Influence of geochemical, organo-petrographical and palynofacies assemblages on hydrocarbon generation: A study from upper Oligocene coal and shale of the Makum Coal Basin, Assam, India

Abstract The Makum Coal Basin of Northeast India comprises important coal and shale reserves of upper Oligocene age. A total of twenty samples were collected to evaluate the kerogen type, hydrocarbon generation potential, and thermal maturity through geochemically, organo-petrographically and palynofacies analyses. The samples are characterized by high TOC (total organic carbon) and large S2 (remaining heavier hydrocarbons), meet the required standards as good to excellent generation potential. The coal and shale are marked by mixed Type III-II kerogen with minor occurrence of Type I kerogen, moderate to high hydrogen index (HI) values and are considered to generate thermogenic gas with some condensate oil. The reflectance values indicate samples are mostly immature and close to the oil birth line. The amount of Vt60 grains also suggests that the samples can produce some condensate oil along with thermogenic gas. The Rock-Eval pyrolysis (REP) data stands with the interpretation through petrographic, palynofacies and Fourier Transform Infrared spectroscopy (FTIR) in determining kerogen type and generation potential. The generation potential is also controlled by the degree of organic matters reactivity. The estimated reactivity indicates that besides vitrinite and liptinite, up to 70 vol% of the inertinite macerals have the potential to generate hydrocarbon. The occurrence of a substantial amount of reactive macerals, i.e. collotelinite, vitrodetrinite, collodetrinte, corpogelinite, alginite, resinite and bituminite within the samples makes them more potential to generate hydrocarbons. The diagram proposed based on our data of macerals and palynofacies assemblages may also be a convenient tool in detecting kerogen type(s) and nature of hydrocarbons.