Integrated log analysis of Cretaceous sedimentary sequence of Ramnad ... · Integrated log analysis...

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.

D. Srikant et al. / International Journal of Petroleum and Geoscience Engineering (IJPGE) 2 (1): 34-61, 2014

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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

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