RESEARCH ARTICLES CURRENT SCIENCE, VOL. 95, NO. 3, 10 AUGUST 2008 361 *For correspondence. (e-mail: dayaisotope@yahoo.com) Geo-microbial prospecting method for hydrocarbon exploration in Vengannapalli Village, Cuddapah Basin, India M. A. Rasheed, M. Veena Prasanna, T. Satish Kumar, D. J. Patil and A. M. Dayal* National Geophysical Research Institute, Uppal Road, Hyderabad 500 606, India Geo-microbial prospecting for hydrocarbons is an ex- ploration method based on the seepage of light gase- ous hydrocarbons from oil/gas reservoirs towards the surface and their utilization by hydrocarbon-oxidizing bacteria. The detection of anomalous populations of methane, ethane and propane-oxidizing bacteria in the surface soils or sediments, helps to evaluate the pros- pects for hydrocarbon exploration. Recently, natural gas leakage from boreholes was reported in Vengannapalli Village, Potuluru Mandal, Anantapur District, Andhra Pradesh. Geo-microbial survey has been carried out in Vengannapalli Village with a view to investigate whether the leakage of natural gas is associated with some geo- microbial anomalies that can establish an upward migration of hydrocarbons from the deep subsurface source. In the present study, we report high bacterial counts for methane (5.4 × 10 5 cfu/g), ethane (5.5 × 10 5 cfu/g) and propane-oxidizing bacteria (4.6 × 10 4 cfu/g) in soil samples collected from Vengannapalli Village. The adsorbed light gaseous hydrocarbon analyses show the presence of moderate to low concentrations of methane (12.4–222.8 ppb), ethane (3–35.9 ppb), propane (2– 24.3 ppb) and butane (1–18.4 ppb) in the soil samples. Carbon isotope analyses of few soil samples show enriched δ 13 C 1 values of methane ranging from –35.0 to –10.6‰, indicative of methane oxidation. Geo-microbial prospecting method coupled with adsorbed soil gas and carbon isotope studies, suggest that hydrocarbon micro-seepage of subsurface origin is present in the area and indicate that the area is worth visiting for conventional petroleum exploration. Keywords: Hydrocarbons, hydrocarbon-oxidizing bac- teria, micro-seepage, microbial prospecting. A microbial prospecting method for hydrocarbon research and exploration has been in use for the last five decades. It is an exploration method based on the premise that the light gaseous hydrocarbons, namely methane (C 1 ), ethane (C 2 ), propane (C 3 ) and butane (C 4 ) migrate upward from subsurface petroleum accumulations by diffusion and ef- fusion 1 , and are utilized by a variety of microorganisms present in the sub-soil ecosystem. The methane, ethane, propane and butane-oxidizing bacteria exclusively use these gases as carbon source for their metabolic activities and growth. These bacteria are mostly found enriched in the shallow soils/sediments above hydrocarbon-bearing struc- tures and can differentiate between hydrocarbon prospective and non-prospective areas 2 . The isolation and enumera- tion of specific C 2+ alkane-oxidizing bacteria are used as indirect petroleum prospecting method 3 . Application of hydrocarbon-oxidizing bacteria in petroleum exploration has been discussed in great detail 3–7 . Microbial anomalies have been proved to be reliable indicators of oil and gas in the sub-surface 8 . A direct and positive relationship bet- ween the microbial population and hydrocarbon concen- tration in the soils has been observed in various producing reservoirs worldwide 8–12 . The methane-oxidizing bacteria are usually predominant over gas fields as the gas reser- voirs are commonly dominated by methane 13 . Thermogenic processes produce methane and substantial amounts of other saturated hydrocarbons by irreversible reaction of residual organic matter or kerogen 14 . Ethane, propane and butane are assumed to be originated from the migration of thermogenically produced petroleum from depth and are usually not associated with generation in shallow soils 15 . These light hydrocarbons are utilized by the phylogeneti- cally diverse group of bacteria belonging to genera Bre- vibacterium, Corynebacterium, Flavobacterium, Myco- bacterium, Nocardia, Pseudomonas and Rhodococcus 16–18 . A microbial prospecting method involves isolation and enumeration of various groups of methane, ethane, pro- pane and butane-oxidizing bacteria in sub-soil strata for the delineations of hydrocarbon prospect in an area. The abundance of these hydrocarbon-oxidizing bacteria in the soils yields hydrocarbon signature on the surface and hence they are considered as indicator microbes 19 . Based on a study conducted by Wagner et al. 17 , oil and gas fields were identified using Microbial Prospecting for Oil and Gas (MPOG) method and the success rate has been re- ported to be 90%. This method can be integrated with geological, geochemical and geophysical methods to evaluate the hydrocarbon prospect of an area and to pri- oritize the drilling locations, thereby reducing drilling risks and achieving higher success in petroleum exploration 8 . In the present study, the microbial prospecting method has been applied in Vengannapalli Village, Cuddapah Basin, for evaluating the prospectivity for hydrocarbon explora-