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Abstract: Structural Features on the West Africian Continental Slope Delineated by 3-D Seismic Coherency


Coherency technology applied in the evaluation of slope areas in offshore blocks of West Africa provides dramatic images of near water bottom and subsurface structures/hazards. These structures/hazards include pock mark-like features, faulting, channels, shale ridges, shale diapirs and large-scale slump blocks (slides).

The pock mark-like features are shallow (200-250ms below sea floor), circular to oblong depressions, 100 to 200m in diameter, which commonly overlie faults and subsurface meandering channels. The origin of these features is associated with the migration of gas via counter regional faults and fractures and faults resulting from differential compaction of the channel/channel margins.

Massive (5-6km X 50km)low coherence subsurface shale ridges are oriented orthogonally to the present shelf edge. Randomly oriented high coherency zones within the ridges indicate that the ridges are texturally heterogeneous. Some high coherency zones still retain a stratigraphic signature and are interpreted as blocks of the adjacent substrate that were incorporated into the ridges. This is supported by the irregular ridge boundaries as depicted in the 3-D coherency visualization of the ridges. Shale diapirs, associated with the ridges, are observed in various stages of development.

Large (5km X 8km) subsurface blocks exhibit linear and chaotic textures representing compressional faulting resulting from downslope movement.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria