Anomalously Pressured Gas in the Rocky Mountain Laramide Basins

By

Ronald C. Surdam, Zunsheng Jiao, and Yuri Ganshin

Innovative Discovery Technologies, Laramie, WY

Anomalously pressured, gas-charged energy resources can be effectively detected and delineated using sonic and seismic interval velocity evaluations. In such evaluations, it is important to begin by creating, from a combination of sonic and seismic data, a velocity interpretation that approximates the present-day rock/fluid velocity characteristics. Next, the ideal regional normal velocity/depth function and trends are determined and removed from the observed present-day velocity/depth profile. This operation (1) isolates the regional velocity inversion surface (i.e., the pressure surface boundary that separates normally pressured rock/fluids above, from anomalously pressured rock/fluids below), and (2) the anomalously slow, gas-charged rock/fluid volumes below the regional velocity inversion surface. In the Rocky Mountain Laramie Basins (RMLB), numerous observations suggest that anomalously slow velocity domains indicate the presence of a significant gas phase in the fluid system.

In the Wind River Basin, like all the RMLB, known anomalously pressured, commercial gas accumulations are characterized by anomalously slow velocity domains. Using velocity evaluations, the anomalously pressured gas assets can be detected using sonic logs (1-D), seismic lines (2-D), and seismic volumes (3-D). Examples from the Wind River Basin will be used to illustrate the isolation of anomalously pressured, commercial gas assets in one, two, and three dimensions.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming