Abstract: Gas Migrating from Basin-Center Low-Permeability Hydrocarbon Accumulations in the Rocky Mountain Basins: Where Does It Go?

Ronald C. Johnson, Dudley D. Rice, Thomas D. Fouch

Isotopically heavy (methane ^dgr¹³C -40 to -30 per mil), thermally mature gases are produced in many marginally mature to immature (R_m 0.5 to 0.8) lower Tertiary reservoirs in the Wind River basin of Wyoming, the Piceance basin of Colorado, and the Uinta basin of Utah. These gases probably migrated from deeper, overpressured basin-center hydrocarbon accumulations. The migrated gases are isotopically similar but commonly much drier chemically than gas in basin-center accumulations, suggesting that some of the C₂₊ fraction was left behind during migration. Predominantly vertical migration of nonassociated gas has occurred at the Madden, Pavillion-Muddy Ridge, and East Riverton Dome fields in the Wind River basin, at the Natural Buttes field in the Uinta asin, and at the Rulison and Sulfur Creek fields in the Piceance basin. Vertical migration distances are as much as 10,000 ft or more, even though many of these fields are not highly faulted. In contrast, associated gases at the Red Wash field in the Uinta basin have migrated laterally tens of miles along hydrodynamic gradients from the Altamont-Bluebell area.

Migration of gases from the basin-centered accumulations is retarded by thick Tertiary lacustrine shale sequences, such as the Paleocene Waltman Shale in the Wind River basin and the lower part of the Eocene Green River Formation in the Piceance basin. The isotopically light (methane ^dgr¹³C -42 to -50 per mil) gases above these shales are typically associated with waxy oil.

Both gas and oil were probably derived from the lacustrine shales. Determining migration pathways from basin-centered hydrocarbon accumulations will help in exploring for hydrocarbons in shallow reservoirs. In addition, large resources of gas are present in basin-center accumulations, although reservoirs are tight.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994