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Impact of Mobile Shale on Structural and Stratigraphic Evolution of the Niger Delta


Wiener, Richard W., Michael G. Mann, Valerie Goggin, David M. Advocate, M. Terry Angelich, ExxonMobil Exploration Company, Houston, TX


Deformation of mobile shale substrate is dynamically related to evolution of trap types, depositional facies and paleogeographic elements in the Niger Delta. The progenitor of mobile shale was deposited in deep water in the Early Tertiary, and tectonically mobilized during Late Tertiary deformation in a linked extensional-contractional system. Seismic trans­parency, low velocity, low density, and ductile deformation style characterize the mobile shale and indicate it is overpressured and mechanically weak. Understanding the properties and history of the mobile shale is important because of its impact on trap geometry, bed and fault seal risking, and reservoir distribution associated with structural development.

The mobile shale has a major control on structural styles and sedimentation in the Niger Delta. Updip deltaic sedimentation and associated extension is accommodated by shale withdrawal. Downdip contractional trap types that involve mobile shale include detachment folds, shear fault bend and fault propagation folds, and tear faults. The mobile shale zone contains stratigraphic surfaces that form regional detachments, which link extension, con­traction, and strike-slip in this gravity-driven system.

Structural features associated with mobile shale deformation controlled distribution of paleogeographic elements. The downdip limit of extension marks the paleo-shelf edge, and the downdip limit of contraction marks a major gradient change to undeformed lower slope / basin floor plain. These paleobathymetric features migrated basinwards with time and con­trolled deposition such that distributive deposits formed in the unstructured basin plain, ponded minibasin and channelized deposits formed on the slope where contractional defor­mation prevailed, and lowstand deltas formed along the shelf margin.