Direct hydrocarbon detection using comparative P-wave and S-wave seismic sections

Two field experiments in the Sacramento basin and one in the Green River basin demonstrate that comparative P-wave and S-wave CDP seismic sections can be used to detect gas directly in sandstone reservoirs. The lines in the Sacramento basin were shot over producing gas fields known to correlate with amplitude anomalies on P-wave sections. Reflections of comparable strength are present on the P-wave and S-wave sections at lithologic boundaries, but gas‐saturated zones correlating with P-wave bright spots show no equivalent S-wave amplitude anomalies. The responses are consistent with laboratory observations that P-wave velocity is more sensitive to the introduction of gas into liquid‐saturated pore space than S-wave velocity. The line in the Green River basin was shot over a relatively deep gas field producing from an overpressured reservoir not associated with a conventional P-wave bright spot. The P-wave reflection strength of the reservoir is about 50 percent greater than the S-wave reflection strength, whereas the P and S strengths of other major reflectors are comparable. The three field tests show that an S-wave section validates a P-wave bright spot attributed to gas saturation when there is no anomalous amplitude at the equivalent S-wave event and that the technique is useful for verification of subtle as well as strong amplitude anomalies.