Cenozoic evolution of mobile shales and fluid escape features, offshore western Niger Delta.

Description

Numerous studies of the offshore Niger Delta have documented substrate-related collapse systems based on widely spaced 2D seismic data that generally lack detailed age calibration, but shale evolution through time remains poorly understood. This is investigated in this study through a detailed interpretation of an industry 3D seismic survey and calibration with biostratigraphic data. Our results show an overall thinning of stratigraphy towards mobile shale in the northeast of the study area at least since the Burdigalian. This suggests the onset of shale deformation in the Burdigalian, contrary to suggestions that it initiated in the Tortonian or later. Increase in sedimentation rates over the late Eocene-Serravallian is interpreted to have contributed to early generation of overpressure and progradation of the offshore western Niger Delta siliciclastic wedge. This is inferred to have contributed to shale deformation and release of overpressured fluids and solid particles through mud volcanoes and sea floor pockmarks. While mud volcanoes and giant pockmarks provide clue on deeper fluid migration, smaller pockmarks above normal faults, buried submarine channels and MTDs, suggest shallower fluid migration. The presence of buckle folds support previous observations that the active compressional zone of the Niger Delta was in the present-day continental slope during the late Eocene before migrating to the outer fold-thrust-belt in the Pliocene. This study provides new insight into the long-term control (18 million years) of mobile shales on the stratigraphic evolution of the offshore western Niger Delta.

Abstract

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