Detection of Anomalously Pressured Gas Reservoirs:

2-D Seismic Surveys in the Wind River Basin, Wyoming

By

Yuri Ganshin, Ronald C. Surdam, and Zunsheng Jiao

Innovative Discovery Technologies, Laramie, WY

Anomalously pressured gas (APG) accumulations are a huge, poorly explored energy resource. This study focuses on APG reservoirs in the Tertiary lower Fort Union and Cretaceous formations (6,000-20,000 ft depth) in the Wind River Basin (WRB). USGS estimates indicate the WRB (~8,500 mi2) contains ~900 Tcf of APG, but cumulative production is <1 Tcf. Despite increased natural gas exploration activity, APG development is hindered by a lack of new exploration and exploitation technologies and techniques.

Conventional semblance velocity analyses often do not provide sufficient resolution to detect velocity reversals associated with APG. This study used an enhanced seismic data processing technique _ automated, continuous (sample-by-sample) calculation of the interval velocity field _ with many potential exploration applications, including reservoir detection, characterization, and monitoring. Using synthetic models, this technique is shown to easily detect the presence of low-velocity layers as thin as a quarter of a wavelength (although it does not resolve bounding interfaces). It has been applied to a variety of on-land seismic datasets acquired over APG reservoirs.

Sonic logs provide direct evidence of the correlation between gas-charged rocks and measured anomalous velocities. Constructing a detailed velocity field for a seismic profile results in more detailed delineation of the velocity-inversion surface and anomalous domains beneath it, which significantly reduces uncertainty. Results of continuous velocity measurements are demonstrated on 2-D seismic P-wave reflection data from the WRB, and indicate that the presence of gas correlates with anomalously slow velocity. Integrating velocity data with other geophysical and geological data can further reduce uncertainty.

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