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GRC Transactions, Vol. 32, 2008

Enhanced Geothermal Resources in NE Deccan Province, India

Varun Chandrasekhar and D. Chandrasekharam GeoSyndicate Power Pvt. Ltd. Indian Institute of Technology Bombay, India

Keywords the Deccan and He isotopes do not support mantle involvement Deccan basalts, Enhanced Geothermal System, Narmada, in the evolution of these thermal springs. Process of serpentiniza- Tapi, granite tion of the Deccan basalt flows at some depth is indicated by high amount H2 present in the thermal gases (Minissale et al., 2000). The temperature of the thermal springs along the west coast vary Abstract from 47 - 72 °C while along the SONATA the temperature of the thermal springs vary from 35 to 98 °C (Verma and Banerjee, 1992, Geothermal provinces within the Deccan basalt province de- Chandrasekharam and Antu, 1995, Minissale et al., 2000). It has rive the heat from high heat producing granites that underlie the been reported basalt cover. Fluids circulating within these granites issue at the that heat source surface with temperatures varying from 57 to 98 °C. These granites for these sprigs form the basement rocks in NE part of the Deccan basalt province. is the granites Subsurface lithological information deduced from geophysical located below and geological data reveal excellent EGS sites in this province. t h e D e c c a n volcanic flows Deccan Volcanic Province (Minissale et al., 2000). In the The Deccan volcanic province is one of the most important and present paper large flood basalt provinces of the world that has drawn much attention from the scientists’ world over during the last two decades. The basalt flows attain maximum thickness along the west coast and thin- out along their NE and southern fringes. Although it is not known what lies below the flows along the western margin, fairly good information is available on the rocks over which these flows fringe out along its NE and southern margins (Verma and Ba- nerjee, 1992, Subbarao et al.,1994). The thickness of the flows over such margins vary from 400-600 m ( Chandrasekharam, 2003 and references therein). A large number of thermal springs occur along (a) (b) its western ( along the west coast fault) Figure 1a. Deccan volcanic province showing the location of thermal springs along Tapi river. and N-NE flank (Son-Narmada Tapi lin- Figure 1b. N and NE fringes of the Deccan volcanic province showing the relationship between the Deccan eament: SONATA). The geochemistry of basalts and the Archean granites and younger and Neo-Proterozoic sediments. DV: Deccan Volcanics; G & thermal waters and thermal gases indicate G: Bundelkhand and Baster granites; S: Recent alluvium. Subsurface section shown in Figure 2 is deduced a deep meteoric water circulation below along X-X’ line (modified after Chandrasekharam, 2003 and Verma and Banerjee, 1992).

71 Chandrasekhar and Chandrasekharam

we shall report the subsurface section of a part of the north- Reactivation of these faults gave rise to regional graben structure eastern Deccan volcanic province based on the geophysical and that formed the loci for the later sedimentary rocks such as those geological information and identify enhanced geothermal sites represented by the Gondwana group of rocks (Acharyya and for future exploration and exploitation. The present paper attains Roy, 2000). Although Kaila et al (1985), based on Deep Seismic significance in the light of the current debate on global climate Sounding (DSS) profiles, interpreted extension of these faults to change, clean development mechanism and future energy security/ mantle depths, thermal gas geochemistry (3He/4He ratios) from independence to the country. the SONATA geothermal province indicate crustal signature thus ruling out the possibility of these faults extending to mantle depth What Lies Below the Deccan Basalts? (Minissale et al., 2000). The Bundelkhand granites encompass granites of two ages. The older granite is of 3.5 billion years old The Deccan basalts, with an approximate thickness of 2.5 km and the younger one is of 2.5 billion years old. The age of the around Mumbai, tapers to less than 500 m towards eastern and Baster granite is 2.3 billion years. The Baster granite contains large southern periphery. Although sub-Deccan lithological informa- enclaves of granite batholiths of age varying from 1684- 1816 tion is lacking, from the lithology around its periphery, it may be million years (Gupta et al., 1993). Both the Bundelkhand and inferred that towards the eastern and southern fringes the flows Baster granites have very high radioactive minerals (K: 1.4-5.3% over lie the granites and granites gneiss of Archean age and the U: 3-11 ppm and Th: 6-56 ppm; Gupta et al., 1993, Menon et al., Neo-Proterozoic sediments. The Bundelkand granites north of 2003). As a result of their high radioactive mineral content, these Narmada River and the Baster granites exposed towards the south granites are generating radiogenic heat thus resulting in high heat of Narmada River are the two major granite batholiths that under- flow anomaly and geothermal gradient in this region. The heat lie the basalts in this region (Figure 1). Thus it is apparent that a production by these granites vary from 3.4 to 7.4 µWm-3 while major volume of the lavas have occupied a basinal structure in the heat flow value over the region varies from 41 to 82 mW/ the central region thus attaining a large thickness of 2.5 km. The m2 (Gupta et al., 1993, Menon et al., 2003). In fact, the entire geochemical signature of the thermal springs that flow through SONATA geothermal provinces are characterized by such high the Deccan basalts indicate deep circulation within the flows or heat flow anomaly and geothermal gradient (Chandrasekharam, within the granites with out the involvement of magmatic or mantle 2000, 2005, Roy and Rao, 2000). Using the heat flow values, heat sources ( Minissale et al., 2000). Regional faults like the geothermal gradient, thermal conductivity of the granites, crustal west coast and Narmada-Tapi fault and associated satellite faults density and specific heat of the granites, resource base temperature (Chandrasekharam, 1985) aid in deep circulation of the meteoric of the SONATA geothermal province ( below the Deccan basalts) water. Geothermal gradient and heat flow value along the west was estimated following the criteria suggested by Rowley (1982). coast vary from 47 to 59 °C/km and 75 to 129 mW/m2 respectively This method gave temperature values of 150 to 210 °C at depths and along the SONATA the geothermal gradient and heat flow varying between 1.5 to 4 km (Varun Chandrasekhar and Chan- value vary from 60 to 90 °C/km and 67 to 219 mW/m2 respectively drasekharam, 2007). Magnetotelluric investigations conducted (Ravisanker, 1987, Chandrasekharam, 2000, 2005). The highest across SONATA geothermal province ( X-X’ in Figure 1) indicate geothermal gradient of 90 °C/km and heat flow value of 120 mW/ granite batholiths below the Deccan basalts ( Rao et al., 2004). m2 were reported within the Tattapani geothermal province where Further, based on 2D ray tracing technique, Sridhar and Tewari the thermal springs recorded the highest temperature of 97 °C. (2001) identified a graben filled with 2 km thick sediments across Fluid circulation models and water rock interaction experimental SONATA towards NE of the line X-X’ shown in Figure 1. These investigation suggest fluid circulation to depth of 3 km within sediments recorded a velocity of 3.2 km/sec. The Deccan basalts the granites with reservoir temperature varying from 135-217 °C with velocity of 4.8 to 4.9 km/sec overlie the sedimentary structure (Minissale et al., 2000, Chandrasekharam and Antu, 1995). and high heat producing granites with velocity of 5.7 to 6 km/ sec lie below the sediments. These sediments are the Gondwana High Heat Generating Formation interpreted through DSS (Kaila et al., 1985). Granites Below the Deccan Basalts

The Bundelkhand and the Baster granites are the two promi- nent granite bodies that lie below the Deccan basalts in the Eastern fringes and these granites form the basement rock in this region. The contact between these two granite bodies is represented by E-W and ENE-WSW faults. One such prominent fault is the Figure 2. Subsurface lithology and structure across Tapi lineament. Section is drawn along x-x’ shown in Figure Narmada fault along which the 1. Seismic wave velocities are shown for each formation. F-F: Deep seated faults (modified after Kaila et al, 1985, Narmada River flows (Figure 1). Chandrasekharam et al., 2006).

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EGS Below Deccan Basalts Chandrasekharam, D. 2000. Geothermal energy resources of India- Coun- try update. Proceedings, World Geothermal Congress 2000, (Eds) E. Iglesias, D. Blackwell, T.Hunt, J.Lund, S.Tamanyu and K. Kimbara. Based on the above discussed geophysical and geochemical 133-145. data, subsurface structure, across the line X-X’ shown in Figure Chandrasekharam, D. 2003. “Deccan Flood Bassalts” in Indian Continental 1, is deduced (Figure 2). This structure concurs with the magneto Lithoisphere: Emerging Research Trends. (Eds) T.N.Mahadevan and B.R telluric interpretation given by Rao et al., (2004) Arora and K.R Gupta. Geol Soc.India Memoir. 53, 197-214. A thick blanket of granite with high heat producing capacity Chandrasekharam, D. 2005. Geothermal Energy Resources of India: Past and underlying an insulated cover of low heat conducting rock forma- the Present. Proceedings World Geothermal Congress Antalya, Turkey, tion indicate a enhanced geothermal system below the Deccan. 24-29 April 2005, 5 pp. Deep circulating hot fluid system, in the geothermal provinces of Chandrasekharam, D., Varun Chandrasekhar, and Alam, M.A. 2006 Geo- SONATA (Minissale et al., 2000), confirm the existence of such thermal Energy Resources of India: Ongoing and Future Developments. high heat generating granites below the Deccan basalts. As ob- Proceedings, AAPG International Conference Perth, Australia. served earlier (Varun Chandrasekhar and Chandrasekharam, 2007) Gowd, T.N. and Srirama Rao, S.V. 1992. Tectonic Stress Field in the Indian these granites are under NNE-ENE oriented SHmax (Gowd and Subcontinent. Journal of Geophysical Research. 97, 11879-11888. Srirama Rao, 1992) with SHmax increasing at the rate of 55 MPa/ km. As indicated in Figure 2, artificial thermal reservoir (s) can Gupta, M.L., Sundar, A., Sharma, S.R. and Singh, S.B. 1993. Heat flow in the Bastar Craton, central Indian Shield: implications for thermal character- be created at depth > 1 km in these granites. Future technological istics of Proterozoic cratons. Phy. Earth and Planet. Inter. 78, 23-31. innovation in drilling and hydro-fracturing techniques will make these sites best suited for developing large scale EGS and reduce Kaila, K. L., Reddy, P.R., Dixit, M.M. and Koteswara Rao, P. 1985. Crustal structure across Narmada-Son lineament, Central India, from deep seismic power deficit in the country. soundings, J. Geol. Soc. India. 26, 465- 480. Menon, R., P. Senthil Kumar, G. Koti Reddy, and R. Srinivasan, 2003. Conclusions Radiogenic heat production of Late Archean Bundelkhand granite and some Proterozoic gneisses and granitoids of central India. Current Sci- Sites suitable for developing EGS exists in NE part of the Deccan. ence. 85, 634- 638. Considering the volume and aerial extent of the high heat generating Minissale, A., Vaselli, O., Chandrasekharam, D., Magro, G., Tassi, F. and granites, substantial amount of power can be generated from the Casiglia, A. 2000. Origin and evolution of “intercratonic” thermal above province and offset carbon trade and minimize carbon dioxide fluids from central-western Peninsular India. Earth. Planet. Sci. Lett. emission in future. Further, the distribution of these high heat generat- 181, 377-394. ing granites is advantageous to supply off grid power to clusters of Rao, C.K., Ogawa, Y., Gokarn, S.G. and Gupta1, G. 2004. Electromagnetic rural villages at affordable cost, considering the advancement taking imaging of magma across the Narmada Son lineament, central India. place in drilling and hydraulic fracturing technology Earth Planets Space. 56, 229–238. Roy, S. and R.U.M. Rao, 2000. Heat flow in the Indian shield. Journal of Acknowledgement Geophysical Research. 105, 25587-25604. We thank Steve Bjornstad for reviewing the manuscript. Rowley, J.C., 1982. World geothermal resources in L.M.Edwards, G.V.Chilingar, H.H. Rieke III and W.H. Ferti (eds) Handbook of geother- mal energy. Gulf Publishing Company, London. 44-176. References Subbarao, K.V., Chandrasekharam, D., Navaneethakrishnan, P. and Hooper, Acharyya, S. K. and Roy, A. 2000. Tectonothermal history of the Central P.R. 1994. Stratigraphy and structure of parts of the central Deccan Indian Tectonic Zone and reactivation of major faults/shear zones. J. basalt province: Eruptive models. In K.V.Subbarao (ed). “Volcan- Geol. Soc. India. 55, 239–256. ism” (B.P. Radhakrishna volume), Wiley Eastern Ltd, New Delhi, 296-321. Chandrasekharam, D. 1985. Structure and evolution of the western continental margin of India deduced from gravity, seismic, geo- Varun Chandrasekhar and Chandrasekharam, D. 2007. Enhanced Geo- magnetic and geochronological studies. Phy. Earth. Planet. Interiors. thermal Resources: Indian Scenario. Geother. Res. Council Trans. 31, 41, 186-198 . 271-273. Chandrasekharam, D. and Antu, M.C. 1995. Geochemistry of Tattapani Verma, R.K. and Banerjee, P. 1992. Nature of continental crust along the thermal springs, Madhya Pradesh, India: Field and experimental in- Narmada-Son Lineament inferred from gravity and deep seismic sounding vestigations. Geothermics. 24, 553-559 . data. Tectonophy. 202, 375-397.

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