Geo-microbial and light gaseous hydrocarbon anomalies in the near surface soils of Deccan Syneclise Basin, India: Implications to hydrocarbon resource potential

Abstract

Deccan Syneclise Basin is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. Surface geochemical prospecting method was applied in Deccan Syneclise for evaluation of hydrocarbon resource potential of the basin. The gas chromatographic analyses of adsorbed soil gasses showed the presence of C1 to C4 hydrocarbons. The concentrations of adsorbed soil gasses ranged for methane (C1)=1 to 483ppb, ethane (C2)=1 to 83ppb, propane (C3)=1 to 38ppb, butane (nC4)=1 to 60ppb and ΣC2+=1 to 129ppb, respectively. The scatter plots between C1 and C4 components depict linear trend indicating that all the gasses are derived from the same source.The carbon isotope composition of adsorbed methane was measured using Gas Chromatography–Combustion-Isotope Ratio Mass Spectrometry (GC–C-IRMS). The values for methane δ13C1 varied from −39.9 to −19.9‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gasses derived from thermogenic sources. The propane oxidizing bacteria (POB) ranged between 1.0×102 and 12.8×105cfu/g of soil. The adsorbed light gaseous hydrocarbons and propane oxidizing bacterial anomalies found in and around Aurangabad, Ahmednagar, Shirdi, Akola, Bhusawal and Indore areas are considered to be promising for future hydrocarbon research and exploration.