A Strategy to Reduce Risk in Searching for Gas Accumulations in Tight-Gas Sand

Ronald C. Surdam and Zun Sheng. Wyoming State Geological Survey, P.O. Box 1347, Laramie, WY 82073, phone: 307 766 2286, fax: 307 766 2605, [email protected]

In Rock Mountain Laramide Basins, many commercial gas accumulations occurring in the lower Tertiary/Mesozoic strategraphic sections are characterized by the following critical attributes:

1) anomalously pressured (both over- and underpressured), but can appear to be normally pressured;

2) occur beneath a regional velocity inversion surface;

3) compartmentalized and gas-charged;

4) located at intersections of reservoirs facies and gas-charged domains;

5) productive intersections are typically enhanced by a combination of structural, stratigraphic, and diagenetic elements;

6) the reservoir facies commonly are considered tight-gas sands;

7) no apparent meteoric water connection.

In order to maximize risk reduction in searching for these types of gas accumulations, the highest priority tasks are: 1) evaluating gas distribution in the fluid system; and, 2) determining the distribution of reservoir facies in the rock system. Our work in the past 10+ years has documented that the distribution of gas-charged domains can be effectively delineated by detailed sonic/seismic velocity evaluations. More recent work suggests that a variety of seismic attributes can be used to adequately define the distribution of reservoir facies. Thus, at present, there are tools to determine the spatial intersections of gas-charged domains and reservoir facies. This approach has been tested in six known gas-producing fields in the Wind River Basin. The results have been positive; and, in each field, the productive intersections of reservoir facies and gas-charged domains were accurately located.