Abstract

The Greater Green River Basin of Wyoming, Colorado, and Utah contains large amounts of gas in overpressured low-permeability Cretaceous and Tertiary reservoirs. The stratigraphic sequence in which these reservoirs occur is as much as 14,000 ft (4,267 m) thick and includes the Lower Cretaceous Cloverly Formation through the Lower Tertiary Fort Union Formation.

The accumulation of thermogenic gas in these low-permeability reservoirs, at rates greater than it is lost, causes fluid (gas) pressure to rise above regional hydrostatic pressure. Thus, in the study area, all overpressured reservoirs are gas bearing. The gas-bearing overpressured sandstone reservoirs occupy the deeper parts of the basin, down-dip from water-bearing normal-pressured reservoirs. Structural and stratigraphic trapping aspects in these unconventional reservoirs are not as important as in conventional reservoirs; the top of overpressuring cuts across structural and stratigraphic boundaries. The source of the gas is predominantly type III organic matter in the interbedded coal and carbonaceous lithologies and type II and III organic matter in the interbedded marine shales.

For resource appraisal purposes, the gas-bearing interval was subdivided into five stratigraphic plays and the volume of gas-in-place for each play was estimated using a probabilistic analysis. Not included were normally-pressured "transition zones" where interbedded gas and water-bearing reservoirs occur shallower than and / or updip of the overpressured rocks. The estimate of the total gas-in-place resource of these plays ranges from 3,611 to 6,837 Tcf (trillion cubic feet) with 5,063 Tcf as the mean estimate.

Because of additional uncertainty related to economic recovery factors, estimation of recoverable volumes is inherently less precise than estimation of in-place volumes. Two of the most important factors influencing the volume of recoverable gas are price and technology. Therefore, recoverable gas was estimated for each play under two cases: (1) current technology, with a gas price of five dollars per thousand cubic feet, and (2) future technology, with no dollar limit. For the current technology case, the estimate of total recoverable gas ranges from 27 to 148 Tcf, with 73 Tcf as the mean estimate. For the future technology case, the estimate of total recoverable gas ranges from 189 to 816 Tcf, with 433 Tcf as the mean estimate.