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Distinguishing Water Saturation Changes from Porosity or Clay Content Changes Using Multicomponent Seismic Data
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 Rps/Rpp as a partial gas indicator (PGI), where Rps and Rpp are defined as the change in the P-SV and P-P reflection coefficients, respectively. The target portion of the reservoir is compared to an inferred background portion of the reservoir, which is assumed to be 100% water saturated.
The Rps/Rpp 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 Rps/Rpp 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 Rps/Rpp is an effective direct hydrocarbon indicator and PGI for all three classes of gas reservoirs (Rutherford and Willams, 1989) at both shale/sand and sand/shale interfaces. The three classes of reservoirs are classified based on their acoustic impedance contrasts with their overlaying shales. The Rps/Rpp 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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