A detailed field project report about the Semliki basin in Ntoroko District, Albertine Grabben.

Abstract

This report presents the findings of a field study conducted between 8th and 19th June 2025 in the Semliki Basin, located within the Albertine Graben of the East African Rift System (EARS), Ntoroko District, southwestern Uganda. The study was carried out by third-year students of the Bachelor of Science in Petroleum Geoscience and Production programme at Makerere University, under the coordination of the Department of Geology and Petroleum Studies. The Semliki Basin is a half-graben rift structure covering approximately 1,200 km², bounded by the Rwenzori Mountain escarpments to the southeast, Lake Albert to the north, and the Semliki and Lamia rivers to the west. Fieldwork focused on the Kichwamba, Kisegi Hill, Kibuku roadcut, Turaco well sites, and Sempaya Hot Springs, employing methods including stratigraphic logging, structural measurements, grain size analysis, paleocurrent determination, facies analysis, and geophysical data interpretation. Basement lithology consists primarily of high-grade Proterozoic metamorphic rocks including granite gneisses, migmatites, and quartzites, which are heavily jointed and fractured. Overlying these basement rocks are seven Miocene-to-Recent sedimentary formations; the Kisegi, Kasande, Kakara, Oluka, Nyaburugo, Nyakabingo, and Nyabusosi formations; comprising sandstones, siltstones, mudstones, conglomerates, and clays deposited in fluvial, deltaic, and lacustrine environments. Structural analysis revealed NE-SW-trending normal faults, joints, foliations, and folds consistent with the extensional tectonics of the Albertine Graben. Primary sedimentary structures including cross-bedding, laminations, ripple marks, and mud cracks were used to reconstruct paleocurrent directions and depositional environments. Geophysical studies, including Bouguer gravity anomaly maps, total magnetic intensity maps, seismic section interpretation, and petrophysical wireline log analysis from the Turaco 1, 2, and 3 wells, confirmed the basin's asymmetric half-graben architecture and the presence of hydrocarbon-bearing intervals. The Semliki Basin demonstrates all the critical elements of a viable petroleum system: lacustrine source rocks with significant organic content (Kasande Formation), porous and permeable sandstone reservoir rocks (Kisegi Formation), impermeable clay top seals, and structural and stratigraphic traps associated with the basin's fault architecture. Evidence of active oil generation is provided by the Kibuku oil seep, while the Sempaya Hot Springs in the Buranga Geothermal Field indicate elevated geothermal gradients that accelerate source rock maturation. Overall, the Semliki Basin holds substantial potential for hydrocarbon exploration, warranting continued geological and geophysical investigation.