It is difficult to predict whether gas saturation is low or high in reservoir pore spaces prior to drilling. When reservoirs include lateral porosity or clay content changes, this task is even more difficult. However, the problem is easier to address with high-quality multicomponent seismic data. This paper proposes to use DRps/DRpp as a partial gas indicator (PGI), where DRps and DRpp are defined as the change in the P-SV and P-P reflection coefficients, respectively. The target portion of the reservoir is compared with an inferred background portion of the reservoir, which is assumed to be 100% water saturated.

The DRps/DRpp ratio behaves quite differently for high and low gas saturations, as shown by theoretical reflection coefficient computation for a range of examples using the Zoeppritz and Gassmann's equations. The responses of DRps/DRpp to porosity and clay content changes are computed using empirical relationships among velocities, porosity and clay content (Wang and Nur, 1992; Han et al., 1986; Eberhart-Phillips et al., 1989; Castagna et al., 1985). The ratio is insensitive to the magnitude of porosity or clay content changes, and this behavior is very different from the variations in the ratio associated with changes in gas saturation.

Theoretical reflection coefficient computation, modeling, and synthetic seismograms show that DRps/DRpp is an effective direct hydrocarbon indicator and PGI for all three classes of gas reservoirs (Rutherford and Williams, 1989) at both shale/sand and sand/shale interfaces. The three classes of reservoirs are classified based on their acoustic impedance contrasts with their overlying shales. The DRps/DRpp ratio can distinguish water saturation changes from porosity or clay content changes and separate regions of high gas saturation from low saturation areas.

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