Amplitude variations with offset of pressure-seal reflections

Knowledge of pressure compartments is essential for identifying potential risks in advance of drilling and in understanding the mechanism of hydrocarbon trapping and migration. These compartments are bounded by pressure seals which, under certain conditions, can be detected and characterized with seismic reflection data. Use of prestack seismic data requires the analysis of reflections generated at the pressure seals, such as the top of a reservoir, rather than the properties of the rock volume. In this sense, amplitude variations with offset (AVO) techniques can be a good tool to obtain information from such pressure seals or pressure transition zones. In this work, I investigate the AVO response of compartment seals by analyzing the plane-wave reflection coefficients as a function of pore pressure (above and below the seal), incidence angle, and seal thickness. For the case investigated in this work (Berea sandstone with high gas saturation), the AVO effects are important at low effective pressures, that is, when the pore pressure approaches the confining pressure. In shale-free transition zones, the anomaly is, in general, negative, whereas in shale/sandstone sequences, the anomaly can be positive or negative depending on seal thickness and formation pressure, In terms of the four-category classification for AVO crossplotting, the rr anomalies are mainly class IV and class I.