Abstract

The coalbed methane resources in the Upper Cretaceous Mesaverde and Meeteetse Formations of the Wind River Reservation were studied using surface and subsurface information. Coals were cored and desorbed at several localities at depths of less than 1,000 ft in order to study the distribution of coalbed gas at shallow depths in the Reservation. Mesaverde coal resources occur in the Reservation at depths of 9,000 ft or less along an 8 to 14 mile wide zone along the southwest flank of the Wind River Basin and in the North Fork Syncline, west of the main part of the basin. About 4 billion short tons of Mesaverde coal occurs in beds 2 ft thick or greater at depths of 0 to 3,000 ft, 9.4 billion short tons at depths of 3,000 to 6,000 ft, and 9 billion short tons at depths of 6,000 to 9,000 ft. The total thickness of coal in beds 2 ft thick or greater in the Mesaverde Formation varies from less than 20 ft along the western margin of the Wind River Basin to over 100 ft south and east of Riverton in the southeast corner of the Reservation. Total thickness of coal in the Meeteetse Formation varies from about 6 ft to 40 ft but data are sparse.

Methane contents of 0.40 to 3.60 cc/gm (12.8 to 115 cubic ft/ton), recalculated to a dry ash-free basis were desorbed from Mesaverde coals along the southwest flank of the basin at depths of 307 to 818 ft. Little methane was desorbed from Mesaverde coal beds at Alkali Butte, and from Mesaverde and Meeteetse coal beds in the Shotgun Butte area, indicating that degassing of coal beds in the shallow subsurface has occurred locally. Factors that may have contributed to degassing are the presence or absence of a seal, and the amount of sandstone in the coaly interval. Mesaverde coal beds are buried beneath younger strata along the southwest flank of the Wind River Basin. Gas and oil were encountered in sandstones in the coaly interval along the southwest flank of the basin suggesting that these younger rocks act as a seal. In addition, the coaly intervals tested at Alkali Butte and in the Shotgun Butte area, where coals contained little gas, contain a much higher percentage of sandstones than coaly intervals that tested significant amounts of gas.

Low, average, and high estimates of in-place coalbed gas for Mesaverde coals in the Reservation were made for the following three depth ranges: 300 to 3,000 ft, 3,000 to 6,000 ft, and 6,000 to 9,000 ft. No attempt was made to estimate coalbed gas in the Meeteetse Formation because of the lack of data. The total tonnage of coal in the Mesaverde Formation at each depth range was estimated from isopach maps and overburden maps. These values were multiplied by estimates of coalbed gas in cubic feet per ton for each depth range to provide the in-place resource numbers. Desorption results from a drilling program were used to estimate cubic feet per ton in the 300 to 3,000 ft depth range. Data from other Rocky Mountain basins were used to estimate methane contents in coals at 3,000 to 6,000 ft and 6,000 to 9,000 ft because there are no data on methane contents for coals in the Wind River Basin in these depth ranges. For coals in the main part of the Wind River Basin in the 300 to 3,000 ft depth range, the low, average, and high in-place gas estimates are 110,269, and 604 billion cubic feet (bcf). For the 3,000 to 6,000 ft depth range, low, average, and high in-place gas estimates are 1.05, 1.75, and 2.45 trillion cubic feet (tcf), and for the 6,000 to 9,000 depth range, the estimates are 1.46, 2.30 and 3.14 tcf. For coals in the North Fork Syncline in the 300 to 3,000 ft depth range, the low, average, and high estimates are 37, 91, and 205 bcf of gas in place. In the 3,000 to 6,000 ft rang, the estimates are 268, 447, and 626 bcf of gas in place. Only a fraction of this gas can probably be recovered. There is only one producing coalbed methane well in the Wind River Basin and the information needed to estimate recovery factors is not yet available.