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International Journal of Petroleum and Geoscience Engineering (IJPGE) 2 (1): 34-61, 2014 ISSN 2289-4713 © Academic Research Online Publisher
Research paper
Integrated log analysis of Cretaceous sedimentary sequence of Ramnad
sub–basin, Cauvery Basin, Southern India
D. Srikant a,*
, P. Shanmugam b
a Petroleum Engineering Program, Department of Ocean Engineering,
b Indian Institute of Technology - Madras, Chennai - 600036, India
*Corresponding author. Tel: +91 – 9566102352
Email address: [email protected]
A b s t r a c t
Keywords:
Petroleum exploration,
Litho-bio-stratigraphy,
Paleobathymetry,
Depositional environment,
Source rock study,
Ramnad basin.
In this paper, a detailed study has been undertaken by considering Seismic,
Geology, Geophysical/Petrophysical and Geochemical data acquired in the drilled
wells in order to have good understanding on the geological set up for building the
suitable petroleum system and reservoir characterization for optimum reserve
estimation and economic exploitation of proven hydrocarbons. Lithostratigrahic
and biostratigrahic correlations have been examined. Paleobathymetry,
depositional environment, reservoir rock composition, source rock studies and
pore pressure studies have been carried out in general considering the well data of
few representative wells drilled in the Ramnad sub basin. The inferred lithology
in Nannilam formation is feldspathic sands associated with montmorillonite, mica
and mixed clay, where as in Bhuvanagiri, lithology is mainly calcareous sandstone
associated with silt and clay minerals namely chlorite, kaolinite and mixed clay.
Depositional environment in Bhuvanagiri and Nannilam formations is found to be
marine and coastal respectively, and this may be due to variations in
paleobathymetry levels at the time of sediment deposition. Source rock studies
inferred that Andimadam shale is the source rock for Nannilam and Bhuvanagiri
reservoirs. This is rich in organic content with early maturation. The pore pressure
and temperature studies indicate that there is no high pressure and temperature
zone in this area and hence drilling and logging can be carried out under normal
temperature and pressure regimes. This study forms ground work for carrying out
the detailed field study in each fields of the Ramnad sub basin that will enable for
augmenting the estimated/proven hydrocarbon reserves, locating the bypass/left
over/missed hydrocarbons and also providing suitable solutions for economic
exploitation.
Accepted:09March 2014 © Academic Research Online Publisher. All rights reserved.
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1. Introduction
Ever since the presence of thick succession of marine cretaceous sediments established by Blanford
during the 18-19th century, Cauvery basin has drawn the attention of Geologists [1], Geophysicists,
Geochemists and Petrophysicists in India and abroad . This basin has gained considerable importance
both academically as well as commercially. Several exploration efforts in terms of Gravity, Magnetic,
Seismic and Borehole logging surveys have been put resulting in discovery of good number of
discrete and commercially viable oil and gas pools. However, a good understanding of the
Geological/Geophysical set up for framing the petroleum system suited to this basin is essential for
not only maximizing reserves estimated but also for economic recovery of estimated/proved
hydrocarbon reserves.
Exploration of hydrocarbons in Indian basins has been intensified since the introduction of New
Exploratory Licensing Policy (NELP) in early 90s to meet the ever increasing energy demand.
Ramnad sub basin lying on the southern part of Cauvery Basin has been proved to contain huge gas
reserves. Based on initial exploratory findings four discrete gas fields namely Kanjirangudi,
Perungulam, Periyapattinam and Ramanavalasai have been discovered and proven suitable for
commercial exploitation.
In order to understand the depositional setting, sediment distribution and hydrocarbon habitat, a study
has been undertaken with a synergetic approach using the geological, geophysical and geochemical
data acquired in the area falling in two dip lines and one strike line Fig. 1. Well data and log data
acquired in well nos. C-1, P-1 (dipline – 1), K-12, K-5A and R-1 (dipline – 2) and P-1, XP-1
(strikeline – 1) are considered for understanding the lithostratigraphy, biostratigraphy,
paleobathymetry, pore pressure distribution and rock matrix and clay mineralogy for detailed
reservoir analysis. The log correlations for the wells falling in the area under study are made using the
well logs namely GR, Resistivity and porosity logs [2]. The porosity logs include RHOB, NPHI and
DT. For understanding lithostratigraphy the respective markers on well logs of each well are
indentified and correlated. For understanding the levels of lithostratigraphy and biostratigraphy the
respective markers on well logs of each well are indentified and correlated. NGS data recorded in
some of the wells are used for determining the clay mineral composition and type of depositional
environment in Nannilam and Bhuvanagiri formations. Acoustic impedance and formation water
salinity studies in Nannilam and Bhuvanagiri formations bring out the clear understanding on
demarcation of hydrocarbon horizons from the rest of the formations like shale and water bearing. For
determining the type of source rock and its level of organic maturity (LOM), log determined TOC
values are compared with laboratory determined values. Finally, for understanding the pore pressure
distribution in this area D-Exponant, Sigma plots and shale compaction profiles using resistivity and
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sonic well logs are studied. The details of the methods adopted in this study and observed results are
discussed in the following sections.
2. Study Area – Background
The Cauvery [3] basin is a pericratonic / rift basin extending along the East Coast of southern part of
India covering an area of about 0.15 million sq.km comprising onland (25,000 sq.km) and offshore
areas (125,000 sq km). The sediments fill are about five to six kilometers in thickness and ranging in
age from Late Jurassic (Gondwana) to recent. It is divided into a number of sub-parallel horsts and
grabens, trending in a general NE-SW direction and further sub-divided into four sub-basins namely
Ariyalur-Pondicherry in the north, Tranquebar Depression, Nagapattinam Depression, Thanjavur
Depression in middle part and Ramanad and Gulf of Mannar Depression in the southern part.
Ramnad sub basin is the southernmost sub basin trending towards NE-SW direction and is flanked by
Pattukkottai-Mannargudi Ridge on NW and Mandapam Delft Ridge on SE. The onland part of the
basin extends into the Palk Bay offshore in the northeastern part and Gulf of Mannar in the south. As
on date more than 40 exploratory wells and 15development wells have been drilled. Out of 17
prospects explored, commercial quantities of gas were discovered in the sandstone reservoirs of
Nannilam, Bhuvanagiri and Kamalapuram formations. The sandstone reservoirs of Nannilarn
Formation mainly produced gas in the fields of Periyapattinam, Perungulam, Ramanavalasai, Palk
Bay Shallow-1 and Kanjirangudi. Bhuvanagiri Formation also produced gas in commercial quantities
in the wells of Periyapattinam, Ramanavalasai, and Kanjirangudi.
3. Data and Methods
Open hole log data namely SP, GR, CALIPER, RESISTIVITY, DENSITY, NEUTRON, SONIC
TRAVEL TIME and special logs like Spectral gamma ray (DSL),formation pressure data
(SFT/RFT/MDT/RCI) were acquired in the wells drilled in the area covered by diplines – 1 and 2 and
strikeline-1. Well data including MLU data collected at drill sites of all the wells are considered for
validating the log data results. Geology and geochemical (TOC, Tmax) reports issued for this area are
considered for ascertaining the reservoir rock properties. Seismic sections on two diplines and one
strikeline loaded with correlation logs recorded in representative wells are considered for correlation
of different litho units.
The log correlation technique is adopted for understanding the variations in litho-stratigraphy and bio-
stratigraphy. The cross plot technique is adopted for determining the rock matrix and clay mineral
composition using the open hole well log data in Nannilam and Bhuvanagiri formations. NGS log
data wherever recorded is used for determining the mineral composition and depositional
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environment. Acoustic impedance estimated using sonic and density logs are cross plotted with water
saturation (Sw) for distinguishing the hydrocarbon horizons. Cutting and core (CC and SWC) data
reports are used for validating the rock matrix and clay mineral composition determined using well
logs. Passey method (∆logR) is applied for determining the TOC using well logs which is later
validated with the lab reports. Shale compaction profiles are generated using Rt and sonic well logs
and these are correlated with D – exponent and sigma logs for understanding the pore pressure
distribution. Formation water salinity is estimated by cross-plotting NPHI and Rt on log–log sheet
using the Picket plot. The Th/U versus Th/K plots are generated for determining the environment of
deposition. The Th/K versus PE plots is generated for determination of clay type.
Passey method is an established method for determination of total organic carbon (TOC) in organic
rich rocks [4]. In this technique porosity logs namely sonic log, density log and neutron log are
overlain on resistivity log preferably with deep resistivity log. Among this sonic resistivity overlain
technique is more reliable and widely used in the industry and is named as ∆logR technique.
In organic lean rocks the scales are adjusted such that the sonic and resistivity curves are overlain with
each other. In organic rich rocks, due to the presence of kerogen, sonic log reads higher travel time
and resistivity log reads higher resistivity because kerogen is more resistive compared to formation
water which is present in the pores of the formation. The separation in organic rich rocks [5] between
resistivity and sonic log is called ∆logR. Wireline logs can be used to identify source rocks and serve
as an indicator for the source rock potential provided the source rocks have minimum thickness within
the resolution of the measurements being made and they are sufficiently rich in organic matter.
The algebraic expression for the calculated ΔlogR from the Sonic Vs Resistivity, Neutron Vs
Resistivity and Density Vs Resistivity overlays are as follows:
ΔlogR Sonic = log10 (R/ R-baseline) + 0.02 (Dt – Dt-baseline)
ΔlogR Neutron = log10 (R /R-baseline) + 4.0 (φN – φN-baseline
ΔlogR Density = log10 (R/ R-baseline) – 2.5 (ρb – ρb-baseline)
The ΔlogR separation is linearly related to the TOC content and is a function of maturity. The
empirical equation for calculating TOC content in organic rich rocks from ΔlogR is:
TOC = (ΔlogR) * 10 (2.297 – 0.1688 * LOM), where TOC is the total organic carbon content (wt %)
and LOM is the measured level of maturity. LOM is obtained from the vitrinite reflectance (VRo) or
thermal alteration index by using the maturation indicators.
The advantages of this method are that the wireline methods for estimating the organic matter content
have the advantage of economy, readily available sources of data and the continuous sampling of a
vertically heterogeneous shale section. The limitations are that in hydrocarbon reservoir rocks large
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separation occurs between porosity and resistivity log even though there is no kerogen in that interval.
This can be identified and eliminated by using the gamma ray response.
4. Results
Figures 1a, 1b and 1c present the seismic sections covering two diplines and one strike line loaded
with correlation logs recorded in representative wells. All the drilled wells in the study area (Fig. 2) as
on today cover four hydrocarbon proven fields namely, Kanjirangudi, Perungulam, Periyapattinam
and Ramanavalasai and two dry areas namely Uchapuli and Chomaitangi. The structure contour map
for Nannilam sand-2X and Gas iso - pay map for the same sand is presented in (Fig. 3). It can be
observed that the sand-2X top is at shallowest level in well no. K-12 (1879m) and is deepened
towards K-5A in NE direction with top depth at 1957m. Gas pay thickness is maximum in well no. K-
12 compared to other wells and decreasing in NE direction towards K-5A with minimum value of pay
thickness as 10.5m. Regional GWC (Gas Water Contact) is seen at 1967m. The structure contour map
for Bhuvanagiri sand-1X and Gas iso - pay map for the same sand is presented in Fig. 4. The top of
sand-1X in Bhuvanagiri formation is at shallowest depth i.e. 2077.5m and is deepening further in NE
direction towards K-5A with maximum sand top value at 2168.5m. It can be noticed from the Gas iso
- pay contour map of Bhuvanagiri formation that the pay thickness in sand-1X is maximum (17m) in
well no. K-12 and decreases in NE direction with zero pay in well no. K-5A. The regional GWC (Gas
Water Contact) is observed at 2115.5m.
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Fig.1. a: Seismic section showing wells C-1 and P-1. b: Seismic section showing wells K-12, K-5A and R-1.
C - 1
XLM
XVG XLM
XVG
P - 1
K - 12
XLM
XVG
K – 5A R - 1
XLM XLM
XVG
(a)
(b)
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Fig.1. c: Seismic wells showing wells P-1 and XP-1.
Fig. 2: A map showing different wells in Ramnad Palk-bay sub–basin.
P - 1 XP - 1
XLM
XVG
XLM XVG
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Fig. 3: Contour map showing the Nannilam formation.
Fig. 4: Contour map showing the Bhuvanagiri formation.
Briefly, the various geological, geophysical and geochemical data acquired in the study area, where
two dip lines and one strike line are shown in Figs. 1a, 1b and 1c. The dipline – I is oriented in EW
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direction passing through wells C-1 and P-1. The well C-1 is located on western flank of Ramnad sub
basin, while P-1 is at the central depression. The formations Portonova (Campanian to
Maastrichitian), Nannilam (Santonian to Campanian) and Bhuvanagiri (Cenomanian to Turonian) are
encountered structurally at shallow depth in well C-1 compared to P-1 (Fig. 5).The thickness of
Portonova shale increases towards P-1 indicating deeper bathyal environment. The thickness of
Nannilam is less in C-1 compared to P-1. Nannilam sand is charged in P-1 due to the presence of
cross faults connecting the source rock with the reservoir rock. The gross pay sand thickness in P-1 is
25m. Kudavasal (Coniacean to Santonian) formation is thicker in C-1 compared to P-1. However,
Bhuvanagiri sands are deposited extensively in the region with varying thickness. The Bhuvanagiri
sands are not charged in both the wells (C-1 and P-1) due to the absence of favourable conditions
namely structural elements. The gross pay sand thickness in P-1 is 25m.
Fig. 5: Correlation of wells falling on Dipline – 1 (C-1 – first three plots; P-1 – last three plots).
Dipline – II is oriented in WE direction passing through wells K-12, K-5A and R-1. The log
correlation along the wells K-12, K-5A and R-1 shows the thickness of tertiary sequence increasing
towards R-1. Portonova shale is structurally at a shallower depth in K-12 compared to other two wells
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K-5A and R-1 (Fig. 6). The thickness of Portonova shale increases towards west at K-12 and K-5A
suggesting deeper bathyal environment compared to R-1. Nannilam formation in this area is
structurally higher in K-12 and K-5A vis – a – vis R-1. The thickness of Nannilam sand in the well R
- 1 is more and it is water bearing due to its structurally deeper position. The sands in other two wells
(K-12 and K-5A) are charged with hydrocarbons. The thickness of Kudavasal shale is drastically
reduced in R-1 compared to other two wells, indicating proximal wedge of this sequence towards
Mandapam delft horst. Even though all the three wells K-12, K-5A and R-1 were penetrated partly at
the top of Bhuvanagiri formation, the thickness of Bhuvanagiri is more in K-12, K-5A as these wells
are in the basinal area as seen in seismic dipline – II. However, the Bhuvanagiri sands do not have any
entrapment condition in these wells except oil indication in R-1. The seismic dipline – II suggests that
the basement is dipping towards west and the well R-1 is drilled on the flank of the hanging wall of
Madanam horst.
Fig. 6: Correlation of wells falling on Dipline – 2 (K-12 - first three plots; K-5A – second three plots; R-1 –
third three plots).
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Strike line passing through P-1 and XP-1 is oriented along N–S direction which is parallel to the axis
of Ramnad sub basin. In seismic strike line the cretaceous–tertiary boundary is inclined towards south
suggesting a depocenter towards south during tertiary era. This is also clearly seen in log correlation
passing through wells P-1 and XP-1 (Fig. 7). The depocenter during early cretaceous period shifted
towards P-1 resulting in increase of thickness of early cretaceous sequence towards P-1. The
Nannilam formation is structurally down at P-1 compared to XP-1. The Nannilam sands of both P-1
and XP-1 are charged indicating a favourable migratory fairway along this direction. The Kudavasal
and Bhuvanagiri formations are structurally down in P-1 vis – a – vis XP-1. As a result, the
Bhuvanagiri formation of XP-1 area is charged with hydrocarbon.
Fig. 7: Correlation of wells falling on strikeline – 1 (P-1 – first three plots; XP-1 – last three plots).
In general the Nannilam formation is charged in Perungulam, Kanjirangudi and Periyapattinam area,
whereas the Bhuvanagiri formation is charged only in Periyapattinam area.
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4.1. Spectral gamma ray log analysis
The spectral gamma ray [6] log provides a large amount of data that can help discriminate between
depositional environments. Clay bearing rocks with high total gamma ray readings are not only
related to clay fraction but also due to presence of uranium–radium series isotopes of organic origin.
If the spectral gamma log indicates presence of potassium and thorium together with uranium it may
be said that the potassium and thorium contributions are associated with the clay content of the shaly
carbonate, while the uranium is associated with some organic source which was deposited in a
reducing environment that favours the conservation of organic material. High potassium and high
thorium values together with low uranium indicates an oxidized environment which is not a
favourable environment for conservation of organic material. Organic matter is good at concentrating
uranium. If this is deposited in reducing environment it can be preserved and transformed to
hydrocarbons. Thorium salts are easily soluble in water. Thus, in marine deposits TURA (Thorium/
Uranium ratio) is minimal up to 2 and 2-8 in coastal environment and more than 8 in continental
environments. In Nannilam formation Th/U ratio ranging from 3-20 indicates that the depositional
environment is coastal in nature. In Bhuvanagiri formation Th/U ratio is mostly within the range of 0-
1 indicating that the depositional environment is of reducing nature, which is nothing but marine.
4.2. Nannilam Formation (Santonian to Campanian)
The Lithology [7] plots shown in (Fig. 8) infer that the rock matrix in Nannilam formation is
dominated by quartz. A few number of points falling close to the limestone indicate that the rock
matrix is a composition of higher grain density minerals in addition to quartz. The points falling in the
N-W direction of the sandstone line are found to be influenced by the presence of gas. The lithology
(Density-Neutron) cross plot (Fig. 9) infers that Nannilam clay is dominated by montmorillonite. It
can be noticed in NGS (Th vs K) and PE vs Th/K cross plots that clay mineralogy is mainly
represented by mixed clay (Illite, Mica and Muscovite). As per the core studies, the Nannilam
formation is characterized by the presence of feldspathic sands in addition to mica and mixed clay
associated with 20% porosity and these data are very much reflected in cross plots (Figs. 8 and 9).
Also, predominance of glauconite mineral has been observed as detrital composition. Reservoir
characters [8] are found to be good in Nannilam formation.
Fig. 10 shows the NGS log – log plot of Th/K on x – axis and Th/U on y – axis. Most of the points are
clustered on the north western part of the graph. Th/K ratio is in the range of 1 to 5 and Th/U ratio is
in the range of 3 to 10. The Th/U ratio ranging from 3-20 indicates that the depositional environment
is coastal in nature. This may probably be due to the shallow level of paleobathymetry at the time of
deposition of sediments in Nannilam formation. The sedimentary processes operating at the time of
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deposition of reservoir sands of Nannilam Formation are dominantly gravity driven with
intermittently traction currents deposited sediments. It is observed in the area along NW to SE that
there is a gradual variation in depositional processes as well as in nature of sediment supply.
Sedimentological analysis (not shown) revealed that the sediments deposited at Kanjirangudi and
Koluvur area in NW slopes area are deposited dominantly by sandy debris flows, whereas sediments
at the middle section at Ramanavalasai and Periyapattinam are deposited by mixed processes of sandy
and muddy debris flow deposits. In SE slope at Perungulam sediments are deposited by dominantly
muddy debris flow and minor slump flows. The sediments supply in this part of area might have been
from the NW direction. The sediments at Uchipuli area are deposited by dominantly slump processes.
At this point sediment supply might have been from the SE provenance. The sediments are
characterized by the presence of few well rounded quartz grains along with subangular to subrounded
which indicate that sediments were originally transported by external drainage / area with
considerable length of transportation probably in fluvial regimes and deposited in shelf or at the
shallow marine area. These shelf sediments were again reworked by gravity driven processes and
deposited as debris and muddy debris flows in deep water setting.
Fig. 8: Identification of rock matrix composition using RHOB – NPHI cross-plot of the Nannilam formation.
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Fig. 9: Identification of clay mineral composition of the Nannilam formation.
Fig. 10: Plots showing the depositional environment of the Nannilam formation.
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4.3. Bhuvanagiri Formation (Turonian to Coniacian)
In the matrix density plot shown in (Fig. 11) most of the points are distributed close to limestone line.
In (Fig. 12), most of the data points are falling below the quartz line. This indicates that rock matrix is
of calcareous sandstone. As per geological findings the formation rock matrix consists of sand and
fine grained silts associated with calcareous material and this account for the points lying around the
limestone matrix line (Fig. 11). As per cutting and core sample observations, sands in Bhuvanagiri are
shaly/silty. The productivity of these sands is less due to silty/shalyness of the formation. The inferred
clay mineralogy from the cross-plots (NGS and Lithology) presented in (Fig. 12) is a composition of
mixed clay and chlorite. It has been found in (Fig. 13) that Th/U ratio is mostly within the range of 0-
1 indicating that the depositional environment is of reducing nature, which is nothing but marine. This
may be due to the deeper paleobathymetry levels (water depth 415m) during the deposition of
sediments in the Bhuvanagiri formation.
Fig. 11: Identification of rock matrix composition of the Bhuvanagiri formation.
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Fig. 12: Identification of clay mineral composition.
Fig. 13: Plots showing the depositional environment of the Bhuvanagiri formation.
Well # KJ-11Bhuvanagiri ( 2175- 2189m)Thorium / Uranium ratio histogramvalue is less than 2 indicates marineenvironment.
Th / U Ratio ( )4 8 12 160 20
Fre
qu
en
cy (
%)
500
100
# Points Total: 170
Start Depth: 2188 m # Points Plotted: 88Stop Depth: 2175.05 m # Points Absent: 0Sampling Rate: 0.0762 m # Points Cut: 82
X Max Value: 0.888883 # > X Scale Max: 0X Min Value: 0.440053 # < X Scale Min: 0
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A double logarithmic plot of resistivity measurement on the x – axis versus porosity measurement on
the y – axis is called the pickett plot. The plot is based on taking the logarithm of the Archie’s
equation
log Rt = - m log Φ + log aRw – 2log Sw
Points of constant water saturation (Sw) will plot on a straight line with negative slope of value ‘m’.
Water zones define the lower most line on the plot marked as red line in (Fig. 14). Since Sw = 100%,
the water resistivity can be determined from a point on the line. After establishing the water line other
parallel lines are drawn for different Sw = 75%, 50% and 25%.
The pickett plots (Fig. 14) generated using Resistivity –Density combination infers that the formation
water resistivity is 0.1 ohmm for Nannilam formation at 170°F and 0.14 ohmm for Bhuvanagiri
formation at 180°F. The water salinities from production testing for Nannilam formation are
30000ppm and for Bhuvanagiri formation is 24500ppm.
Fig. 14: Formation water resistivity of the Nannilam and Bhuvanagiri formations.
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5. Acoustic impedance
Acoustic impedance has been computed (Unit: gm.m./cc.sec) by using density and sonic logs to study
the variation of impedance in shale, gas and water bearing sands. The cross-plot of impedance vs.
water-saturation (Fig. 15) for the Nannilam section indicates a clear difference in impedance values
for gas and water bearing sections; for gas, sand impedance range from 19500 - 25000 and for water
sand values are between 25000 -30000. The acoustic impedance vs. density cross plot (Fig. 16)
isolates Nannilam sands from Kudavalasal shale defining the shale-sand boundary. It also clearly
indentifies the low density shales in the shale formation. Acoustic impedance against gas bearing
sands is less compared to water bearing sands due to decrease in bulk density and decrease in acoustic
velocity. This is an alternative method for identification of gas bearing sands. This method is useful
when resistivity logs are affected due to environmental conditions. From the graph it is implied that
acoustic impedance is directly proportional to water saturation in gas bearing sands. The limitation of
this method is that it cannot be used against oil bearing sands.
Fig. 15: Identification of gas sands from acoustic impedance versus water saturation.
Acoustic Impedance VS Water saturation
Fig.14
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Fig. 16: Identification of gas sands from acoustic impedance and density log response.
6. Source rock study
For the hydrocarbons found in Nannilam and Bhuvanagiri formations the source rock is Andimadam
shale [9]. Extensive studies have been carried out for determining the maturity of source rock in terms
of TOC (total organic carbon) [9]. The estimated TOC in this source rock is of the order 1.0 to 3.8
indicating that the organic richness of the source is good to very good. TOC log is constructed using
Passey method (∆logR).
The laboratory determined TOC values are plotted on the TOC log generated by Passey method (Fig.
17). It [10] has been found that the computed TOC log is in agreement with lab determined TOC
values. The hydrocarbons are generated from deeper sediments, migrated through cross faults and
accumulated in Bhuvanagiri and Nannilam sands.
Acoustic Impedance VS Density Log ResponseFig.15
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Fig. 17: Source rock analysis from well logs and lab measurements.
7. Pore Pressure Study
The formation pore pressure [11] is defined as the pressure acting upon the fluids (water, oil and gas)
in the pore space of the formation. There are various techniques developed to predict the overpressure
horizon. The Sigma log and Resistivity and Sonic well log data are the precious and good indicators
for identifying the pore pressure in the formation. The relationship of Sigma log, resistivity log and
sonic log with the pore pressure is described below.
7.1. Sigma Log
Sigma Log √σ◦ = F √σ t’
√σ t’ represents rock strength which is a function of drilling parameters namely
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Weight on bit (WOB)
Rate of Penetration (ROP)
Revolution per minute (RPM)
Bit size
‘F’ represents over balance correction
Sigma Log √σ◦ is a function of rock strength
The significant characteristic of the sigma log is its ability to function as a valid indicator of formation
pressure gradient. Any deviation in the sigma log towards left from the normal trend indicates the
presence of over pressure zone and deviation to the right indicates the presence of over compaction.
Sigma log generated in (Fig. 18) indicates normal pressure regime.
7.2. Resistivity Log Analysis
The analysis of shale resistivity using wireline log data is one of the oldest methods for detecting the
abnormal pore pressure. Formation resistivity depends on porosity, the type of the fluid within the
pore space and its ionic strength. Under normal compaction conditions, an increase in shale resistivity
with depth corresponds to a reduction in porosity. An anomalous change in formation pressure is
usually associated with a shift in the normal compaction trend, indicated on an electric log by a
reduction in resistivity associated with an increase in porosity. There is no decrease in the resistivity
with increase in depth indicating normal compaction.
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Fig. 18: Pore pressure distribution in Ramnad Palk – bay sub basin from the D-Exponent and
Shale Compaction plot.
7.3. Acoustic (Sonic) Log Analysis
It is possible to record the transit time of elastic waves through formation over pre-determined
distances. The log is useful for estimating porosity and measuring sound energy transmission
characteristics for use in differentiating fluid content. The recorded interval transit time is in micro-
seconds/ft. In a given formation, it depends upon lithology, degree of compaction, porosity and fluid
content in the pore space. This device has got the advantage of being largely unaffected by changes in
hole size. So this log is an effective tool among the other logs for identification of overpressure zones.
For formation pressure evaluation, the transit time of shales is plotted versus depth on linear-log
paper. In a normal pressure environment, all data points fall along the normal compaction trend line.
However, in the case of overpressured shales, the transit time would increase above the normal ∆t
values. This increase is indicative of an increase in porosity which is normal in the case of over
pressured shale section. There is no increase in the sonic transit time indicating normal compaction.
A plot of D – exponent, sigma log, resistivity log and sonic log are generated and shown in (Fig. 18).
The D – exponent indicates that as a drill bit bores a hole into the earth, it will gradually experience
denser formations and therefore slower rate of penetration. The general trend is normally a gradual
slowing rate of penetration. The basic drillability exponent was published which relates the action of
tricone bit teeth to an inherent characteristic of the rock, the drillability, or 'd' :
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d = log10(R/60N)/log10(12W/106D) where : R=ROP (ft/hr) N=RPM (rev/min) W=WOB (lbs) D=bit
size (ins).
It is evident from these plots that there is no significant over pressure zone that can hamper future
drilling and logging jobs in this area as all the plots show the normal trend. Hence, it may be
concluded that the future drilling and logging of wells in this sub-basin can be carried out in the
normal pressure regime.
8. Rock physics study
The latest full wave sonic tool combines new dipole-based technology with the latest monopole
developments into one system, providing the best method available today for obtaining borehole
compressional, shear and Stoneley slowness.
Dipole technology allows borehole shear measurements to be made in “soft” rock as well as “hard”
rock formations. Limited by borehole physics, monopole tools can only detect shear velocities that are
faster than the borehole fluid velocity or in hard rocks only. Dipole tools overcome this fluid velocity
barrier. Key applications for the full wave sonic measurement, besides traditional uses for
compressional data include: (1) Mechanical property analysis: Applications include well bore
stability, perforation stability or sanding analysis, and hydraulic fracture height prediction, (2)
Formation evaluation: Applications include gas detection, natural fracture detection and evaluation
and qualitative indications of permeability, (3) Geophysical interpretation: Applications include
synthetic seismograms, and calibration of inputs to amplitude variation with offset (AVO) analysis,
and (4) Formation shear anisotropy: Combining anisotropy with other input from Petrophysics,
geology and reservoir engineering may reveal a connection between aligned features and paths of
fluid flow. A plot on a 5 – 0 scale of Vp/Vs and Poisson ratios on a scale of 0 – 0.5 differentiates
between sand, shale and hydrocarbon zones (Fig. 19). The curves overlay in water bearing sand zone
and they are separated by 1 – 2 divisions in shale. In hydrocarbon zone Poisson ratio decrease and
Vp/Vs also decrease. It shows a crossover against hydrocarbon zones. This plot of Poisson ratio and
ratio of Vp/Vs clearly differentiates between sand and shales.
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Fig. 19: Plot of poisson's ratio and ratio of Vp/Vs.
9. Discussion
The interpretation and analysis of data are summarized as follows:
I. C-1 is structurally up w.r.t P-1 at all levels in dipline-1. The Nannilam and Bhuvanagiri
formations are structurally higher in well C-1 and these formations in this well are devoid of
hydrocarbons. This may be probably due to the absence of charging mechanism. The
thickness of Portonova shale increases towards P–1 indicating deeper bathyal environment.
II. In dipline-2 Kudavasal shale lying between Nannilam and Bhuvanagiri formations is
moderately thick in K-12 and K-5A, but it is thinned into a thin shale streak in R-1, paving for
thickening of the Nannilam and Bhuvanagiri formations. All of the three wells are found to be
hydrocarbon bearing. The log correlation along the wells K–12, K–5A and R–1 shows the
thickness of tertiary sequence increase towards R–1. The thickness of Portonova shale
increase towards west suggesting deeper bathyal environment compared to R–1. The
thickness of Kudavasal shale is drastically reduced in R–1 compared to other two wells
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indicating proximal wedge of this sequence towards Mandapam delft horst. The seismic
dipline–II suggests that the basement is dipping towards west and the well R–1 is drilled on
the flank of the hanging wall of Madanam horst.
III. In seismic strike line, the Cretaceous–Tertiary boundary is inclined towards south suggesting
a depocenter towards south during the Tertiary era. The depocenter during early cretaceous
period shifts towards P–1 resulting in increase of thickness of early Cretaceous sequence
towards P–1. The Kudavasal and Bhuvanagiri formations are structurally down in P–1 vis – a
– vis XP–1. As a result, the Bhuvanagiri formation of XP–1 area is charged with
hydrocarbon.
IV. Integrated study of log correlation and structural contour maps particularly in Kangirangudi
field infers that the thickness of sand2X of Nannilam formation and sand-1X of Bhuvanagiri
formation are maximum in well K-12 compared to other wells and decrease in the NE
direction towards K-5A in which sand thickness is minimum in both the formations. Similar
is the case of Gas pay sand thickness. The inferred Regional GWCs in Nannilam and
Bhuvanagiri formations from contour maps are XX67m and XX15.5m respectively.
V. The integrated study of well logs and core data infers that the rock matrix in Nannilam
formation is mainly sandstone associated with clay minerals feldspar, mica, montmorrillonite
and mixed clay. In Bhuvanagiri formation, the inferred rock matrix is calcareous sandstone
along with siltstone and clay mineralogy is combination of Kaolinite, chlorite and mixed clay.
VI. Formation water resistivity from Pickett plots for Nannilam and Bhuvanagiri formations are
0.1 ohmm at 170°F and 0.14 ohmm at 180°F respectively. The water salinities from
production testing for Nannilam formation are 30000ppm and for Bhuvanagiri formation is
24500ppm.
VII. NGS log analysis infers that the depositional environment in Nannilam formation is coastal in
nature as Th/U varies in the range of 3 to 20 indicating transition from marine to non-marine
(Oxidizing) environment. In Bhuvanagiri formation, the NGS ratio Th/U maintained less than
2 indicating that the depositional environment is of purely marine nature.
VIII. Accoustic impedance vs Density cross-plots enable to distinguish hydrocarbon horizons and
also isolate low density shales from the rest of the formation.
IX. Source rock for Nannilam and Bhuvanagiri formations is the Andimadam shale. TOC values
estimated in geochemical studies correlate well with the log determined TOC values.
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X. Finally, the pore pressure studies using D-Exponent and Sigma logs along with shale
compaction plots using resistivity and sonic logs infer that the formations (Nannilam and
Bhuvanagiri) under study come under the normal pressure regime, and hence future drilling
and logging jobs in this area can be carried out under the normal pressure regime.
10. Conclusion
This study brings out the salient features of the Ramnad sub-basin, which is the important part of the
Cauvery basin in view of the presence of huge hydrocarbon resources. This covers four hydrocarbon
proven fields namely, Kajirangudi, Periyapattinam, Perungulam and Ramanavalasai. The sands in the
Nannilam and Bhuvanagiri formations are main gas producers in this Ramnad sub-basin. The
Nannilam formation is characterized by feldspathic sands along with clay mineral composition of
montmorrillonite, mica and mixed clay. The sands in Bhuvanagiri formation are mainly calcareous
nature associated with mica, Illite, mixed clay and partly Biotite and Kaolinite as clay constituents.
The various well logging techniques, discussed in this paper for identifying the hydrocarbon horizons,
isolating low density shales, and determining the depositional environment in general for basin as a
whole, can be used for studying the geological and petrophysical characteristics of each field of the
Ramnad sub-basin. It may be concluded from the source rock studies that the source rock is the
Andimadam formation which is very rich in organic content and is early in maturation stage
indicating good source potential. It is also inferred from the present studies that the pore pressures in
Nannilam and Bhuvanagiri formations are within the normal pressure regime, and hence there is no
threat of high pressures that can hamper future drilling and logging jobs.
Form this study; it is found that in the northern part of the sub–basin the deeper bathyal environment
was in the eastern part, whereas in the southern part of the sub–basin the deeper bathyal environment
shifted to the west due to Mandapam delft Ridge. The depocenter was in the northern part of the sub-
basin during early Cretaceous, whereas in the tertiary era the depocenter shifted towards south.
Crossplots of NGS logs revealed that Nannilam (Santonian to Campanian) formation was deposited in
the coastal environment and Bhuvanagiri (Turonian to Coniacian) formation was deposited in the
marine environment. A new approach for determination of gas zones established by cross plotting
acoustic impedance versus water saturation gave further information on these aspects.
The present work is a foundation for carrying out the field wise reservoir characterization, in terms of
framing the suitable petrophysical model with detailed studies on source rock typing, correlating and
tying up the lithologic and formation boundaries observed from seismic sections, geological
interpretation and well log interpretation. Understanding the depositional environment at different
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stratigraphic levels along with rock matrix and clay mineral compositions in each field of the Ramnad
sub-basin is an important part of this work. The work involves estimation of effective porosity,
hydrocarbon saturation and net pay thickness which are critical inputs for the realistic estimation of
hydrocarbon reserves. Finally, it involves studies of the contour maps generated for pay sand top
depths, effective porosity, hydrocarbon saturation, permeability, and isopay thickness maps for
understanding spatial distribution of reservoir parameters and thereby identifying prospective areas
for hydrocarbon accumulation. These studies will provide key inputs for production planning and
searching for suitable solutions for optimum and economic recovery of proved hydrocarbons in this
basin.
Acknowledgements
The authors wish to thank the Basin Manager (Cauvery Basin) of the Oil and Natural Gas Corporation
Limited (ONGC) for providing the well data, log data and other data for the Ramnad–Palk Bay sub-
basin. We would like to thank Dr.B.A.Rao (Ex-ONGCian) for his help during this study. The authors
would like to acknowledge the support provided by IIT Madras, Chennai – 600036.
References
[1] Pandey J, Singh NP, Krishna BR, Sharma DD, Parikh S, Nath S., Lithostratigraphy of Indian
Petroliferous basins, KDMIPE, ONGC, Dehradun 1993 (ONGC Unpublished Report).
[2] Dewan JT., Modern Open Hole logs Interpretation 1983. Penn well Books, Penn well Publishing
Company, Tulsa Oklahoma.
[3] Avasthi DN, Raju VVV, Kashettyar BYA., Case history of Geophysical surveys for oil in Cauvery
basin 1980, ONGC Report, India.
[4] Passey QR, Creaney S, Kulla JB, Moretti FJ, Stroud JD., A practical model for organic richness
from porosity and resistivity logs, AAPG Bulletin, 1990; 74: 1777-1794.
[5] Herron SL., A Total Organic Carbon Log for Source Rock Evaluation. The Log Analyst,
November-December, 1987.
[6] Brock J., Applied Open Hole Log Analysis, Gulf Publishing Company Book Division Houston,
London, Paris, Tokyo, 1986; 2.
[7] Burke J.A., The Litho-Porosity Cross-plots; Trans. Tenth Ann. Logging Symp., 1969. Advanced
Log Interpretation Principles, 1990 by M/S Schlumberger Asia.
[8] Janardhranan M., Sumathi A., Kashyap A., Goswamy B.G., Prasad J., New insights on
hydrocarbon charge in Albian and older sediments in Ramnad sub basin, Cauvery basin, India 1982.
RGL, NGC, ONGC Bulletin, 1982: 45.
D. Srikant et al. / International Journal of Petroleum and Geoscience Engineering (IJPGE) 2 (1): 34-61, 2014
61 | P a g e
[9] Schmoker J.W., Determination of Organic-Matter Content of Appalachian Devonian Shales from
Gamma-Ray Logs, AAPG Bulletin, 1981;63(9).
[10] Meyer B.L., Nederlof MH. Identification of Source Rocks on Wireline Logs by
Density/Resistivity and Sonic Transit Time/Resistivity Crossplots, AAPG Bulletin, 1984; 68(2):121-
129.
[11] Sahay B., Pressure regimes in oil and gas exploration. New Delhi Allied Publishers Ltd 1994.
[12] Asquith., Basic Well Log Analysis for Geologists, American Association of Petroleum
Geologists Tulsa, Oklahoma USA, 1982.
[13] Fertl W.H., Chillngar G.V., Total Organic Carbon Content Determined from Well Logs. SPE
Formation Evaluation, June 1988.