Structural Style and Reservoir Distribution in Deep-Water Niger Delta: A Case Study of “Nanny Field”

Abstract

Aim: The structural styles and its effect on reservoir distribution in deepwater Niger Delta was analyzed using information derived from 3D seismic data and well logs interpretation with a view to facilitating a better implication of faulting with respect to determining prospectively potential within “NANNY” Field. Methodology: The data analysis was carried out using Nanny-1 well log, Well tie to seismic was conducted and five (5) mapable sequence boundaries were identified and interpreted across “NANNY” Field. Fault interpretation and framework modeling was developed for the field. Results: The results from seismic interpretation and well log data show that in the Inner Fold and Thrust Belt synthesis of the structural province is characterized by complex; broad scale thrust cored anticlines and imbricates structures that are widely spaced. This spacing creates accommodation space for reservoir development. This region also has multiple levels of detachment surfaces which improves hydrocarbon charge. The analysis of the Transition zone reveals that the structural province is typified by large areas of little or no formation. Prospects in the transition zone have well defined four way closures with large size of containers from drilled well results. The outcome of well log data analysis and fault interpretation on seismic reveals that the Outer Fold and Thrust Belt where “NANNY” Field lies, is dominated by smaller and less prominent sets of closely spaced imbricated thrust faults with shallow single detachment surface. This structural province is characterized by active and intense faulting style, some of which are still active as evidenced by the footprint on the sea floor. The regional petro-physical study using data from thirty wells randomly selected across the three structural provinces shows that in deepwater Niger Delta, porosity decreases with depth. This result shows that shallow reservoirs have higher porosity and permeability than reservoirs that are emplaced deeper stratigraphically. Conclusion: This study reveals that thrust faults in the structural province are closely spaced and active with little or no seal strength thereby preventing the focusing of hydrocarbon. Hence, prospectively within the “NANNY” Field area will likely have increasing geological risk arising from fault seals, limited fetch area, and small sizes of container and limited volume of source rock (Akata).