A report of Geologic Field mapping project of Semliki basin in the albertine Graben, Ntoroko District.

Abstract

The area of study was Semliki basin located in the southern part of the Albertine graben, Ntoroko district, western Uganda. Semliki area lies onshore south of Lake Albert; it is bounded by the escarpment to the east and south and by rivers Lamia and Semliki to the west. It covers approximately 1200km. The aim of this study was to enable us apply the geological, geophysical and other knowledge to understand the geology and analyze the petroleum system of the area, study the environments of deposition and the depositional processes that took place in the area. The Semliki basin is generally filled with Middle Miocene to recent age sediments exhibiting a fining upward sequence (characteristic of fluvial systems) from conglomerates (overlying the basement rock) to sands to silts to clays. It is structurally wedge shaped and majorly comprises faults, joints, laminations and bedding planes, cross beds, foliations, quartz veins, unconformities, soft sediment deformation structures among others. The southern part of this basin has the highest elevation but the smallest thickness of accumulated sediments whereas the depocentre (5km thickness of sediments) is in the northern part of the Semliki basin. We collected vast structural and stratigraphic data that we then analyzed to make conclusions about the petroleum potential of Semliki basin. The study area comprised of fluvial, deltaic and lacustrine sediments. The basin was affected by intensive tectonic activity, that led to the formation of various types of structures with in the basement that propagate to the sediments such as faults, joints, folds, veins and banding. Sedimentary structures observed included cross bedding, unconformities, faults, mud diapirs and deformation bands. Mud diapirs, faults, plunging folds would be good structural seals. The presence of hot springs within the area indicated the presence of high enough geothermal gradients for maturation of hydrocarbons. Sedimentology, stratigraphy and tectonics are applied to develop a full understanding of the rocks and the sediments that fill Semliki basin. This information was used to interpret the geologic history and evaluate the economic importance of these rocks (e.g Boggs, 1995). We concluded that the study area had a good petroleum system characterized with all the required system elements that is source rocks (organic rich shales), reservoir rocks (thick sandstones), seals (shales and evaporites), traps (faults, diapirs) and migration pathways.