INFERRED STRUCTURAL-STRATIGRAPHIC EVOLUTION OF THE LATE MIOCENE EASTERN NIGER DELTA: EVIDENCE FROM SEQUENTIAL RESTORATION

Four sub-regional structural cross-sections have been constructed across the shelf and upper slope regions of the easternmost parts of the Niger Delta (50-80 km long). These profiles are constrained by seismic data and numerous well penetrations, however the structural and stratigraphic geometry of the Upper Miocene becomes less certain at > 3300 m depth which is below most well depths and beneath the clear, contiguous seismic imaging. Structural restoration offers a means for testing the feasibility and consistency of deep interpretations. Restoration transformations involve inclined-normal-shear (reverse-drag structures), vertical-shear (mobile shale withdrawal), slip-line shear (margins of shale diapirs), residual pure shear (distributed layer parallel stretching), and inclined-reverse-shear (contractional fold-thrust structures). Section construction, restoration, and decompaction used LithoTect software.

Deep structural interpretation and sub-well control stratigraphic correlation is assessed and modified iteratively by the process of restoration. Final restoration to a sequence of four Mio-Pliocene datums (4.4, 4.9, 5.7, 7.8 Ma) allows the average strain rate to be computed for each interval. Sequential back-stripping of decompacted layers allows average sedimentation rates to be determined. For the eastern Niger Delta, the restorations show: a) since the Late Miocene average extensional strain rates within the actively extending outer shelf-upper slope region range between (9x10^-16 – 7x10^-15 s^-1), b) higher strain rates correlate with higher sedimentation rates, c) strain rates and net magnitudes of extension diminish to the east as Niger Delta progradation contacts the bathymetric high of the Cameroon Line.