SMITH, G.C. and GIDLOW, P.M. 1987, Weighted Stacking for Rock Property Estimation and Detection of Gas, Geophysical Prospecting 35,993-1014.Amplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo-stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties.The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P-wave reflection coefficients with the angle of incidence of a P-wave as a function of the P-wave velocities, the S-wave velocities and the densities above and below an interface.Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time-and offset-variant weights to the data samples before stacking. The properties which can be displayed by geo-stack are: P-wave reflectivity (or true zero-offset reflectivity), S-wave reflectivity, and the reflectivity of P-wave velocity divided by S-wave velocity (or ' pseudo-Poisson's ratio reflectivity '). If assumptions are made about the relation between P-wave velocity and S-wave velocity for water-bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas.

The Geostack technique is a method of analyzing seismic amplitude variation with offset (AVO) information. One of the outputs of the analysis is a set of direct hydrocarbon indicator traces called "fluid factor" traces. The fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the "mudrock line." The mudrock line is the line on a crossplot of P-wave velocity against S-wave velocity on which water-saturated sandstones, shales, and siltstones lie. Some of the rock types that lie off the mudrock line are gas-saturated sandstones, carbonates, and igneous rocks. In the absence of carbonates and igneous rocks, high amplitude reflections on fluid factor traces would be expected to represent gas-saturated sandstones. Of course, this relationship does not apply exactly in nature, and the extent to which the mudrock line model applies varies from area to area. However, it is a useful model in many basins of the world, including the one studied here. Geostack processing has been done on a 3-D seismic data set over the Mossel Bay gas field on the southern continental shelf of South Africa. We found that anomalously high amplitude fluid factor reflections occurred at the top and base of the gas-reservoir sandstone. Maps were made of the amplitude of these fluid factor reflections, and it was found that the high amplitude values were restricted mainly to the gas field area as determined by drilling. The highest amplitudes were found to be located roughly in the areas of best reservoir quality (i.e., highest porosity) in areas where the reservoir is relatively thick.

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