ABSTRACT
Several techniques are used for the exploration of hydrocarbons, of which; the geochemical techniques involving the microbiological technique use the principle of detecting the light hydrocarbon seepage activities for indication of sub-surface petroleum accumulations. A survey was carried out to characterize the light gaseous hydrocarbons seeping in oil and gas fields of Krishna-Godavari basin of Andhra Pradesh. A set of 50 sub-soil samples were collected at depths of about 3 m for geochemical analyses and 1m for microbiological analysis. The microbial prospecting studies showed the presence of high bacterial population for methane 2.5 x 102 to 6.0 x 106 cfu g-1, propane 1x102 to 8.0 x 106 cfu g-1 in soil samples. The adsorbed soil gas analysis showed the presence of moderate to low concentrations of methane (26 to 139 ppb), ethane (0 to 17 ppb), propane (0 to 8 ppb), butane (0 to 5 ppb) and pentane (0 to 2 ppb) in the soil samples of the study area. Carbon isotope analysis for methane (´13C1) ranging from -36.6 to -22.7‰ Pee Dee Belemnite (PDB) suggests these gases are of thermogenic origin. Geo-microbial prospecting method coupled with adsorbed soil gas and carbon isotope ratio analysis have thus shown good correlation with existing oil/ gas fields of Krishna-Godavari basin.