Sequence Stratigraphic Architecture of the Sedimentary Succession in the South-Eastern Sedimentary Basin: A Potential for Energy Exploration and Development

Egbu, Ogechi C.¹
 Obi, Gordian C.²
 Nwajide, Sunny C.³
 Okogbue, Celestine O.⁴

The Campanian-Eocene sedimentary succession within the Anambra basin is analyzed to establish a sequence stratigraphic framework and provide new insight into the basin evolution and reservoir development. Sequence stratigraphic correlation of borehole and outcrop sections reveals four major lithofacies assemblages that determine the reservoir flow units and seals within the Campanian-Eocene succession. These include: fluvial and tidally-influenced fluvial facies; estuarine bay-fill delta; estuarine central basin and marine shale facies, and progradational shoreface-foreshore facies. The facies assemblages are arranged into seven basinward-stepping depositional sequences in which sandstone reservoir members encased by marine shale progressively overstep down-dip. Each of the sandstone reservoirs displays a progradational character, with (1) a lower abruptly progradational, wave-dominated shoreface facies that is overlain basinward by (2) tidally influenced cross bedded sandstone facies. This basinward architecture is interpreted to be a response to conditions of decreasing rate of addition of accommodation space in relation to sediment supply, which followed periods of reduced asymmetrical subsidence of the Anambra platform. The intervening marine shales reflect periods of slightly increasing accommodation and coastal transgression. Stratigraphic correlation shows that the reservoir sandstones are laterally continuous and locally compartmentalized. The estuarine central basin and marine shale facies provide for bottom seals, lateral seals and top seals. The over all consistency recorded in facies organization and distribution in the Campanian-Eocene sequences in the Anambra basin, and the progressive southward migration of depositional systems, suggest that the episodic and asymmetrical subsidence of the Campanian-Maastrichtian Anambra basin progressed into the Paleogene.