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Abstract

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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