The Niger Delta is a Paleogene to Recent, wave-dominated delta situated in the Gulf of Guinea and extending into the northern JDZ. Following the Mesozoic rifting of the Atlantic, sedimentation began with Albian drift deposits. Sediments filled the Benue Trough and by Late Eocene time began to prograde across the existing continental slope into the deep sea. Continued seaward progradation since the Eocene has extended the continental margin to its present position.
Modern seismic data and improved models of sand distribution indicate that in places prospective acreage can extend up to 300 km from the present-day coastline of Nigeria. Extensive regional 2D and 3D multi-client seismic data shot by a number of seismic contractors provide a high quality regional dataset that has enabled unprecedented insight into the tectono-stratigraphic evolution of the Niger Delta and especially the deep-water province. The total sedimentary prism, an area of some 140,000 km2, has a maximum stratigraphic thickness of about 12 km.
STRATIGRAPHY The stratigraphy of the Niger Delta is divided into three diachronous units of Eocene to Recent age that form a major regressive cycle. The uppermost unit, the Benin Fm, comprises continental/fluviatile and backswamp deposits up to 2500m thick. These are underlain by the Agbada Fm of paralic, brackish to marine, coastal and fluvio-marine deposits, organized into coarsening upwards ‘offlap’ cycles. The underlying Akata Fm, comprises up to 6500m of marine pro-delta clays. Shales of the Akata Fm are overpressured and have deformed in response to delta progradation. These shales facilitate regional decollement for updip extension and downdip compression. Shales of the Akata Fm constitute a world-class source rock. Deepwater turbidite sands also exist within this formation.
STRUCTURAL AND DEPOSITIONAL HISTORY Diapiric shale structures began forming by Late Miocene time in response to lateral shale withdrawal from beneath the advancing deltaic load, combined with compressional uplift and folding of pro-delta strata. During the Pliocene and Pleistocene time, these structures were buried by the prograding delta and extensional growth faulting commenced. Subsidence within the depobelts ceased episodically, at which time alluvial sands advanced rapidly across the delta top, concurrent with a basinward shift in deposition and thereby creating seaward-stepping depocentres.
Extensive gravity tectonism has deformed sediments over the continental slope and the resulting folding, faulting and diapirism have created intraslope basins 10 to 25 km wide, filled with thick sequences of ponded sediments that represent a wide range of depositional processes. Submarine canyons cut across these deformed zones and give rise to aggradational channel/levee systems which are distributaries for large deep-sea fans. Transport and deposition of terrigenous sediments beyond the shelf have been accomplished mainly by turbidites and mass transport deposits (slumps, debris flows).
During periods of of low sea-level, deltas migrated seaward to the shelf edge and large amounts of sediments were transported to the slope, rise and deep-sea fans by turbidity currents and related mass flows via submarine canyons. Turbidity flows were confined within deep leveed channels on the upper and middle fan, but spread laterally outwards as sheet flows on the lower fan dispersing large amounts of coarse sediments across broad areas.
