Depositional Setting, Origin and Evolution of Organic Matter in Eocene Kopili Formation, Shillong Plateau, India

ABSTRACT The Late Eocene Kopili Formation in the Shillong Plateau, India, features a sequence of alternating strata, including shale, siltstone, and sandstone, with thick sandstone beds at the top. This study investigates the carbonaceous shales within these layers by analyzing palynofacies assemblages, maceral composition, pyrolytic hydrocarbons, and biomarkers. The palynofacies are predominantly rich in phytoclasts, exhibiting significant compositional variation across three distinct palynofacies units, which reflect fluctuations in depositional settings within a nearshore fluvio-deltaic environment. The organic matters are mostly represented by telovitrinite and inertinite macerals, with a complete absence of liptinite, indicating minimal humification and redox processes during early diagenesis. The kerogen types identified are predominantly Type III and IV, sourced mainly from terrestrial plants. Biomarker analysis reveals a significant contribution from severely biodegraded terrestrial angiosperms. The depositional environment is characterized as clay-rich, oxic to dysoxic-anoxic, and highly proximal, with variable water depths. Rock-Eval pyrolysis indicates an average Total Organic Carbon (TOC) of 0.64%, suggesting moderate to low hydrocarbon generation potential for the Kopili Formation. Various maturity indicators, such as vitrinite reflectance, TAI, Tmax and, biomarker maturity indices, suggest that a significant portion of the organic material is in an immature to early mature stage. In contrast to similar sequences found in the Assam and Bengal shelves, the Kopili Formation shows a reduced hydrocarbon potential, likely resulting from its deposition in a shallower, more oxygen-rich environment.