10 th 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 Cauvery Basin, ONGC, Chennai. E.Mail: giri_m1960@yahoo.com
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 2
FIG:2a 3D-Perspective Of Basement Horizon Mattur-Pundi Area 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) 3
Fig.3: Structure contour map of Tanjore sub basin showing fault pattern at basement level 4
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. 5
Fig.4: Cluster diagram showing high gamma ray and low resistivity values in Pundi wells in comparison to Mattur wells 6
Fig.5 Composite log showing high GR values, low Rt values, high Potassium, Thorium contents in PU-D well 7
Fig.6 Composite log showing low GR values high Rt values, low Potassium, Thorium contents in MT-K well 8
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 9
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. 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) 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. 10