Analytical Study of Methane Hydrate Stability Zones and Gas Hydrate Resource Assessment in the Deepwater Niger Delta Slope, Offshore South South Nigeria

Gas hydrates represent a potentially significant unconventional hydrocarbon resource in deepwater environments, and the Nigerian deepwater slope in the South South offshore zone contains geological conditions broadly consistent with methane hydrate stability. This study analytically investigates methane hydrate stability zones and conducts a preliminary gas hydrate resource assessment for the deepwater Niger Delta slope. Thermodynamic stability calculations using the van der Waals-Platteeuw model were performed using measured seawater bottom temperature profiles, bathymetric data, and subbottom sediment temperature gradients derived from borehole temperature logs from deepwater wells. Hydrate stability zone thickness was computed as a function of water depth for the depth range of 400 to 2,500 metres. Published bottom-simulating reflector mapping from industry and academic 3D seismic datasets over the deepwater slope was compiled to define the areal extent of the hydrate stability zone. Sediment core data from Ocean Drilling Programme cruises in the Gulf of Guinea were used to constrain organic matter content and methanogenic potential for gas generation calculations. Total methane hydrate in-place estimates following the U.S. Department of Energy Methane Hydrate Inventory approach yield P90 to P10 estimates of 12 to 67 trillion cubic feet in the evaluated deepwater slope polygon. Geohazard implications for conventional deepwater drilling and pipeline routing through the hydrate stability zone are assessed. Keywords: methane hydrate, stability zone, gas hydrate resource, deepwater Niger Delta, South South Nigeria.