The Piceance Basin is a geologic structural basin in northwestern Colorado, in the United States. It includes geologic formations from Cambrian to Holocene in age, but the thickest section is made up of rocks from the Cretaceous Period. The basin contains reserves of coal, natural gas, and oil shale. The name likely derives from the Shoshoni word /piasonittsi/ meaning “tall grass” (/pia-/ ‘big’ and /soni-/ ‘grass’). Hydrocarbon resources In 2016 USGS released an assessment of the resources of the Mancos Shale of the Piceance Basin in Colorado and Utah, "a total of assessed technically recoverable mean resources of 74 million barrels of shale oil, 66.3 trillion cubic feet of gas, and 45 million barrels of natural gas liquids." Natural gas The basin has come to increasing public attention in recent years because of widespread drilling to extract natural gas. The primary target of gas development has been the Williams Fork Formation of the Mesaverde Group, of Cretaceous age. The Williams Fork is a several-thousand-foot thick section of shale, sandstone and coal deposited in a coastal plain environment. The formation has long been known to contain natural gas. The sandstone reservoirs have low permeability and limited areal extent, however, which made gas wells uneconomic in the past. In 1969 an atomic device was detonated as part of a scientific experiment in a well drilled into the Williams Fork Formation southwest of Rifle, Colorado to determine whether the device would fracture rock and enable extraction of natural gas in commercial quantities. Project Rulison, as it was called, was successful in liberating natural gas in commercial quantities, but was abandoned when the gas was determined to be unusable due to radioactivity. Advances in hydraulic fracturing within the past decade, along with higher gas prices, have made gas wells broadly economic in the area. In 2007 the basin contained five of the top 50 US gas fields in proved reserves (Grand Valley #16, Parachute #24, Mamm Creek #27, Rulison #29, and Piceance Creek #46). Oil shale Increasing demand for energy resources has spurred interest in energy alternatives such as oil shale. The Piceance Basin contains one of the thickest and richest oil shale deposits in the world and is the focus of most on-going oil shale research and development extraction projects in the U.S. The Piceance Basin has an estimated 1.525 trillion barrels of in-place oil shale resources, and an estimated 43.3 billion tons