ABSTRACT
The Niger Delta, a notable clastic accumulation along the Western African passive margin, exemplifies a vibrant geological environment forged by enduring sedimentary phenomena. This investigation delves into the intricacies of sedimentology within the Agbada Formation situated in the Niger Delta, with particular emphasis on the Delta field. Encompassing an expanse of more than 5000 feet within the Delta field, the Agbada Formation showcases discernible lithological variances, traceable to a sequence stratigraphic structure distinguished by five significant sequence boundaries. These demarcations, prominently visualized in seismic cross sections, arise from the cascading movements that carved steeper slopes due to structural instabilities within the clastic prism of the Niger Delta. Noteworthy are the foundational deposits above these incised regions, exhibiting the meandering migration of substantial turbidite channels. Within these successions, the sedimentary chronicle unveils a captivating saga of forward expansion. Ascending sequences of coarsening beds, spanning hundreds of feet in breadth, chronicle the advancement of deltas into ravines sculpted by turbidite processes and onto downward-faulted segments. Higher in the sequences, slender yet more sustained seismic reflections harmonize with chunky and finer-ascending well-log configurations, signifying deposition in coastal, transitional, and fluvial settings. To conduct this examination, well log data for the 36 boreholes and the seismic dataset were input into Landmark StratworksTM and SeisworksTM, respectively. Ten stratigraphic horizons and significant faults were charted. Five of the stratigraphic horizons represented substantial erosion surfaces, while the remaining five were nearly level surfaces interposed between these erosional planes. The relative temporal sequences of the mapped horizons were established through correlation with the SPDC biostratigraphic dossier. The investigation unveils a correlation between instances of structural collapse within the clastic prism of the Niger Delta and the forward extension of sediments in the Agbada Formation, concurrent with the loading of underlying Akata Formation shales. Moreover, it posits that the pace of this forward extension might have been influenced by third-order eustatic sea level fluctuations. Interestingly, the impact of deformation during sedimentation on the patterns of sediment transport and deposition is most conspicuous in the earlier segments of the Agbada Formation. This encompasses erosions into the base of listric normal faults, redirections of channelized flow paths across faults, and the thinning of deposits over the summits of rollover anticlines on the displaced fault blocks. However, such structural influences diminish in prominence in later segments, with canyon erosions along the boundaries between sequences becoming more conspicuous. This hints at a shift of sediment accumulation towards the basin and the structural collapse of the clastic prism as accommodation space was filled in the Delta field region.
