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GRC Transactions, Vol. 33, 2009
Geothermal Systems in India
Varun Chandrasekhar1 and D. Chandrasekharam1,2
1
GeoSyndicate Power Private Ltd., Mumbai India • varun@geosyndicate.com
Dept. Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India
dchandra@iitb.ac.in
2
indicated by frequent earthquakes of magnitude varying from 3.5
to 6 (Chadha, 1992). Similar faults evolved due to the formation
of graben and horst structures along the coast are seen off the west
coast within the continental margin of India (Chandrasekharam,
1985). Although subsurface lithological information within the
coastal Deccan Flood basalts is lacking, recent bore hole information from a core drilled off the shore of Mumbai ( The Bombay
High Oil fields) reveal the presence of 1438 Ma granite below
the volcanic flows. The Th content in this granite varies from
18 to 21 ppm and is similar to some of the high heat generating
Keywords
Wet geothermal systems, granites, helium isotope, Deccan
flood basalt, mantle
Abstract
The wet geothermal systems in India are associated with deep
seated faults/rifts and collision tectonics. Although these systems
are located in different lithological formations, they are dominantly controlled by high heat generating granites of different ages.
The helium isotope ratio of the thermal gases from all the thermal
provinces strongly suggests their association with such granites.
Thus all the seven thermal systems indicate a natural enhanced
geothermal systems (EGS) operating with in this subcontinent thus
giving an excellent opportunity to understand and strategically
plan large scale EGS projects in the near future.
Overview
The geothermal provinces in India are associated either with
deep seated rift systems, like East Africa or, are associated with
continental collision zones (Figure 1). The temperature of the
thermal springs measured at the surface varies from 47 to 98 °C.
One common feature of all these provinces, as described below,
is that they circulate within the continental crust and are hosted
by high heat generating granites.
West Coast Geothermal Province
The west coast geothermal province is located within the Deccan Flood Basalt province and all thermal springs are located along
a line parallel to the western coast of India. The flood basalts attain
a thickness of 2.5 km in this part of the province and are traversed
by several N-S trending faults and dykes (Chandrasekharam,
1985, Chandrasekharam, 2003). The major tectonic feature that
controls the flow of the thermal springs is the west coast fault that is
closely associated with the evolution of the Deccan volcanism and
reactivated several times subsequent to the Deccan volcanism as
Figure 1. Geothermal provinces in India.
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Chandrasekhar and Chandrasekharam
granites reported earlier (Chandrasekharam and Chandrasekhar,
2008). However the presence of granites below the Deccan basalts
along the coast is not continuous as indicated by positive gravity
anomalies over several regions of the coast. These positive gravity anomalies indicate foundering of the curst and exposing the
basic crust below the granitic crust (Negi et al, 1992) thus giving
rise to graben structures. High 4He (> 2 % v/v) and low 3He/4He
(R= R/Ra, Ra the ratio in the atmosphere; 0.12 to 0.21) reported
in the thermal gases from this region (Minissale et al, 2000) indicate minimum involvement of mantle component in controlling
the chemical composition of the gases. Thus major circulation of
the thermal water is occurring in the granite basement below the
Deccan flood basalt flows. This inference is supported by the occurrence of thermal spring with highest surface temperature (72
°C, Ramanathan and Chandrasekharam, 1997) in the southern part
of the coast near Rajapur. This thermal spring is issuing through
a granite inlier within the basalt flow. This granite appears to be
the Closepet granite that lies below the Deccan flood basalt floes
towards southern boundary of the Deccan volcanic province. The
Closepet granite, according to a recent study, is termed as fertile
granite due to high concentration of Th (10-43 ppm), U (3-21 ppm)
thus providing heat to the circulating thermal water ( Senthilkumar
and Sethuraman, 2003).
thermal gases ( Minissale et al., 2003). The high total helium
content (~1 % v/v) apparently is being contributed by the Godhra
granites to the circulating fluids.
Sonata
The SONATA geothermal province is controlled by the mid
continental rift termed as the Son-Narmada-Tapi lineament
(SONATA) trending WSW-ENE. The well known Tattapani geothermal springs are located at the eastern edge of the SONATA
and are related to the Balarampur fault system(Chandrasekharam
and Antu 1995). The springs flow through Archean metamorphic
Formations consisting of quartzites, schists, gneisses intruded
by granites, pegmatites and amphibolites (Joga Rao et al., 1986).
The Gondwana Formation (sandstone) lies over these rock. A thin
veneer of Deccan flood basalt flows covers the entire region. The
SONATA is a focus of several earthquakes of moderate magnitude.
The surface temperature of the SONATA thermal springs vary
from 30 to 93 °C (Tattapani). The thermal gases are characterized by very high helium content (0.54 to 7 % v/v; Minissale et
al, 2000) and low 3He/4He ratio. Though a deep seismic sounding
(DSS) investigation inferred the presence of the SONATA fracture
extending to mantle depth ( Kaila et al 1981) the helium data is
not supporting such inference and the main heat source for the
thermal waters is the high heat generating Baster and Bundelkhand
granites (3 to 5 µw/m3; geothermal gradient 60 - 90 °C/km and
heat flow 107 mW/m2 ) that form the basement rocks in this region
( Chandrasekharam and Chandrasekhar, 2008), Granite-water
interaction experimental results also confirm the involvement
of granite and circulation of the thermal water to depth of 2 km
( Chandrasekharam and Antu, 1995) resulting in high fluoride
content of 20 ppm in Tattapani thermal waters, the highest value
registered by thermal waters in India. A schematic flow path of
the Tattapani thermal waters is shown in Figure 3.
Similarly in other areas of the SONATA geothermal province
(e.g. Salbadri, Figure 1) magneto telluric investigation reveal the
presence of similar high heat generating granite intrusives ( Chandrasekharam and Prasad, 1998, Rao et al., 2004, Chandrasekharam
and Chandrasekhar, 2008).
Cambay Province
The Cambay geothermal province is represented by 22 thermal springs (35-93 °C) that are located within a wide range of
lithology varying in age from Archean to Quaternary (Figure 2.).
The Cambay is bounded by two deep seated N-S trending faults
enclosing a 3 to 4 km deep sedimentary basin overlying the Deccan flood basalts. The Deccan flood basalts form a thin veneer
over the granitic basement (Figure 2) A deep seismic sounding
investigation infers the extension of these faults to mantle depths
(Kaila et al., 1981). The Cambay basin is a foci of major alkaline
magmatism (Sheth and Chandrasekharam, 1997). Granite intrusives like the 955 Ma old Godhra granite, out crop within the basin
near Tuwa where thermal springs ( Figure 1) with highest issuing
temperature of 93 °C is located. The other thermal springs within
the basin have temperatures lower than the Tuwa thermal spring.
The anomalous geothermal gradient (70°C/km) and heat flow
value (67-93 mW/m2) in this region is attributed to such major
plutonic activities and upwarping of the mantle related to the
Deccan volcanism (Negi et al., 1992,, Chandrasekharam, 2005).
In spite of the presence deep seated faults on either side of the
Cambay basin, the contribution by the mantle to the geothermal
system is small as indicated by low 3He/4He ratio ( 0.27) in the
Godavari Province
The Godavari geothermal province is located within the
Godavari rift trending NW-SE direction (Figure1). The issuing
temperature of the thermal waters in this province varies from 43
to 68 °C. The subsurface geology deduced from bore hole informa-
Figure 2. Subsurface geology of the Cambay basin deduced from seismic
refraction and DSS profile. The numbers indicate seismic velocity in km/s
(modified after Tiwari et al, 1995, Kaila, 1981).
Figure 3. Circulation pattern of thermal waters in Tattapani geothermal
site. ‘F’ : Faults (Modified after Chandrasekharam and Bundschuh, 2008).
608
Chandrasekhar and Chandrasekharam
tion from the oil industry indicate a thick sedimentary formation
(Gondwana) capping the basement high heat generating granite
(3.9 w/m3). Both this granite as well as the rifted structure is
responsible for high heat flow (52-100 mW/m2) in this region. The
subsurface lithology and the thermal gradient clearly demonstrate
the importance of the granite in this geothermal province. The
granite outcrop can be seen at several places within the basin.
The relationship between the granite and geothermal gradient is
shown in figure 4.
these younger granites is supported by anomalously high helium
content in the thermal gases samples from this province( ~ 11473
ppm, Hoke et al, 2000). Both Puga and Yangbajing geothermal
sites are located within these granites (Chandrasekharam and
Bundschuh, 2008).
Wet Geothermal Systems and EGS
It is apparent that the wet geothermal systems in India are
controlled by the distribution of high heat generating granites. Although a small mantle helium component is present in the thermals
gases, this component could have been an artifact of mantle material entrapped during the evolution of the crust, as is the case of the
3
He/4He ratio in the Himalayan granites ( Hoke et al., 2000). This
is true in the case of Cambay and SONATA geothermal provinces
where this small mantle helium component may be due to mantle
helium leak through the major fractures and faults extending to
mantle depths. This shows that all the wet geothermal systems in
India in fact represent natural enhanced geothermal systems and
they provide considerable amount of information for selecting sites
for initiating EGS projects in India. It has been shown earlier that
1000 sq km of such high heat generating granite of Ladakh has
the capacity to generate about 61160 x 1012 kWhr of electricity.
This is far greater than the future demand of electricity for Leh
(130 x 109 kWhr, Chandrasekharam and Varun Chandrasekhar,
2008). Considering the total surface exposure of such high heat
generating granite over the Indian subcontinent (150000 sq.km),
and the stress regime of the Indian plate (NNE-ENE oriented
SHmax, Chandrasekharam, 2001) Indian granites will be future
warehouse of EGS. It is estimated that these granites have the
capacity to generate energy equivalent to 3.133 x 1022 BTU.
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