Geology and stratigraphy of the south-eastern Lake Edward basin (Petroleum Exploration Area 4B), Albertine Rift Valley, Uganda

The Lake Edward basin lies within the Albertine Rift Valley of Uganda and the Democratic Republic of Congo which forms the northern end of the western arm of the East African Rift System. It is a frontier petroleum prospective area, which, at the outset of this study, had no exploration wells drilled within it or any deep reflection seismic surveys. There have been some previous studies in the basin, but none produced a geological map subdividing the onshore rift-fill sediments or established a workable stratigraphic framework for them. Between 2007 and 2010, Dominion Uganda Ltd., in collaboration with Trinity College Dublin and the Petroleum Exploration and Production Department of the Ministry of Energy, Uganda, undertook a geological mapping survey of the south-eastern onshore part of the basin, known as petroleum ‘Exploration Area 4B’ (EA4B). Five rift sediment formations were identified and mapped across the area to produce a new geological map of EA4B. Palynological analyses suggest that all exposed rift sediments are (Late to Mid) Pleistocene--Holocene. EA4B is dominated by a north-east to south-west trending fault zone which underwent significant extension within the last 130,000 years to produce a trough, or sub-basin, to the south-east against the rift margin. This trough subsequently filled, initially with ponded swamp clays, followed by coarse fluvial and alluvial clastics. There is field evidence for minor inversion and ‘pop-up’ structures along some footwall crests, suggesting that the neotectonic phase is compressional or transpressional, and this has caused stream rejuvenation and incision.