Geology and Mechanics of the Basin-Centered Gas Accumulation, Piceance Basin, Colorado

Stephen P. Cumella, Bill Barrett Corp, 1099 18th St, Suite 2300, Denver, CO 80202, phone: 303 312-8538, fax: 303 291-0420, [email protected] and Jay Scheevel, Scheevel Geotechnologies, 1866 Deer Park Circle North, Grand Junction, CO 81506.

The Williams Fork Formation in the Piceance Basin contains multi-TCF reserves in a very large basin-centered gas accumulation. Most gas is produced from a continuously gas-saturated 1,500-2,400 ft gross interval in the lower part of the Williams Fork. During maximum burial, gas was generated and expelled from the Cameo coal interval in the lower most part of the Williams Fork. The Cameo is the primary source of Williams Fork gas. Both the lateral and vertical distribution of gas and the distribution of overpressure are directly linked to the migration of the gas as it moves upward and outward from the Cameo deep-basin coals. The overall distribution and pressure of the gas in the Williams Fork is probably the direct result of pore-pressure assisted fracturing and subsequent migration through the induced fracture systems. The first place one might expect fracture assisted migration to occur is within the gas-generative Cameo interval. When gas generation causes critical pore pressure to be exceeded, the rock fractures, and the rate of gas escape from the overpressured rock rapidly increases, stabilizing or reducing its pressure and allowing gas to flow into a lower-pressured adjacent sand body. One might expect this process to be repeated in a daisy-chain fashion, moving outward and upward from the gas generative parts of the Cameo. Ultimately, the ability of a given sand to sustain overpressured conditions will depend on the balance of the rate of gas entry with the rate of gas escape from the sand.