Oil shale extraction refers to the process in which kerogen, a mixture of organic chemical compouns found in oil shales, is converted into synthetic crude oil through the chemical process of pyrolysis. In this process, oil shale is heated in the absence of oxygen to a temperature at which oil shale is decomposed and kerogen is pyrolysed into a petroleum-like condensable shale oil—a form of non-conventional oil—and combustible shale gas (shale gas can also refer to gas occurring naturally in shales). The process also produces a solid residue in form of spent shale (char). Decomposition of oil shale begins at relatively low temperatures (300 °C (570 °F)), but proceeds more rapidly and more completely at higher temperature.