Most oil shales are fine-grained sedimentary rocks containing relatively large amounts of organic matter from which significant amounts of shale oil and combustible gas can be extracted by destructive distillation. One of the largest known locations is the oil shale locked in the 40.000 km2 (16000 sq-mile) Green River Formation in Colorado, Utah, and Wyoming. 37 Oil shale differs from coal whereby the organic matter in shales has a higher atomic Hydrogen to Carbon ratio. Coal also has an organic to inorganic matter ratio of more than 4,75 to 5 while as oil shales have a higher content of sedimentary rock. Sources estimate the world reserves of Oil Shales at more than 2,5 trillion barrels. Oil shales are thought to form when algae and sediment deposit in lakes, lagoons and swamps where an anaerobic (oxygen free) environment prevent the breakdown of organic matter, thus allowing it to accumulate in thick layers. Thet is later covered with overlying rock to be baked under high temperature and pressure. However heat and pressure was lower than in oil and gas reservoirs. The shale can be strip mined and processed with distillation. Extraction with fracturing and heating is still relatively unproven. Companies are experimenting with direct electrical heating rather than e.g. steam injection. Extraction cost is currently around 25-30 USD per barrel.