A report of the geologic and geophysics field work of Semliki Basin in the Albertine Graben, Ntoroko District, Western Uganda.

Abstract

The geological and geophysical field study was undertaken towards the beginning of the seventh semester, and lasted 10 days, from 15th march 2021 to 24th march 2020, under the guidance of academic staff from the Department of Geology and Petroleum Studies, Makerere University. The study area was the Semliki Basin, located in Ntoroko District, Western Uganda, on the southern side of the Albertine Graben. The main objective of the field work was to acquire training on how to collect and interpret stratigraphic, petrographic, sedimentologic and structural data which are very important in hydrocarbon exploration. Semliki sedimentary basin is located in the central domain of the western arm of the Albertine graben. It 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. The basin is asymmetric in nature with a thin layer of sediments in the southern and southeastern parts but increases in thickness towards the north-west in the DRC. Its depocenter coincides with the location where the western bounding Semliki fault meets the basin floor. The basement rocks are dominated by fractured gneisses and granites while sedimentary rocks are dominated by sands and clays. The stratigraphic sequence exhibited in the basin is divided into seven Formations, namely; Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo and Nyabusosi . The provenance of the detritus in the basin was deduced to be in the Rwenzori mountains, with water as the main agent of transport. 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, primary and secondary, that enabled paleocurrent and paleoenvironment reconstruction through characteristic facies associations. These structures include joints, faults, stratification, 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.