tertiary from the Mesozoic sediments below. These

Cretaceous basaltic rocks are known as the Deccan traps.

The basalt is composed of multi-layered extrusive flows that

suggest at least 40 periodic pulses of lava forming inter-trap

deposits (ref. Short and Blair, 1986, Chapter 3). This gener-

ates heterogeneous basalt layers, which makes sub-basaltimaging more difficult. Sub-basalt imaging has become an

important topic in geosciences communities, as targets are

obscured by the distinct elastic wave behaviour of the basalt.

Seismic velocities in basalt (P-wave velocities range from 4 to

6 km/s) are higher than those of surrounding sediments,

resulting in ray turning, wave-mode conversion, and scatter-

ing of seismic waves. A special issue on sub-basalt imaging

(Geophysical Prospecting, May 2003) covered various acqui-

sition and processing challenges in basaltic formations.

Fundamental issues identified are:

1. Multiple events generated in interbedded units of basalts

and between the top of the basalt and the sea floor.

2. Energy scattering and absorption from t he hetero-

geneities.

3. Wave mode conversion at the top of the basalt.

Various au thors have suggested different methods to impro ve

sub-basalt imaging dependent on the area and the associatedproblems. The major concern is when multiples interfere

with primary reflections, which is typically tackled with

Radon filtering (ref. Yilmaz, 2001, Chapter 6; Spitzer et al.,

2003). Loss of frequency content at the interface of basalt

and sedimentary layers caused by the differential scattering

of high frequency components is another problem.

Ziolkowski, et al. (2003) propose to use a low frequency

source (less than 30 Hz) which can allow seismic energy to

penetrate through basalt. Also there is strong seismic wave-

mode conversion at the basalt-sediment interface due to the

high velocity contrast. Primary wave (P-wave) energy is con-

verted to shear wave (S-wave) energy at that int erface. In th isgeological setting, multicomponent data can give a better

image below basalt (Wang and Singh, 2003) by combining P-

wave and S-wave information. H anssen, et al. (2000, 2003)

also conclude that locally converted shear waves (with a

weak signal) are difficult to use for imaging below basalt.

Another concern is the amplitude attenuation (seismic inelas-

tic behaviour) caused by heterogeneities, which should be

compensated for during seismic data analysis. Inverse Q-fil-

tering (Yilmaz, 2001) can be used to compensate the inelas-

tic effects. According to the work referred to, there is no sin-

gle method or processing flow which is robust for sub-basalt

imaging in all geological settings.

Though thick Mesozoic sediments with hydrocarbonprospectivity occur below the Deccan traps, few geoscientific

studies have been carried out so far because of inadequate

seismic data and the sub-basalt imaging problems. Sain et al.

(2002) try to image the sub-basalt Mesozoic sediments using

wide-angle data but only demonstrate large scale features,

whereas for hydrocarbon exploration we are interested in

first breakvolume 22, July 2004

Prospect hunting below Deccan basalt:imaging challenges and solutionsDhananjay Kumar,1,2 Ravi Bastia,2 and Debajyoti Guha2

1 Corresponding address: Dhananjay Kumar, John A. and Katherine G. School of Geosciences, University of Texas Institute

for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759, USA, Email: [email protected] Address: O il & G as Division, Dhirubhai Ambani Knowledge City (DA KC), Thane Belapur Road, N avi Mumbai-400709

India, Email: [email protected], [email protected]


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detailed images of the reservoir. We prop ose three stud ies toachieve this objective:

1) Imaging problems associated with basalt,

2) Solutions to improve the image, and

3) Identification of potential hydrocarbon traps.

This article focusses on the Kutch basin, NW coast of India

(Figure 1). In this basin, the Bhuj forma tion is the ma in reser-

voir, and this is overlain by the thick basalt, which makes

imaging difficult.

Geology and lithology of the Kutch basin

The Kutch basin forms the northern part o f the western con-tinenta l margin of India tha t is classified as a p assive margin.

Three contiguous NW-SE trending major tectonic elements

have been identified in the offshore Kutch basin (Figure 2).

These are:

1) The Kori Comorin depression.

2) The Kori Comorin ridge.

3) The Laxmi-Laccadive depression.

Figure 2 also shows four E-W trending onshore ridges. The

basin is pericratonic and can be sub-divided into NE-SW to

E-W trending and westerly plunging ridges and depressions.

The broad lithology of the Deccan trap associated system is

as follows.

The upp er stratigraphy is composed of Tertiary sediments

dominated by limestone, sand and shale underlain by thick

basalt of Upper-Cretaceous to Palaeozoic, overlying Mesozoic

sediments. The thickness of basalt cover varies from 300 m in

the northwest to 2000 m in the southwest (Figure 1).

Seismic imaging problems and possible solutionsMany geoscientists have studied specific imaging problems

associated with b asalts above reservoirs. Here we discuss theissues related to sub-basalt imaging in the offshore Kutch

basin (Deccan basalt). The basalt in the Kutch basin differs

from many ba salts in oth er parts of the wo rld, as the thick-

ness of the basalt, composed of extrusive lava flows, varies

considerably. Therefore the basin is highly heterogeneous.

Seismic velocities are complex, and together with velocity

inversions introduce multiples, scattering, attenuation and

mode conversion.

Seismic mult iplesIn the offshore Kutch basin, there are three strong seismic

reflection interfaces:1) The sea floor.

2) The top of the basalt.

3) The base of the basalt.


Figure 1Isopach m ap of t he Deccan traps in the Ku tch basin.

The area of interest is highlighted. Indus fan is also a current

area of exploration interest.

Figure 2 Tectonic trends in the Kutch

basin. Three continuous N W-SE trend-

ing tectonic elements in the offshore

Kutch basin and four E-W trending

onshore ridges are shown.


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The strongest reflector is the top of the basalt. Th erefore very

little energy passes through the basalt and this has a reduced

frequency content. Strongly reflected energy from the top of

basalt r eflects many more times at interfaces between the seafloor and basalt before it is recorded. The primary reflection

response from the top of the basalt is stronger than the mul-

tiples, and the multiples are stronger than the reflection

response from the sub-basalt reflectors. Radon filtering is

effective in suppressing multiples. Figure 3 shows the effects

of Radon filtering on seismic data from the Kutch basin.

Above the basalt, multiple events have been reduced signifi-

cantly (Figure 3).

The heterogeneous basalt in the G ulf of Kutch generates

interbed multiples and th e signal to noise ratio deteriorates.

As shown in Figure 3, inside the basalt layers conventional-

ly processed data show better r esolution th an Ra don filtered

data, due to the limitation of the frequency range used for

the par abolic radon transformation (Spitzer et al., 2003). As

P-wave energy penetrates below the basalt, the signalbecomes very weak. In general, multiple energy dominates

over the signal. With Radon filtering, very little improve-

ment below the basalt is observed. Ringing has been reduced

and some amplitudes have been enhan ced (Figure 3). Due to

the complicated velocity model, post-stack time migration

(Figure 4) is not effective. Pre-stack depth migration with an

anisotropy correction is recommended to collapse the scat-

tered energy at the correct position. (Silva and Corcoran,

2002). However, pre-stack depth migration is sensitive to

the velocity (and anisotropy parameters) and is also very

costly.


Figure 3 Improvement in sub-basalt

image with radon transform, a) before

radon filtering, and b) after radon fil-

tering. In the intra-basalt portion of the

radon section (b) overall resolution is

poor, as we limit the frequency range

for the parabolic transformation.

Figure 4 A typical processed seismic

section in the Gulf of Kutch.

Resolution below traptop is poor.

Various horizons have been marked

including the Bhuj formation, which is

the primary zone of interest.


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Scattering, frequency loss and attenuationAt the top of basalt and within the basalt there is a strong

scattering of seismic energy, which reduces both higher fre-

quencies and the strength of the signal (Figure 4). Some low

frequency seismic energy passes through the basalt, with a

consequent loss in resolution. Moderate frequencies (approx.

40 Hz central frequency) are used in present seismic dataacquisition. Seismic energy is highly attenuated in the com-

plex multi-layered basalt formation of the Kutch basin which

reduces the sub-basalt signal. Spitzer et al. (2003) model the

heterogeneous basalt layer with an attenuation factor (Q ) of

30. Low Q indicates high attenuation, 30 is a low Q value.

This effect is normally not considered during seismic data

processing. However, inverse Q filtering can be a solution to

suppress the attenuation effects.

Mode conversionMode conversion of the incident wave (P-wave to S-wave

and vice-versa) occurs at seismic interfaces. At the top of

basalt, non-vertically incident P-wave energy partly converts

to S-wave energy, and there is strong ray turning of P-waves

entering the basalt. Since the P-wave velocity of the overbur-

den sediment is close to the S-wave velocity of the basalt

layer, the geometry for the converted S-waves inside the

basalt is relatively simple. This converted wave (PS-wave)can again convert back to a P-wave (PSP-wave) at the sea

floor before it is recorded. Converted P-waves (PSP- or PSSP-

waves) can be used for sub-basalt imaging (Barzaghi et al.

2002), but in practice they are very weak. For improved res-

olution, multicomponent data recording is required, which is

possible in a marine environment with ocean-bottom

receivers. At present, no multicomponent data are available

in the Kutch basin, however, if used, we expect imaging

improvements should be possible. The combined effects of

the above make sub-basalt imaging difficult. None of the

suggested mitigation meth ods are robust. Seismic wave prop-


Figure 5A 2D geological model of t he Kutch basin. The Bhuj sand formation is the prime reservoir below basalt. This model

is based on the seismic data bearing in mind the tectonic picture of the Gulf of Kutch.

Figure 6 A 3D geological model of theKutch basin. This is the overall geolog-

ical model in the Gulf of Kutch (scale is

approximate). It is based on the gener-

al geology and not on the seismic data.


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agation effects are angle dependent; therefore the effect of

the issues discussed will be dependent on the angle of wave

propagation. Angle stack panels should be analyzed to select

the best image.

Potential hydrocarbon traps, offshore KutchSub-basalt seismic imaging in the Kutch basin has been a

challenge to geoscientists. Two dimensional (Figure 5) and

three dimensional (Figure 6) geological models have been

developed to illustrate possible concepts. Two dimensional

models are based on seismic data b earing in mind th e geolog-

ic and tectonic picture. These models cannot be correlated

exactly with the seismic data due to the poor images. Three

dimensional models are based on the geological information

and not on the seismic data; they show an overall model for

the Kutch basin. There are two phases of rifting in the Kutch

basin (Figure 5), first in the sand-pro ne Kaladon gar, and later

in the Lower Bhuj (also sandy) formations. Between the two

was a calm period of carbonat e deposition in the Jhurio andJumara formations. Carb onate deposits were followed by the

Jhuran (a good source rock), and Lower Bhuj (sand) forma-

tions. With the Upper Bhuj deposition the sag phase com-

menced (still a topic of discussion), followed by the Naliya

and Mun dra (sag phase) formations. The Deccan traps o ver-

ly the sag phase. Exploration targets are the Bhuj sands

below the ba salt.

Discussion and conclusionsThe following seismic acquisition and processing techniques

are suggested for improving the seismic image:

1) Radon filtering for multiple suppression.2) Pre-stack depth migration (with anisotropy correction, if

possible) for correct imaging.

3) Angle domain ana lysis to redu ce the an gle-dependent

wave propagation effects.

4) Use of low-frequency seismic sources to reduce scattering.

5) Inverse-Q filtering to compensate for the effects of atten-

uation.

6) Multi- component seismic data.

Even if only P-wave data with mo derate source frequency are

available, pre-stack depth migration can improve the image

provided good interpretive knowledge has been incorporated

into the velocity model. With long offset seismic experi-ments, rays can penetrate below the basalt, but this gives a

low resolution image and requires the application of higher

order normal move-out corrections during data processing.

Although geoscientists are often mainly concerned with mul-

tiples, it is both the effect of scattering and attenuation due

to heterogeneity and the presence of multiples which has

made sub-basalt imaging difficult.

The Mesozoic sediments below the Deccan basalt are

probable hydrocarbon reservoirs, but imaging is a problem

with the present data and methods available. Scattering at

the top of basalt redistributes the seismic energy, and only

low energy, low frequency signal passes through basalt.

Radon filtering helps improve the image above the top of

the basalt, as there is a high velocity contrast between pri-

mary and multiples, and a good signal to noise ratio. But

below the basalt, Radon filtering gives little improvement.

Long offset pre-stack multicomponent seismic data can help

improve the image. Model-based imaging together with

other discussed methods helped build the reservoir model in

the Kutch basin.

AcknowledgementsWe thank Reliance Industries, Mumbai for allowing publica-

tion of this work, and its geosciences team for its help and

suggestions. We also than k M rinal K Sen, University of Texas

Institute for Geophysics, for his suggestions. The authors

thank reviewers for their constructive suggestions.

References

Barzaghi, L., Calcagni, D., Passolunghi, M., and Sandroni, S.[2002] Faeroe sub-basalt seismic imaging: a new iterative

time processing approach. First Break, 20 , 611-617.

Hanssen, P., Li, X.-Y., and Z iolkowski, A. [2000 ] Converted

waves for sub-basalt imaging? 70 th SEG meeting, Calgary,

Canada, Expanded Abstract, MC 2.3.

Hanssen, P., Ziolkowski, A., and Li, X.-Y. [2003] A quanti-

tative study on the use of converted waves for sub-basalt

imaging. Geophysical Prospecting, 51 , 183-193.

Sain, K., Zelt, C. A., a nd Reddy, P. R. [2002] imaging of sub-

volcanic Mesozoics in the Saurashtra peninsula of India

using traveltime inversion of wide-angle seismic data.

Geophysical Journal International, 150, 820-826.Short, N. M. Sr., and Blair, R. W. Jr. [1986] Geomorphology

from space: A global overview of regional landforms. NASA

publication .

Silva, R., and Corcoran, D. [2002] Sub-basalt imaging via

pre-stack depth migration-an example from the Slyne Basin,

offshore Ireland. First Break, 20 , 295-299.

Spitzer, R., White, R. S., and Christie, P. A. F. [2003]

Enhancing subbasalt reflections using parabolic t-p transfor-

mation. The Leading Edge, 22 , 1184-1201.

Wang, Y., and Satish, S. C. [2003] Separation of P- and S-

wavefields from wide-angle multicomponent OBC data for a

basalt model. Geophysical Prospecting, 51 , 233-245.

Yilmaz, .Z . [2001] Seismic data analysis: processing, inver-

sion, and interpretation of seismic data, SEG publication.

Ziolkowski, A., Hanssen, P., Gatliff, R., Jakubowicz, H.,

Dobson, A., Hampson, G., Li, X., and Liu, E. [2003] Use of

low frequencies for sub-basalt imaging. Geophysical

Prospecting, 51 , 169-182.


2888105 prospecting for subdeccan trap mesozoicformationhosted hydrocarbons

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