Amplitude-versus-offset (AVO) attribute analysis, AVO elastic impedance (EI) inversion, and various acoustic impedance (AI) inversion techniques were applied on the 3-D land seismic data from the Rio Bravo oil and gas field located in the San Joaquin Basin, California. The main objective of this seismic amplitude analysis and interpretation is to detect the presence of gas accumulations in the shale-sand sequences of the Pliocene Etchegoin Formation, through the direct determination of lithology and pore-fluid content from seismic data. The low impedance "I" gas sands encased in shale or other high compressional-wave velocity clastic rocks were easily recognized and tracked laterally through the use of different AI inversion sections. The low AI anomalies roughly correlate with the known gas entry zones as shown in gas producing wells 61X-34 and 41X-34. It is also possible to evaluate the other parts of the field by using the AI color amplitude characteristics of gas producing wells as examples.

The AVO response at the interface between the top of the "I" gas sand and the overlying shale was found to behave as a Class IV AVO anomaly. The AVO anomaly is characterized by relatively large negative reflection coefficients that decrease in magnitude or amplitude with increasing angle of incidence. In terms of the basic attributes, Class IV anomaly is characterized by large negative intercept (A) at zero-incidence angle and positive gradient (B) with increasing angle of incidence. The AVO response of brine sands follow the same characteristics but of smaller magnitude than the gas sands. In well Kernco 11-34, the presence of gas was detected at a depth interval between 5395 ft and 5430 ft by crossplotting the near (0°-15°) - and far (15°-30°)-angle range stacks. The same gas anomaly was detected by the elastic impedance inversion of near-angle stack. The results obtained from the elastic impedance inversion of prestack seismic gathers showed that this approach can be used as a good indicator of lithology and pore-fluid and could be applied to interpret seismic data in addition to the traditional poststack seismic inversion. In summary, all the seismic interpretation techniques that were applied could be used as a routine geophysical tool to evaluate the presence of gas accumulations in these shallow, porous, gas sands to reduce drilling risk.