Seismic analysis of the Niger Delta gravitational detachment system

Abstract

Ductile deformation of overpressured, fine-grained, argillaceous sediments (“mobile shale”) is commonly invoked to explain the deformation style at the base of thin-skinned, gravitational detachment systems. The usage of “mobile shale” arose as a consequence of poor imaging on seismic reflection data, where low-resolution seismic intervals appeared ductile on a seismic scale (a thickening and thinning of the seismic interval). Acquisition of high-quality seismic reflection data from the Niger Delta provides an opportunity to investigate the internal structures within a basal detachment succession that is commonly referred to as the “mobile shale.”

Deformation within the basal detachment succession in down-dip compressional settings is characterised by brittle deformation. Thickening of the basal detachment succession occurs through contractional duplexes and stacked imbricates that have formed within the cores of detachment folds. In up-dip extensional settings, the formation of stacked master detachment faults and detachments, which splay off the pre-existing master detachment fault, incorporates structures that formed in the hanging wall of older, structurally lower detachment faults into the basal detachment succession. Plastic deformation that involves a complete loss of shear strength within the deforming sediment probably does occur. Such processes are invoked to explain the lateral redistribution of strata leading to the formation of a “shale weld” in down-dip compressional settings.

The recognition of fault-related folding within detachment fold cores and the deformation imaged beneath a major listric fault system highlights the fact that end-member structural models do not always adequately capture the structural complexity at the base of gravitational detachment systems. Despite the overpressured signature of basal detachment successions composed of argillaceous sediments, overpressure is not synonymous with a wide-spread ductile deformation style. Therefore, the term “mobile shale” – although widely used – inaccurately represents the styles and types of the deformational processes that occur within basal detachment successions composed of overpressured, argillaceous sediments.