Correlation and Interpretation of Three-Component Survey Seismic Anomalies, Zamora Gas Field, Northern Sacramento Valley, California

J. F. Zaengle, C. W. Frasier

A three-component seismic line acquired over the Zamora gas field, northern Sacramento Valley, California, displays conventional (P-P) and converted-wave (P-SV) seismic anomalies. Well log and seismic modeling results indicate that bright-spot reflection strengths are related to variations in fluid saturation and lithology. The converted-wave reflection strengths and interval velocities provide valuable information that compliments more conventional strategies such as amplitude with offset and polarity analysis. This integrated strategy can improve risk evaluation and well site selection in areas where rapid changes in reservoir quality impact resource evaluation.

Stacked sections of the P-P and P-SV data display high signal-to-noise ratios. Wavelet extractions indicate that the processed P-P data contain a zero-phase wavelet that is exceptionally clean, with little side lobe energy. The extracted wavelet was convolved with the reflectivity series calculated from sonic/density logs to produce synthetic seismic traces. These modeling efforts provided exceptionally good ties between subsurface well control and the seismic field trial data.

The converted-wave (P-SV) data were compressed to P-P travel times using the ARCO mudrock line function, which relates Vs to Vp. This allowed an evaluation of event correlations between the two data sets. A low-porosity carbonate-cemented sandstone shallow in the section displays high-reflectivity events on both the P-P and P-SV sections. The presence of well-developed events on both the conventional and converted-wave sections is due to "hard-streaks" that result in a large increase in both Vp and Vs velocity. The top of the Capay shale has a much sharper P-SV than P-P reflection due to a much larger decrease in Vs than Vp velocity. The gas-productive Starkey sandstone is represented by a P-P anomaly with no corresponding P-SV seismic reflection. The absence of a bright-spot on the c nverted-wave section is due to the presence of gas, which results in a larger decrease in Vp than in Vs velocities. Amplitude with offset models support these conclusions.

P-P and P-SV seismic events on the original time sections were used to calculate Vp/Vs ratios for the principal lithologic units in the Zamora area. Well log correlations to the seismic events and Vp/Vs ratio data indicate that changes in Vp/Vs can be directly related to changes in sand/clay ratio and the consolidation state of the sediments both laterally and vertically in the section. The Vp/Vs ratios determined from seismic event correlations match those derived empirically using sonic logs and the ARCO mudrock line function. This result indicates that the ARCO mudrock line function provides a good first-order approximation of Vs when other shear velocity data are not available.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.