Recognition of Hydrocarbon Microseepage Using Microbial and Adsorbed Soil Gas Indicators in the Petroliferous Region of Krishna-Godavari Basin, India

The present study aims at exploring the possible correlation between adsorbed light gaseous hydrocarbon distribution pattern and the hydrocarbon oxidizing microbes present in the sub-soil samples. To establish the role of the latter in identifying the upward migration of hydrocarbons, especially a known petroliferous Krishna-Godavari Basin has been investigated. Soil samples from oil and gas fields of Tatipaka and Pasarlapudi areas of the basin show the presence of bacterial population for methane (3.46 × 10⁵ cfu/g), ethane (3.85 × 10⁵ cfu/g) and propane (3.04 × 10⁵ cfu/g) oxidizing bacteria in soil samples. Gas chromatographic analyses of adsorbed soil gases show the presence of C₁ to C₄ hydrocarbons. The concentration of adsorbed soil gases ranged for methane (C₁) = 1 to 115 ppb, ethane (C₂) = 1 to 99 ppb, propane (C₃) = 1 to 34 ppb, butane (nC₄) = 1 to 9 ppb and ΣC₂₊ = 1 to 115 ppb. The scatter plots between C₁ and C₄ components depict a linear trend indicating that all gases are from the same source. The total organic carbon (TOC) content of the soil samples ranges from 0.18% to 1.34%. Pearson correlation analysis shows that the concentration of ΣC₂₊ does not show any correlation (r = 0.1) with TOC, suggesting that the adsorbed gases are not derivatives of the organic carbon. Moreover, the values for methane δ¹³C₁ varied from -39.9‰ to -19.9‰ (V-PDP) Vienna PeeDee Belemnite indicate thermogenic origin. The integration of geomicrobial prospecting method together with adsorbed soil gas and carbon isotope studies shows a good correlation with the producing oil and gas fields of Krishna-Godavari Basin.