An overview of organic geochemical indices to evaluate conventional petroleum source rocks: a summary of examples from the Indian Subcontinent

The elemental, isotopic, and molecular (biomarker) compositions of sedimentary organic matter help to evaluate conventional petroleum source rocks. The objectives of this overview article are to discuss commonly used organic geochemical indices for petroleum source rock evaluation, and analyze and synthesize such studies in the Indian Subcontinent. Since most of the fundamental analysis has been carried out in the European and North American Continents, this study covers the gap in the Indian Subcontinent. The organic geochemical indices reviewed are total organic carbon (TOC), Rock-Eval pyrolysis, C/N ratio, carbon isotopic composition, molecule/biomarkers (e.g., <i>n</i>-alkane/aliphatic hydrocarbon, polycyclic aromatic hydrocarbon) which are used for qualitative and quantitative analysis of source rock composition. This study also evaluates the lithostratigraphic characteristics of the source rocks in the Mannar Basin of the Indian Subcontinent. In conclusion, the Mannar petroleum source rocks show Type III and Type II–III dominated gas and oil-gas prone kerogen with fair to good source rock potential. A comparison of organic geochemical indices with their geologic setting would improve interpretations of petroleum source rock characteristics in the Indian Subcontinent.