Geoscience Frontiers 5 (2014) 419e428

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China University of Geosciences (Beijing)

Geoscience Frontiers

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Research paper

Surface geochemical data evaluation and integration with geophysicalobservations for hydrocarbon prospecting, Tapti graben, DeccanSyneclise, India

T. Satish Kumar a,*, A.M. Dayal b, V. Sudarshan c

aCentre of Excellence for Energy Studies, Oil India Ltd, Guwahati 781022, IndiabNational Geophysical Research Institute (CSIR), Hyderabad 500006, IndiacApplied Geochemistry Department, Osmania University, Hyderabad 500006, India

a r t i c l e i n f o

Article history:Received 28 December 2012Received in revised form27 July 2013Accepted 5 August 2013Available online 27 August 2013

Keywords:Adsorbed gasMicroseepageLight hydrocarbonStable isotopeGeophysicalDeccan Syneclise

* Corresponding author.E-mail address: satishkumar.thadoju@gmail.com (T. Sa

Peer-review under responsibility of China University

Production and hosting by Els

1674-9871/$ e see front matter � 2013, China Univerhttp://dx.doi.org/10.1016/j.gsf.2013.08.003

a b s t r a c t

The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significanthydrocarbon discoveries, particularly for Mesozoic sequences, have not been established throughconventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study,near-surface geochemical data are used to understand the petroleum system and also investigate typeof source for hydrocarbons generation of the study area. Soil samples were collected from favorableareas identified by integrated geophysical studies. The compositional and isotopic signatures ofadsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of pe-troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for lighthydrocarbons (C1eC4) and their carbon isotopes from the western part of Tapti graben, DeccanSyneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously highconcentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane(C1 ¼ 518 ppb) and ethane plus higher hydrocarbons (SC2þ ¼ 977 ppb) were observed. Statisticalanalysis shows that samples from 13% of the samples contain anomalously high concentrations oflight hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gasanomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. Thecarbon 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 thesurface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentra-tions were associated with dykes, lineaments and presented on thinner basaltic cover in the studyarea, which probably acts as channel for the micro-seepage of hydrocarbons.

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tish Kumar).

of Geosciences (Beijing)

evier

sity of Geosciences (Beijing) and P

1. Introduction

Visible seepage of oil or hydrocarbon gases in sedimentary ba-sins has played a major role in the discovery of many oil fieldsthroughout the world (Link, 1952). In 1930s, the concept of aprominent macro seep environment in association with commer-cial subsurface hydrocarbons was extended to the micro seep levelby measuring minute surface gas concentrations or secondary soilalteration effects induced by gas seepage. Microseepage of hydro-carbon gases into near-surface environments is often interpreted asa direct indication of the presence of deeper hydrocarbons. Jonesand Drozd (1983) and Severne et al. (1991) provided examples forsuch successful applications of soil gas surveys in exploration. The

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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.SatishKum

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Figure 2. (a) Trap thickness map in Deccan Syneclise with study area (modified after Harin(after DGH, 2006). (c) The Narmada-Tapti seaway of Deccan volcanic province with study a

Table 1Generalized stratigraphy of the Deccan Syneclise Basin (After Deshpande, 1998).

Proterozoic Penganga beds Gadchiroli, RatnagiriLimetsones, shales

Archean Sausar, Sakoli, Nagpur, BhandaraAmagaon, Ratnagiri, SindhudurgUnclassified gneisses

Age Formation/ Anticipatedrange Group thickness (km) Lithology Geographic distribution

Recent– Alluvium Nagpur, Bhandara , Pleistocene Laterite, sand Candrapur, Wardha

Jalgaon,Kolhapur Satara Thane, etc.

Early Paleocene Deccan Traps 1–2 Basalts Most of the state from–Cretaceous west of Nagpur to

Arabian sea coast--------------------------------------------Unconformity----------------------------------------Late Lameta, bagh Arenaceous Nagpur, CandrapurCretacoous beds limestone, Dhule, Gadchiroli

2–3.5 Sandstone YavatmalMiddle Upper shaleTriassic Gondwana

------------------------------------------ Unconformity --------------------------------------Triassic– Nagpur, ChandrapurCarboniferous Lower Yavatmal

Gondwana

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 421

identification of seepage can add valuable information to theexploration process when the diagnosis of gas micro seep envi-ronments can be distinguished and separated from backgrounddata. Under certain conditions, active seepage may be recognizedthrough measurement of concentration and composition of hy-drocarbon gases in soils (Jones and Drozd, 1983; Klusman, 1993).

This paper describes the application of surface geochemicalstudies carried out in sub-trappean of Mesozoic age in Deccan Trapcover, western part of Tapti graben for the prospecting of hydro-carbons. The region has received wide attention for its hydrocarbonprospects because of the thick GondwanaeMesozoic sedimenta-tion of 1e2.75 km buried under the lava flow of Deccan Traps (Kailaet al., 1989; DGH, 2006). Hence an attempt has been made to findthe presence of hydrocarbons in the reported Mesozoic sedimentareas. In the present work, the adsorbed soil gases and their carbonisotope signatures have been analyzed to determine the concen-tration of light hydrocarbons and to identify the seepage and sourceof hydrocarbons.

1.1. Geology of the study area

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

arayana et al., 2007). (b) Map showing Mesozoic sediment thickness of the study arearea (modified after Keller et al., 2009).

Table 2Gas chromatographic analysis of light gaseous hydrocarbons desorbed from soilsamples and summary of statistical values.

Sample Id C1(ppb)

C2

(ppb)C3

(ppb)εC4

(ppb)Sample Id C1

(ppb)C2(ppb)

C3

(ppb)εC4(ppb)

DSS/01/06 15.6 0 0 0 DSS/41/06 5.6 0 0 0DSS/02/06 141.4 28.9 13.7 0 DSS/42/06 44.8 8.1 4.7 0DSS/03/06 35.5 6.9 5.3 5.4 DSS/43/06 181.5 33.7 14.9 5.8DSS/04/06 15.8 1 0 0 DSS/44/06 13.6 0 0 0DSS/05/06 53.6 8 3.6 0 DSS/45/06 27.2 3.3 2.1 0DSS/06/06 4 0 0 0 DSS/46/06 14.3 2.2 1.8 0DSS/07/06 35.9 7.3 4.8 2.8 DSS/47/06 14.7 0 0 0DSS/08/06 26.3 6.3 6.1 7.8 DSS/48/06 4.3 0 0 0DSS/09/06 24.7 3.3 2.6 0 DSS/49/06 18.7 3.5 3.4 2DSS/10/06 24.5 3.7 3.1 0 DSS/50/06 10.6 1.3 0 0DSS/11/06 4.5 0 0 0 DSS/51/06 212.3 48.8 23.7 10.4DSS/12/06 17.1 2.5 1.5 0 DSS/52/06 8.2 0 0 0DSS/13/06 6.2 0 0 0 DSS/53/06 20.8 0 0 0DSS/14/06 69.8 12.7 7.4 0 DSS/54/06 17.2 2.5 0 0DSS/15/06 61.9 12.1 7.9 0 DSS/55/06 8.5 0 0 0DSS/16/06 9.1 0.8 0.7 0 DSS/56/06 35.3 6 2.8 0DSS/16(II)/06 4.3 0 0 0 DSS/57/06 53.3 6.1 0 0DSS/17/06 4.3 0 0 0 DSS/58/06 8 0 0 0DSS/18/06 72.7 14.1 6.6 0 DSS/59/06 7.7 0 0 0DSS/19/06 75.8 12.9 8.6 0 DSS/60/06 6.4 0 0 0DSS/20/06 25.8 3 1.2 0 DSS/61/06 12.6 0 0 0DSS/21/06 42.7 5.9 4.1 0 DSS/62/06 19.5 0 0 0DSS/22/06 39.3 5.9 2.2 0 DSS/63/06 41.5 4.9 3.9 0DSS/23/06 11.9 1.7 1.3 0 DSS/64/06 18.3 2.5 2.5 0DSS/24/06 42.7 7.3 5 0 DSS/65/06 8.9 0 0 0DSS/25/06 4.9 0 0 0 DSS/66/06 4.7 0 0 0DSS/26/06 9.8 1 0 0 DSS/67/06 2.6 0.3 0 0DSS/27/06 24.6 2.9 0 0 DSS/68/06 34.3 6.7 4.2 0DSS/28/06 19.7 0 0 0 DSS/69/06 12.2 0 0 0DSS/29/06 21.3 3 2.5 2.5 DSS/70/06 16.6 0 0 0DSS/30/06 18.4 2.8 2 0 DSS/71/06 16.6 1.3 0 0DSS/31/06 7.6 0.6 0 0 DSS/72/06 7.4 0.6 0 0DSS/32/06 7 0 0 0 DSS/73/06 2.6 0 0 0DSS/33/06 14.2 1.9 1 0 DSS/74/06 2.8 0 0 0DSS/34/06 4.9 0 0 0DSS/35/06 3.7 0 0 0 Mean 26.1 3.9 2.1 0.5DSS/36/06 20.3 1.8 1.1 0 Std.

deviation36.3 7.8 3.9 1.8

DSS/37/06 4.7 0 0 0 Range 209.6 48.8 23.7 10.4DSS/38/06 11 0 0 0 Minimum 2.6 0 0 0DSS/39/06 4.1 0 0 0 Maximum 212.3 48.8 23.7 10.4DSS/40/06 4 0 0 0

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428422

structures, magmatic crustal accretion, sedimentary basins andcomplex geophysical signatures (Fig.1). Major tectonic adjustmentsin various crustal blocks, in Narmada-Tapti graben occurred inPrecambrian/Gondwana times which must have been responsiblefor the formation of Vindhyan and Gondwana sedimentary basins.

Tapti Basin, an intracratonic half graben in western-central India(Guha, 1995) is considered to be Mesozoic marginal marine basin(Biswas, 1987). The generalized stratigraphy of Deccan Traps is givenin Table 1. Tapti Basin forms a linear tract spread over a length of350kmandanaveragewidthof 30kmcoveredbyalluviumofTertiaryto recent with isolated inliers of the Deccan Traps. The alluviumthickness from south to north, in general, extends to a depth ofw200 m to >400 m below mean sea level at places (Ravi Shanker,1987). The Deccan Trap thickness varies considerably from 100 m inthe northeastern part to more than 1500m towards thewest coast ofIndia (Fig. 2a). In the Narmada-Tapti region, a hidden Mesozoic sedi-mentary basin underlying the Deccan Traps has been reported in theform of two grabens separated by a small horst of Satpura hills. In thesouthern part a larger Tapti grabenwith sediment thickness of about2000 m is revealed, whereas in the northern part there is a smallerNarmada graben with sediment thickness of about 1000 m (Kaila,1988). Integrated geophysical studies identified the major sedimen-tary basin in and around Sendhwa, Shirpur, Dhule and Sakri havingvery large thickness of sub-trappeanMesozoic sediments of the orderof 750e2250 m (Fig. 2b) (DGH, 2006). The basement topography isquite undulating in the region and it is deepest in Shirpur andSendhwa region. The sediments of this Mesozoic basin were depos-ited in a larger Mesozoic sea, which extended from Narmada-Taptiregion through Saurashtra, Kutch, up to Sind and Salt Range in theshape of a horseshoe. TheMoho configuration under theDeccan Trapcovered area reveals a depression in the central part extending in anENEeWSW direction, which almost coincides with the region ofhiddenMesozoic basin (Kaila,1988). Both the Narmada-Tapti and thewest coast tectonic belts are characterized by positive gravity anom-alies, high gravity gradients, high heat flow 70e100 mW/M2 andseismicactivity (Arora andReddy,1991) and the tectonichistoryof thebasin indicate the thermal subsidence and burial (Schutter, 2003;Rohrman, 2007) to be significant enough to cause the maturation oforganic rich sediments, which might be favorable conditions for hy-drocarbon generation. Keller et al. (2009) has been reported that, themarine incursionaccompaniedbyplanktic foraminiferaandbrackish-marine ostracods indicates a seaway existed into central India duringthe Maastrichtian to early Paleocene (Fig. 2c). This seaway may havefollowedtheNarmadaandTapti rift zoneswherea seaway isknowntohave existed during the late Cenomanian to Turonian. Therefore, themarine transgression and regression that occurred in western andcentral India before the Deccan volcanism, might have favored thedeposition of organic-rich source rocks. Further, the Deccan Trapvolcanism during late Cretaceousmight have generated the requisitethermal conditions and acted as a catalyst in aMesozoic hydrocarbongeneration process (Biswas and Deshpande, 1983).

1.2. Geophysical evidences

The area has been the subject of extensive geological andgeophysical investigations for many years (West, 1962; Qureshy,1964; Choubey, 1971; Mishra, 1977; Crawford, 1978; Biswas, 1982,1987; Ravi Shanker, 1987a,b; Ravi Shanker, 1991; Kaila andKrishna, 1992; Verma and Banerjee, 1992; Powar, 1993; Singh andMeissner, 1995; Bhattacharji et al., 1996). Integrated geophysicalexploration studies were carried out using seismic refraction,magnetotelluric, deep resistivity sounding and gravity methods bythe CSIR-National Geophysical Research Institute (NGRI) and suc-cessfully delineated subtrapean Mesozoic sediments in Saurashtraand Kutch basins (NGRI, 2004).

Deep seismic studies (DSS) along the Thuadara-Sendhwa-Sindad profile wherein the first arrival refraction data analyzedbased on 2D tracing technique revealed a graben between Narmadaand Tapti. This graben contains 1000e2800 m thick low velocity(3.2e3.6 km s�1) sediments under a thick cover of Deccan traps.And this study also revealed that the graben is bounded by twofaults, one south of Narmada i.e., Barwani-Sukta and the othernorth of Tapti river. The graben seems to be extending towardssouth-east. The low velocity Mesozoic sedimentary rocks with a P-wave velocity of 3.5 km s�1 and thickness ranging from about 0.70to 1.6 km and 0.55e1.1 km lies along the east-west and north-southprofiles, respectively (Murty et al., 2010).

The first layer with a P-wave velocity of 5.15e5.25 km s�1 andthickness varying from 0.7 to 1.5 km represents the Deccan Trapformation along the NarayanpureNandurbar profile. The Trap layervelocity ranges from 4.5 to 5.20 km s�1 and the thickness variesfrom 0.95 to 1.5 km along the KothareSakri profile. The secondlayer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s�1 and thickness ranging from about 0.70to 1.6 km and 0.55e1.1 km along the EeW and NeS profiles,respectively. Presence of a low-velocity zone (LVZ) below the vol-canic rocks in the study area is inferred from the travel-time ‘skip’and amplitude decay of the first arrival refraction data togetherwith the prominent wide-angle reflection phase immediately after

Table 3Pearson correlation coefficients between light hydrocarbon (C1eC4) gases.

C1 C2 C3 εC4

C1 1C2 0.99 1C3 0.95 0.98 1εC4 0.61 0.66 0.69 1

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 423

the first arrivals from the Deccan Traps formation. The basementwith a P-wave velocity of 5.8e6.05 km s�1 lies at a depth rangingfrom 1.5 to 2.45 km along the profiles. The velocity models of theprofiles are similar to each other at the intersection point. The re-sults indicate the existence of a Mesozoic basin in the Narmada-Tapti region of the Deccan Syneclise.

2. Soil sampling and analytical methods

2.1. Soil sampling

Study area is located in the western part of Tapti Basin (Fig. 1),Deccan Syneclise, India, between 74�140e74�590E and 20�530e21�330N. An adsorbed soil gas survey was carried out and a total of75 soil samples were collected from a depth of w2.5 m at an in-terval of 2e3 km along existing roads. Cores were collected using ahollow metal pipe by manual hammering to the required depth.The cores collected were wrapped in aluminum foil and sealed inpoly-metal packs.

2.2. Analysis of soil gases for light hydrocarbons

The light gaseous hydrocarbons were extracted from the soilsamples using a ‘Gas extraction system’ (Horvitz, 1981). The lightgaseous hydrocarbons were desorbed from soil samples by treatingw1 g of wet sieved 63 mm fraction of sample with orthophosphoric

Figure 3. (a) Histogram for methane, ethane, propane and butane concentrations. (b) Pix

acid in vacuum. The desorbed light gaseous hydrocarbons werecollected by water displacement in a graduated tube fitted withrubber septa. The volume of desorbed gas was recorded and 500 mLof the gas was injected into a Varian CP-3380 Gas Chromatograph(GC) equipped with a 1.8 m � 1/800 � 2.00 mm Porapak Q Columnand Flame Ionization Detector. The GC was calibrated using anexternal standard with known concentrations of methane (C1),ethane (C2), propane (C3), i-butane (iC4) and n-butane (nC4). Themoisture content of samples was determined and the gas concen-trations are reported in ppb on dry weight basis. The accuracy ofmeasurement of C1 to C4 component is �1 ppb of gas weight.

2.3. Analysis for carbon isotopes of light hydrocarbons

Carbon isotopic composition of light hydrocarbons (d13C1, d13C2,and d13C3) in soil samples is determined using a GC-C-IRMS, whichcomprises an Agilent 6890 GC coupled to a Finnigan-Delta Plus XPIsotope Ratio Mass Spectrometer via a GC combustion III interface atCSIR-NGRI. OnemL of desorbed gas is injected into the GC in splitlessmode with helium as carrier gas at fixed over temperature of 28 �C.The light hydrocarbon gases eluting from the GC column enter thecombustion reactor maintained at 960 �C where it gets converted toCO2andwater.ANafionmembrane tube isused to removewater, priorto passing the CO2 into the mass spectrometer. Reference standardsare intermixed with samples to monitor instrumental performance.The carbon isotope ratio in the samplewas determined by comparingisotoperatioswith thatof a standard,NISTRM8560 (IAEANGS2)usingISODAT software. The d13C is calculated using the following equation:

d13C ¼ {[(13C/12C)/(13C/12C)] � 1} � 1000.

The carbon isotopic composition is reported in permil (&)relative to the Pee Dee Belemnite (PDB). The precision of the iso-topic analysis is �0.5&.

ler plot for discriminating oil, oil and gas and gas zones using C1/C2 and C1/C3 ratios.

Table 4Approximate empirical range of micro seep compositional ratio for gas, gascondensate, and oil (Jones and Drozd, 1983).

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

Dry gas 100e20 25e50 2e20Gas condensate 20e10 16.5e25 20e60Oil 10e4 10e16.5 60e500

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428424

3. Results and discussion

3.1. Concentrations of light hydrocarbon gases

The concentrations of each of the five organic constituents (C1,C2, C3, iC4, and nC4 alkanes) are expressed in parts per billion (ppb)in all the 75 soil samples. The overall hydrocarbon gas compositionpresence 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 aresummarized in Table 2. Pearson correlation coefficients are used forcomparing the linear relationships between geochemical variables,two at a time (Table 3). Correlation coefficients obtained for C1 vs.C2, C1 vs. C3 and C2 vs. C3, show very high correlation (r �0.9)with each other. This indicates that these hydrocarbons weregenerated from a thermogenic source (genetically related), andbelong to a migration fairway system associated with one sub-surface source (Gevirtz et al., 1983). The geochemical data popu-lation and distribution pattern were analyzed using frequency

Figure 4. Cross plots between methane and ethane, and e

distribution histogram for all the individual hydrocarbons. Thehistograms of C1 to C3 (Fig. 3a) clearly show normal distributionwith a positive skewness which is commonly associated with sur-face geochemical data in petroleum exploration (Tedesco, 1995).

The geochemical signature (gas/condensate/oil) is determinedusing ratios of hydrocarbon constituents detected in the soil gassamples. The compositional signature displayed by C1/C2, C1/C3ratios, as defined by Pixler (1969) is shown in Fig. 3b, which hasbeen used to quantify and qualify compositional signatures in mudlogging applications and has proved to be useful for the

thane and propane indicating the zones for soil gas.

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).

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 425

interpretation of soil gas source. It can be seen that almost all thesamples fall in the oil zone. Out of 75 samples analyzed for lighthydrocarbons desorbed from soil, C1 and C2 were observed in 42samples constituting 55% of the population. Within these 42 sam-ples, 86% of the samples plot in oil zone and 14% in oil and gas zone(Fig. 4a). Similarly, 32 samples show the presence of C2 and C3components out of which only 75% of the samples plot in the oilzone and 25% of the samples fall in the oil and gas zone (Fig. 4b).Approximate Empirical Range of micro seep compositional ratio forgas, gas condensate, and oil (Jones and Drozd, 1983) may be used toconstrain further wet vs. dry gas indications in soil gases (Table 4).The hydrocarbon gases with higher carbon number, i.e., C2 þ gases,including ethane, propane, iso- and n- butanes, are detected in 56%of the samples, which has been found as an effective indicator ofpetroleum deposits (Saunders et al., 1991). Using the yield ratioequation C1/C2 vs. C1/(C2 þ C3), it is observed that 54% of thesamples are of thermogenic gases (Fig. 5) (Bernard et al., 1977). The

Figure 6. (a) Isoconcentration distribution map of methane. (b

compositional signatures associated with these high (C1 and C2þ)concentration regions are in the N and S-E parts (Fig. 6a and b) ofthe study area. Accurate determination of background hydrocarbonconcentrations is paramount. Background hydrocarbon concen-trations are considered to be normal data from sources other thanvertical migration. Therefore, hydrocarbon data above the back-ground level can include some vertical migration data. The mean asexact background value, assume that values above the mean plusone standard deviation are anomalous. Background concentrationsfor methane, ethane and propane are 26.1 ppb, 3.9 ppb and 2.1 ppbrespectively, which are moderately close to the median values. TheC1 and C2þ anomalous maps were prepared based on meanþ1ststandard deviation. Fig. 7a and b shows that large magnitudeanomalies found in sub-trappeanMesozoic sediments i.e., northernregion of ShirpureNardane-Sindkheda-Shendvade and southernregion of Dhule e Khede, where traps are thinner. In addition,significant high concentrations of light hydrocarbons are found

) Isoconcentration distribution map of ethane plus (C2þ).

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

Table 5Compound-specific carbon isotopic compositions of methane, ethane and propanewith light hydrocarbon concentration of the study area.

Sample no. d13C1 (&) d13C2 (&) d13C3 (&) Wetness ratio%[C1/(C2 þ C3)]

DSS/02/06 �30.2 �20.3 �20.3 3.32DSS/18/06 �23.9 �19.8 �16.9 3.51DSS/19/06 �23.3 �18.1 �29.1 3.52DSS/21/06 �27.1 �25.7 e 4.3DSS/22/06 �26.9 �20.6 �25.5 4.8DSS/24/06 �25.9 �18.9 �32.1 3.46DSS/43/06 �29.2 �26.4 �27.1 3.73DSS/57/06 �22.6 �18 �26.5 8.73DSS/63/06 �26.4 �27.1 e 4.71

Figure 8. Compound-specific carbon isotopic compositions of the seven gas samples fromunaltered samples.

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428426

along the northern, southern faults and lineaments indicating thatthe structural features viz., faults and lineaments acting as conduitsfor these light hydrocarbons.

3.2. Carbon isotope enrichment in gases desorbed from soil samples

The carbon isotope composition for the hydrocarbon gases des-orbed from the soil samples are given in Table 5. The enrichmentpatterns for methane, ethane and propane in different samples areshown in Fig. 8. Theoretical considerations and empirical observa-tions suggest that individual gaseous hydrocarbons generated fromcracking processes at the same temperature and maturity shouldshow characteristic carbon isotopic compositions (Chung et al., 1988;

the study area. The DSS/2, DSS/18 and DSS/43 samples are displaying evidence for

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.

T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 427

Berner et al., 1990; James, 1990). Generally, carbon isotopes ofmethane, ethane and propane show d13C1 < d13C2 < d13C3 trend innatural gas. The present study reveals a deviation from this trend andthe presence of propane, which is isotopically lighter than methane,suggests that the soil gases have suffered partial oxidation leading toisotopically heavier methane (Fig. 8). Therefore samples might beresults from heterogeneity in the source organic matter, mixing ofgases from different sources, oxidation of thermogenic gas, or partialdiffusive leakage of the gas reservoir (Laughrey and Baldassare,1998). The Bernard diagram (Bernard et al., 1978) is commonlyused to classify natural gas origins. A modified Bernard diagramcharacterizing soil gases from the western part of Tapti Basin basedon the wetness [C1/(C2 þ C3)] vs. methane d13C is shown in Fig. 9,which falls in thermogenic source.

4. Conclusion

Integrated interpretation of soil gases and carbon isotopes ofmethane, ethane and propane has led to reliable assessment ofmicro-seepage and anomalous zones, which are related to sub-trappean Mesozoic sediments in the Deccan Traps. Light hydro-carbons generated in the Mesozoic sedimentary rocks, migratedthrough the Deccan Traps and become adsorbed in the near sub-surface soils. From the correlation, these hydrocarbons are geneti-cally related, and might have been generated from a thermogenicsource because of the presence of C2 and C3 components. Thissuggests that all the hydrocarbon constituents in the micro-seepsare from the same source, without any prominent secondaryalteration during the path of migration to the surface and theirsubsequent adsorption on the soil. The compositional ratios andPixler plot indicate that most of the samples fall within the oil zone.Carbon isotope signatures suggest that these gases are derived fromoxidation of thermogenic gas of the gas reservoir. The present studyprovides clear evidences of significant concentrations of hydro-carbon, which is economically viable for exploration in the Meso-zoic sedimentary rocks of the western part of Tapti Graben.

Acknowledgments

This paper forms part of the Ph.D research work of Satish Kumar.He is thankful to the Director, NGRI for permitting him to carry out

Ph.D intheDepartmentofAppliedGeochemistry,OsmaniaUniversity,Hyderabad. Satish acknowledge CSIR for the Senior ResearchFellowship.

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