A mixed-base crude oil extracted from mountains of sedimentary shale in Colorado, Utah, and Wyoming by heating at 425–535C (approximately 800–1000F). Two methods can be used: surface mining and excavation. In the first, the shale is bulldozed from beds, crushed, and fed into retorts of either the vertical or horizontal type. In the second, shafts are driven into the mountain and the shale is heated in situ by direct combustion in an interior chamber excavated for the purpose. The in situ method is the more efficient. The oil-bearing component of the shale is called kerogen. Only 20–30 gallons (less than one barrel) of oil is obtained per ton of shale processed, and less than 33% of the total oil content is recoverable. Major obstacles are the vast earth-moving operations necessary, the need for large volumes of cooling water, and disposal of the spent shale. Though rather extensive pilot plant operations have been carried out in recent years, no large-scale production is likely for the indefinite future for both economic and technical reasons.See Kerogen; Oil Shale; Synfuel.
