Sequence Stratigraphy of an Offshore Field, Eastern Niger Delta, Nigeria

M. E Nton and T. B. Esan
University of Ibadan, Ibadan, Nigeria

Sequence stratigraphy has been employed in an offshore field, Eastern Niger delta with the aims of deducing key bounding surfaces, depositional sequences and their corresponding systems tracts. In addition, palaeo-environment of deposition and potential stratigraphic traps for oil and gas are highlighted. These are based on integration of results obtained from seismic profiles, composite wire-line logs and biofacies data of four wells within the field of study. Lithostratigraphic analysis obtained from composite logs indicate the presence of two major units, notably; the paralic Agbada and the predominantly sandy Benin Formations. Seismic reflection configurations, well log shapes and faunal abundance/diversities, reveal inner neritic to supra-littoral palaeo-environments of deposition characteristic of a phase of delta progradation. Also, two sequence boundaries and three maximum flooding surfaces are recognized and used to subdivide the stratigraphic succession into depositional sequences and their corresponding systems tracts. Highstand and transgressive systems tracts are recognized in each of the three depositional sequences. However, the absence of Lowstand systems tracts can be inferred to have resulted from erosion by succeeding Transgressive systems tracts. Marker shales, characterized by Haplophragmoides-24 (6.0Ma) and Bolivina-48 (5.5Ma) were used to date the key bounding surfaces with the aid of the Niger delta chronostratigraphic chart.

Integrated analysis reveals sediment deposition from Late Miocene to Late Pliocene. Arising from the seismic profiles, it can be observed that the area is dominated by growth faults showing a depositional model characterized by expansional fault system and thus constitute the main trapping mechanism.