Figure and Table Captions 

Introduction 

The large tract beneath the Upper Cretaceous-Paleocene Deccan Trap in western India is called Deccan Syneclise (Figure 1). It is one amongst the 26 sedimentary basins of India and is grouped under category IV: i.e., potentially prospective basin. Geophysical studies have revealed hidden Mesozoic sedimentary basins under the Deccan Traps. Surface Geochemical Surveys have been carried out in one of the prospective zones to assess the hydrocarbon prospectivity of the basin. Geochemical prospecting of hydrocarbons identifies the surface or near-surface occurrences of hydrocarbons or their alteration products, which are due to macro/micro seepage of the subsurface hydrocarbon occurrences. The micro/macro seepage is an established phenomenon, and these occur because processes and mechanisms such as diffusion, effusion, and buoyancy allow hydrocarbons to escape from reservoirs and migrate to the surface where they may be retained in the sediments and soils or diffuse into atmosphere or water columns. Microseepage of hydrocarbons has led to the discovery of many important petroleum producing areas in the world.

Deccan Syneclise 

Deccan Syneclise is an intracratonic sedimentary basin covering an area of ~273 x 10³ sq. km. The basin is mostly covered by Deccan Traps, with the exposure of Bagh and Lameta beds in the adjoining areas. The Deccan trap thickness varies largely and is about 100 m in the northeastern part and >1500 m towards the west coast of India. Below the Deccan Traps in the Narmada-Tapti region a hidden Mesozoic basin has been mapped in the form of two grabens separated by a small horst. 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, 1989). Integrated geophysical studies carried out by National Geophysical Research Institute (NGRI), Hyderabad and Directorate General of Hydrocarbons (DGH), New Delhi, have revealed the presence of Mesozoic sediments with thickness of about 1000m to 2500 m in the central part of Narmada-Tapti region (DGH, Annual Report 2003-04). 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 form of horseshoe. The Moho configuration under the Deccan-Trap-covered area reveals the depression in the central part extending in an ENE-WSW direction, which almost coincides with the region of hidden Mesozoic basin (Kaila, 1989). The marine transgressions and regressions that occurred in west-central India before the Deccan volcanicity 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). The generalized stratigraphy of Deccan Syneclise is given in Table 1.

Soil Sampling and Analytical Procedure 

A total of 50 soil samples were collected in part of Deccan Syneclise at an interval of 5 km along existing roads. The sample location map of the area is given in Figure 2. Samples have been collected in the depth range of 1.2 – 3.5 m using manual augers. The soil cores collected were wrapped in aluminum foils and sealed in poly-metal packs.  

One gram of soil sample is reacted under vacuum with orthophosphoric acid to desorb the soil gases. The CO₂ released was trapped in KOH solution and the light gaseous hydrocarbons were collected by water displacement in a graduated tube fitted with rubber septa. The volume of desorbed gases is then recorded, and 500 μl of desorbed gas sample is injected into the Varian CP-3800 Gas Chromatograph fitted with Porapak ‘Q’ column, programmable temperature controller, and flame ionization detector. The GC was calibrated by using an external standard with known concentrations of methane, ethane, propane, i-butane, n-butane, i-pentane, and n-pentane. The quantitative estimate of light gaseous hydrocarbon constituents in each sample was made using peak area measurement as a basis, and the correction for moisture content was applied. The accuracy of measurement of C₁ to C₅ components is < 1 ng/g.

Results and Discussion 

The light gaseous hydrocarbon concentrations (CH₄, C₂H₆, C₃H₈, i-C₄H₁₀, n- C₄H₁₀, i-C₅H₁₂ and n-C₅H₁₂) in soil samples of Deccan Syneclise vary from 3 to 1187 (CH₄), 1 to 633 (C₂H₆), 1 to 504 (C₃H₈), 1 to 123 (i-C₄H₁₀) and 2 to 159(n-C₄H₁₀) in ppb, apart from i-C₅H₁₂ and n-C₅H₁₂ in few samples. The contour map of C₁ and ΣC₂₊ are plotted in Figures 3 and 4 and show that the samples south of Nandurbar are characterized by higher C₁ and ΣC₂₊ values. The crossplots between C₁-C₂, C₁-C₃, C₂-C₃ and C₁-ΣC₂₊, show linear correlation (r >0.8), which indicates that the light gaseous hydrocarbon may have migrated from the same source, and the effect of secondary alteration during their seepage toward the surface may be insignificant.  

Analyses of the gas samples for the measurement of δC¹³ in methane were carried out using Thermo Finnigan Delta Plus XP Isotope Ratio Mass Spectrometer. The δC¹³ values are reported as parts per thousand (‰) relative to the Peedee belemnite (PDB) standard (precision is ±0.3%). δC¹³ in methane lies in the range of –24 to –39.4‰ PDB suggesting a thermogenic origin.  

The presence of C₁-C₅ hydrocarbons in the adsorbed soil gases in the samples collected from part of Deccan Syneclise indicates that hydrocarbon generation has taken place in the basin and gases are derived from thermogenic source (Klusman, 1993; Kumar et al., 2004; Schumacher and Abrams, 1996). The geochemical studies suggest that this part of Deccan Syneclise may prove to be a warm area for future hydrocarbon exploration and exploitation.

Conclusions 

Evidence of generation of hydrocarbons derived from possible thermogenic source beneath the Deccan Traps may open new vistas for commercial discovery of oil/gas in the Mesozoic of India.

Acknowledgements 

The authors thank the Director, National Geophysical Research Institute, India, for granting permission to publish this work. The first author gratefully acknowledges Council of Scientific and Industrial Research, India, for the award of Senior Research Fellowship and Hardy Exploration and Production (India) Inc. for constant encouragement.

References 

Biswas, S.K., and Deshpande, S.V., 1983, Geology and hydrocarbon prospects of Kutch, Saurashtra and Narmada basins, in Bhandari, L.L., et al. (eds.), Petroliferous Basins of India, p. 111-126.

Bois, C., Bouche, P., and Pelet, R. 1982, Global geologic history and distribution of hydrocarbon reserves: AAPG Bulletin, v. 66, p. 1248-1270.

Gombos, Andrew M., Jr., Powell, William G., and Norton, Ian O., 1995, The tectonic evolution of western India and its impact on hydrocarbon occurrences: an overview, Sedimentary Geology, v. 96, p. 119-129.

Kaila, K.L., 1989, Mapping the thickness of Deccan Trap flows in India from DSS studies and inferences about a hidden Mesozoic Basin in the Narmada – Tapti region, in Subbarao, K.V. (ed.) Deccan Flood Basalts: Geological Society of India Memoir 10.

Klusman, R.W., 1993, Soil gas and related methods for natural resource exploration: John Wiley & Sons, England, 473 p.

Kumar, B., Patil, D.J., Kalpana, G., and Vishnu Vardhan, C. 2004, Geochemical prospecting of hydrocarbons in frontier basins of India: Search and Discovery Article #10073 (2004): Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Dallas, Texas, April 18-21, 2004.

Oil Infraline- Oil and Gas Exploration and Production in India, A reference book, 2006, 503 p.

Schumacher, D., and Abrams, M.A., (eds.), 1996, Hydrocarbon migration and its near surface expression: AAPG Memoir 66, 446 p.