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