PETROLEUM SOURCE ROCK EVALUATION OF UPPER EOCENE KOPILI SHALE, BENGAL BASIN, BANGLADESH

The upper Eocene Kopili Shale, composed of dark gray to black fossiliferous shale with a few limestone beds, is exposed locally along the eastern bank of the Dauki River, Sylhet Trough, in northeastern Bangladesh. This shale is also known in the subsurface at Bogra, northwest Bangladesh. Controversy exists regarding the source-rock for Miocene gas reservoirs in the Bengal Basin, and the source-rock potential of the Kopili Shale in Bangladesh is unknown. Kopili-equivalent shale in India, however, has proven source-rock potential. Accordingly, this research focuses on Kopili source-rock potential in Bangladesh.

Thin sections reveal foraminifera, bioturbation, pyrite framboids, pinstripe bedding, and ripple bedding, suggesting shallow marine deposition. Rock-Eval pyrolysis yields high T_max values and pronounced S₂ peaks indicating that Kopili strata are mature enough to have generated thermogenic gas. Outcrop samples from the northeast are in the peak oil to dry gas window but have limited source potential because TOC is < 1%. Core samples from the northwest are in the dry gas window but also are organic-lean. Organic petrologic analyses under reflected white and blue light reveal macerals such as liptodetrinite, cutinite, bituminite, vitrodetrinite, and inertodetrinite. Liptinitic macerals have faint brown fluorescence or are nonfluorescent, which, along with measured vitrinite reflectance values, indicate high thermal maturity.

Ongoing research on organic geochemistry and thermal maturity of the Kopili Shale will help better assess its prospects as a source for hydrocarbons in Neogene reservoirs of Bangladesh.