Petrophysical and Rock Physics Analysis of Gas Sand Reservoirs in the Western Niger Delta Deepwater Using Seismic Inversion

Deepwater gas sand reservoirs in the western Niger Delta margin remain challenging to characterize due to the complex interplay of diagenetic alteration, fluid substitution effects, and limited well coverage. This study conducts petrophysical and rock physics analysis of gas sand reservoirs in the western Niger Delta deepwater using seismic inversion, with data processing performed at the Nnamdi Azikiwe University Applied Geophysics Laboratory. Pre-stack seismic data from two 3D surveys were loaded into Petrel and Kingdom Suite software for processing and interpretation. Four wells penetrating deepwater gas sands provided impedance, velocity, and density logs used as calibration inputs for model-based and simultaneous seismic inversion. Rock physics templates constructed from well log crossplots differentiated gas sands, brine sands, and shales in acoustic impedance and Vp/Vs space. Lambda-rho and mu-rho attributes derived from simultaneous inversion highlight gas-bearing zones as low lambda-rho and moderate mu-rho anomalies. Gas sands in the primary reservoir interval exhibit P-impedance contrasts of 12 to 20 percent relative to encasing shale, enabling reliable identification in the inverted volume. Sweet spot mapping on co-rendered attribute volumes outlines three discrete gas sand bodies with estimated gross rock volumes of 8 to 47 million cubic metres. Fluid substitution modeling using Gassmann equations confirms the predicted seismic responses for gas-saturated scenarios. These results provide a quantitative characterization framework for deepwater gas sand exploration and development planning.

Keywords: seismic inversion, rock physics, gas sand, Niger Delta deepwater, Gassmann