THREE-DIMENSIONAL MODELING OF THE DEEPWATER NIGER DELTA USING CRITICAL TAPER WEDGE MECHANICS

We present a new implementation of critical taper wedge mechanics to model the three-dimensional geometry of the compressive toe of the deepwater Niger Delta. Contractional deformation in the Niger Delta is driven by gravitational collapse of shelf sediments in the hinterland, creating a 600 km long fold-and-thrust belt that is similar, in many respects, to its counterparts in accretionary margins. In the Niger Delta, contractional deformation is accommodated on folds and thrust faults that sole into a regional detachment in the Akata Formation, an overpressured marine shale sequence. This detachment is imaged in regional seismic reflection profiles to be dipping toward the shelf, while the bathymetric slope is away from the shelf, consistent with a critical taper wedge model. As in the tectonic settings, an influx of sediments into the Delta causes the wedge to internally deform by Coulomb failure in order to maintain a constant taper angle.

We use these observations to create a three-dimensional model of the Niger delta fold-and-thrust belt using critical taper wedge mechanics. Regional 2-D seismic reflection profiles, detailed bathymetric data, and maps of the main detachment are used to define the regional geometry of the wedge. We supplement this dataset with deep-water well pressure information and fault maps to investigate spatial variations in the internal and basal strength of the wedge, including the distribution of fluid overpressures. Moreover, we find that topographic variations in the regional detachment directly influence the bathymetric slope of the toe of the Niger delta, in excellent agreement with the theory.