Prospectivity in ultradeep water: the case for petroleum generation and migration within the outer parts of the Niger Delta apron

Abstract

AbstractThe interpretation of 2-D and 3-D seismic data acquired over the ultradeep water (1500–4000 m), lower delta-slope region of the Niger Delta indicates the continuation of the principal stratigraphic subdivisions out onto the continental rise. The Niger Delta sediment apron is still over 4 km in thickness in 4000 m of water. The continuation of a thick sediment pile into ultradeep water, coupled with the existence of a proven petroleum system on the higher parts of the slope, extends the potential region for mature Palaeogene and possibly older, oilsource rocks, up to and beyond the limit of current drilling technology. Contrary to previous models, the Miocene-Holocene Agbada Formation does not thin out oceanward relative to the underlying Akata Formation. The continuation of this seismic-event-based division of the Tertiary succession into and throughout the ultradeep-water province indicates an abrupt change in depositional style with the arrival of major submarine fans.The collapse of the Niger Delta sediment cone above overpressured muds of the Akata Formation is manifested in the ultradeep-water region as thrusting. The style of deformation varies across the area, depending on the level of detachment within the Akata Formation. The thrusting is divided into compartments by transfer zones against which toe-thrusts terminate or are offset. The toe-thrusts can be seen to have antecedents in the framework of rifts that predated breakup. These rift elements are imaged beneath the western delta region and reflect possible extent of extended continental crust in this area.The lower slope is traversed by large turbidite channels that extend from the upper parts of the slope to beyond the outermost toe-thrusts. These channels provide a route for sand to be brought into these water depths from the shelf and from mud-diapir-bounded mini-basins higher on the slope.The existence of underlying, presumed Late Cretaceous rift elements beneath parts of the ultradeep-water areas raise the possibility of additional source rocks within the ultradeep waters of the Niger Delta.