The largest shale oil reserves in the world are in the Green River Formation, spread out across Utah, Wyoming and Colorado, with the richest deposits in Colorado. Aside from these, California was once thought to have the largest oil shale deposit in the United States, with the Monterey Shale Formation.
The Green River Formation is a large area containing many shale oil deposits. There are three main oil fields, with the largest, by land area, located in Wyoming. However, the richest deposits are believed to be in Colorado. The entire formation is estimated to contain 3 trillion barrels worth of oil, of which 800 billion barrels are recoverable. Aside from the Green River states, California's Monterey Shale was once believed to be the biggest U.S. shale oil deposit, but as of 2013, only 4 percent of the deposit's 13 billion barrels are recoverable.