Evolution of Structures and Depositional Facies Associated with Shale Tectonics, Miocene-Pleistocene, Offshore, Niger Delta

Shale deformation can have an enormous impact on the sedimentary facies overlying deforming shale units, giving rise to various depositional, structural and geomorphological elements. These features impact considerably on petroleum systems as they often have direct bearing on hydrocarbon migration, accumulation and trapping. Understanding the spatial and temporal evolution of structures and depositional facies above shale detachments has implications for hydrocarbon exploration and exploitation along margins prone to shale tectonics.

Using a 782 km2 3D seismic and well data from offshore Niger Delta, we have imaged various stratigraphic, structural and geomorphological features produced in a 4.5s TWT thick Miocene-Pleistocene sedimentary sequence overlying a deformed pre-Miocene marine shale unit.The Early-Mid Miocene interval overlying the shale unit consists of antiform and synform structures that probably formed in response to the lateral withdrawal and migration of mobile shale as well as the compressional uplift and folding of pro-delta strata. The antiform structures are thrust-cored folds and are associated with thrust faulting. The Mid-Miocene to Pliocene interval is characterized mainly by mass transport deposits (MTDs) and erosional channels. The MTDs, marked by low to high amplitude, sub-parallel to distorted seismic reflections, occur in single to amalgamated units, of 80 to 400ms TWT thickness and are truncated by mud chimneys of post-dated mud volcanism. The Pleistocene interval is characterized by sub-parallel to parallel reflections, buried mud volcanoes, fluid structures and stacked channels. The Miocene-Pleistocene sequence is truncated by a post-dated episode of fluidized mud extrusion, marked by mud chimneys reaching 3km in diameter. The mud volcanism, which formed mud cones, collapse vent structures and radial faults on the sea-floor indicate the continuous influence of the mobile shale unit on the structural, stratigraphic as well as the geomorphological evolution of the Neogene to Recent sedimentary facies.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California