Log Responses of Basement Rocks in Mattur-Pundi Areas, … · 2017. 11. 23. · contents in Pundi wells have been observed. 3. Based on the differences Mattur area is mostly gneissic

Cauvery Basin, ONGC, Chennai.

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10th Biennial International Conference & Exposition

P 288

Log Responses of Basement Rocks in Mattur-Pundi Areas,

Tanjore Sub Basin, Cauvery Basin, India.

M.Giridhar*, N.C.Das and B.K.Choudhary

Summary

Tanjore sub basin situated on the western margin of the Cauvery Basin was evolved during Late Jurassic as a result of rifting

of Gondwanaland. Basement rocks are mostly of gneissic and at places granitic in composition in Tanjore sub basin where

hydrocarbon has been produced. Extensive study on the effects of compositional variations on log responses of igneous and

metamorphic rocks was done by Peching et al., (2005). Similar analysis has been attempted in Mattur-Pundi areas to

understand log responses of basement rocks. Comparative study between the logs of Mattur-Pundi areas shows that in Mattur

area resistivities are higher with low gamma ray values compared to Pundi area where the gamma ray values are higher with

low resistivity values although both the fields produced hydrocarbon from Basement. Spectral gamma ray log shows high

Potassium and Thorium contents in Pundi area compared to Mattur area. The variation in log character of these two areas

may be due to compositional variation of rock types being mostly gneissic in Mattur area and granitic in Pundi area. As the

reservoir character of granitic rocks are generally better than gneissic rocks the understanding of log characters in relation

to the type of basement would help in designing exploration/production testing strategy.

Keywords: Gneisses, granites, Mattur, Pundi, resistivity and radioactive logs

Introduction

Electrical and radioactive logs are recorded routinely in

all the wells drilled during hydrocarbon exploration.

Evaluation of logs is done primarily for sedimentary rocks

to know the lithology, hydrocarbon saturation and

porosity. A lot of literature is available on the formation

evaluation of sedimentary rocks, whereas for the

basement rocks the evaluation of logs to understand the

type of basement based on the log response is scanty.

Peching, et.al. (2005) have compiled an extensive data of

log measurements of the boreholes drilled into crystalline

basement of European and North American provinces.

Study was focussed to understand compositional

variations of igneous and metamorphic rocks. From the

logs potassium and neutron porosity are particularly

helpful in distinguishing different types of gneisses and

igneous rocks. The proportions of amphibole/pyroxene,

mica-feldspar has influence on log responses. The

gamma-ray values cover a large value range (40-550)

within the plutonic rocks and gneisses. The average

gamma ray values of plutonic rocks are about 250API for

granites/aplites and about 350API for the

Fig.1: Tectonic map of Cauvery Basin showing different sub

basins and ridges

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syenites/monzonites and are significantly higher than the

gamma ray mean values of all gneisses(70-200API). Mean

electical resistivities for plutonic rocks (2.1-3.9 Ohm.m.)

are lower than for ortho gneisses(3.2-4.5 Ohm.m.) as

reported in their study.

Tako Koning(2012) after extensive study on the basement

rocks of Asian countries has reported that quartzites and

granites form optimum reservoirs. Weathered “rotten”

granites can also be excellent reservoirs. Rocks such as

schists and gneisses are less attractive and tend to smear

and not fracture when subjected to tectonic stresses. The

high mafic content of schists also negates the creation of

secondary porosity by weathering. In this paper an attempt

was made to understand lithological variation between

basement rocks based on the log responses in Tanjore sub

basin.

Basement configuration of Tanjore sub basin

Cauvery basi was evolved as a result of rift-drift

phenomenon of Indian plate from Gondwana land during

Late Jurassic-Early Cretaceous with taphrogenic

fragmentation of Archean Basement which resulted in the

development of faults and has divided the basin in to horsts

and grabens/half grabens. In Tanjore sub-basin which lies

on the western part of the Cauvery basin(Fig.1), the

western margin fault has served as foot wall and hanging

wall has moved down giving the half graben structure.

There are four prominent lows at basement level, one NE

of Vadakkukkottai-1, one around orathanadu-1, two lows

north of Sattanur-1 and the basement is shallower towards

south, east and northern parts of the sub-basin(Figs.2,2a).

Generalized stratigraphy of Tanjore sub basin showing

hydrocarbon occurrences is shown in Table 1.

Fig 2: 3D-Perspective of Basement Horizon – Thanjavur Acreage

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Basement faults are aligned in NE-SW direction in the

main basinal low whereas they are aligned in the direction

of ENE-WSW in the northern part in Mattur-Pundi areas.

Southernmost part of the basin is also dissected by number

of smaller faults which are aligned in NW-SE direction.

The basement in Tanjore sub-basin is mostly granitic

gneiss or granite with schistose rocks. The hanging wall is

dissected into a series of small half grabens by east-hading

antithetic faults trending NE-SW towards eastern side and

parallel to the Pattukottai-Mannargudi ridge. Towards

eastern side the basement is steep close to Pattukkottai

ridge and becomes gentle towards central parts and deep

towards western side giving half graben structure.

The depth of basement is shallowest towards north around

Mattur-Pundi areas(1000-1200m) and shallowest towards

south around Krishnapuram area ( <2300m) whereas it is

deepest towards north of Sattanur (5500-6000m),

Vadakkukkottai low (>5000m) and Orathandu

low(>5000m) and the depth increases towards west(Fig.3)

FIG:2a 3D-Perspective Of Basement Horizon – Mattur-Pundi Area

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Fig.3: Structure contour map of Tanjore sub basin showing fault pattern at basement level

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Compositional variation of logs in Basement rocks

Around 30 wells have penetrated Basement rocks in

Tanjore sub basin. Only those wells which have penetrated

more than 80m have been considered for the study

representing true basement. Basement rocks are mostly

Biotite-Hornblende gneisses in composition with granites

at places (Vadatheru area).Even phyllites have been

reported(Mattur-6). Since in Mattur-Pundi wells a

considerable thickness has been drilled a comparison

between these two areas has been taken for the

study(Table.2) The gamma ray values for Mattur wells are

low and vary between 20-50 API compared to Pundi wells

( 60-125 API). Resistivity values for Mattur wells vary

between 80-200 Ohm.m. and for Pundi wells between 20-

50 Ohm.m. having considerable differences between these

areas(Figs.5,6,).The spectral gamma ray also corroborates

with this observation having low potassium(K) and

Thorium(Th) contents in Mattur-K well compared with

PU-D well which has high Potassium and Thorium

contents(Figs.7,8) The densities vary between 2.7-2.8 g/cc

for Mattur area whereas for Pundi area it is between 2.6-

2.7. The neutron porosities are better for Pundi area than

Mattur area. Scatter diagram between gamma ray and

resistivity values show difference between two

areas(Fig.4).Incidentally Pundi-B has produced

12,000tons of oil in comparision to Mattur-B which

produced only 2000tons of oil supporting that in Pundi area

the basement is granitic in nature with better fracture

development. Alangudi-A well which is close to Mattur

area where 400m basement has been drilled is

characterised by garnetiferous biotite gneiss the gamma

ray values range between 30-40API and has an average

resistivity value of 200 Ohm.m supporting above

observation. Similar studies can be attempted for the

entire basin where the basement rock has been drilled for

considerable depths.

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Fig.4: Cluster diagram showing high gamma ray and low resistivity values in Pundi wells in

comparison to Mattur wells

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Fig.5 Composite log showing high GR values, low Rt values, high Potassium, Thorium contents in PU-D well

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Fig.6 Composite log showing low GR values high Rt values, low Potassium, Thorium contents in MT-K well

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Fig.7: Depth vs Potassium plots showing high potassium content in Pundi well

Fig.8: Depth vs Thorium plots showing high Thorium content in Pundi well

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Conclusions

1. Gamma ray and Resistivity values between Mattur

and Pundi areas show variation in Basement rocks.

2. Low gamma ray, high resistivity, low potassium and

Thorium contents in Mattur wells and high gamma

ray, low resistivity, high potassium and Thorium

contents in Pundi wells have been observed.

3. Based on the differences Mattur area is mostly

gneissic in composition and in Pundi area is mostly

granitic in composition.

4. As quartzites and granites serve as good reservoir

rocks compared to gneissic rocks a detailed study as

regards the nature of Basement would help in

designing exploration and production strategy as

quantities of hydrocarbon accumulations may vary

depending on the type of rock.

Acknowledgements

The authors express their thanks to Dr.B.S.Josyulu,

Executive Director-Basin Manager, Cauvery Basin for

encouragement, cooperation to carry out such studies.

Authors express their gratitude to Mr. S. Prabakaran, GM

(Geol)-Block Manager for going through the manuscript

and making suggestions. They are grateful to SPG

authorities for conducting the conference and allowing the

paper to be presented in the proceedings.

References

Peching,R, Delius,H and Bartetzko, A(2005)” Effect of

compositional variations on log responses of igneous and

metamorphic rocks. II:acid and intermediate rocks”

Petrophysical Properties of Crystalline

Rocks(eds).Geological Society, London, Special

Publications,240,279-300.

Tako Koning(2012) “Best practices for exploring and

producing oil and gas from fractured and weathered

basement: Examples from Asia” Search and Discovery

article AAPG International conference and Exhibition,

Singapore, September,16-19,2012.

Well completion reports, laboratory reports submitted by

different agencies of ONGC (Unpublished data)