Geological Characterization of the Miocene–Pliocene Succession in the Semliki Basin, Uganda: Implications for Hydrocarbon Exploration and Drilling in the East African Rift System

The Albertine Graben, an active sedimentary petroliferous basin, has gained global attention as the unexplored areas are recently being targeted for hydrocarbon prospectivity. Here, we present the first detailed geological investigation of the Upper Miocene–Pliocene clastic interval in the southern Lake Albert, part of the Semliki Basin. We employed an integrated approach that includes source rock evaluation, reservoir characterization, pore pressure, and geomechanical evaluation. Thermal maturity analyzed from vitrinite reflectance, spore color index, and Rock–Eval Tmax indicates that the Lower Kasande–Kakara shales are into the early catagenetic maturity, and the onset of oil window occurs at around 2550 m. With 1.8–2.4% total organic carbon content and dead carbon-free hydrogen index of ~ 600 mg S2/g TOC, a Type I/II oil-prone source rock was inferred, while the state of the thermal maturity reflects on the relatively low free oil yields associated with a poor oil production index. The quartz arenite reservoirs of the Upper Miocene Kasande–Kakara Formation exhibit excellent petrophysical characteristics and possess pore pressure gradients of 0.17–0.24 psi/ft (1 psi/ft = 22.6206 Mpa/km) in two distinct zones (2040.7–2221.5 m and 2554.7–2730 m) with gross vertical thickness of ~ 206 m. The region belongs to a normal faulting tectonic regime where the vertical stress gradient is 0.91–0.93 psi/ft with a lower bound of minimum horizontal stress gradient interpreted as 0.62 psi/ft. The hydrostatically pressured Miocene shales have higher shear failure gradients and exhibited extensive wellbore failures. The implications of geological characterization in both hydrocarbon exploration and future drilling in the basin are envisaged in this research.