Petroleum source-rock evaluation of upper Eocene Kopili Shale, Bengal Basin, Bangladesh

Abstract

The upper Eocene Kopili Shale occurs throughout the Bengal Basin, including in the northwestern Indian platform and deeper basin areas (e.g., Sylhet Trough) of Bangladesh. Mudrocks presumed to be equivalent to the Kopili Shale in India are known hydrocarbon source rocks. However, the source-rock potential of the Kopili Shale in Bangladesh is not well established, thus prompting the current study of abundance, character, and maturity of organic matter in Kopili Shale samples from the Bengal Basin.Organic petrologic observations and Rock-Eval pyrolysis data indicate that organic matter in the Kopili Shale is largely terrigenous, including an admixture of type I/II (liptodetrinite, cutinite, bituminite), type III (vitrodetrinite), and type IV (inertodetrinite) macerals. Mean vitrinite reflectance values (Ro=0.86–1.32%) and a single reliable Tmax value (433°C) indicate that organic matter from all sampled sections is thermally mature. Total organic carbon (TOC) contents of samples from core and outcrop are generally low (<0.6%) and thus reflect relatively poor hydrocarbon-source potential. However, TOC values of ~1.0% and S2 values from one section indicate that source potential is locally higher in the Sylhet Trough area.An understanding of differences in Kopili source-rock potential between India and parts of the Bengal Basin will require more comprehensive comparative facies analyses.