oil, and a solid residue. This usually takes place between and . The process of decomposition begins at relatively low temperatures () but proceeds more rapidly and more completely at higher temperatures. In-situ processing involves heating the oil shale underground. Such technologies can potentially extract more oil from a given area of land than ex-situ processes, since they can access the material at greater depths than surface mines can. Several companies have patented methods for in-situ retorting. However, most of these methods remain in the experimental phase. Two in-situ processes could be used: true in-situ processing does not involve mining the oil shale, while modified in-situ processing involves removing part of the oil shale and bringing it to the surface for modified in-situ retorting in order to create permeability for gas flow in a rubble chimney. Explosives rubblize the oil-shale deposit. Hundreds of patents for oil shale retorting technologies exist; however, only a few dozen have undergone testing. By 2006, only four technologies remained in commercial use: Kiviter, Galoter, Fushun, and Petrosix. Applications and products Oil shale is utilized as a fuel for thermal power-plants, burning it (like coal) to drive steam turbines; some of these plants employ the resulting heat for district heating of homes and businesses. In addition to its use as a fuel, oil shale may also serve in the production of specialty carbon fibers, adsorbent carbons, carbon black, phenols, resins, glues, tanning agents, mastic, road bitumen, cement, bricks, construction and decorative blocks, soil-additives, fertilizers, rock-wool insulation, glass, and pharmaceutical products. However, oil shale use for production of these items remains small or only in experimental development. Some oil shales yield sulfur, ammonia, alumina, soda ash, uranium, and nahcolite as shale-oil extraction byproducts. Between 1946 and 1952, a marine type of Dictyonema shale served for uranium production in Sillamäe, Estonia, and between 1950 and 1989 Sweden used alum shale for the same purposes. Oil shale gas has served as a substitute for natural gas, but , producing oil shale gas as a natural-gas substitute remained economically infeasible. The shale oil derived from oil shale does not directly substitute for crude oil in all applications. It may contain higher concentrations of olefins, oxygen, and nitrogen than conventional crude oil. Some shale oils may have higher sulfur or arsenic content. By comparison with West Texas Intermediate, the benchmark standard for crude oil in the futures-contract market, the Green River