A report on the geologic and stratigraphic logging project of Semiliki Basin-Albertine Graben in Ntoroko District Western Uganda

Abstract

The geologic and stratigraphic logging project was conducted in Semliki basin, Ntoroko district, western Uganda. Semliki basin is a pull apart basin located in the central domain of the western arm of the Albertine graben. The main aim of the field study was to acquire field training on how to collect and interpret stratigraphic, petrographic, sedimentologic and structural data which are important in hydrocarbon exploration. Some of the materials used during the study included a geologic hammer, Grain Size Scale, Jacob staff, handheld GPS, notebook and a geologic compass. The methods involved desk Study conducted at camp, fieldwork, sedimentary logging exercise, data collection, discussions, analysis and interpretation. The Semliki basin is covered by sediments that represent Middle Miocene to recent exhibiting a fining upward sequence underlain by possible Jurassic to Early Tertiary age sediments resting unconformably over the basement rocks. The stratigraphic sequence exhibited in the basin is divided into seven Formations (in decreasing age) namely; the Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo and Nyabusosi formations. Four lithofacies were identified indicating different depositional environments (fluvial, deltaic and lacustrine) and are ranging from very fine- very coarse grained sediments and cobbles/ pebble size cemented together in conglomerates. The basin is affected by intensive tectonic activity, that led to the formation of vast types of structures that include faults, joints, bedding and plunging folds among others. Geophysical data of Semliki basin and shallow, high resolution (150-200 Hz) seismic data from an intraslope salt withdrawal mini basin in the Gulf of Mexico were availed for interpretation. This data was later analyzed by softwares such as Oasis montaj, sedilog, and Teclog64.The sediments of the Semliki Basin represent a petroleum play for hydrocarbon accumulations as all elements of a petroleum system were identified in the field for example presence of oil seeps in Kibuku indicate organic rich source rocks an indicator of an active system thus great petroleum potential. Information from the field visit to Sempaya hot spring indicate the presence of high geothermal gradients of over 67°C/km for the maturation and generation of hydrocarbons at shallow depths. The geophysical logs also showed the presence of hydrocarbons due to the cross overs observed between the neutron porosity and density logs. It was concluded that the study area had a fair petroleum system (working petroleum play).