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STRATIGRAPHIC/STRUCTURAL
FRAMEWORK
The Niger delta basin covers an area of about
75,000 km2 and is composed of an overall regressive clastic sequence that
reaches a maximum thickness of 9,000 to 12,000 m (29,500 to 39,400 ft).
The delta developed with a balance between of sedimentation rates and
subsidence. The resulting sedimentation patterns appear to have been
influenced by the structural configuration and tectonics of the basement.
The evolution of the basin is strongly tied to spreading in the North and South
Atlantic oceans and the subsequent break-up of Gondwana. Regional tectonic
and the stratigraphic framework of the associated rift systems provide the
setting for the development of the Tertiary Niger delta.
The Niger delta built out in a series of
depo-centers,
extending over the continental edge onto oceanic crust. At least three
major depositional cycles have been identified spanning from late early
Cretaceous through the Cenozoic. Onset of the initial cycle occurred
during the mid-Cretaceous Albian and was terminated by the Santonian compression
event. The next cycle began with the Santonian/Campanian transgression and
continued through the Late Cretaceous regressive phase. The third
mega-cycle is a multiplex that was initiated by the widespread Paleocene
transgression and continued through the Cenozoic progradation of the Niger
delta. This Cenozoic progradation is the focus for hydrocarbon exploration
in the basin.
The Cenozoic sediment build-up was accompanied by
synsedimentary tectonics normal to the progradation resulting in a series of
seven parallel, fault-bounded depositional belts, becoming progressively younger
from north to south. The depo-belt architecture is overprinted by the same
stratigraphic sequence (from top to bottom) and consists of:
1) Conti-nental coarse-clastic Benin Formation,
2) Paralic, transitional to marine, interbedded sandstone and shale (clay) of
the Agbada Formation, and
3) Massive, marine shale section of the Akata Formation.
4) Formations of the stratigraphic succession are time transgressive and become
progressively younger as the delta progrades southward.
Syndepositional tectonics, regional sedimentary
patterns, shale ridges or diapirism, and basement influences on the Tertiary
overburden strongly affect the structuring of the Niger delta sediments.
The wealth of structural features that occur in the delta are triggered by
syndepositional tectonics and associated sediment loading. The mechanics
of the major growth-fault development can be directly correlated with
differences in depositional rates and subsidence. These mechanics produced
the mega-structural or depo-belt systems, macro-structures, and “teepee” or
back-to-back (diapiric) structures that are major factors in the development of
the hydrocarbon habitat of the delta.
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