Abstract: Sequence Stratigraphic Framework of the Niger Delta Basin

ADESIDA, A.A., T.J.A REIJERS, C.S. NWAJIDE

Over 12,000 m of the Tertiary Niger Delta sedimentary fill has been subdivided into three broad and strongly diachronous facies units; the Benin Formation (continental, delta-top), the Agbada Formation (shallow marine, delta-front) and the Akata Formation (deeper marine pro-delta). The sediments accumulated during various increments of delta progradation from the southern termination of the Anambra basin into the Gulf of Guinea in equatorial West Africa. Regular marine transgressive-regressive cycles and a gradually evolving coastline controlled sedimentation dispersal, whereas growth faults determined subsidence, and growth-fault associated sedimentation created a series of basinwards growing 50-60 km wide depobelts in which petroleum traps formed, giving rise to one of the most prolific hydrocarbon provinces of the world.

Subdivision into three formations has proven to be insufficient for optimal stratigraphic application, hence various informal stratigraphic subdivisions have been proposed over time, in which local and regionally mappable reservoir units in various fields have been differently and inconsistently named. To set a first step towards bringing order in this unsatisfactory situation this paper proposes formal establishment of regional megasequences as a base for lithostratigraphy throughout the Niger Delta.

Based on sequence stratigraphic principles, a chronostratigraphically constrained framework is feasible, in which sedimentologic and biostratigraphic units are integrated with sequence stratigraphic events and chronostratigraphic boundaries. Such a scheme simultaneously elucidates the stratigraphic evolution of the delta and brings order to the chaotic nomenclature.

Eleven discrete regional depositional cycles are reflected by delta-wide genetic composite (mega)sequences with a transgressive basal (para)sequence set (TST), locally followed by heterogeneous coarse siliciclastics (para)sequence sets (HST). The various depobelts show development of the delta-wide genetic composite (mega)sequences through time. In these, interacting eustatic sea-level fluctuations of several frequencies are reflected during periods of variable subsidence rates. Each combination of changing parameters leaves a characteristic mark in the sequence. Each period of delta-wide marine transgression shows a retreat of some 70-100 km of the paleo-shoreline in a landward direction. Globally such retreats coincide with periods of major eustatic sealevel rises.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria