Sequence stratigraphy of Niger Delta, Delta field, offshore Nigeria

The Niger Delta clastic wedge formed along the West Africa passive margin. This wedge has been divided into three formations that reflect long-term progradation: 1) pro-delta shales of the Akata Formation (Paleocene to Recent), 2) deltaic and paralic facies of the Agbada Formation (Eocene to Recent) and 3) fluviatile facies of the Benin Formation (Oligocene-Recent). This study combines a three-dimensional seismic image with well log data from Delta field to describe lithic variations of the Agbada Formation and develop a sequence stratigraphic framework. The 5000-feet thick Agbada Formation in Delta field is divided by five major sequence boundaries, each observed in seismic cross sections to significantly truncate underlying strata. Sequence boundaries developed as mass flows eroded slopes steepened by the structural collapse of the Niger Delta clastic wedge. Basal deposits directly overlying areas of deepest incision along sequence boundaries formed by the migration of large, sinuous turbidite channels. Upward-coarsening sets of inclined beds, hundreds of feet thick, record progradation of deltas into turbidite-carved canyons and onto down faulted blocks. Thinner, more continuous seismic reflections higher within sequences are associated with blocky and upward-fining well-log patterns interpreted to reflect deposition in shoreline, paralic, and fluvial environments. Episodes of structural collapse of the Niger Delta clastic wedge appear to be associated with progradation of Agbada Formation sediments and the loading of underlying Akata Formation shales. Progradation may have been more rapid during third order eustatic sea level falls. Effects of syn-sedimentary deformation on patterns of sediment transport and deposition are more pronounced in lower sequences within the Agbada Formation, and include: 1) incision into foot walls of listric normal faults, 2) abrupt reorientation of channelized flow pathways across faults, and 3) thinning of deposits across crests of rollover anticlines on down thrown fault blocks. Structural controls on deposition are less pronounced within younger sequences and canyon incisions along sequence boundaries are more pronounced, suggesting that the locus of sediment accumulation and structural collapse of the clastic wedge moved farther basinward as accommodation was filled in the area of Delta field.