A recent paper documents a new method of evaluating bright spots or other direct hydrocarbon indicators (DHIs). The technique involves the qualitative comparison of compressional (P)-wave and shear (S)-wave¹ seismic data. In practice, such a comparison offers a viable means of evaluating DHIs previously observed on P-wave data. The application of SH-wave seismic data for evaluation of DHIs was documented with a case study of P- and SH-wave data from the Putah Sink field of central California. As a second case history, this paper presents an interpretation of P- and SH-wave seismic data from the Myrnam field of Alberta.

Shear waves differ from compressional waves in both the direction of particle motion relative to the direction of wave propagation and in the rock properties that control the wave velocity. A P-wave is an elastic wave in which the particle motion is perpendicular to the direction of wave propagation. Because of this relationship between P- and S-waves, the velocities of the two are functions of different rock properties.

Consideration of the elastic properties that control the velocity of P- and S-waves in a rock indicates that P-waves are sensitive to the type of pore fluid present within a rock whereas S-waves are only affected slightly by changes in fluid type. Thus, if the presence of gas within a reservoir rock gives rise to an anomalous seismic expression on P-wave data, a DHI, there will be no comparable expression on S-wave data. However, a P-wave anomaly generated by a lithologic feature, a false DHI, will have a corresponding S-wave anomaly. One consequence of this relationship is that it is possible to evaluate the potential of P-wave DHIs through a comparison of P- and S-wave seismic data recorded over a prospect.

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