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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 50 (2000), Pages 249-258

Distinguishing Water Saturation Changes from Porosity or Clay Content Changes Using Multicomponent Seismic Data

Fuping Zhu, Richard L. Gibson, Jr., Joel S. Watkins, Sung H. Yuh


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 delta.gif (844 bytes)Rps/delta.gif (844 bytes)Rpp as a partial gas indicator (PGI), where delta.gif (844 bytes)Rps and delta.gif (844 bytes)Rpp are defined as the change in the P-Previous HitSVNext Hit 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 delta.gif (844 bytes)Rps/delta.gif (844 bytes)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 Previous HitZoeppritzNext Hit and Gassmann's Previous HitequationsTop. The responses of delta.gif (844 bytes)Rps/delta.gif (844 bytes)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 delta.gif (844 bytes)Rps/delta.gif (844 bytes)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 delta.gif (844 bytes)Rps/delta.gif (844 bytes)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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