Role of Shale Deformation in the Structural Development of a Deepwater Gravitational System in the Niger Delta

This paper presents a three-dimensional (3D) seismic-based case study (similar to 1,200 km(2)) from the deepwater Niger Delta to examine the role of shale deformation in the structural development of a deepwater gravitational system. Tectono-stratigraphic interpretation reveals that this system consists of two sets of major fold-thrusts laterally separated by a central oblique detachment fold. A prominent shale thick beneath these structures is believed to have originated from tectonic deformation rather than a pretectonic thick, due to its complex internal structures. Seismic mapping of the growth units indicates synchronous initiation of the oblique detachment fold with the main thrusts and gradual growth in response to thickening shales. Downslope gravitational contraction is not considered the direct cause for the oblique shale-detachment fold. Evidence from the 3D shale distribution and deformation styles within the shale unit reveals that shales that were squeezed out of adjacent shale-thinning areas "flowed" laterally into the detachment-fold core. Based on the spatial variation in structural deformation and growth strata distribution, this study proposes a model that considers differential contraction and differential loading from syntectonic sediments as two key factors leading to the 3D shale redistribution which ultimately determines the deformation styles and evolution history within the overburden. Additionally, seismic imaging within the shale unit recognizes various internal structures ranging from hundreds to thousands of meters in scale, and confirms what has been suggested in previous studies, that redistribution of shales occurred through a combination of multiscale brittle failures, ductile folding, and plastic flows.