The petroleum potential of the Arangi and Kajrahat Limestone formations from the Semri Group, Chopan, Uttar Pradesh, India

The Lower Vindhyan (Semri) Group is largely composed of thick unmetamorphosed and undeformed sedimentary sequences of Late Palaeoproterozoic age. A detailed geochemical and palynological study has been carried out for the black shale lithounits of the Arangi and Kajrahat Limestone formations exposed near Chopan town, Sonbhadra district, Uttar Pradesh for evaluating their hydrocarbon potential and thermal maturity. The organic geochemical findings are mainly based on proximate analysis, ultimate analysis and Rock-Eval pyrolysis and indicates that the studied shale samples of both the formations have, due to high maturity, only a fair to poor hydrocarbon potential. Organic-walled microfossils recovered indicate high thermal maturity of the organic matter based on Acritarch Alteration Index (AAI) which is consistent with elevated T_max and very low hydrogen index (HI) values. Thus, only a low potential for generation of dry gaseous hydrocarbons remains for these highly matured samples, whereas the original kerogen type can be either type I or II.