Basin-Centered and Conventional Gas Accumulations in the Williams Fork Formation, Piceance Basin, Colorado

Cumella, Stephen¹ (1) Bill Barrett Corp, Evergreen, CO

Most gas production in the Piceance Basin is from a continuously gas-saturated interval in the lower two-thirds of the Williams Fork Formation. Most of the gas reservoirs in this interval are discontinuous fluvial sandstones. Sandstones completed in this interval commonly produce very little water; a typical well produces less than five barrels of water per million cubic feet of gas. The continuously gas-saturated interval is probably the result of pervasive fracturing that has allowed gas to migrate from the coals in the lower part of the Williams Fork up to the top of continuous gas saturation. The pervasive fracturing likely resulted from overpressuring as the rate of gas generation from the coals exceeded the rate that gas could migrate upwards and laterally away from the coals. Above the continuously gas saturated interval, both gas- and water-bearing sandstones are present in what is commonly referred to as a transition zone. Gas in the transition zone was probably conventionally trapped as gas migrating upward along major fracture and fault zones encountered sandstones that had the potential to trap gas in stratigraphic and structural traps. Downdip water contacts can be identified in some of these accumulations, especially in those with better reservoir quality. However, in many instances downdip water is absent, perhaps due to regional uplift and resulting gas expansion in discontinuous sandstone reservoirs that are tightly sealed. Completing high-water producing intervals can seriously impact the economics of the wells, but differentiating wet zones from gas zones on logs is difficult.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana