Oil shale
Oil shale gas is produced by retorting (pyrolysis) of oil shale. In the pyrolysis process, oil shale is heated until its kerogen decomposes into vapors of a petroleum-like condensable shale oil, non-condensible combustible oil shale gas, and spent shale—a solid residue. The process is the same as the shale oil extraction and oil shale gas usually occurs as a byproduct of shale oil production. The ratio of oil shale gas to shale oil depends on retorting temperature and as a rule, increases by the rise of temperature.