Geo-microbial prospecting for hydrocarbons is an exploration method based on the seepage of light gaseous hydrocarbons from oil/gas reservoirs to the surface and their utilization by hydrocarbon oxidizing bacteria. These bacteria utilize hydrocarbon gases as their sole source of food and are found to be enriched in the near surface soils/sediments above the oil and gas reservoirs. The detection of anomalous populations of n-pentane and n-hexane oxidizing bacteria in the surface soils can help to evaluate the prospects for hydrocarbon exploration. A geo-microbial survey has been carried out in the Bikaner Nagaur basin to investigate the prospects for hydrocarbon exploration. In the present study, bacterial counts for n-pentane utilizing bacteria range between 2.0×10 2 and 1.26×10 6 cfu/gm and n-hexane utilizing bacteria range between 2.0×10 2 and 1.21×10 6 cfu/gm. The bacterial concentration distribution maps show four distinct anomalies in the study area. The possibility of discovering oil or gas reservoirs using the microbiological method is emphasized by the fact that the hydrocarbon-oxidizing bacteria range between 10 3 and 10 6 cfu/gm in soil/sediment receiving hydrocarbon micro-seepages. In t he present study area of the Bikaner Nagaur basin, n-pentane and n-hexane utilizing bacteria are found between 10 5 and 10 6 cfu/gm of soil sample, which is significant and thereby substantiates the seepage of lighter hydrocarbon accumulations from oil and gas reservoirs. Geo-microbial prospecting studies suggest that hydrocarbon micro-seepage of subsurface origin is present in the study area and indicate that the area has positive prospects for petroleum exploration.

Light hydrocarbons in soil have been used as direct indicators in geochemical hydrocarbon exploration, which remains an unconventional path in the petroleum industry. The occurrence of adsorbed soil gases, methane and heavier homologues were recorded in the near-surface soil samples collected from Kutch-Saurashtra, India. Soil gas alkanes were interpreted to be derived from deep-seated hydrocarbon sources and have migrated to the surface through structural discontinuities. The source of hydrocarbons is assessed to be thermogenic and could have been primarily derived from humic organic matter with partial contribution from sapropelic matter. Gas chromatographic analyses of hydrocarbons desorbed from soil samples through acid extraction technique showed the presence of methane through n-butane and the observed concentrations (in ppb) vary from: methane (C 1) from 4-291, ethane (C 2) from 0-84, propane (C 3) from 0-37, i-butane (iC 4) from 0-5 and n-butane (nC 4) from 0-4. Carbon isotopes measured for methane and ethane by GC-C-IRMS, range between −42.9 to −13.3 (Pee Dee Belemnite-PDB) and −21.2 to −12.4 (PDB), respectively. The increased occurrence of hydrocarbons in the areas near Anjar of Kutch and the area south to Rajkot of Saurashtra signifies the area potential for oil and gas.

Direct correlation have been observed between certain trace element and hydrocarbon anomalies in the near subsurface soils of Vindhyan basin, India. This relationship with hydrocarbon is very useful in hydrocarbon exploration. 52 soil samples from Vindhyan basin were collected from a depth of 2.5m. All the soil samples were analyzed for light hydrocarbon, isotope and trace element concentrations. The adsorbed light gaseous hydrocarbon analyses show the presence of methane (8 - 328 ppb), ethane (0 - 27 ppb) and propane (0 -11 ppb) respectively and these values indicate the presence of hydrocarbon micro-seepage in the study area. The carbon isotopic values determined for methane and ethane for these soil samples are (-26.41 to -47.70 ‰ PDB) and (-20.07 to -35.30 ‰ PDB) respectively and they are thermogenic in nature. The trace element concentrations of nickel (33-220 ppm), vanadium (72-226 ppm), copper (20- 131 ppm), chromium (94-205 ppm), zinc (66-561 ppm) and cobalt (9-39 ppm) have higher than the normal concentrations in soils. Trace element concentrations are used to plot with the data obtained from light gaseous hydrocarbon concentrations and carbon isotopic values of soil samples of the Vindhyan basin. Trace element anomalies have been observed around the hydrocarbon anomalies in the study area.

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