A vast unexploited source of hydrocarbons is oil shales; i.e., shales rich in kerogen. The Schlumberger Oilfield Glossary defines kerogen as “the naturally occurring, solid, insoluble organic matter that occurs in source rocks and can yield oil upon heating.” The original organic constituents of kerogen are algae and woody plant material. Kerogens have a high molecular weight relative to bitumen or soluble organic matter. Bitumen forms from kerogen during petroleum generation. Estimates vary as to how much oil is contained in oil shale reserves. The United States Office of Naval Petroleum and Oil Shale Reserves estimates some 1.6 trillion barrels of oil are contained in oil shales around the world, with 60–70% of reserves (1.0–1.2 trillion barrels) in the United States. Most U.S. oil shale is concentrated in the Green River Formation in Wyoming, Utah, and Colorado (covering 16,000 square miles).