A report on the geologic and stratigraphic logging project of the Semliki basin in the Albertine Graben, Ntoroko District, Western Uganda

Abstract

The Semliki Basin, located in the western branch of the East African Rift System, includes the Kibuku area and is part of Block 3 of Uganda‘s Albertine Graben under a Petroleum Exploration License. This basin is shared by Uganda and the Democratic Republic of the Congo. The Ugandan portion lies in the southern part of Lake Albert and the adjacent land to the south. The geological foundation beneath the rift (the pre-rift basement) is made up of both felsic and mafic rocks. After the rift formed, the overlying rock layers (post-rift lithology) included altered granites, granitic gneisses, and sedimentary rocks—mainly deposited by rivers and lakes. Both types of rocks are thought to have originated from the nearby Rwenzori Mountains. Stratigraphically, the sediments are classified into five main formations. From oldest to youngest, these are the Kasande (bottom), Nyaburogo, Oluka, Karaka, and Katorogo (top) formations. Studies of the basin and sediment types suggest that the Kibuku area‘s sediments, especially those exposed along road cuts, were deposited in environments such as rivers, lakes, and deltas. Pebbles found in the conglomerates that rest on the basement rocks closely match the mineral composition of the Rwenzori Mountains, indicating that the sediments likely came from there. The Semliki Basin is considered a potential petroleum system, showing essential features for oil accumulation: possible reservoirs, source rocks, seals, traps, and migration routes for hydrocarbons. It is a sub-basin within the broader Albertine Graben, which is Uganda‘s most promising region for oil and gas exploration. The basin holds sediments from the mid-Miocene to the present, lying over older layers that may date back to the Jurassic or Early Tertiary, and these rest unevenly on the basement rocks. The main types of sediment in the basin are sandstones, siltstones, and claystones, mostly deposited by rivers, lakes, and deltas. The basement consists of igneous and high-grade metamorphic rocks like granites and granite gneiss. To evaluate the petroleum potential of the basin, detailed structural and stratigraphic data were collected and analyzed. Due to significant tectonic activity in the region, both the basement and sedimentary rocks display a variety of geological features, including folds, bedding planes, cross-bedding, unconformities, joints, veins, and faults. The early development of oil-generating organic matter in these young sediments is supported by surface features like the Sempaya Hot Springs. Overall, the Semliki Basin has been confirmed as a potential petroleum play, with all the critical elements of a functioning petroleum system in place