Mobile Shale in the Niger Delta: Characteristics, Structure, and Evolution
Published:January 01, 2011
Richard W. Wiener, Michael G. Mann, Michael Terry Angelich, Joseph B. Molyneux, 2011. "Mobile Shale in the Niger Delta: Characteristics, Structure, and Evolution", Shale Tectonics, Lesli J. Wood
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Mobile shale exerts fundamental controls on regional structural styles and sedimentation in the Niger Delta and also impacts trap geometry, reservoir distribution, fluid flow, and seal capacity at the prospect level. In the Niger Delta, overpressured shale forms a detachment zone for normal faults, detachment folds, and fold-thrust structures in a linked extensional-contractional system. The updip extensional zone is characterized by landward-dipping and seaward-dipping normal faults superposed on older contractional detachment folds. The thick mobile shale province is characterized by high-relief, shale-cored detachment folds and minibasins. Downdip contractional structural provinces include thrusted mobile-shale, low-to moderate-relief detachment folds, and a fold-thrust belt. Mobile shale is characterized by ductile deformation style, transparent seismic facies, and anomalously low density and seismic velocity. Detachment folding and thrusting are interpreted to characterize mobile shale deformation more so than diapirism.
Cross section restorations show a balance of extension and contraction through time, a basinward progradation of extensional and contractional systems, and an evolution of the mobile shale substrate. Shale withdrawal in half grabens and minibasins is balanced downdip by tectonic thickening in cores of detachment folds.
Paleogeographic reconstructions of the shelf, slope, and basin show structural controls on paleogeography and basinward progradation. The shelf margin was localized by major ex-tensional systems, whereas the slope-to-basin transition was controlled by the contractional structural front. Deltaic deposits formed along the shelf margin. Slope depositional systems included minibasins and confined-channel systems influenced by paleostructures, whereas basin-floor deposits were not affected by paleostructuresandhenceweremoreweakly confined.
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The phenomenon of rocks moving under their own means has always fascinated both scientists and nonscientists alike. The 2006 AAPG Hedberg Conference on Mobile Shale Basins was held in response to a need to gather industry and academic communities in a common forum to address the very existence of mobile shales. Stimulating and informative discussions at that Conference led to this special volume on shale tectonics. AAPG Memoir 93 documents shale tectonics from a variety of basins around the world, including the southern Beaufort Sea; the Krishna-Godavari Basin, India; eastern offshore Trinidad; offshore Brunei; and along the westernmost portion of the Mediterranean Sea. The book also provides information on the petrographic framework, behavior, geometries, and geodynamic models of shales. Publication of this Memoir coincides with a growing interest in shales as hydrocarbon reservoirs, and will add to the body of literature that significantly addresses both extrusive and intrusive shales.