Geo-microbiological studies in conjunction with different geo-scientific studies for the evaluation of hydrocarbon prospects in Proterozoic Vindhyan Basin, India

The Proterozoic Vindhyan Basin is considered to be prospective for hydrocarbons and is grouped under category III sedimentary basins of India. The major part of the study area is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets, surface geochemical prospecting based on microseepage of hydrocarbons from subsurface accumulations, which could be advantageous in such areas. Geo-microbiological study was carried out in conjunction with different geo-scientific studies to characterize the seeping natural gases and to evaluate the hydrocarbon prospects of the study area. Total 70 soil samples were collected in reconnaissance pattern, and the study revealed the presence of high bacterial counts for methane (3.7×105 cfu/g), ethane (3.2×105 cfu/g), propane (3.0×105 cfu/g) and butane oxidizing bacteria (2.3×105 cfu/g), which signify the seepage of hydrocarbons from the subsurface. The compositional characteristics of the hydrocarbon gases desorbed from soil samples of study area indicate the presence of light gaseous hydrocarbons methane through butane, thus suggesting that hydrocarbon generation has taken place in the basin probably of thermogenic origin. The trend followed by the hydrocarbons C1>C2>C3>iC4>nC4 further confirms the petroliferous nature of the gases. The stable carbon isotope values of light hydrocarbons desorbed form the soil samples indicate a clear signature of thermogenic gas. The concentrations of TOC and TIC in the soil samples of Sagar vary from 0.09% to 0.93% and 0.003% to 0.82% respectively. The TOC and TIC correlates poorly with the adsorbed soil gases (r=0.3, r=0.2), representing a lack of association of adsorbed soil gases with the surfacial organic matter thus indicate seepage related surface anomalies. The study revealed the presence of high concentrations of hydrocarbon oxidizing bacteria and adsorbed soil gases near Sagar area of Vindhyan Basin. The integrated geo-scientific studies showed excellent correlation and suggests that the Sagar area is considered to be potential for hydrocarbon generation and entrapment.