Unveiling Class IV AVO Responses. A Case Study from Peninsular Malaysia

Amplitude Versus Offset (AVO) responses play a critical role in subsurface characterization as they are indicative of relative variations of elastic properties within the subsurface. This paper embarks on a journey to unravel potential scenarios leading to class IV AVO seismic anomalies observed at depth of around 3000m bsl in the Malay Basin, offshore Malaysia. Seismic AVO classes are visualized on crossplots of VpVs (ratio of compressional and shear velocities) versus acoustic impedance with overlayed rock physics templates ( Odegaard and Avseth, 2004 ). By overlaying elastic responses with rock physics templates, we represent in a compact cross plot expected petrophysical properties such as volume of shales, porosity and fluid saturation, and reservoir conditions such as effective pressure and diagenetic effects. Rock physics analysis suggests that these anomalies may be associated with overpressured sandy or shaly layers, or may corresponds to the base of a seismically hard sand. While integration with other geological information may help reduce the number of possible outcomes, modelling helps capture the possible alternative scenarios that honour the seismic data.