Steve R. Lawrence and Ian P. Hutchinson
Geoscience, RPS Energy, Henley-on-Thames, United Kingdom
Several phases of rifting have affected the interior of Sub-Saharan Africa during the Phanerozoic. These took place in the Triassic (Late Karoo), the Cretaceous (Early-Mid and Late) and Tertiary (Mio-Pliocene-Recent of the East African Rift System or EARS). These rift episodes have different directions of extension meaning that rift trends often intersect. Conversely, rifting is strongly controlled by basement structure so that, if local conditions are right, there will be a propensity for the coincidence of rifts even if rifting is oblique to the prevailing direction of extension. These possibilities lead to the probability of ‘hidden’ rifts. In addition, in areas where there is a paucity of sub-surface data there is the possibility of ‘hidden’ rifts through ignorance.
We review regional rift models. Karoo rifting has been related to ‘continental escape’ where cratonic blocks have moved differentially along transcontinental shear-zones. The multi-phase Cretaceous rifting is related to regional NE-SW extension. Models for the EARS invoke mantle-driven ‘active’ rifting. We use these models to predict ‘hidden’ rifts in Central and Southern Africa.
The petroleum prospectivity of these interfering rift systems is evaluated on the basis of potential source rock development in the Karoo and Cretaceous rift sections. Lower Karoo (Permian) deltaic/paludal shales have a dominantly gas-prone character and where appropriately buried represent a huge, untapped gas resource. Late Jurassic/Early Cretaceous, Stanleyville lacustrine shales, sampled in Northeast DRC, are organically very rich and oil-prone. These would represent a source of oil resources where buried by superimposed Cretaceous or EARS rifts.