OIL SHALE

any fine-grained sedimentary rock containing solid organic matter that yields significant quantities of oil when heated. This shale oil, commonly called syncrude, is a potentially valuable fossil fuel. Oil shales vary in their mineral composition. For example, clay minerals predominate in true shales, while other minerals (e.g., dolomite and calcite) occur in appreciable but subordinate amounts in the carbonates. In all shale types, layers of the constituent minerals alternate with layers of darker organic matter. This organic material consists chiefly of kerogen, an insoluble product of bacterially altered plant and animal detritus. Oil shales occur in certain types of geologic environments. Thick deposits are commonly found in the basins of former large lakes, especially those of tectonic origin. Thinner but more extensive deposits tend to occur in shallow marine environments. Some oil shales also can be found in former bogs and lagoons; these are generally associated with coal seams. To be of commercial significance, an oil shale must have a large amount of kerogen, sufficient to provide more energy than is required to process the rock. If the kerogen content of a shale is 2.5 percent by weight, its total calorific value will be consumed during processing. Accordingly, oil shales with a kerogen content below the threshold value of 2.5 percent cannot be used as a source of energy. In the United States, only oil shales that yield 42 litres of oil per metric ton (10 gallons per short ton) are considered commercial grade. The production of shale oil begins with the mining of the shale. The shale is then crushed and fed into a retorting unit in which the rock is heated to roughly 500 C (930 F). The intense heat breaks the chemical bonds of the kerogen macromolecule, thereby liberating small molecules of liquid and gaseous hydrocarbons, along with various compounds of nitrogen, sulfur, and oxygen. In another method, the retorting occurs undergound (in situ). The shale is fractured below the surface with explosives and subsequently set afire. The hydrocarbons released by the rock are pumped to the surface and collected. Recovering oil from shale is much more expensive than producing light-to-medium and heavy oils. There are also serious drawbacks to current methods of extraction and refining. Vast amounts of water are required, rendering refining impracticable in arid lands where water is at a premium. Also, large amounts of carcinogenic wastes are produced during the refining process, and the mining process itself has deleterious environmental effects. Thus, exploitation of shale oil on a wide scale will probably not occur until other petroleum resources have been nearly depleted. Very few countries have oil-shale processing plants. Estonia, China, and Brazil have refinery facilities for producing relatively limited quantities of syncrude, while the U.S. government operates an experimental plant in Parachute Creek, Colo. any sedimentary rock containing various amounts of solid organic material that yields hydrocarbons, along with a variety of solid products, when subjected to pyrolysisa treatment that consists of heating the rock to about 500 C. The liquid oil extracted from oil shale as well as that derived from tar sands is referred to as syncrude. Most of the solid by-products of oil shale are unusable wastes, but a few of them have commercial value. These include sulfur, ammonia, alumina, soda ash, and nahcolite (a material that can be used in an industrial air-pollution control procedure known as stack-gas scrubbing). Additional reading Current information concerning the characteristic properties of oil shales, their sources, and special problems of exploitation may be found in the proceedings of meetings, such as Oil Shale Symposium Proceedings (annual); material originating at symposia chaired by Paul B. Tarman, Synthetic Fuels from Oil Shale (1980), Synthetic Fuels from Oil Shale II (1982), and Synthetic Fuels from Oil Shale and Tar Sands (1983); and H.C. Stauffer (ed.), Oil Shale, Tar Sands, and Related Materials (1981), symposium papers, including essays on oil shale cracking and retorting. General works include Ken P. Chong and John Ward Smith (eds.), Mechanics of Oil Shale (1984), a collection of summary papers on the exploitation of oil shales; T.F. Yen and George V. Chilingarian (eds.), Oil Shale (1976), background essays on different aspects of oil shale technology and science; Paul L. Russell, History of Western Oil Shale (1980); and Perry Nowacki (ed.), Oil Shale Technical Data Handbook (1981). The transitional character of kerogen rocks and their limnological and stratigraphical properties are treated in Bernard Durand (ed.), Kerogen: Insoluble Organic Matter from Sedimentary Rocks (1980); Bartholomew Nagy and Umberto Colombo (eds.), Fundamental Aspects of Petroleum Geochemistry (1967); and A.I. Levorsen, Geology of Petroleum, 2nd rev. ed. (1967).Data on world distribution, exploitation, and technology are included in Ferdinand Mayer, Weltatlas Erdl und Erdgas, 2nd ed. (1976); and T.F. Yen (ed.), Science and Technology of Oil Shale (1976). Information also is presented in Zeitschrift fr Angewandte Geologie (monthly); Erdl-Erdgas (monthly); and Oil and Gas Journal (weekly).