Coal-bed Methane Show Within the Wind River Indian Reservation Enhances Gas Prospects in Deeper Portions of the Wind River Basin, Wyoming
BARKER, CHARLES E., RONALD C. JOHNSON, MARK J. PAWLEWICZ, BONNIE L. CRYSDALE, and ARTHUR C. CLARK, U.S. Geological Survey, Denver, CO
Desorbed gas volume from coal core when corrected for lost gas and to standard conditions (STP) indicates a total coal-bed methane (CBM) content of about 2 cc/g (100 scf/ton) in the Cretaceous Mesaverde Formation from depths of 470 to 590 ft in the southeast quarter of Sec. 32, T2S, R2E, northwest of Hudson, Wyoming. The desorbed gas is 97.3-99.7% methane with d13C1 signatures of -47 to -53(sigma), suggesting a degraded biogenic gas or a mixture of biogenic and thermogenic gas. In this area, the total thickness of coal beds ranges up to 30 ft with a vitrinite reflectance (Rm) of about 0.5%. These coals (considering only individual beds over 2 ft thick) thicken to a total of about 100 ft at approximately 3500 ft depth in the Riverton Dome area some 12 mi east of Hudson.
Cores taken in other shallow (< 1,000 ft deep) coals in the Mesaverde and Meeteetse Formations (also about 0.5% Rm) near Alkali Butte (southeast quarter of Sec. 29, T15, R6E) and near Uelton mine (center of Sec. 30, T6N, R2E), contained no significant gas. The water table at these sites is near surface; so natural desorption is apparently not now occurring, but sometime in the past such an event possibly depleted the CBM. Even though these two wells did not yield gas, the gas find near Hudson is significant; it suggests that more deeply buried coals should have good CBM potential because of the additional potential for preservation and because the content of thermogenic methane increases with depth and Rm. The shallow coals around the southwest edge of the basin are marginally matu e with an Rm of 0.4-0.5%, which indicates that gas, if present, will be early biogenic gas that must have been preserved during burial and water table fluctuations.
The burial depth and rank of Cretaceous coal generally increases to the northeast within the Wind River basin, reaching 18,500 ft and
2.0% Rm in a trough along the northeastern margin. In the deep trough the Cretaceous coals are generally too deep to be considered a good CBM prospect. However, Tertiary to Holocene folding coupled with erosion of at least 3,000 ft of rock across most of the basin (and in some areas even more) has resulted in significant exhumation of some coals.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)