Semliki Basin Field Study Report

Abstract

This report is as a result of a field excursion carried out in the Semliki basin, Albertine graben in Western Uganda. The study was focused on the description and analysis of the depositional environments and processes that took place in the Semliki area. Semliki sedimentary basin is a pull apart basin formed when the Albertine graben, a product of active rifting, underwent transtensional strike-slip deformation that was controlled by already existing NE-SW graben-forming normal faults. Both Uganda and the Democratic Republic of Congo share the Semliki basin. The Ugandan portion of the Semliki basin covers the southern part of the Lake Albert, plus a landward area to the south of the lake. The area’s pre-rift basement lithology consists of fractured granites and granitic gneisses, felsic and mafic rocks and the post-rift lithology is made up of sedimentary rock mainly sands and clays of the fluvial and lacustrine nature believed to have their source of provenance in the Rwenzori ranges. The stratigraphic sequence of the sediments exhibited in the basin is divided into seven Formations, namely; Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo and Nyabusosi in that order of ascent. It was observed that environments in the basin shifted over time from alluvial plain, lacustrine, deltaic plain in semi-arid, humid and tropical climatic conditions; and as such enabled the deposition of the distinctive Formations. The rock record in Semliki preserved several structures that enabled paleocurrent and paleoenvironment reconstruction These structures include joints, faults, stratification, flower structures, among others. 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.