Abstract

The Agbada Formation in Robertkiri field is a 2.7-km (9000-ft)-thick succession of Miocene shallow-marine and nonmarine deposits formed as the continental margin of the Niger Delta structurally collapsed under accumulating sediments. A high-resolution sequence-stratigraphic framework for these strata was constructed by combining data from 20 well logs and a 1400-km2 (550-mi2) seismic volume. Syndepositional structural collapse occurred along a succession of major, cuspate, offshore-dipping normal faults and associated antithetic faults and rollover anticlines within downdropped blocks. Six fourth-order sequences, each hundreds of meters thick, formed during episodic progradation and retrogradation of deltaic shorelines. Their development was complicated by the thickening of deposits across major growth faults and away from the crests of adjacent rollover anticlines. Successively younger sequences became thinner, more laterally uniform in thickness, and less structurally deformed and contain less growth strata above downdropped fault blocks. Erosion along successively younger sequence boundaries became shallower and broader as accommodation declined and more sediment was bypassed basinward. Areas of deepest sequence boundary incision and most rapid structural deformation shifted basinward over time with regional delta progradation. Stratigraphic patterns across successive downdropped fault blocks suggest that collapse of this continental margin occurred under prograding deltaic deposits on the shelf instead of within lowstand intraslope basins. Differences in the development of successive sequences reflect gradually slowing rates of structural collapse as underlying mobile shale is depleted, allowing deltaic deposition to shift farther basinward.

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