Oil shale is most commonly defined a sedimentary rock containing organic matter rich in hydrogen, known as kerogen. When the rock is heated, the organic matter releases petroleum-like liquids. Oil shale deposits are widely distributed around the world – some 600 deposits in more than 30 countries are known, with resources of the associated shale oil totaling almost 500 billion tonnes, or approximately 3.2 trillion barrels. It is estimated that over 60% of this amount – roughly two trillion barrels – is located in the United States.
Oil shale deposits in the United States are predominantly in the Green River Formation, which is spread through parts of Colorado, Wyoming and Utah. Using a geology-based assessment methodology, the U.S. Geological Service estimates that 1.525 trillion barrels of oil are in place in seventeen oil shale zones in the Eocene Green River Formation in the Piceance Basin of western Colorado.
Oil shale can be used for several purposes: to obtain heat by direct combustion (for example, in the generation of electricity); to produce shale oil, and as a source of other valuable chemicals.
The oil shale industry dates back to late 17th century Scotland, where oil was produced from heating oil shale. Oil shale pyrolysis was developed in France, where in 1832, a method for producing lighting oil was realized. During the 19th century oil shale thermal processing factories also operated in Australia, the United States, Brazil, Germany, and Scotland. During the 20th century oil shale processing factories were built in several countries, including China and Israel. However, later most of them were closed, largely due to the rapid development of crude oil industry.