AVO analysis of VSP and well log data

Craig A. Coulombe, Robert R. Stewart, Michael J. Jones

This study uses VSP and well log data to analyze the AVO response of a subsurface gas-saturated carbonate zone. Analysis of the well log data show that the gas-saturated zone is dolomitized with a porosity as high as 18 %. The VSP data consist of three surveys; a zero-offset VSP; an offset VSP, and a multioffset VSP. The P-SV converted-wave reflections from the offset VSP and multioffset VSP were found to correlate well with the pure P-wave reflections from each VSP. Forward modeling was used to analyze the AVO response of the gas-saturated zone. It was found that there is extreme tuning in the reservoir zone, causing the combined AVO response of the layers within 1/2 of the seismic wavelength to be very complicated. It was also found that a better match of the forward model data with the field multioffset VSP data could be made by lowering the P-wave velocity in the reservoir zone.