Surface Geochemical Data Evaluation and Integration with Geophysical Observations for Hydrocarbon Prospecting, Tapti Graben, Deccan Syneclise, India

Geoscience Frontiers 5 (2014) 419e428

Contents lists available at ScienceDirect

China University of Geosciences (Beijing) Geoscience Frontiers

journal homepage: www.elsevier.com/locate/gsf

Research paper Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting, Tapti graben, Deccan Syneclise, India

T. Satish Kumar a,*, A.M. Dayal b, V. Sudarshan c a Centre of Excellence for Energy Studies, Oil India Ltd, Guwahati 781022, India b National Geophysical Research Institute (CSIR), Hyderabad 500006, India c Applied Geochemistry Department, Osmania University, Hyderabad 500006, India article info abstract

Article history: The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant Received 28 December 2012 hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through Received in revised form conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, 27 July 2013 near-surface geochemical data are used to understand the petroleum system and also investigate type Accepted 5 August 2013 of source for hydrocarbons generation of the study area. Soil samples were collected from favorable Available online 27 August 2013 areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of pe- Keywords: Adsorbed gas troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light e Microseepage hydrocarbons (C1 C4) and their carbon isotopes from the western part of Tapti graben, Deccan Light hydrocarbon Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high Stable isotope concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane Geophysical (C1 ¼ 518 ppb) and ethane plus higher hydrocarbons (SC2þ ¼ 977 ppb) were observed. Statistical Deccan Syneclise analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from 22.5& to 30.2& PDB, 18.0& to 27.1& PDB and 16.9&e32.1& PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentrations were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons. Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Visible seepage of oil or hydrocarbon gases in sedimentary basins has played a major role in the discovery of many oil ﬁelds throughout the world (Link, 1952). In 1930s, the concept of a

* Corresponding author. prominent macro seep environment in association with commer- E-mail address: [email protected] (T. Satish Kumar). cial subsurface hydrocarbons was extended to the micro seep level by measuring minute surface gas concentrations or secondary soil Peer-review under responsibility of China University of Geosciences (Beijing) alteration effects induced by gas seepage. Microseepage of hydrocarbon gases into near-surface environments is often interpreted as a direct indication of the presence of deeper hydrocarbons. Jones and Drozd (1983) and Severne et al. (1991) provided examples for Production and hosting by Elsevier such successful applications of soil gas surveys in exploration. The

1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.08.003 420 .Sts ua ta./Gocec rnir 21)419 (2014) 5 Frontiers Geoscience / al. et Kumar Satish T. e 428

Figure 1. Adsorbed soil sample locations are plotted on geology of the study area, Tapti Graben, Deccan Syneclise (modiﬁed after Geological Survey of India, 2001). T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 421

Table 1 identification of seepage can add valuable information to the Generalized stratigraphy of the Deccan Syneclise Basin (After Deshpande, 1998). exploration process when the diagnosis of gas micro seep envi- Age Formation/ Anticipated ronments can be distinguished and separated from background range Group thickness (km) Lithology Geographic distribution data. Under certain conditions, active seepage may be recognized through measurement of concentration and composition of hy- Recent– Alluvium Nagpur, Bhandara , Pleistocene Laterite, sand Candrapur, Wardha drocarbon gases in soils (Jones and Drozd, 1983; Klusman, 1993). Jalgaon, This paper describes the application of surface geochemical Kolhapur Satara Thane, etc. Early Paleocene Deccan Traps 1–2 Basalts Most of the state from studies carried out in sub-trappean of Mesozoic age in Deccan Trap –Cretaceous west of Nagpur to cover, western part of Tapti graben for the prospecting of hydro- Arabian sea coast ------Unconformity------carbons. The region has received wide attention for its hydrocarbon Late Lameta, bagh Arenaceous Nagpur, Candrapur prospects because of the thick GondwanaeMesozoic sedimenta- Cretacoous beds limestone, Dhule, Gadchiroli tion of 1e2.75 km buried under the lava flow of Deccan Traps (Kaila 2–3.5 Sandstone Yavatmal Middle Upper shale et al., 1989; DGH, 2006). Hence an attempt has been made to find Triassic Gondwana the presence of hydrocarbons in the reported Mesozoic sediment ------Unconformity ------Triassic– Nagpur, Chandrapur areas. In the present work, the adsorbed soil gases and their carbon Carboniferous Lower Yavatmal isotope signatures have been analyzed to determine the concen- Gondwana tration of light hydrocarbons and to identify the seepage and source Proterozoic Penganga beds Gadchiroli, Ratnagiri Limetsones, shales of hydrocarbons. Archean Sausar, Sakoli, Nagpur, Bhandara Amagaon, Ratnagiri, Sindhudurg Unclassified gneisses 1.1. Geology of the study area

The Narmada-Tapti rift system, which constitutes the western part of the NSL (Narmada-Son Lineament), is covered by a thick pile of Deccan lava ﬂows and is characterized by several hidden tectonic

Figure 2. (a) Trap thickness map in Deccan Syneclise with study area (modified after Harinarayana et al., 2007). (b) Map showing Mesozoic sediment thickness of the study area (after DGH, 2006). (c) The Narmada-Tapti seaway of Deccan volcanic province with study area (modified after Keller et al., 2009). 422 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 structures, magmatic crustal accretion, sedimentary basins and Deep seismic studies (DSS) along the Thuadara-Sendhwa- complex geophysical signatures (Fig. 1). Major tectonic adjustments Sindad profile wherein the first arrival refraction data analyzed in various crustal blocks, in Narmada-Tapti graben occurred in based on 2D tracing technique revealed a graben between Narmada Precambrian/Gondwana times which must have been responsible and Tapti. This graben contains 1000e2800 m thick low velocity for the formation of Vindhyan and Gondwana sedimentary basins. (3.2e3.6 km s 1) sediments under a thick cover of Deccan traps. Tapti Basin, an intracratonic half graben in western-central India And this study also revealed that the graben is bounded by two (Guha, 1995) is considered to be Mesozoic marginal marine basin faults, one south of Narmada i.e., Barwani-Sukta and the other (Biswas, 1987). The generalized stratigraphy of Deccan Traps is given north of Tapti river. The graben seems to be extending towards in Table 1. Tapti Basin forms a linear tract spread over a length of south-east. The low velocity Mesozoic sedimentary rocks with a P- 350 km and an average width of 30 km covered by alluvium of Tertiary wave velocity of 3.5 km s 1 and thickness ranging from about 0.70 to recent with isolated inliers of the Deccan Traps. The alluvium to 1.6 km and 0.55e1.1 km lies along the east-west and north-south thickness from south to north, in general, extends to a depth of profiles, respectively (Murty et al., 2010). w200 m to >400 m below mean sea level at places (Ravi Shanker, The first layer with a P-wave velocity of 5.15e5.25 km s 1 and 1987). The Deccan Trap thickness varies considerably from 100 m in thickness varying from 0.7 to 1.5 km represents the Deccan Trap the northeastern part to more than 1500 m towards the west coast of formation along the NarayanpureNandurbar profile. The Trap layer India (Fig. 2a). In the Narmada-Tapti region, a hidden Mesozoic sedi- velocity ranges from 4.5 to 5.20 km s 1 and the thickness varies mentary basin underlying the Deccan Traps has been reported in the from 0.95 to 1.5 km along the KothareSakri profile. The second form of two grabens separated by a small horst of Satpura hills. In the layer represents the low velocity Mesozoic sediments with a P- southern part a larger Tapti graben with sediment thickness of about wave velocity of 3.5 km s 1 and thickness ranging from about 0.70 2000 m is revealed, whereas in the northern part there is a smaller to 1.6 km and 0.55e1.1 km along the EeW and NeSprofiles, Narmada graben with sediment thickness of about 1000 m (Kaila, respectively. Presence of a low-velocity zone (LVZ) below the vol- 1988). Integrated geophysical studies identified the major sedimen- canic rocks in the study area is inferred from the travel-time ‘skip’ tary basin in and around Sendhwa, Shirpur, Dhule and Sakri having and amplitude decay of the first arrival refraction data together very large thickness of sub-trappean Mesozoic sediments of the order with the prominent wide-angle reflection phase immediately after of 750e2250 m (Fig. 2b) (DGH, 2006). The basement topography is quite undulating in the region and it is deepest in Shirpur and Table 2 Sendhwa region. The sediments of this Mesozoic basin were depos- Gas chromatographic analysis of light gaseous hydrocarbons desorbed from soil samples and summary of statistical values. ited in a larger Mesozoic sea, which extended from Narmada-Tapti ε ε region through Saurashtra, Kutch, up to Sind and Salt Range in the Sample Id C1 C2 C3 C4 Sample Id C1 C2 C3 C4 shape of a horseshoe. The Moho configuration under the Deccan Trap (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) covered area reveals a depression in the central part extending in an DSS/01/06 15.6 0 0 0 DSS/41/06 5.6 0 0 0 ENEeWSW direction, which almost coincides with the region of DSS/02/06 141.4 28.9 13.7 0 DSS/42/06 44.8 8.1 4.7 0 DSS/03/06 35.5 6.9 5.3 5.4 DSS/43/06 181.5 33.7 14.9 5.8 hidden Mesozoic basin (Kaila, 1988). Both the Narmada-Tapti and the DSS/04/06 15.8 1 0 0 DSS/44/06 13.6 0 0 0 west coast tectonic belts are characterized by positive gravity anom- DSS/05/06 53.6 8 3.6 0 DSS/45/06 27.2 3.3 2.1 0 alies, high gravity gradients, high heat flow 70e100 mW/M2 and DSS/06/06 4 0 0 0 DSS/46/06 14.3 2.2 1.8 0 seismic activity (Arora and Reddy,1991) and the tectonic history of the DSS/07/06 35.9 7.3 4.8 2.8 DSS/47/06 14.7 0 0 0 DSS/08/06 26.3 6.3 6.1 7.8 DSS/48/06 4.3 0 0 0 basin indicate the thermal subsidence and burial (Schutter, 2003; DSS/09/06 24.7 3.3 2.6 0 DSS/49/06 18.7 3.5 3.4 2 Rohrman, 2007)tobesignificant enough to cause the maturation of DSS/10/06 24.5 3.7 3.1 0 DSS/50/06 10.6 1.3 0 0 organic rich sediments, which might be favorable conditions for hy- DSS/11/06 4.5 0 0 0 DSS/51/06 212.3 48.8 23.7 10.4 drocarbon generation. Keller et al. (2009) has been reported that, the DSS/12/06 17.1 2.5 1.5 0 DSS/52/06 8.2 0 0 0 marine incursion accompanied by planktic foraminifera and brackish- DSS/13/06 6.2 0 0 0 DSS/53/06 20.8 0 0 0 DSS/14/06 69.8 12.7 7.4 0 DSS/54/06 17.2 2.5 0 0 marine ostracods indicates a seaway existed into central India during DSS/15/06 61.9 12.1 7.9 0 DSS/55/06 8.5 0 0 0 the Maastrichtian to early Paleocene (Fig. 2c). This seaway may have DSS/16/06 9.1 0.8 0.7 0 DSS/56/06 35.3 6 2.8 0 followed the Narmadaand Tapti rift zones where a seaway isknownto DSS/16(II)/06 4.3 0 0 0 DSS/57/06 53.3 6.1 0 0 have existed during the late Cenomanian to Turonian. Therefore, the DSS/17/06 4.3 0 0 0 DSS/58/06 8 0 0 0 DSS/18/06 72.7 14.1 6.6 0 DSS/59/06 7.7 0 0 0 marine transgression and regression that occurred in western and DSS/19/06 75.8 12.9 8.6 0 DSS/60/06 6.4 0 0 0 central India before the Deccan volcanism, might have favored the DSS/20/06 25.8 3 1.2 0 DSS/61/06 12.6 0 0 0 deposition of organic-rich source rocks. Further, the Deccan Trap DSS/21/06 42.7 5.9 4.1 0 DSS/62/06 19.5 0 0 0 volcanism during late Cretaceous might have generated the requisite DSS/22/06 39.3 5.9 2.2 0 DSS/63/06 41.5 4.9 3.9 0 thermal conditions and acted as a catalyst in a Mesozoic hydrocarbon DSS/23/06 11.9 1.7 1.3 0 DSS/64/06 18.3 2.5 2.5 0 DSS/24/06 42.7 7.3 5 0 DSS/65/06 8.9 0 0 0 generation process (Biswas and Deshpande, 1983). DSS/25/06 4.9 0 0 0 DSS/66/06 4.7 0 0 0 DSS/26/06 9.8 1 0 0 DSS/67/06 2.6 0.3 0 0 1.2. Geophysical evidences DSS/27/06 24.6 2.9 0 0 DSS/68/06 34.3 6.7 4.2 0 DSS/28/06 19.7 0 0 0 DSS/69/06 12.2 0 0 0 DSS/29/06 21.3 3 2.5 2.5 DSS/70/06 16.6 0 0 0 The area has been the subject of extensive geological and DSS/30/06 18.4 2.8 2 0 DSS/71/06 16.6 1.3 0 0 geophysical investigations for many years (West, 1962; Qureshy, DSS/31/06 7.6 0.6 0 0 DSS/72/06 7.4 0.6 0 0 1964; Choubey, 1971; Mishra, 1977; Crawford, 1978; Biswas, 1982, DSS/32/06 7 0 0 0 DSS/73/06 2.6 0 0 0 1987; Ravi Shanker, 1987a,b; Ravi Shanker, 1991; Kaila and DSS/33/06 14.2 1.9 1 0 DSS/74/06 2.8 0 0 0 Krishna, 1992; Verma and Banerjee, 1992; Powar, 1993; Singh and DSS/34/06 4.9 0 0 0 DSS/35/06 3.7 0 0 0 Mean 26.1 3.9 2.1 0.5 Meissner, 1995; Bhattacharji et al., 1996). Integrated geophysical DSS/36/06 20.3 1.8 1.1 0 Std. 36.3 7.8 3.9 1.8 exploration studies were carried out using seismic refraction, deviation magnetotelluric, deep resistivity sounding and gravity methods by DSS/37/06 4.7 0 0 0 Range 209.6 48.8 23.7 10.4 the CSIR-National Geophysical Research Institute (NGRI) and suc- DSS/38/06 11 0 0 0 Minimum 2.6 0 0 0 DSS/39/06 4.1 0 0 0 Maximum 212.3 48.8 23.7 10.4 cessfully delineated subtrapean Mesozoic sediments in Saurashtra DSS/40/06 4 0 0 0 and Kutch basins (NGRI, 2004). T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 423

Table 3 acid in vacuum. The desorbed light gaseous hydrocarbons were ﬁ e Pearson correlation coef cients between light hydrocarbon (C1 C4) gases. collected by water displacement in a graduated tube ﬁtted with m C1 C2 C3 εC4 rubber septa. The volume of desorbed gas was recorded and 500 L

C1 1 of the gas was injected into a Varian CP-3380 Gas Chromatograph 00 C2 0.99 1 (GC) equipped with a 1.8 m 1/8 2.00 mm Porapak Q Column C3 0.95 0.98 1 and Flame Ionization Detector. The GC was calibrated using an εC4 0.61 0.66 0.69 1 external standard with known concentrations of methane (C1), ethane (C2), propane (C3), i-butane (iC4) and n-butane (nC4). The the first arrivals from the Deccan Traps formation. The basement moisture content of samples was determined and the gas concen- with a P-wave velocity of 5.8e6.05 km s 1 lies at a depth ranging trations are reported in ppb on dry weight basis. The accuracy of from 1.5 to 2.45 km along the profiles. The velocity models of the measurement of C1 to C4 component is 1 ppb of gas weight. profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada- 2.3. Analysis for carbon isotopes of light hydrocarbons Tapti region of the Deccan Syneclise. 13 13 Carbon isotopic composition of light hydrocarbons (d C1, d C2, 13 2. Soil sampling and analytical methods and d C3) in soil samples is determined using a GC-C-IRMS, which comprises an Agilent 6890 GC coupled to a Finnigan-Delta Plus XP 2.1. Soil sampling Isotope Ratio Mass Spectrometer via a GC combustion III interface at CSIR-NGRI. One mL of desorbed gas is injected into the GC in splitless Study area is located in the western part of Tapti Basin (Fig. 1), mode with helium as carrier gas at fixed over temperature of 28 C. 0 0 0 Deccan Syneclise, India, between 74 14 e74 59 E and 20 53 e The light hydrocarbon gases eluting from the GC column enter the 0 21 33 N. An adsorbed soil gas survey was carried out and a total of combustion reactor maintained at 960 Cwhereitgetsconvertedto w 75 soil samples were collected from a depth of 2.5 m at an in- CO2 and water. A Nafion membrane tube is used to remove water, prior e terval of 2 3 km along existing roads. Cores were collected using a to passing the CO2 into the mass spectrometer. Reference standards hollow metal pipe by manual hammering to the required depth. are intermixed with samples to monitor instrumental performance. The cores collected were wrapped in aluminum foil and sealed in The carbon isotope ratio in the sample was determined by comparing poly-metal packs. isotoperatios with thatof a standard, NIST RM 8560 (IAEA NGS2) using ISODAT software. The d13C is calculated using the following equation: 2.2. Analysis of soil gases for light hydrocarbons d13C ¼ {[(13C/12C)/(13C/12C)] 1} 1000. The light gaseous hydrocarbons were extracted from the soil samples using a ‘Gas extraction system’ (Horvitz, 1981). The light The carbon isotopic composition is reported in permil (&) gaseous hydrocarbons were desorbed from soil samples by treating relative to the Pee Dee Belemnite (PDB). The precision of the iso- w1 g of wet sieved 63 mm fraction of sample with orthophosphoric topic analysis is 0.5&.

Figure 3. (a) Histogram for methane, ethane, propane and butane concentrations. (b) Pixler plot for discriminating oil, oil and gas and gas zones using C1/C2 and C1/C3 ratios. 424 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428

3. Results and discussion Table 4 Approximate empirical range of micro seep compositional ratio for gas, gas condensate, and oil (Jones and Drozd, 1983). 3.1. Concentrations of light hydrocarbon gases

Hydrocarbon composition C1/C2 C2/C3 10 C3/C1 1000

The concentrations of each of the five organic constituents (C1, Dry gas 100e20 25e50 2e20 Gas condensate 20e10 16.5e25 20e60 C2,C3,iC4, and nC4 alkanes) are expressed in parts per billion (ppb) e e e in all the 75 soil samples. The overall hydrocarbon gas composition Oil 10 41016.5 60 500 presence in the total samples is: C1 80.2%, C2 11.8%, C3 6.4%, iC4 0.2% and nC4 1.2%. The detail statistics of each soil gas constituent are distribution histogram for all the individual hydrocarbons. The summarized in Table 2. Pearson correlation coefficients are used for histograms of C1 to C3 (Fig. 3a) clearly show normal distribution comparing the linear relationships between geochemical variables, with a positive skewness which is commonly associated with sur- two at a time (Table 3). Correlation coefficients obtained for C1 vs. face geochemical data in petroleum exploration (Tedesco, 1995). C2,C1 vs. C3 and C2 vs. C3, show very high correlation (r 0.9) The geochemical signature (gas/condensate/oil) is determined with each other. This indicates that these hydrocarbons were using ratios of hydrocarbon constituents detected in the soil gas generated from a thermogenic source (genetically related), and samples. The compositional signature displayed by C1/C2,C1/C3 belong to a migration fairway system associated with one sub- ratios, as defined by Pixler (1969) is shown in Fig. 3b, which has surface source (Gevirtz et al., 1983). The geochemical data popu- been used to quantify and qualify compositional signatures in mud lation and distribution pattern were analyzed using frequency logging applications and has proved to be useful for the

Figure 4. Cross plots between methane and ethane, and ethane and propane indicating the zones for soil gas. T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 425

Figure 5. Yield ratio between C1/C2 vs. C1/(C2 þ C3), to observed possible source for light hydrocarbons (modiﬁed after Bernard et al., 1977).

interpretation of soil gas source. It can be seen that almost all the compositional signatures associated with these high (C1 and C2þ) samples fall in the oil zone. Out of 75 samples analyzed for light concentration regions are in the N and S-E parts (Fig. 6a and b) of hydrocarbons desorbed from soil, C1 and C2 were observed in 42 the study area. Accurate determination of background hydrocarbon samples constituting 55% of the population. Within these 42 sam- concentrations is paramount. Background hydrocarbon concen- ples, 86% of the samples plot in oil zone and 14% in oil and gas zone trations are considered to be normal data from sources other than (Fig. 4a). Similarly, 32 samples show the presence of C2 and C3 vertical migration. Therefore, hydrocarbon data above the back- components out of which only 75% of the samples plot in the oil ground level can include some vertical migration data. The mean as zone and 25% of the samples fall in the oil and gas zone (Fig. 4b). exact background value, assume that values above the mean plus Approximate Empirical Range of micro seep compositional ratio for one standard deviation are anomalous. Background concentrations gas, gas condensate, and oil (Jones and Drozd, 1983) may be used to for methane, ethane and propane are 26.1 ppb, 3.9 ppb and 2.1 ppb constrain further wet vs. dry gas indications in soil gases (Table 4). respectively, which are moderately close to the median values. The The hydrocarbon gases with higher carbon number, i.e., C2 þ gases, C1 and C2þ anomalous maps were prepared based on meanþ1st including ethane, propane, iso- and n- butanes, are detected in 56% standard deviation. Fig. 7a and b shows that large magnitude of the samples, which has been found as an effective indicator of anomalies found in sub-trappean Mesozoic sediments i.e., northern petroleum deposits (Saunders et al., 1991). Using the yield ratio region of ShirpureNardane-Sindkheda-Shendvade and southern equation C1/C2 vs. C1/(C2 þ C3), it is observed that 54% of the region of Dhule e Khede, where traps are thinner. In addition, samples are of thermogenic gases (Fig. 5)(Bernard et al., 1977). The signiﬁcant high concentrations of light hydrocarbons are found

Figure 6. (a) Isoconcentration distribution map of methane. (b) Isoconcentration distribution map of ethane plus (C2þ). 426 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428

Figure 7. (a) Anomaly distribution map of methane. (b) Anomaly distribution map of ethane plus (C2þ).

Table 5 along the northern, southern faults and lineaments indicating that Compound-speciﬁc carbon isotopic compositions of methane, ethane and propane the structural features viz., faults and lineaments acting as conduits with light hydrocarbon concentration of the study area. for these light hydrocarbons. 13 13 13 Sample no. d C1 (&) d C2 (&) d C3 (&) Wetness ratio% [C /(C þ C )] 1 2 3 3.2. Carbon isotope enrichment in gases desorbed from soil samples DSS/02/06 30.2 20.3 20.3 3.32 DSS/18/06 23.9 19.8 16.9 3.51 DSS/19/06 23.3 18.1 29.1 3.52 The carbon isotope composition for the hydrocarbon gases des- DSS/21/06 27.1 25.7 e 4.3 orbed from the soil samples are given in Table 5. The enrichment DSS/22/06 26.9 20.6 25.5 4.8 patterns for methane, ethane and propane in different samples are DSS/24/06 25.9 18.9 32.1 3.46 shown in Fig. 8. Theoretical considerations and empirical observa- DSS/43/06 29.2 26.4 27.1 3.73 tions suggest that individual gaseous hydrocarbons generated from DSS/57/06 22.6 18 26.5 8.73 DSS/63/06 26.4 27.1 e 4.71 cracking processes at the same temperature and maturity should show characteristic carbon isotopic compositions (Chung et al., 1988;

Figure 8. Compound-speciﬁc carbon isotopic compositions of the seven gas samples from the study area. The DSS/2, DSS/18 and DSS/43 samples are displaying evidence for unaltered samples. T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 427

Figure 9. Methane carbon isotope ratio compared to wetness [(C1/C2 þ C3)] (Bernard et al., 1978) and indicating possible source for the light hydrocarbons.

Berner et al., 1990; James, 1990). Generally, carbon isotopes of Ph.D in the Departmentof Applied Geochemistry, Osmania University, 13 13 13 methane, ethane and propane show d C1 < d C2 < d C3 trend in Hyderabad. Satish acknowledge CSIR for the Senior Research natural gas. The present study reveals a deviation from this trend and Fellowship. the presence of propane, which is isotopically lighter than methane, suggests that the soil gases have suffered partial oxidation leading to References isotopically heavier methane (Fig. 8). Therefore samples might be results from heterogeneity in the source organic matter, mixing of Arora, B.R., Reddy, C.D., 1991. Magnetovariational study over a seismically active gases from different sources, oxidation of thermogenic gas, or partial area in the Deccan Trap province of western India. Physics of the Earth and Planetary Interiors 66, 118e131. diffusive leakage of the gas reservoir (Laughrey and Baldassare, Bernard, B.B., Brooks, J.M., Sackett, W.M., 1977. A geochemical model for charac- 1998). The Bernard diagram (Bernard et al., 1978) is commonly terization of hydrocarbon gas sources in marine sediments. In: Proceedings 9th used to classify natural gas origins. A modified Bernard diagram Offshore Technology Conference, Houston, TX, p. 435. Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texas characterizing soil gases from the western part of Tapti Basin based continental shelf and slope sediments. Journal of Geophysical Research 83, 13 on the wetness [C1/(C2 þ C3)] vs. methane d C is shown in Fig. 9, 4053e4061. which falls in thermogenic source. Berner, U., Dumke, I., Faber, E., Poggenburg, J., 1990. Organic geochemistry of surface sediments of the Sulu Trench/Philippines. In: Proceedings of the Ocean Drilling Program, Initial Reports 124, pp. 113e118. 4. Conclusion Bhattacharji, S., Chatterjee, N., Wampler, J.M., Nayak, P.N., Deshmukh, S.S., 1996. Indian intraplate and continental margin rifting, lithospheric extension, and mantle upwelling in Deccan flood basalt volcanism near the K/T boundary: Integrated interpretation of soil gases and carbon isotopes of evidence from Mafic Dike Swarms. Journal of Geology 104, 379e398. methane, ethane and propane has led to reliable assessment of Biswas, S.K., 1982. Rift basins in the western margin of India and their hydrocarbon micro-seepage and anomalous zones, which are related to sub- prospects. American Association Petroleum Geologists Bulletin 66 (10),1497e1513. Biswas, S.K., 1987. Regional tectonic framework, structure evolution of the western trappean Mesozoic sediments in the Deccan Traps. Light hydro- marginal basins of India. Tectonophysics 135, 307e327. carbons generated in the Mesozoic sedimentary rocks, migrated Biswas, S.K., Deshpande, S.V., 1983. Geology and hydrocarbon prospects of Kutch, through the Deccan Traps and become adsorbed in the near sub- Saurashtra and Narmada basins. In: Bhandari, L.L., et al. (Eds.), Petroliferous e surface soils. From the correlation, these hydrocarbons are geneti- Basins of India, pp. 111 126. Choubey, V.D., 1971. PreeDeccan Trap topography in Central India and crustal cally related, and might have been generated from a thermogenic warping in relation to Narmada rift structure and volcanic activity. Bulletin of e source because of the presence of C2 and C3 components. This Volcanology 35, 660 685. suggests that all the hydrocarbon constituents in the micro-seeps Chung, H.M., Gromly, J.R., Squires, R.M., 1988. Origin of gaseous hydrocarbons in subsurface environments: theoretical considerations of carbon isotope distri- are from the same source, without any prominent secondary bution. Chemical Geology 71, 97e103. alteration during the path of migration to the surface and their Crawford, A.R., 1978. Narmada-Son lineament of India traced into Madagascar. subsequent adsorption on the soil. The compositional ratios and Journal of the Geological Society of India 19 (4), 144e153. Deshpande, G.G., 1998. Geology of Maharashtra, (1st edn.): Geological Society of Pixler plot indicate that most of the samples fall within the oil zone. India, Bangalore, p. 223. Carbon isotope signatures suggest that these gases are derived from DGH, 2006. New Exploration Licensing Policy (NELP) VI. Directorate General of oxidation of thermogenic gas of the gas reservoir. The present study Hydrocarbons, India. Retrieved from www.dghindia.org. fi Gevirtz, J.L., Carey, B.D., Blanco, S.R.,1983. Surface geochemical exploration in the North provides clear evidences of signi cant concentrations of hydro- Sea. Methodology with an example. In: Brook, J. (Ed.), Petroleum Geochemistry and carbon, which is economically viable for exploration in the Meso- Exploration of Europe, Geological Society, Special Publ., vol. 12, pp. 35e50. zoic sedimentary rocks of the western part of Tapti Graben. Guha, S., 1995. Tectonic framework and evolution of the Tapi basin e an intracratonic half graben in western central India. Indian Minerals 49, 61e78. Harinarayana,T., Patro,B.P.K., Veeraswamy, K., Manoj, C.,Naganjaneyulu, K., Murthy, D.N., Acknowledgments Virupakshi, G., 2007.Regionalgeoelectic structurebeneath Deccan Volcanic Province of the Indian subcontinent using magnetotellurics. Tectonophysics 445, 60e80. Horvitz, L., 1981. Hydrocarbon prospecting after forty years. In: Gottleib, B.M. (Ed.), This paper forms part of the Ph.D research work of Satish Kumar. Unconventional Methods in Exploration for Petroleum and Natural Gas II. He is thankful to the Director, NGRI for permitting him to carry out Southern Methodist University Press, Dallas, TX, pp. 83e95. 428 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428

James, A.T., 1990. Correlation of reservoir gases using the carbon isotopic compo- technical report NGRI, 2004. Study of Mesozoic Sediments for Hydrocarbon sitions of wet gas components. American Association of Petroleum Geologist Exploration (restricted). Bulletin 74, 1441e1458. Pixler, B.O., 1969. Formation evaluation by analysis of hydrocarbon ratios. Journal of Jones, V.T., Drozd, R.J., 1983. Predictions of oil and gas potential by near-surface Petroleum Technology 21, 665e670. geochemistry. American Association of Petroleum Geologists Bulletin 67, Powar, K.B., 1993. Geomorphological evolution of Konkan coastal belt and adjoining 932e952. Sahyadri uplands with reference to Quaternary uplift. Current Science 64, Kaila, K.L., 1988. Mapping the thickness of Deccan trap flows in India from DSS 793e796. studies and inferences about a hidden Mesozoic basin in the Narmada e Tapti Qureshy, M.N., 1964. Gravity anomalies as related to regional tectonics of peninsular region. In: Subbarao, K.V. (Ed.), Deccan Flood Basal, 10. Geological Society of India. In: Report on 22nd International Geological Congress, New Delhi, India Memoir, pp. 91e116. pp. 490e506. Kaila, K.L., Rao, I.B.P., Rao, P.K., Rao, N.M., Krishna, V.G., Sridhar, A.R., 1989. DSS Ravi Shanker, 1987. Status of Geothermal Exploration in Maharashtra and Madhya studies over Deccan traps along the Thuadara-Sendhwa-Sindad profile across Pradesh. Geological Survey of India, Records, 115, pp. 7e29. Narmada-Son lineament, India. In: Mereu, R.F., Mueller, S., Fountain, D.M. (Eds.), Ravi Shanker, 1991. Thermal and crustal structure of “SONATA. A zone of mid Properties and Processes of Earth’s Lower Crust, 51. American Geophysical continental rifting in Indian shield. Journal of the Geological Society of India 37, Union Geophysical Monograph, pp. 127e141. 211e220. Kaila, K.L., Krishna, V.G., 1992. Deep seismic sounding studies in India and major Ravi shanker, 1987b. Neotectonic activity along the Tapi e Satpura lineament in discoveries. Current Science 62, 117e154. central India. Indian Minerals 41, 19e30. Keller, G., Adatte, T., Bajpai, S., Mohabey, D.M., Widdowson, M., Khosla, A., Rohrman, M., 2007. Prospectivity of volcanic basins: trap delineation and acreage Sharma, R., Gertsch, B., Fleitmann, D., Sahni, A., Khosla, S.C., 2009. KeT transi- de-risking. Association of Petroleum Geologists Bulletin 91, 915e939. tion in Deccan Traps of central India marks major marine Seaway across India. Saunders, D.F., Burson, K.R., Thompson, C.K., 1991. Observed relation of soil mag- Earth and Planetary Science Letters 282, 10e23. netic e susceptibility and soil-gas hydrocarbon analyses to subsurface hydro- Klusman, R.W., 1993. Soil Gas and Related Methods for Natural Resource Explora- carbon accumulation. Association of Petroleum Geologists Bulletin 75, tion. Wiley and Sons Publishers, New York, 483pp. 389e408. Laughrey, C.D., Baldassare, F.J., 1998. Geochemistry and origin of some natural Schutter, S.R., 2003. Occurrences of hydrocarbons in and around igneous rocks. In: gases in the Plateau province of the central Appalachian basin, Pennsylvania Petford, N., Mccaffrey, K.J.W. (Eds.), Hydrocarbons in Crystalline Rocks. and Ohio. American Association of Petroleum Geologists Bulletin 82, Geological Society of London, Special Publications 214, pp. 35e68. 317e335. Severne, B.C., Langford, G.D., Fullagar, P.K., 1991. Surface geochemical exploration in Link, W.K., 1952. Significance of oil and gas seeps in world oil exploration. Associ- Australia: case histories from the Eromanga and Canning Basins. APGE Bulleting ation of Petroleum Geologists Bulletin 36, 1505e1540. 7, 88e115. Mishra, D.C., 1977. Possible extension of the Narmada-Son lineament towards Singh, A.P., Meissner, R., 1995. Crustal configuration of the NarmadaeTapti region Murray ridge (Arabian Sea) and the eastern syn-taxial bend of the Himalayas. (India) from gravity studies. Journal of Geodynamics 20, 111e127. Earth and Planetary Science Letter 36, 301e308. Tedesco, Steven A., 1995. Surface Geochemistry in Petroleum Exploration. Chapman Murty, A.S.N., Rejendra Prasad, B., Koteswara Rao, P., Raju, S., Sateesh, T., 2010. and Hall Publisher, pp. 159e191. Delineation of subtrappean Mesozoic sediments in Deccan Syneclise, India, Verma, R.K., Banerjee, P., 1992. Nature of continental crust along the NarmadaeSon using travel-time inversion of seismic refraction and wide-angle reflection data. lineament inferred from gravity and deep seismic sounding data. Tectonophy- Pure and Applied Geophysics 167, 233e251. http://dx.doi.org/10.1007/s00024- sics 202, 375e397. 010-0050-z. West, W.D., 1962. The line of Narmada-Son valley. Current Science 31, 143e144.