Facies analysis and sequence stratigraphy of the South East Lake Edward Basin, South Western Uganda

Abstract

The south-east Lake Edward basin is the onshore part of the Block 4B exploration area which was licensed to Dominion Petroleum Limited in 2007. This area lies within the western branch of the East African Rift System (EARS). Dominion Petroleum Ltd is carrying out intensive oil exploration in the Lake Edward Basin whereby the researcher participated in some components of this exploration. This study impinges upon the economic potential of the Albertine graben, principally its oil reserves. It attempts to settle the stratigraphic problems within this part of the EARS with the aim of evaluating its petroleum potential. From previous reports, several formal lithostratigraphic names can be seen. However, it is hard to tell how these differ internally. During this study, a lithofacies approach was employed to look at exposed sections at a higher resolution than just recognizing formations. This approach proved flexible in studying sedimentation in fluvial dominated, tectonically controlled system with abrupt lithofacies changes. Using a combination of field surveying and section measurements, X-Ray diffraction measurements, organic, major and trace element geochemistry, it was possible to designate exposed strata into lithofacies and characterize stratigraphic intervals using the defined facies, depict presence of petroleum source somewhere in the basin, deduce sedimentary environments and tectonic events that have occurred within the basin. Results from this study have shown that rift-fill sediments in the southeast Lake Edward basin are dominated by alluvial fans and fluvial distributary fan complexes. Five individual fan complexes can be recognized fringing the edge of the rift and these merge distally towards the present day lake shoreline. Within the fan complexes, five broad Lithofacies can be recognised. All these fall under two formations i.e. Bwambara and Queen Elizabeth. Potential source, reservoir and seal intervals can be identified within these fan complexes and sequences from outcrop studies. The ongoing petroleum exploration has also produced the first confirmed occurrence of relict oil in porous tufa limestones and on the surface of Lake Edward indicating presence of a working petroleum kitchen some where in the basin. Structurally, the studied part of the basin is dominated by down-to-the east normal faults controlled by basin bounding faults to the east and west (DRC).