Light gaseous hydrocarbon anomalies in the near surface soils of Proterozoic Cuddapah Basin: Implications for hydrocarbon prospects

The Proterozoic Cuddapah Basin, South India is grouped under Category IV sedimentary basins of India by Directorate General of Hydrocarbons (DGH), India having possible existence of hydrocarbons. Reconnaissance surface geochemical survey for adsorbed soil gas analysis was conducted in Cuddapah Basin for initial evaluation of hydrocarbon resource potential of the basin. The gas chromatographic analyses of adsorbed soil gases show presence of C1 to C4 hydrocarbons. The scatter plots between C1 to C4 components depict linear trend indicating that all the gases are derived from the same source and the hydrocarbon ratios of C3/C1 ⁎ 1000 between 60–500 and 20–60 suggests thermogenic origin for these gases. Further, XRD analyses demonstrated increased presence of calcite in non-anomalous samples compared to anomalous samples and also lack of correlation between ΣC2+ and soil TIC-TOC indicates seepage related surface anomalies. An important observation from this study was that the distribution of ΣC2+ anomalies over Nandhyal and Koilkuntla coincide with the favorable subsurface structural features and thick sediment deposits (6000–9000 m) defined by geophysical methods in these areas, signifying the potential for hydrocarbon generation and entrapment in the basin.