USGS Assessment: Oil and Gas in the Mancos Shale of Colorado's Piceance Basin (2016)

The Castle, an outcrop of the Niobrara Member of the Mancos Shale near Delta, Colorado. Joshua Hicks, USGS.

Using a geology-based method, the USGS estimates that 66.3 trillion cubic feet of gas, 74 million barrels of shale oil and 45 million barrels of natural gas liquids are technically recoverable (mean estimates) from the Late Cretaceous Mancos Shale of the Piceance Basin, part of the Uinta-Piceance Province in Colorado and Utah.

Why a new assessment

• Last done in 2003, as part of an assessment of the whole Uinta-Piceance Province.
• Since then, more than 2,000 wells have been drilled and completed in the Mancos Shale of the basin.
• New data: a USGS research core drilled in the southern basin added geologic and geochemical detail.

The Uinta-Piceance Province and the Mancos Shale assessment units. Base: National Park Service. USGS.

The rock

• Setting: deposited in a foreland basin of the Cretaceous Western Interior Seaway, which at its largest stretched from the present Arctic Ocean to the Gulf of Mexico and from central Utah to western Minnesota. The Laramide orogeny later broke the basin into Rocky Mountain basins, including the Piceance.
• Scale: more than 29 million years of deposition, and more than 4,000 feet thick across most of the basin — mostly mudrocks with land-derived silt and sand and biogenic calcite, laid down offshore.
• Richest part: the lower half, with the most calcite and organic matter (Type II and mixed II/III kerogen, up to 4.9 weight percent). The upper half holds scattered siltstone and sandstone bodies from ancient shorefaces.

How the gas and oil are held

The Mancos Shale is the main source rock of the Mancos/Mowry Total Petroleum System. Its hydrocarbons moved into tight reservoirs within the formation and into conventional reservoirs above and below it (including along the Douglas Creek arch); some stayed in the calcareous, organic-rich Niobrara Member.

• Tight reservoirs: basal offshore siltstones and sandstones; the Prairie Canyon Member (including the Mancos "B" zone); and shoreface sandstones in the upper Mancos and the Iles Formation.
• Production: shallower tight gas comes from hydraulically fractured vertical and directional wells, reported together with the overlying Williams Fork Formation; deeper tight gas and oil come mostly from fractured horizontal wells.

Assessment units

Unlike 2003, when the Mancos and Mowry were one interval across the whole province, this assessment covers only the Piceance Basin and splits the shale vertically into units with different geology, drilling methods and well yields:

Results (mean estimates)

Ranges are the F95 to F5 estimates. All the assessed resources are continuous (unconventional).

Sources

Based on Assessment of Continuous (Unconventional) Oil and Gas Resources in the Late Cretaceous Mancos Shale of the Piceance Basin, Uinta-Piceance Province, Colorado and Utah, 2016, by the Mancos Shale Assessment Team (Sarah J. Hawkins, task leader, and others), USGS Fact Sheet 2016–3030, U.S. Geological Survey (DOI), citing Cobban and others (1994), Johnson and Rice (1990), Hawkins and others (2012), Kirschbaum (2003), Cole and others (1997), Lillis and others (2003) and Peters and Cassa (1994); a work of the United States government in the public domain. The map, photograph and results table come from the fact sheet's PDF. More: USGS Energy Resources Program.