Due to the contrasts in the physical properties of rocks, seismic amplitudes vary with source-receiver offset. How seismic amplitudes change with offset has been described quantitatively by the Zoeppritz equations and other modifications to the equations. Quantitative modeling and analysis of amplitude variation with offset (AVO) is useful in discriminating between different reservoir lithology and fluids.
In this work, amplitude variation with offset modeling and analysis was carried out using data obtained from offshore Niger Delta comprising gamma ray, caliper, resistivity, p-wave, density and neutron logs, and a 60-fold pre stack time migrated CDP gathers with offset range of 400 m to 6,000 m. A shear wave sonic log was transformed from the sonic P-wave log using the Castagna's relation. The reservoir was identified on the basis of its relatively low GR values and high resistivity. The study focused on one reservoirat well depths 1325m to 1535m, but detailed ray-tracing was done at the target reservoir depth of 1480m to 1495m (15m reservoir interval).. Prior to the modeling, a Fluid Replacement Modeling (FRM) was carried out to remove the effects of mud filtrate invasion into the reservoir.
The FRM result shows increase in shear wave velocity and decrease in Poisson's ratio across the reservoir, which is indicative of the presence of gas in the reservoir. AVO modeling carried out at the reservoir top shows negative intercept andnegativegradient; the intercept becoming more and more negativeas the offset increased, indicating a Class 2 AVO response.