Geological and Geophysical Field Study in the Semliki Basin, Ntoroko district.

Abstract

Following the discovery of commercial reserves of oil in the Albertine graben, several research teams have conducted surveys to explore other parts of the graben whose potential is not known. Semliki basin being one of the six basins so far name within the Albertine graben stands a chance for exploration for petroleum in Uganda. In geological situations, the structure of the basin and thickness of sediment are always considered when conducting the various surveys. Hence the petroleum potential of the study area (Semliki basin) is assessed on a basis of the typical elements and processes of a petroleum system. Analyzing the effects of tectonism and structures in the lithologies of the basin in addition to the way sediments contribute to the petroleum play was very crucial for this investigation. With an aim of acquiring field training on how to collect and interpret stratigraphic, petrographic, sedimentologic and structural data determining the petroleum potential of the Semliki basin which lies in the south of Lake Albert, and the field study lasted a period of two weeks. The NW-SE normal faults encountered at the basin margins were used in sediment routing for the basin and analysis of potential structural traps in the area. From field observations different lithologies, stratigraphic arrangements, structures were envisaged each with a possible contribution to the petroleum potential of the basin. Results from geophysical data interpretation identified positive flower structures within the basin. A relationship to the main mode of normal faulting (main rift direction) was indicated by the NW-SE trending faults at Kichwamba and the basement exposure at the quarry. Foliation in granite gneiss whose trend is WNW-ESE was found to be perpendicular to the compressional forces. To justify the fact that hydrocarbons were already generated even after concluding that the potential source rocks of the Kasande Formation were at a shallow depth which could not allow for generation of hydrocarbons, the geothermal potential pf the basin was used. Having temperature gradients over 67°C/km the Sempaya hot springs among other players in the geothermal fields of this basin account for the maturation and generation of hydrocarbons at shallow depths. The group logging practice along the roadcut at Kibuku mainly covered sediments of the Kisegi Formation, Kasande Formation and Kakara Formation with an overall thickness of over 103 metres. Fluvial, lacustrine, and alluvial environments of deposition were deduced from the facies interrelationships and sediment structures. Paleo current directions from indicators such as cross-beds were used in deducing processes of transgression and regression for near shore environments. The sediments in the Semliki basin represent a petroleum play for hydrocarbon accumulation for which we have most of the necessary elements of a working petroleum system.