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Research paper
Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting, Tapti graben, Deccan Syneclise, India
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T. Satish Kumara,*, A.M. Dayalb, V. Sudarshanc
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
Article history:
Received 28 December 2012
Received in revised form
27 July 2013
Accepted 5 August 2013
Available online 27 August 2013
Keywords: Adsorbed gas Microseepage Light hydrocarbon Stable isotope Geophysical Deccan Syneclise
ABSTRACT
The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional 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 type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of petroleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons (C1—C4) and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane (C1 = 518 ppb) and ethane plus higher hydrocarbons (SC2+ = 977 ppb) were observed. Statistical 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&—32.1& 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.
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1. Introduction
* Corresponding author.
E-mail address: satishkumar.thadoju@gmail.com (T. Satish Kumar). Peer-review under responsibility of China University of Geosciences (Beijing)
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 prominent macro seep environment in association with commercial subsurface hydrocarbons was extended to the micro seep level by measuring minute surface gas concentrations or secondary soil 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 such 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).
identification of seepage can add valuable information to the exploration process when the diagnosis of gas micro seep environments can be distinguished and separated from background data. Under certain conditions, active seepage may be recognized through measurement of concentration and composition of hydrocarbon gases in soils (Jones and Drozd, 1983; Klusman, 1993).
This paper describes the application of surface geochemical studies carried out in sub-trappean of Mesozoic age in Deccan Trap cover, western part of Tapti graben for the prospecting of hydrocarbons. The region has received wide attention for its hydrocarbon prospects because of the thick Gondwana—Mesozoic sedimentation of 1—2.75 km buried under the lava flow of Deccan Traps (Kaila et al., 1989; DGH, 2006). Hence an attempt has been made to find the presence of hydrocarbons in the reported Mesozoic sediment areas. In the present work, the adsorbed soil gases and their carbon isotope signatures have been analyzed to determine the concentration of light hydrocarbons and to identify the seepage and source of hydrocarbons.
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
72°E 74° 76° 78°
Table 1
Generalized stratigraphy of the Deccan Syneclise Basin (After Deshpande, 1998).
Formation/ Group
Anticipated
thickness (km) Lithology Geographic distribution
Recent- Alluvium Pleistocene Laterite, sand
EarlyPaleocene DeccanTraps 1-2 -Cretaceous
Nagpur, Bhandara, Candrapur, Wardha
Kolhapur Satara Thane, etc. Most of the state from west of Nagpur to Arabian sea coast
Late Lameta, bagh
Cretacoous beds
Middle Triassic
Upper Gondwana
-Unconformity--
Arenaceous limestone, Sandstone shale
Nagpur, Candrapur Dhule, Gadchiroli Yavatmal
Triassic-Carboniferous Lower
Gondwana Proterozoic Penganga beds
Limetsones, shales Archean Sausar, Sakoli,
Amagaon,
Unclassified gneisses
Unconformity -
Nagpur, Chandrapur Yavatmal
Gadchiroli, Ratnagiri
Nagpur, Bhandara Ratnagiri, Sindhudurg
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).
structures, magmatic crustal accretion, sedimentary basins and complex geophysical signatures (Fig. 1). Major tectonic adjustments in various crustal blocks, in Narmada-Tapti graben occurred in Precambrian/Gondwana times which must have been responsible for 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 given in Table 1. Tapti Basin forms a linear tract spread over a length of 350 km and an average width of 30 km covered by alluvium of Tertiary to recent with isolated inliers of the Deccan Traps. The alluvium thickness from south to north, in general, extends to a depth of ~200 m to >400 m below mean sea level at places (Ravi Shanker,
1987). The Deccan Trap thickness varies considerably from 100 m in the northeastern part to more than 1500 m towards the west coast of India (Fig. 2a). In the Narmada-Tapti region, a hidden Mesozoic sedimentary basin underlying the Deccan Traps has been reported in the form of two grabens separated by a small horst of Satpura hills. In the southern part a larger Tapti graben with sediment thickness of about 2000 m is revealed, whereas in the northern part there is a smaller Narmada graben with sediment thickness of about 1000 m (Kaila,
1988). Integrated geophysical studies identified the major sedimentary basin in and around Sendhwa, Shirpur, Dhule and Sakri having very large thickness of sub-trappean Mesozoic sediments of the order of 750—2250 m (Fig. 2b) (DGH, 2006). The basement topography is quite undulating in the region and it is deepest in Shirpur and Sendhwa region. The sediments of this Mesozoic basin were deposited in a larger Mesozoic sea, which extended from Narmada-Tapti region through Saurashtra, Kutch, up to Sind and Salt Range in the shape of a horseshoe. The Moho configuration under the Deccan Trap covered area reveals a depression in the central part extending in an ENE—WSW direction, which almost coincides with the region of hidden Mesozoic basin (Kaila, 1988). Both the Narmada-Tapti and the west coast tectonic belts are characterized by positive gravity anomalies, high gravity gradients, high heat flow 70—100 mW/M2 and seismic activity (Arora and Reddy, 1991) and the tectonic history of the basin indicate the thermal subsidence and burial (Schutter, 2003; Rohrman, 2007) to be significant enough to cause the maturation of organic rich sediments, which might be favorable conditions for hydrocarbon generation. Keller et al. (2009) has been reported that, the marine incursion accompanied by planktic foraminifera and brackish-marine ostracods indicates a seaway existed into central India during the Maastrichtian to early Paleocene (Fig. 2c). This seaway may have followed the Narmada and Tapti rift zones where a seaway is known to have existed during the late Cenomanian to Turonian. Therefore, the marine transgression and regression that occurred in western and central India before the Deccan volcanism, might have favored the deposition of organic-rich source rocks. Further, the Deccan Trap volcanism during late Cretaceous might have generated the requisite thermal conditions and acted as a catalyst in a Mesozoic hydrocarbon generation process (Biswas and Deshpande, 1983).
1.2. Geophysical evidences
The area has been the subject of extensive geological and geophysical 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 and Krishna, 1992; Verma and Banerjee, 1992; Powar, 1993; Singh and Meissner, 1995; Bhattacharji et al., 1996). Integrated geophysical exploration studies were carried out using seismic refraction, magnetotelluric, deep resistivity sounding and gravity methods by the CSIR-National Geophysical Research Institute (NGRI) and successfully delineated subtrapean Mesozoic sediments in Saurashtra and Kutch basins (NGRI, 2004).
Deep seismic studies (DSS) along the Thuadara-Sendhwa-Sindad profile wherein the first arrival refraction data analyzed based on 2D tracing technique revealed a graben between Narmada and Tapti. This graben contains 1000—2800 m thick low velocity (3.2—3.6 km s_1) sediments under a thick cover of Deccan traps. And this study also revealed that the graben is bounded by two faults, one south of Narmada i.e., Barwani-Sukta and the other north of Tapti river. The graben seems to be extending towards south-east. The low velocity Mesozoic sedimentary rocks with a P-wave velocity of 3.5 km s_1 and thickness ranging from about 0.70 to 1.6 km and 0.55—1.1 km lies along the east-west and north-south profiles, respectively (Murty et al., 2010).
The first layer with a P-wave velocity of 5.15—5.25 km s_1 and thickness varying from 0.7 to 1.5 km represents the Deccan Trap formation along the Narayanpur—Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s_1 and the thickness varies from 0.95 to 1.5 km along the Kothar—Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s_1 and thickness ranging from about 0.70 to 1.6 km and 0.55—1.1 km along the E—W and N—S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time 'skip' and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after
Table 2
Gas chromatographic analysis of light gaseous hydrocarbons desorbed from soil samples and summary of statistical values.
Sample Id C1 C2 C3 eC4 Sample Id C1 C2 C3 eC4
(ppb) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb)
DSS/01/06 15.6 0 0 0 DSS/41/06 5.6 0 0 0
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
DSS/04/06 15.8 1 0 0 DSS/44/06 13.6 0 0 0
DSS/05/06 53.6 8 3.6 0 DSS/45/06 27.2 3.3 2.1 0
DSS/06/06 4 0 0 0 DSS/46/06 14.3 2.2 1.8 0
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
DSS/09/06 24.7 3.3 2.6 0 DSS/49/06 18.7 3.5 3.4 2
DSS/10/06 24.5 3.7 3.1 0 DSS/50/06 10.6 1.3 0 0
DSS/11/06 4.5 0 0 0 DSS/51/06 212.3 48.8 23.7 10.4
DSS/12/06 17.1 2.5 1.5 0 DSS/52/06 8.2 0 0 0
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
DSS/15/06 61.9 12.1 7.9 0 DSS/55/06 8.5 0 0 0
DSS/16/06 9.1 0.8 0.7 0 DSS/56/06 35.3 6 2.8 0
DSS/16(II)/06 4.3 0 0 0 DSS/57/06 53.3 6.1 0 0
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
DSS/19/06 75.8 12.9 8.6 0 DSS/60/06 6.4 0 0 0
DSS/20/06 25.8 3 1.2 0 DSS/61/06 12.6 0 0 0
DSS/21/06 42.7 5.9 4.1 0 DSS/62/06 19.5 0 0 0
DSS/22/06 39.3 5.9 2.2 0 DSS/63/06 41.5 4.9 3.9 0
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
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
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
DSS/30/06 18.4 2.8 2 0 DSS/71/06 16.6 1.3 0 0
DSS/31/06 7.6 0.6 0 0 DSS/72/06 7.4 0.6 0 0
DSS/32/06 7 0 0 0 DSS/73/06 2.6 0 0 0
DSS/33/06 14.2 1.9 1 0 DSS/74/06 2.8 0 0 0
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
DSS/36/06 20.3 1.8 1.1 0 Std. 36.3 7.8 3.9 1.8
deviation
DSS/37/06 4.7 0 0 0 Range 209.6 48.8 23.7 10.4
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
DSS/40/06 4 0 0 0
Table 3
Pearson correlation coefficients between light hydrocarbon (C1-C4) gases.
C1 C2 C3 eC4
C2 0.99 1
C3 0.95 0.98 1
8C4 0.61 0.66 0.69 1
the first arrivals from the Deccan Traps formation. The basement with a P-wave velocity of 5.8—6.05 km s—1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results 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°14'—74°59'E and 20°53'— 21°33'N. An adsorbed soil gas survey was carried out and a total of 75 soil samples were collected from a depth of ~2.5 m at an interval of 2—3 km along existing roads. Cores were collected using a hollow metal pipe by manual hammering to the required depth. The cores collected were wrapped in aluminum foil and sealed in poly-metal packs.
2.2. Analysis of soil gases for light hydrocarbons
The light gaseous hydrocarbons were extracted from the soil samples using a 'Gas extraction system' (Horvitz, 1981). The light gaseous hydrocarbons were desorbed from soil samples by treating ~ 1 g of wet sieved 63 mm fraction of sample with orthophosphoric
acid in vacuum. The desorbed light gaseous hydrocarbons were collected by water displacement in a graduated tube fitted with rubber septa. The volume of desorbed gas was recorded and 500 pL of the gas was injected into a Varian CP-3380 Gas Chromatograph (GC) equipped with a 1.8 m x 1/8" x 2.00 mm Porapak Q Column and Flame Ionization Detector. The GC was calibrated using an external standard with known concentrations of methane (C1), ethane (C2), propane (C3), i-butane (iC4) and n-butane (nC4). The moisture content of samples was determined and the gas concentrations are reported in ppb on dry weight basis. The accuracy of measurement 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 (513C1, d13C2, and 513C3) in soil samples is determined using a GC-C-IRMS, which comprises an Agilent 6890 GC coupled to a Finnigan-Delta Plus XP 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 mode with helium as carrier gas at fixed over temperature of 28 °C. The light hydrocarbon gases eluting from the GC column enter the combustion reactor maintained at 960 °C where it gets converted to CO2 and water. A Nafion membrane tube is used to remove water, prior to passing the CO2 into the mass spectrometer. Reference standards are intermixed with samples to monitor instrumental performance. The carbon isotope ratio in the sample was determined by comparing isotope ratios with that of a standard, NIST RM 8560 (IAEA NGS2) using ISODAT software. The 513C is calculated using the following equation:
<513C = {[(13C/12C)/(13C/12C)] — 1} x 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&.
Figure 3. (a) Histogram for methane, ethane, propane and butane concentrations. (b) Pixler plot for discriminating oil, oil and gas and gas zones using Ci/C2 and C1/C3 ratios.
3. Results and discussion
3.1. Concentrations of light hydrocarbon gases
The concentrations of each of the five organic constituents (Ci, C2, C3, iC4, and nC4 alkanes) are expressed in parts per billion (ppb) in all the 75 soil samples. The overall hydrocarbon gas composition 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 summarized in Table 2. Pearson correlation coefficients are used for comparing 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 were generated from a thermogenic source (genetically related), and belong to a migration fairway system associated with one subsurface source (Gevirtz et al., 1983). The geochemical data population and distribution pattern were analyzed using frequency
Table 4
Approximate empirical range of micro seep compositional ratio for gas, gas condensate, and oil (Jones and Drozd, 1983).
Hydrocarbon composition C1/C2 C2/C3 x 10 C3/C1 x 1000
Dry gas 100—20 25—50 2—20
Gas condensate 20—10 16.5—25 20—60
Oil 10—4 10—16.5 60—500
distribution histogram for all the individual hydrocarbons. The histograms of Ci to C3 (Fig. 3a) clearly show normal distribution with a positive skewness which is commonly associated with surface geochemical data in petroleum exploration (Tedesco, 1995).
The geochemical signature (gas/condensate/oil) is determined using ratios of hydrocarbon constituents detected in the soil gas samples. The compositional signature displayed by C1/C2, C1/C3 ratios, as defined by Pixler (1969) is shown in Fig. 3b, which has been used to quantify and qualify compositional signatures in mud 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.
Biogenic gas
Thermogenic gas ♦ » * ♦ ♦ ♦♦ ♦ «
0 5 10 15 20
Cl/(C2+C3)
Figure 5. Yield ratio between C1/C2 vs. C1/(C2 + 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 samples fall in the oil zone. Out of 75 samples analyzed for light hydrocarbons desorbed from soil, C1 and C2 were observed in 42 samples constituting 55% of the population. Within these 42 samples, 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 C3 components out of which only 75% of the samples plot in the oil zone and 25% of the samples fall in the oil and gas zone (Fig. 4b). Approximate Empirical Range of micro seep compositional ratio for gas, gas condensate, and oil (Jones and Drozd, 1983) may be used to constrain further wet vs. dry gas indications in soil gases (Table 4). The hydrocarbon gases with higher carbon number, i.e., C2 + gases, including ethane, propane, iso- and n- butanes, are detected in 56% of the samples, which has been found as an effective indicator of petroleum deposits (Saunders et al., 1991). Using the yield ratio equation C1/C2 vs. C1/(C2 + C3), it is observed that 54% of the samples are of thermogenic gases (Fig. 5) (Bernard et al., 1977). The
compositional signatures associated with these high (C1 and C2+) concentration regions are in the N and S-E parts (Fig. 6a and b) of the study area. Accurate determination of background hydrocarbon concentrations is paramount. Background hydrocarbon concentrations are considered to be normal data from sources other than vertical migration. Therefore, hydrocarbon data above the background level can include some vertical migration data. The mean as exact background value, assume that values above the mean plus one standard deviation are anomalous. Background concentrations for methane, ethane and propane are 26.1 ppb, 3.9 ppb and 2.1 ppb respectively, which are moderately close to the median values. The C1 and C2+ anomalous maps were prepared based on mean+1st standard deviation. Fig. 7a and b shows that large magnitude anomalies found in sub-trappean Mesozoic sediments i.e., northern region of Shirpur—Nardane-Sindkheda-Shendvade and southern region of Dhule — Khede, where traps are thinner. In addition, significant high concentrations of light hydrocarbons are found
Figure 6. (a) Isoconcentration distribution map of methane. (b) Isoconcentration distribution map of ethane plus (C2+).
74.5 74.6
Figure 7. (a) Anomaly distribution map of methane. (b) Anomaly distribution map of ethane plus (C2+).
Table 5
Compound-specific carbon isotopic compositions of methane, ethane and propane with light hydrocarbon concentration of the study area.
Sample no.
d C1 (&)
d C2 (&) d C3 (&)
Wetness ratio% [C1/(C2 + C3)]
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
DSS/21/06 -27.1 -25.7 — 4.3
DSS/22/06 -26.9 -20.6 -25.5 4.8
DSS/24/06 -25.9 -18.9 -32.1 3.46
DSS/43/06 -29.2 -26.4 -27.1 3.73
DSS/57/06 -22.6 -18 -26.5 8.73
DSS/63/06 -26.4 -27.1 — 4.71
along the northern, southern faults and lineaments indicating that the structural features viz., faults and lineaments acting as conduits for 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 enrichment patterns for methane, ethane and propane in different samples are shown in Fig. 8. Theoretical considerations and empirical observations suggest that individual gaseous hydrocarbons generated from 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.
Figure 9. Methane carbon isotope ratio compared to wetness [(C1/C2 + C3)] (Bernard et al., 1978) and indicating possible source for the light hydrocarbons.
Berner et al., 1990; James, 1990). Generally, carbon isotopes of methane, ethane and propane show 513C1 < d13C2 < 513C3 trend in natural gas. The present study reveals a deviation from this trend and the presence of propane, which is isotopically lighter than methane, suggests that the soil gases have suffered partial oxidation leading to isotopically heavier methane (Fig. 8). Therefore samples might be results from heterogeneity in the source organic matter, mixing of gases from different sources, oxidation of thermogenic gas, or partial diffusive leakage of the gas reservoir (Laughrey and Baldassare, 1998). The Bernard diagram (Bernard et al., 1978) is commonly used to classify natural gas origins. A modified Bernard diagram characterizing soil gases from the western part of Tapti Basin based on the wetness [C1/(C2 + C3)] vs. methane S13C is shown in Fig. 9, which falls in thermogenic source.
4. Conclusion
Integrated interpretation of soil gases and carbon isotopes of methane, ethane and propane has led to reliable assessment of micro-seepage and anomalous zones, which are related to sub-trappean Mesozoic sediments in the Deccan Traps. Light hydrocarbons generated in the Mesozoic sedimentary rocks, migrated through the Deccan Traps and become adsorbed in the near subsurface soils. From the correlation, these hydrocarbons are genetically related, and might have been generated from a thermogenic source because of the presence of C2 and C3 components. This suggests that all the hydrocarbon constituents in the micro-seeps are from the same source, without any prominent secondary alteration during the path of migration to the surface and their subsequent adsorption on the soil. The compositional ratios and Pixler plot indicate that most of the samples fall within the oil zone. Carbon isotope signatures suggest that these gases are derived from oxidation of thermogenic gas of the gas reservoir. The present study provides clear evidences of significant concentrations of hydrocarbon, 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 in the Department of Applied Geochemistry, Osmania University, Hyderabad. Satish acknowledge CSIR for the Senior Research Fellowship.
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