A report on the geologic and stratigraphic logging project of Semiliki Basin-Albertine Graben in Ntoroko District, Western Uganda from 1st July to 9th July, 2024

Abstract

The geologic and stratigraphic logging project was conducted from July 1st to 9th, 2024, in the Semliki Basin, Ntoroko District, located in the central domain of the western arm of the Albertine Graben, western Uganda. The basin is a structurally complex pull-apart basin formed by transtensional strike-slip deformation along pre-existing NE-SW trending normal faults during active rifting of the Albertine Graben. The primary objective of the field study was to examine the basin's lithological, structural, sedimentological, and stratigraphic characteristics to reconstruct depositional environments and intended to apply the geological, geophysical and other relevant theoretical knowledge attained during the course of the program to study the petroleum system of the Semliki basin. The fieldwork involved sedimentary logging, structural measurements, depositional system reconstruction, and stream sediment analysis. Field methods were supplemented by geophysical data interpretation and software-based post-field analysis using tools such as Oasis Montaj, SedLog 3.0, and TecLog64. Some of the materials used during the study included a geologic hammer, Grain Size Scale, Jacob staff, handheld GPS, notebook and a geologic compass. Sediment lithologies found in area comprise of Sandstones, Siltstones, clays exhibiting structures like beds and laminations and fining upward sequences. From the sequences present the sediments are of fluvial, lacustrine and deltaic sediments Seven major formations are present in decreasing age in the Semliki basin: Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo, and Nyabusosi. These comprise primarily fluvial, lacustrine, and deltaic deposits. The stratigraphic succession, ranging from the Middle Miocene to recent, unconformably overlies possible Jurassic to Early Tertiary sediments and ultimately the crystalline basement composed of granites, gneisses, and amphibolites. Structural features, including faults, joints, bedding, cross-bedding, laminations, unconformities, mud diapirs, and folds, reflect the intense tectonism that has shaped the basin. Evidence collected supports the presence of an active petroleum system. Observations of oil seeps at Kibuku, mature source rock indicators, high geothermal gradients (>67°C/km) near Sempaya hot springs, and quality reservoir and seal rocks all indicate that the Semliki Basin is a viable hydrocarbon exploration target. This study successfully integrated field observations with geophysical and stratigraphic data to build a comprehensive understanding of the basin’s petroleum potential and geologic evolution.