Chapter 4. Evidence of working petroleum play systems: oil seeps and slicks

Maturation and accumulation of economic quantities of hydrocarbons are a common sedimentary component of continental rift basins around the world, both ancient and modern. Thus, groundwater is not the only fluid that may migrate along major rift faults away from basinal depocentres. De-risking petroleum exploration in rift basins requires demonstrating the presence of working petroleum play systems, and the presence of leaking hydrocarbons to the surface is a primary way of demonstrating they are operating. Oil seeps have been known in Lake Albert since 1925 and the Kingfisher, Mputa–Waraga and Jobi–Rii petroleum play systems are due to go into full oil production. Lake Edward, however, has remained a virgin frontier exploration area. Tufa–travertine limestones along the eastern rift-bounding fault of the Rwenzori Mountains have yielded organic residues which gas chromatography–mass spectrometry analyses indicate have a mature, oil-like distribution. Biomarkers suggest a marine shale source rock, the origin of which is at odds with both oils from Lake Albert and generation within the present East African Rift System. A Lake Edward Slick Survey also sampled oil on the water surface that has the same biomarker signatures as the tufa–travertine-derived residues. Together they suggest maturation from a deeper pre-Neogene (?Mesozoic–Paleogene) rift sequence.