Reconstruction of Subsurface Depositional History of Onshore Niger Delta through Electro- and Seismo-sequence Analyses

Electro-sequence and seismo-sequence analyses approach was employed to investigate the subsurface depositional processes in the onshore portion of the Niger Delta Basin. The study aimed at establishing the sedimentary facies, their succession and environments of deposition with a view to reconstruct the subsurface depositional history of the study area.

Facies were delineated through interpretation of the logs (gamma ray, neutron, density, and resistivity logs) from wells within the field. Subsurface trend of these facies was studied through well correlation which revealed the subsurface architecture of the study area. The different depositional systems were deduced by electrosequence analysis using gamma ray log trends and diagnostic shapes. Stratigraphic and depositional features were investigated through attribute analysis.

Four major depositional sequences were recognized in the study area, gradually dipping north-east. The sequence began with shale facies of shallow marine environment formed at the base, followed upward by sandy-shale facies and heterolithic facies of deltaic environment, and finally capped by sandstone facies of fluviodeltaic/continental origin. Electrosequence analysis revealed five major depositional systems: Shoreface, Tidal Channels, Distributary Channels, Deltas and Fluvial Systems, with twelve cycles of deposition. Seismic attribute analysis revealed local variation in the lithologies and facies distribution. The most striking depositional features observed were sinuous channel complexes

The study concluded that Onshore Niger Delta is characterized by a typical prograding delta architecture with a shallowing-up pattern in which a series of strata consistently showed evidence of the younger beds being deposited in shallower water than the older beds they overly. The younger beds tend to be of finer sediments (of delta slope region) than the older beds (of river mouth bars). The depositional systems revealed information which was stimulated from the start of deposition to the present day, and was used to carefully reconstruct the paleo-depositional history of the study area.