Structural and Stratigraphic Styles of Growth Faulted Shallow Marine Deposits in an Eastern Offshore Depobelt Oil Field, Niger Delta, Nigeria

Raphael Oaikhena Oyanyan; Olugbenga Oluseyi Oshinowo

doi:7150261

Seismic data analysis, wells logs motifs interpretations and field-wide wells correlations along dip and strike directions within sequence stratigraphic framework, were done to determine the structural and stratigraphic styles of shallow marine deposits. Structural analysis of seismic data showed that the studied field is an anticlinal downthrown block of a northeast - southwest trending boundary growth fault characterized by crestal and flanks synthetic growth faults, antithetic fault, counter regional growth faults and shale ridges. The combination of faults suggests a transition between a simple rollover anticline and a collapse crest structure, with trapping style interpreted as faulted anticlinal closure. Growth fault scarp that marks the cessation of the effects of growth faults subsidence on sediment stratal geometry formation triggers differential erosion in the eastern zones of the field. Sequence stratigraphic analysis of well-tied seismic data and well-logs identified five depositional sequences. The relationships between relative sea level at paleo-continental shelf and shelf-edge elevation, paleo-seafloor gradient and position of paleo-shoreline relative to shelf-edge, as well as energy flux determined the type of systems tracts in each depositional sequence. Vertical upward change in systems tracts thickness and paleo-topographic gradient reflect simultaneous decrease in rate of growth fault subsidence and increase in sedimentation rate over time. Resistivity logs indicate the occurrence of hydrocarbon in systems tracts reservoirs deposited during high rate of growth faults subsidence, when the rate of accommodation creation was high irrespective of the global sea level condition. Hydrocarbon-water contacts correlation indicates that the growth faults have high sealing capacity by structurally compartmentalizing hydrocarbon occurrences in reservoirs.

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