Sequence stratigraphy of Niger Delta, Robertkiri field, onshore Nigeria

Deposits of Robertkiri field, in the central offshore area of Niger Delta, comprise a 4 km thick succession of Pliocene to Miocene non-marine and shallow marine deposits. A sequence stratigraphic framework for Robertkiri field strata was constructed by combining data from 20 well logs and a seismic volume spanning 1400 km2. Major sequences, hundreds of meters thick, define layers of reservoir and sealing strata formed during episodic progradation and retrogradation of deltaic shorelines. These deposits progress upward from fine-grained prodelta and deep water shales of the Akata Formation through paralic sandstone-shale units of the Agbada Formation and finally to sandy non-marine deposits of the Benin Formation. The Agbada Formation is divided into six third-order sequences starting at the first seismic reflection that can be mapped across the seismic volume. The Agbada Formation under Robertkiri field is complexly deformed across a succession of major, cuspate, offshore-dipping, normal faults, and associated antithetic faults and rollover anticlines within down-dropped blocks. Thickening of intervals between some reflections across major faults and away from the crests of adjacent rollover anticlines suggest syndepositional displacement. Relationships between major faults and the thickness of transparent seismic facies that comprise lower parts of the seismic record suggest faulting was associated with movement of undercompacted shales within the Akata and lower Agbada Formations. Robertkiri field is located along the proximal margin of the Coastal Swamp I depobelt, a subbasin within the Niger Delta clastic wedge formed by margin collapse into underlying undercompacted shale. Accommodation and sequence development in this setting is controlled by both structural faulting and sea level fluctuations. Upsection, sequences become thinner, more laterally uniform in thickness, less structurally deformed and contain less growth strata. Erosion along sequence boundaries becomes progressively shallower and broader, as accommodation under Robertkiri field declined and more sediment was bypassed basinward. Incisions along the base of older sequences (>100 m) is greater than 3rd order sea level falls reported to occur during the Miocene, which suggests that there were local areas of tectonic uplift within this dominantly extensional setting.