Sequence stratigraphy of the Pliocene-Pleistocene strata and shelf-margin deltas of the eastern Niger Delta, Nigeria.

摘要

Sequence stratigraphic analysis of the Pliocene-Pleistocene strata in the eastern part of the Niger Delta from shelf to slope have been carried out using 2,000 km of high resolution 2D and 4,000 km2 of 3D seismic data. The study employed seismic stratigraphic techniques integrated with wireline and biostratigraphic data from 16 wells to document the depositional systems, facies, and stratigraphic architecture/style of third- and higher order stratigraphic units. Additionally, the linkage between shelf margin depositional systems and processes and deepwater deposits was also documented.;An additional control on depositional systems and stratigraphic units is sediment supply. High rates of sediment supply allowed sedimentation to keep pace with structural collapse and subsidence. It also allowed deltas to prograde basinward once accommodation is filled, even during periods of background relative sea-level rise. In this setting, because of high sediment supply and structural collapse under sediment loading, sediment failure, and transportation by gravity is the primary mechanism for sand transfer from the shelf/shelf margin to the deepwater environment. This is contrary to existing sand transfer models. Also, accommodation controlled by local fault subsidence is a key component in determining whether sediment is sequestered on the shelf margin, or able to bypass the shelf margin to deepwater provinces. This has implications for existing sequence stratigraphic models for unstable progradational margins.;The Pliocene-Pleistocene strata in the eastern part of the Niger Delta occur within a 1.6 km thick succession containing five third-order maximum flooding surface-bounded genetic sequences, located in the offshore and slope I "subbasin". The subbasins, or depobelts, were formed by continental margin collapse due to sediment loading of the underlying undercompacted mobile shale. The collapse created basinward and landward dipping listric faults which mark the proximal and distal ends of the subbasin, and complicates the sequence stratigraphy of these strata. Specifically, because these faults are synsedimentary, the rate and timing of fault activity (reactivation) or subsidence varies along strike and dip resulting in variable accommodation. The effect of the along-strike variation in accommodation is the along strike variation in genetic sequence thickness, depositional systems, and systems tracts between coeval stratigraphic units at the third- and higher order levels.