Sequence Stratigraphy of Robertkiri Field, Niger Delta, Nigeria

Olusola A. Magbagbeola and Brian J. Willis
Texas A&M University, College Station, TX

Deposits of Robertkiri field, in the central area of Niger Delta, comprise a 4 km thick succession of Pliocene to Miocene shallow marine and non-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. These deposits progress upward from Akata Formation marine shales through paralic sandstone-shale units of the Agbada Formation and finally to the sandy non-marine Benin Formation. The Agbada Formation is divided into six third-order sequences, hundreds of meters thick, formed during episodic progradation and retrogradation of deltaic shorelines. The Agbada Formation 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. Robertkiri field is located along the proximal margin of the Coastal Swamp I depobelt, one of a succession of subbasins 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. Successive sequences become thinner, more laterally uniform in thickness, less structurally deformed and contain less growth strata. Erosion along successive sequence boundaries becomes shallower and broader, as accommodation declined and more sediment was bypassed basinward. Collapse of this continental margin occurred under prograding deltaic deposits on the shelf, rather than under base of slope lowstand wedges.