An assessment of in-place gas resources in low-permeability Upper Cretaceous and lower Tertiary sandstone reservoirs, Wind River Basin, Wyoming

Abstract

A volumetric approach was used to estimate in-place gas in the Upper Cretaceous through lower Tertiary age basin-centered, low-permeability gas accumulation in the Wind River Basin of central Wyoming. The accumulation was subdivided into a total of 22 plays. Eight different stratigraphic units were considered. These units were then subdivided into as many as three plays based on variations in thermal maturity, presentday formation temperatures, and in the case of the lower member of the Paleocene Fort Union Formation, the presence or absence of an overlying shale seal. Highly overpressured plays occur where present-day formation temperatures exceed 300° F. Pressure gradients in these plays average about 0.73 pound per square inch per foot of depth (psi per ft). Moderately overpressured plays (0.52 psi/ft average pressure gradient) occur where present-day thermal maturity using vitrinite reflectance is 1.1% or greater. Transition plays occur where present-day vitrinite reflectance values are between 1.1% and 0.73%. These plays include a combination of tight gas reservoirs, gas reservoirs with conventional permeabilities, and water-bearing reservoirs,The lower member of the Paleocene Fort Union Formation, the stratigraphically youngest unit included in the basin-centered gas accumulation, is unique in that gas occurs where thermal maturates are as low as a vitrinite reflectance of 0.5 to 0.6%. The overlying lacustrine Waltman Shale Member of the Fort Union Formation appears to be acting as a seal inhibiting the vertical migration of gas out of the basin-centered accumulation. The lower member was divided into two plays, the first where the overlying Waltman Shale is present, and the second where the lacustrine shale has been replaced marginward by deltaic and fluvial deposits. The seal is assumed to be absent in the second play.A combination of overburden maps and isopach maps of total sandstone in beds 10 ft thick or greater is used to subdivide each play into as many as 120 subplay areas, each with a unique sandstone thickness and average depth. Point estimates were made of each attribute for each subplay using the maps, and a point estimate or mean estimate of in-place gas for each subplay was then calculated. The gas in-place in each of the subplays in a play was summed to derive a mean estimate of total gas in-place for a play. Estimates were then made of the ranges for each play attribute at the 95th and 5th percentile levels. The attributes were treated as independent continuous random variables, and ranges of gas in-place for each subplay were then calculated using probability theory. The subplays were aggregated assuming perfect positive correlation to calculate the range of gas in-place for each play. Finally, all of the plays were aggregated assuming perfect positive correlation to assess the total in-place gas in all of the plays in the Wind River Basin.Estimates of mean gas in-place for the twenty two plays are: 1) Upper Cretaceous Frontier Formation highly overpressured-118. trillion cubic feet (tcf);...

Reserves in Western Basins Part Iv: Wind River Basin

“…The balance of generation and loss is thought to occur at 200( F, thus overpressure occurs when temperatures exceed this threshold. The pressure gradient used by Johnson (1996b) for the moderately overpressured interval is 0.52 psi/foot.…”

“…This depth appears to coincide with the depth to 300( F. The latter was used by Johnson to define highly over-pressured regime subplay. The pressure gradient used by Johnson (1996b) for the highly overpressured interval is 0.727 psi/foot.…”

“…Dr. Johnson provided as many as he could that were not restricted by the provider and numerous additional mud logs were acquired from the Denver Well Log Library. Mud logs that were not duplicates of those provided by Dr. Johnson, were examined for mud weights and the depths of first use of 10 pound/gallon and 12 pound/gallon mud were added to the data file of the mud weight depths obtained from Johnson (1996b). Additionally, the mud logs were examined for characteristics of gas shows and sand occurrence (via drilling breaks).…”

“…The primary indicator of the top of overpressure according to Johnson (1996b) is the use of 10 lb./gal. mud by drillers to counter formation pressure.…”

“…Key reports were obtained from the USGS (Johnson et al, 1996a and1996b) and the Wyoming Geological Association.…”