Evaluation of Sonic Velocity Anomalies Associated with Basin-Centered Gas Acumulations in the Greater Green River Basin, Wyoming

Mario A. Gutierrez, Brent A. Couzens, and Neil R. Braunsdorf
Shell International Exploration and Production Inc, Houston, TX

This investigation was aimed at improving the understanding of elastic wave velocity behavior in Tight Gas Sands (TGS) and associated rocks. The specific focus of this study was a determination of the cause(s) of observed P-wave velocity slowdowns often associated with basin-centered gas accumulations. The study focused on Cretaceous-age rocks in the Greater Green River Basin (GGRB), Wyoming. Our multidisciplinary approach involved: (1) an assessment of literature, sonic log interpretation methodologies, and acoustic measures (core datasets) for TGS and associated rocks, and, (2) an integrated pressure-velocity-log data analysis in key data areas. Study results indicate that variations in the sonic velocity log response are related predominantly to lithology. In contrast, gas saturation and abnormal pressures have a comparatively small impact on P-wave velocities at both the regional and field scales. We were unable to confirm the findings of previous studies purporting that seismic velocity could be used to detect abnormal pressures or gas-saturated rocks in the GGRB and call into question the use of velocity anomalies as an exploration tool in the search for gas accumulations in the GGRB. The significant influence of lithology on P-wave velocity response suggests we may apply seismic velocities and related attributes, such as acoustic impedance, to lithology identification in the GGRB.