Field study of the Semliki Basin in the Albertine Graben, Karugutu Subcounty – Ntoroko District Makerere University.

Abstract

This report details the field study conducted in the Semliki Basin, Albertine Graben (Western Uganda), focusing on stratigraphic, sedimentologic, structural, and geophysical data acquisition and interpretation for hydrocarbon exploration potential. Fieldwork involved lithologic description, structural measurement, facies analysis, and visits to key sites including the Buranga geothermal field (Sempaya hot springs) and the Turaco drilling site. The basement comprises low- grade metamorphic rocks of the Buganda-Toro System. Overlying sediments consist of fluvial- lacustrine/deltaic sequences featuring cross-bedded coarse-to-fine sands, silts, and clays, exhibiting coarsening/fining-upward cycles, laminations, mud cracks, and an unconformable contact with the basement. Provenance analysis indicates sediment sourcing from the Rwenzori Mountains. Structural analysis revealed significant rift-related features. The basement exhibits foliation, joints, and faults. Sediments display intraformational faults (predominantly normal), joints, cross-bedding, and slump structures. Structural trends (faults, joints, paleocurrents from rose diagrams) align with the overall Albertine Rift trend. Seismic interpretation identified rift structures including rollover anticlines, faulted rollovers, and tilted fault blocks. Geophysical investigations (gravity, magnetics) indicate the basin's depocenter lies towards the Congo side, corroborated by litho-logs and seismic data showing increased argillaceous content basin ward. Wireline log analysis (Turaco wells: Gamma Ray, Resistivity, Density, Neutron, Sonic) aided formation evaluation. The presence of Sempaya hot springs signifies high subsurface heat flow. Discussion integrates findings to assess the petroleum system. Observed oil seeps (Kibuku area) confirm active hydrocarbon generation. The high geothermal gradient, evidenced by hot springs, likely caused early maturation of Cenozoic source rocks. Reservoir potential exists within the sandy facies, while structural traps (fault blocks, anticlines) and sealing shales are evident. Paleocurrent and facies analysis supports a fluvial-lacustrine/deltaic depositional environment conducive to source and reservoir development. It is concluded that the Semliki Basin possesses a working petroleum play. All essential petroleum system elements (source, reservoir, seal, trap, maturation, migration) are inferred or observed, rendering the basin highly prospective for hydrocarbon exploration. Recommendations focus on further targeted seismic surveys and more field geology trip.