Oil shale

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Oil shale is a fine-grained sedimentary rock, containing significant amounts of kerogen (a solid mixture of organic chemical compounds), from which liquid hydrocarbons can be manufactured. The name oil shale is something of a misnomer as the rock is not necessarily a shale and the hydrocarbon in it is not truly oil. Deposits of oil shale are located around the world, including major deposits in the United States. Global deposits are estimated as equivalent to 2.8–3.3 trillion (2.8–3.3 x 10¹²) barrels of recoverable oil.

The kerogen in oil shale can be converted to synthetic crude oil through the chemical process of pyrolysis. When heated to a sufficiently high temperature a vapor is driven off which can be distilled (retorted) to yield a petroleum-like shale oil—a form of non-conventional oil—and combustible shale gas (shale gas can also refer to gas occurring naturally in shales). Oil shale can also be burnt directly as a low-grade fuel for power generation and heating purposes, and can be used as a raw material in the chemical and construction materials industries.

Oil shale has gained attention as an energy resource as the price of conventional sources of petroleum has risen, and as a way to secure independence from external suppliers of energy.

Geology

Oil shale is an organic-rich sedimentary rock, which belongs to the group of sapropel fuels. It is differentiated from bitumen-impregnated rocks (tar sands and petroleum reservoir rocks), humic coals and carbonaceous shale. In oil shales, the kerogen has not yet been naturally cooked into petroleum by heat and pressure.

Oil shales vary considerably in their mineral content, chemical composition, age, type of kerogen, and depositional history, and are derived from a number of different organisms. One classification scheme is based on their composition, dividing them into three categories: Carbonate-rich shales, siliceous shales, and cannel shales. Another classification is based on kerogen type, which is a function of the hydrogen, carbon, and oxygen content of their original organic matter. Oil shales are also classified as terrestrial, lacustrine, or marine, based on the environment where the initial biomass was deposited.

Reserves

Although oil shale resources occur in many countries, only 33 countries possess deposits of possible economic value. The largest deposits in the world are found in the United States in the Green River basin, which covers portions of Colorado, Utah, and Wyoming; about 70 % of this resource is located on federally-owned or managed land. Total world resources of oil shale were estimated in 2005 at 411 gigatons (411 x 10⁹ tonnes), which is enough to yield 2.8 to 3.3 trillion (2.8 to 3.3 x 10¹²) U.S. barrels. Among those, the United States accounts for 62 % of world resources; together, the United States, Russia and Brazil account for 86 % of the world's resources in terms of shale oil content. These figures are considered tentative, as several deposits have not yet been explored.

History

Oil shale has been in use since ancient times, since it will generally burn without any processing. However, modern industrial oil shale mining began in 1837 in Autun, France, followed by Scotland, Germany, and several other countries. The first oil shale retort was constructed in the United States in 1855. The oil shale industry started growing just before World War I, but was abandoned in most countries after World War II because of high processing costs and the availability of cheaper petroleum. Following the 1973 oil crisis, world production of oil shale reached a peak of 46 million tonnes in 1980 before falling to about 16 million tonnes in 2000, due to competition from conventional petroleum which had remained at under US$30/barrel. The global oil shale industry began to revive in the mid-1990s. In 2003, an oil shale development program was restarted in the United States, and a commercial leasing program for oil shale and tar sand was introduced in 2005.

Extraction and processing

Oil shale is usually mined and then transported to be processed elsewhere (ex situ), although several newer technologies extract its useful components underground (in situ). Several mining methods are used, which all begin by fragmenting the oil shale, so that it may be transported to a power plant or a retorting facility. The most-often used methods of surface mining are open pit mining and strip mining, while underground mining of oil shale employs the room-and-pillar method.

After access to the shale is gained, either on-site or off-site, its kerogen is converted to synthetic crude oil and shale gas through the chemical process of pyrolysis. Most conversion technologies involve heating shale in the absence of oxygen to a temperature at which kerogen is decomposed (pyrolysed) into gas, condensable oil, and a solid residue; this takes place usually at to Other technologies are based on the solvent processes of reactive fluids.

Industry

Currently, oil shale is used industrially in Brazil, China, Estonia and to some extent in Germany, Israel, and Russia. At the beginning of the 21st century, several additional countries were assessing their reserves or had built experimental production plants, while others had phased out their oil shale industry. Oil shales are used for oil production in Estonia, Brazil, and China, for power generation in Estonia, China, Israel, and Germany, for cement production in Estonia, Germany, and China, and by chemical industries in China, Estonia and Russia."); }