Survey of Energy Resources 2007
Definition of Oil Shale
Most oil shales are fine-grained sedimentary rocks containing relatively large amounts of organic matter (known as 'kerogen') from which significant amounts of shale oil and combustible gas can be extracted by destructive distillation. Included in most definitions of 'oil shale', either stated or implied, is the potential for the profitable extraction of shale oil and combustible gas or for burning as a fuel.
The organic matter in oil shale is composed chiefly of carbon, hydrogen, oxygen, and small amounts of sulphur and nitrogen. It forms a complex macromolecular structure that is insoluble in common organic solvents (e.g. carbon disulphide). The organic matter (OM) is mixed with varied amounts of mineral matter (MM) consisting of fine-grained silicate and carbonate minerals. The ratio of OM:MM for commercial grades of oil shale is about 0.75:5 to 1.5:5. Small amounts of bitumen that are soluble in organic solvents are present in some oil shales. Because of its insolubility, the organic matter must be retorted at temperatures of about 500°C to decompose it into shale oil and gas. Some organic carbon remains with the shale residue after retorting but can be burned to obtain additional energy. Oil shale differs from coal whereby the organic matter in coal has a lower atomic H:C ratio, and the OM:MM ratio of coal is usually greater than 4.75:5.