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Hydrocarbon Prospects in Sub-Trappean Mesozoic Deccan Syneclise, India: Evidence from Surface Geochemical Prospecting
By
C. Vishnu Vardhan1*, B. Kumar1, C.J. Kumanan2, Devleena Mani1, and D. J. Patil1
Search and Discovery Article #10143 (2008)
Posted February 10, 2008
1National
Geophysical
Research Institute, Hyderabad (
[email protected] )
2Center for Remote Sensing, Bharathidasan Universtiy, Tiruchirappalli
*Presently with Hardy Exploration and Production (India) Inc. Chennai ( [email protected] )
The
Mesozoic sediments contribute around 54% of the oil and 44% of the gas reserves
of the world (Bois et al. 1982). Indian Mesozoic basins occupy an area of about
400 x 103 sq. km and are characterized as frontier basins under
category II - IV. These basins are mostly overlain by the Deccan Traps of Late
Cretaceous age and are least explored. Deccan Syneclise is one of the largest
Mesozoic basins in India, covering an area of ~ 273 x 103 sq. km.
Geophysical
studies have inferred hidden sub-trappean Mesozoic sediments with
thickness up to 2.5 km. It is considered that requisite heat generation due to
Deccan Trap volcanism soon after the Cretaceous sedimentation may have acted as
a catalyst in hydrocarbon generation. Surface geochemical prospecting surveys
along with carbon isotopic studies have been used to assess hydrocarbon
generation potential of this part of the basin.
The area adjoining Nandurbar beneath
Narmada-Tapti region of Deccan Syneclise was selected for geochemical studies
where
geophysical
studies have shown considerable sediment thickness.
Surface
geochemical studies indicate the generation of light gaseous hydrocarbons, C1
and ΣC2+ in the range of 3 to 1187 ppb and 1 to 1449 ppb,
respectively. The carbon isotopic signatures of selected soil gas samples (δC13
CH4 in the range of –24 to –39.4‰ PDB) suggest thermogenic origin.
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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.
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
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.
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.
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.
The
authors thank the Director, National
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.
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