Abstract: An Assessment of In-Place Gas Resources in Low-Permeability Upper Cretaceous and Lower Tertiary Sandstone Reservoirs, Wind River Basin, Wyoming

JOHNSON, RONALD C., THOMAS M. FINN, ROBERT A. CROVELLI, RICHARD H. BALAY, AND WILLIAM R. KEEFER

A basin-centered, low-permeability gas accumulation occurs in rocks of Late Cretaceous and early Tertiary age in the Wind River Basin of central Wyoming. The central part of the accumulation is overpressured while the marginal or transition zone appears to be normally pressured to underpressured. The over-pressured part of the accumulation (pressure gradients average about 0.52 psi/ft) occurs in rocks with a vitrinite reflectance (Rm) of about 1.1% or greater while the marginal transition zone occurs in rocks with a thermal maturity of about Rm 0.73 to 1.1%. Pressure gradients of more than 0.73 psi/ft were measured in the central core of the overpressured part, where present-day formation temperatures exceed about 300 degrees F.

For a total of 22 plays, a volumetric approach was used to estimate in-place gas resources in each stratigraphic unit within the overpressured and transitional parts of the accumulation. Each play was subdivided into as many as 110 subplay areas. Mean estimates were made of each hydrocarbon-volume attribute for each subplay to calculate the mean estimate of in-place gas for each play. Estimates were also made of the ranges for each volume attribute at the 95th and 5th fractile levels, in order to calculate the range of in-place gas for each subplay using probability theory. The subplays were aggregated assuming perfect positive correlation to calculate the estimated range of gas in each play. Finally, all of the plays were aggregated assuming perfect positive correlation to assess total in-place gas in all of the plays. The mean estimated in-place gas for all 22 plays is 995 tcf, with a 95% chance of at least 603 tcf of gas in place and a 5% chance of at least 1,530 tcf of gas in place.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado