A field study of the Semliki Sedimentary Basin in the Albertine Graben, Western Uganda.

Abstract

The study is conducted in the Semliki basin which is within the Albertine graben. The target areas were Kichwamba, Kibuku area, the Turaco structure. The major study areas included the East African Rift System (EARS), drainage and drainage reversals, braided river systems, depositional environments, facies analysis as well as geophysical and petrophysical data interpretation using software such as Oasis Montaj and TechLog. The Semliki Basin covers an area of approximately 740 km2 in the Ugandan portion of the Albertine Graben. It comprises the Semliki Flats and the adjacent Toro Plain, immediately southwest of Lake Albert. It is bordered to the south east by a steep fault escarpment rising almost 1,000 m to the northernmost spur of the Rwenzori Mountains. The area is distinct from the surrounding areas of Uganda and the DRC because of its low elevation of about 650 m above mean sea level compared with about 1,100 to 1,500 m for the adjoining rift shoulders to the east and 1,500 to 1,800 m to the west. The lithology of the Semliki Basin has been classified into two and these are; the Basement lithology and the sediment lithology. The Basement lithology mainly consists of low grade metamorphic rocks which are a part of the Buganda-Toro system and include granitic gneisses, amphibolites, and quartzites. The rock record in Semliki preserved several structures, primary and secondary, that enabled paleocurrent and paleoenvironment reconstruction through characteristic facies associations. These structures include joints, faults, stratification and flower structures, among others. However, the structural lineaments generally manifest as short discontinuous arrays in a given trend. The basin was observed to have all the components of a working petroleum system. Kasande Formation providing a potential source rock, Kisegi and Kakara with potential reservoir targets while Oluka, Nyakabingo and Kasande are potential seals. It is also well endowed with both structural (rollover & compressional anticlines, tilted fault blocks) and stratigraphic traps (unconformities and pinchouts) as well as extensive fault arrays providing migration pathways. A high geothermal gradient facilitated early maturation of the source rock. Structural synthesis revealed timing of migration that was favorable for accumulation of petroleum. Finally, presence of an oil seepage in Kibuku is an indicator of an active system thus great petroleum potential.