Geologic and stratigraphic logging report of the Semliki Basin Albertine Graben in ntoroko.

Abstract

This report presents a comprehensive geological and stratigraphic study of the Semliki Basin, a half-graben within the western arm of the East African Rift System (EARS), located south and east of Lake Albert in Ntoroko District, Western Uganda. The basin has its shallow end in Uganda and deeper end in Congo, and sediment thickness exceeding 6 km. The field study was conducted as a field training exercise for fourth-year Petroleum Geoscience students at Makerere University targeting areas like Kichwamba, Kibuku Quarry, Itojo, Makondo Fault, and the Turaco well site. It aimed at building skills in lithological description, stratigraphic logging, facies analysis, structural interpretation, and geophysical data assessment, while investigating the basin’s depositional environments, sedimentological processes, sequence stratigraphy, and petroleum potential. Its basement of the Semliki basin consists of high-grade metamorphic rocks like granitic gneisses and migmatites, overlain by sedimentary sequences classified into seven formations based on lithological characteristics and age. These include Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo, and Nyabusosi (Lukaye, 2016). Stratigraphic logs revealed fluvio-lacustrine sequences, including sandstones, silts and clays, with fining-upward and coarsening-upward sequences, cross-bedding, cross-stratification, and blocky clays, indicating deposition in fluvial-deltaic to lacustrine settings influenced by intense tectonic activity. Tectonic forces formed diverse structures, including faults, joints, bedding, laminations, mud diapirs, plunging folds, and normal faulting, observed on outcrops. Geophysical data, including magnetic and gravity surveys, confirmed the basin’s sedimentary nature, while petrophysical analysis of the Turaco-2 well identified moderate-quality sandstone reservoirs interbedded with thick clays and evaporites, suggesting reservoir potential. The presence of Sempaya hot springs and oil seeps in the Kibuku area indicates a high geothermal gradient and organic-rich source rocks, supporting source rock maturation and an active petroleum system. The study confirms the Semliki Basin contains all essential components of a petroleum system including source rock, reservoir, structural traps, and seal. Future exploration should prioritize high-resolution 3D seismic surveys, basin modeling, and geochemical analysis to assess gas origins and trap viability. The basin’s geothermal potential, driven by high heat flow and active faulting, also warrants further investigation.