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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
Wiener, Richard W., Michael G. Mann, Michael Terry Angelich, and Joseph B. Molyneux,
DOI:10.1306/13231313M933423
2010 by The American Association of Petroleum Geologists.
Mobile Shale in the Niger Delta: Characteristics, Structure, and Evolution
Richard W. Wiener,1 Michael G. Mann,2 Michael Terry Angelich,3 Joseph B. Molyneux4
1ExxonMobil International Limited, Leatherhead, Surrey, United Kingdom
2ExxonMobil Exploration Co., Houston, Texas, U.S.A.
3ExxonMobil Exploration Co., Houston, Texas, U.S.A.
4ExxonMobil Exploration Co., Houston, Texas, U.S.A.
ACKNOWLEDGMENTS
The authors thank ExxonMobil Exploration Company for the permission to publish and for underwriting publication costs. We also thank Fugro-Robertson for the permission to publish gravity data. We acknowledge Paradigm Inc. for the use of GeoSec software for cross
section
construction and balancing, as well as LCT for the software for 2-D gravity modeling, Paradigm Inc. for the GeoDepth software for 3-D velocity model building, and Schlumberbger for the use of the Geoframe software for seismic interpretation. We also acknowledge CGG Veritas United Kingdom for permission to publish their seismic data. The gravity modeling was performed by Terry Angelich, and the seismic velocity analysis and depth conversion were performed by Joe Molyneux. David Advocate of ExxonMobil Exploration and Valerie Goggin of BP Exploration (formerly with ExxonMobil Exploration) assisted in the paleogeographic reconstructions. The article benefited from reviews by Martin Jackson, Peter Bentham, Chris Shaw, Pam Darwin, Kevin Biddle, and Malcolm Wilson.
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 extensional 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 paleostructures and hence were more weakly confined.
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