Preliminary Assessment of the Petroleum Source-Rock Potential of Upper Eocene Kopili Shale, Bengal Basin, Bangladesh

Abstract

The upper Eocene Kopili Shale occurs throughout the northern end of the Bengal Basin, including on the northwestern Indian Platform and in deeper basin areas (e.g., Sylhet Trough) of northeastern Bangladesh. The Kopili-equivalent mudrocks in India, interpreted as shallow-marine to lagoonal deposits, are hydrocarbon source rocks for the Sylhet-Kopili/Barail-Tipam composite petroleum system of Assam, India. In the current study, thin section petrography, organic petrologic analysis, XRD and XRF techniques, as well as field observations of the Kopili Shale were used to characterize this mudrock in various parts of Bangladesh. In addition, geochemical analyses such as TOC analysis, Rock-Eval pyrolysis, and vitrinite reflectance studies were used to assess organic richness, type, and thermal maturity of the Kopili Shale. Petrographic thin section analyses reveal localized skeletal grains (e.g., foraminifera), bioturbate fabrics, pyrite framboids, sand lenses and flame structures, suggesting deposition in shallow marine environments characterized by at least periodically oxygenated bottom waters and sulfidic pore waters. XRD and XRF results reveal high quartz content in silt-rich Kopili Shale. Organic petrologic observations and limited reliable Rock-Eval pyrolysis data indicate that organic matter in the Kopili Shale is largely terrestrial, including an admixture of type 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 are thermally mature. However, mean total organic carbon (TOC) contents for samples from the northwestern core sections and two of three northeastern outcrop sections are generally low (<0.6%) and, thus, reflect relatively poor hydrocarbon-source potential. As an exception, TOC values (mean = 1.0%) and Rock-Eval parameters (S2) for samples from the remaining outcrop section (Sripur section) suggest a somewhat higher potential. Taken together, results indicate that the Kopili Shale of the Bengal Basin is a silty mudrock, deposited in a shallow marine environment, and has limited hydrocarbon source potential compared to presumed equivalent shales of the Kopili Formation in the Assam basin of India. Further studies are required to understand the variable source potential and migration pathways of the hydrocarbons generated from Paleogene mudrocks in the region.