A federal agency has increased by more than 40-fold its estimate of natural gas that could be extracted from the Piceance Basin in Western Colorado.

In fact, the Mancos Shale formation in the basin contains the second-largest assessment of natural gas ever conducted by the U.S. Geological Society.

The USGS estimated Mancos Shale in the Piceance Basin contains 66 trillion cubic feet of natural gas, 74 million barrels of oil and 45 million barrels of natural gas liquids. The estimate is for undiscovered, but technically recoverable resources. The USGS estimated in a 2003 assessment Mancos Shale in the Piceance Basin contained 1.6 trillion cubic feet of shale natural gas.

By comparison, the Marcellus Shale formation in the Appalachian Basin in an eight-state region of the Eastern and Midwest United States contains 84 trillion cubic feet of natural gas. The Barnett Shale formation in Texas contains 53 billion cubic feet of gas, the USGS estimated in earlier assessments.

The USGS updated the 13-year-old assessment of Mancos Shale formation in the basin as part of an effort to reassess natural gas and oil accumulations in the United States, said Sarah Hawkins, lead author of the latest assessment. “In the last decade, new drilling in the Mancos Shale provided additional geologic data and required a revision of our previous assessment of technically recoverable, undiscovered oil and gas.”

Since 2003, more than 2,000 wells have been drilled and completed in one or more intervals within the Mancos Shale in the basin. In addition, the USGS Energy Resources Program drilled a research well in the southern Piceance Basin that provided new information used to refine the 2003 assessment.

According to the USGS, Mancos Shale is more than 4,000 feet thick in the Piceance Basin and contains intervals that act as the source rock for shale gas and oil, meaning petroleum was generated in the formation. Some of the oil and gas migrated out of the source rock and into tight reservoirs within Mancos Shale, as well as into conventional reservoirs both above and below the formation. Oil and gas also remained in continuous shale gas and shale oil reservoirs within the Mancos.

Natural gas in the younger, shallower parts of the Mancos Shale is produced primarily from vertical and directional wells in which reservoirs have been hydraulically fractured. Shale oil and gas in the older and deeper intervals of the Mancos Shale are produced mostly from horizontal wells that have been hydraulically fractured.