GEOCHEMICAL EVALUATION OF SOURCE-ROCK POTENTIAL FOR THE EOCENE-OLIGOCENE TOTAL PETROLEUM SYSTEM OF THE BENGAL BASIN, BANGLADESH

Controversy exists regarding the Eocene-Oligocene petroleum system in the Bengal Basin, Bangladesh. The upper Eocene Kopili Shale is composed of dark gray to black fossiliferous shale with subordinate limestone beds, and is considered to be of shallow marine origin.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 geochemical analysis of source-rock potential of the Kopili Shale in Bangladesh. 

Rock-Eval pyrolysis yields high T_max (431-506°) values and pronounced S₂ peaks indicating that Kopili strata in the Bengal Basin are mature enough to have generated thermogenic gas. The type of organic matter found in the Kopili shale is Type-III, which suggests that the source rock is gas-prone. Outcrop samples from the northeast are in the peak oil window, but owing to low TOC content have limited source potential. Core samples of Kopili Shale from the northwest are in the dry gas window but also are organic-lean. Vitrinite reflectance analyses under reflected white and blue light reveal tiny macerals in the outcrop sample of Kopili Shale suggesting extensive weathering and erosion. Geochemical analyses show that Kopili Shale in the northeast is mature and lies in the oil window, and has low production indices. However, oil and gas expelled from the Kopili Shale could have accumulated in the deeper part of the basin if any traps formed during the Oligocene and early Miocene. Kopili Shale from the northwest is in the dry gas window, and has low production indices. This explains why wells in the northwest were dry. X-ray diffraction and X-ray fluorescence data reveal variations in mineralogy and fracturability of the Kopili Shale, which has implications for evaluating unconventional shale-gas prospects. XRD results show that these shales contain varying amounts of quartz and sulfide minerals (e.g., pyrite). High quartz content (~50%) suggests high brittleness, a factor favorable for shale-gas production. Ongoing research on organic geochemistry and thermal maturity of the Kopili Shale will help better assess its prospects as a source for hydrocarbons in Paleogene-Neogene reservoirs of the Bengal basin.