Fault seal analysis of the Lake Edward Basin South Western Uganda

Abstract

Fault seal analysis was carried out in the Lake Edward basin that lies within the Albertine graben between Uganda and the Democratic Republic of Congo. This work was aimed at assessing the fault seal potential of the faults in the basin in relation to the petroleum potential of the basin. To achieve this objective, reservoir and sealing rocks were identified within three possible reservoir formations (Edward, Kazinga and Kiruruma), the entrapment style of the basin was determined and fault properties were evaluated. Reservoir and sealing rocks were identified using the gamma-ray log. The entrapment styles were determined from 2D seismic data interpretation. Fault property evaluation was done from fault juxtaposition analysis, throw analysis, clay content analysis and shale gouge ratio analysis to determine the sealing nature of the faults. In the Kiruruma Formation, most rock units are reservoir rocks while an increase in sealing rock units is seen within the Edward and Kiruruma Formations. Structural traps including anticlines, tilted fault blocks and positive flower structures are found to be present in the basin. Tilted fault block traps are the dominant traps in the basin. Sand/sand juxtaposition prevailed within the Kiruruma Formation while sand/shale juxtaposition dominated the Edward and Kazinga Formations. Fault throws within the basin are generally high (350 m - 400 m) and the shale gouge ratio was established to be < 20% within all three Formations. These results indicate that the faults within the basin do seal within the Edward and Kazinga Formations. The sealing is mainly through sand-shale juxtaposition and not due to a membrane seal. There is no sealing within the Kiruruma Formation (leaking zone) The findings from this research, therefore, greatly reduce the risk associated with exploration in this basin. The availability of proven traps with good sealing is an indication of a strong ability to hold hydrocarbons. Uncertainty due to hydrocarbon leakages has been sufficiently reduced since most traps occur within the Edward and Kazinga Formations which contain sealing faults.