Subsidence and Thermal History Effect on Source Rock Maturity of Semliki Basin, Albertine Graben, Uganda

Abstract

The research study focused on the analysis of subsidence and thermal history effects on source rock maturity in the Semliki Basin. The study utilised the back stripping approach and was accomplished using PetroMod Software. The research utilized previous data sets obtained from geochemical reports and final well data reports to generate a basin model of eight (08) Sedimentary Formations ranging in age from Lower Miocene to Pleistocene sediments i.e. , Surface, Nyabusozi-Nyakabingo, Nyaburogo, Oluka, Kakara, Kasande, Kisegi and Basement formations. The general trend of subsidence observed is a rapid deepening (13.0 to 11.5 Ma and 3.5 to 2.0 Ma) during the main and later rift phases respectively, followed by an exponential decline in the rate of tectonic subsidence. The maximum heat flow (77.59 mWm − 2 ) at 3.5 Ma corresponds to the Late Pliocene to Pleistocene thus deep burial conditions and represents maximum paleo-temperatures. The medium/dark shales of the Kasande-Kakara formations (2070 - 2100 m and 2480 - 2488 m) are the most prospective source rocks and comprise organic matter (TOC c.2.0%) and an oil-prone Type I/II source rock quality. The source rock maturity plots indicate that the source rock entered the early oil zone at 2.0 Ma whereby the top of the oil generation window is observed with Ro 0.5%, corresponding to the Kasande-Kakara formations at a depth of 2400 - 2500 m. This corresponds to the second subsidence event; however, there was no commencement of oil generation. Though limited by the scope of the data set, this study allowed for a quantification of tectonic subsidence and thermal maturity effects on the identified source rock horizons of the Semliki Basin sediments.