Blaizot, Marc René, Total EP, Paris, France
From Oligocene to Pliocene, both the
A pronounced African uplift took place during the Oligocene. The subsequent erosion and the development of both the Niger and Congo river systems, which probably for the first time found direct access to the Atlantic, led to massive sediment loading and subsidence at the Atlantic margins. This is well demonstrated by a series of Oligocene-Pliocene isopach and paleogeographical maps.
The Tertiary eustatic phases, which created common regressive periods along the African margin, also had a major impact. Each regression is marked by huge sediment influxes and the development of classic gravity slide tectonics. These are typified by tensional regimes on the platform/upper-slope and compression regimes in the lower slope areas. Schematically:
Tensional zones contain roll-overs and tilted fault blocks - Compression zones exhibit swarms of short-wave asymmetric folds in the delta toe - Between the tensional and compressional areas, large zones of rafted (translated) turtle back structures typically develop. -Between the gravity slides en-echelon trans-tensional or trans-pressional faults or folds (transverse zones) are created.
The Upper Oligocene, Middle Miocene and Uppermost Miocene times were the key periods for the deposition and deformation of local turbidite reservoirs. Of particular exploration interest are the rafted zones where ribbon-like sand rich meanders are frequently trapped. Abyssal plains ahead or hollows between the front delta-toe folds can accumulate large thickness of lobe-like or ponded sands.
We describe several traps from this environment: - Large roll-over or tilted blocks in the tensional areas like Bengo or Amenam Fields - Narrow elongated anticlines in compressional zones like Kuro or Cola Fields - Large turtle-backs in the rafted areas like Akpo, Girassol or Gindungo Fields, - Complex extrados-faulted anticlines in transverse areas like Acacia or Usan or Fields.