ABSTRACT: The Deep Green River Basin, Wyoming: An Unconventional, Overpressured Gas Accumulation

PENLEY, GARY N., and ROBERT K. MERRILL, Unocal North American Oil and Gas Division, Oklahoma City, OK

In the deep Green River basin between the Moxa arch and Rock Springs uplift, and downdip from gas/condensate production, several lines of evidence indicate that the Cretaceous Muddy/Dakota sandstones contain unconventional reservoirs that are gas saturated and overpressured, comprising a single accumulation some 30 mi wide by 60 mi long. Drill-stem tests recover gas to surface with no water and indicate a pressure gradient as high as 0.63 psi/ft. Pressure-depth

plots generated from drilling mud weights show the transition from normal to overpressure and indicate a pressure gradient as high as 0.75 psi/ft. The overpressured zone is typically encountered at about 14,000 ft. The potentiometric surface in the Green River basin is highest in the center of the basin, demonstrating that any fluid flow would be directed outward from the basin center toward the margins. Calculated water saturations in the basin center are lower than or equal to those on the Moxa arch despite the occurrence of water updip along the flanks of the basin. Source rock vitrinite data and burial history modeling indicate that the Mowry/Shell Creek and Thermopolis formations are still well within the gas generating window and still generating hydrocarbons within the basin. T ese hydrocarbons migrate into the adjacent Muddy/Dakota reservoirs with 3 to 12% porosity and form the fluid pressuring phase within the Green River basin. Potential recoverable reserves could be as high as 20 tcf.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)