Oil shale is an organic-richfine-grainedsedimentary rock containingkerogen (a solid mixture oforganic chemical compounds) from which liquidhydrocarbons can be produced. In addition to kerogen, general composition of oil shales constitutes inorganic substance andbitumens. Based on their deposition environment, oil shales are classified as marine, lacustrine and terrestrial oil shales.[1][2] Oil shales differ from oil-bearing shales, shale deposits that containpetroleum (tight oil) that is sometimes produced from drilled wells. Examples of oil-bearing shales are theBakken Formation,Pierre Shale,Niobrara Formation, andEagle Ford Formation.[3] Accordingly, shale oil produced from oil shale should not be confused with tight oil, which is also frequently called shale oil.[3][4][5]
A 2016 estimate of globaldeposits set the total world resources of oil shale equivalent of 6.05 trillion barrels (962 billion cubic metres) ofoil in place.[6] Oil shale has gained attention as a potential abundant source of oil.[7][8] However, the various attempts to develop oil shale deposits have had limited success. Only Estonia and China have well-established oil shale industries, and Brazil, Germany, and Russia utilize oil shale to some extent.[9]
Oil shale, an organic-rich sedimentary rock, belongs to the group ofsapropelfuels.[13] It does not have a definite geological definition nor a specific chemical formula, and its seams do not always have discrete boundaries. Oil shales vary considerably in their mineral content, chemical composition, age, type of kerogen, and depositional history, and not all oil shales would necessarily be classified asshales in the strict sense.[14][15] According to thepetrologist Adrian C. Hutton of theUniversity of Wollongong, oil shales are not "geological nor geochemically distinctive rock but rather 'economic' term".[16] Their common defining feature is lowsolubility in low-boiling organic solvents and generation of liquid organic products onthermal decomposition.[17] Geologists can classify oil shales on the basis of their composition ascarbonate-rich shales,siliceous shales, orcannel shales.[18]
Oil shale differs frombitumen-impregnated rocks (other so-calledunconventional resources such asoil sands and petroleum reservoir rocks),humic coals andcarbonaceous shale. While oil sands do originate from thebiodegradation of oil, heat and pressure have not (yet) transformed the kerogen in oil shale into petroleum, which means itsmaturation does not exceed earlymesocatagenetic.[17][19][20] Oil shales differ also from oil-bearing shales, shale deposits that contain tight oil that is sometimes produced from drilled wells. Examples of oil-bearing shales are theBakken Formation,Pierre Shale,Niobrara Formation, andEagle Ford Formation.[3] Accordingly, shale oil produced from oil shale should not be confused with tight oil, which is called also frequently shale oil.[3][4][5]
Photomicrographs showing a cannel coal (top) 100% organic matrix and a rich oil shale (bottom) with relatively low mineral content
General composition of oil shales constitutes inorganic matrix, bitumens, and kerogen. While the bitumen portion of oil shales is soluble incarbon disulfide, the kerogen portion is insoluble in carbon disulfide and may containiron,vanadium,nickel,molybdenum, anduranium.[21][22] Oil shale contains a lower percentage of organic matter thancoal. In commercial grades of oil shale the ratio of organic matter to mineral matter lies approximately between 0.75:5 and 1.5:5. At the same time, the organic matter in oil shale has anatomic ratio of hydrogen to carbon (H/C) approximately 1.2 to 1.8 times lower than for crude oil and about 1.5 to 3 times higher than for coals.[13][23][24] The organic components of oil shale derive from a variety of organisms, such as the remains ofalgae,spores,pollen,plant cuticles and corky fragments ofherbaceous and woody plants, and cellular debris from other aquatic and land plants.[23][25] Some deposits contain significantfossils; Germany'sMessel Pit has the status of a UNESCOWorld Heritage Site. The mineral matter in oil shale includes various fine-grainedsilicates andcarbonates.[1][13] Inorganic matrix can containquartz,feldspar,clay (mainlyillite andchlorite), carbonate (calcite anddolomite),pyrite and some other minerals.[22]
Another classification, known as the van Krevelen diagram, assigns kerogen types, depending on thehydrogen,carbon, andoxygen content of oil shales' original organic matter.[15] The most commonly used classification of oil shales, developed between 1987 and 1991 by Adrian C. Hutton, adaptspetrographic terms from coal terminology. This classification designates oil shales as terrestrial, lacustrine (lake-bottom-deposited), or marine (ocean bottom-deposited), based on the environment of the initialbiomass deposit.[1][2] Known oil shales are predominantly of aquatic (marine, lacustrine) origin.[17][2] Hutton's classification scheme has proven useful in estimating the yield and composition of the extracted oil.[26]
Fossils in Ordovician oil shale (kukersite), northern Estonia
As source rocks for most conventionaloil reservoirs, oil shale deposits are found in all world oil provinces, although most of them are too deep to be exploited economically.[27] As with all oil and gas resources, analysts distinguish between oil shale resources and oil shale reserves. "Resources" refer to all oil shale deposits, while "reserves" represent those deposits from which producers can extract oil shale economically using existing technology. Since extraction technologies develop continuously, planners can only estimate the amount of recoverable kerogen.[10][1] Although resources of oil shale occur in many countries, only 33 countries possess known deposits of potential economic value.[28][29] Well-explored deposits, potentially classifiable as reserves, include theGreen River deposits in thewestern United States, the Tertiary deposits inQueensland, Australia, deposits in Sweden and Estonia, the El-Lajjun deposit in Jordan, and deposits in France, Germany, Brazil, China, southern Mongolia and Russia. These deposits have given rise to expectations of yielding at least 40 liters of shale oil per tonne of oil shale, using theFischer Assay.[1][15]
A 2016 estimate set the total world resources of oil shale equivalent to yield of 6.05 trillion barrels (962 billion cubic metres) of shale oil, with the largest resource deposits in the United States accounting more than 80% of the world total resource.[6] For comparison, at the same time the world's provenoil reserves are estimated to be 1.6976 trillion barrels (269.90 billion cubic metres).[30] The largest deposits in the world occur in the United States in the Green River Formation, which covers portions ofColorado,Utah, andWyoming; about 70% of this resource lies on land owned or managed by the United States federal government.[31] Deposits in the United States constitute more than 80% of world resources; other significant resource holders being China, Russia, and Brazil.[6] The amount of economically recoverable oil shale is unknown.[27]
Production of oil shale in millions of metric tons, from 1880 to 2010. Source: Pierre Allix, Alan K. Burnham.[32]
Humans have used oil shale as a fuel since prehistoric times, since it generally burns without any processing.[33] Around 3000 BC, "rock oil" was used inMesopotamia for road construction and making architectural adhesives.[34]Britons of theIron Age used tractable oil shales to fashioncists for burial,[35] or just polish it to create ornaments.[36]
In the 10th century, the Arab physicianMasawaih al-Mardini (Mesue the Younger) described amethod of extraction of oil from "some kind of bituminous shale".[37] The first patent for extracting oil from oil shale was British Crown Patent 330 granted in 1694 to Martin Eele, Thomas Hancock and William Portlock, who had "found a way to extract and make great quantities of pitch, tarr, and oyle out of a sort of stone".[34][38][39]
Modern industrial mining of oil shale began in 1837 inAutun, France, followed by exploitation in Scotland, Germany, and several other countries.[40][41] Operations during the 19th century focused on the production ofkerosene, lamp oil, andparaffin; these products helped supply the growing demand for lighting that arose during theIndustrial Revolution, supplied from Scottish oil shales.[42] Fuel oil, lubricating oil and grease, andammonium sulfate were also produced.[43] Scottish production peaked in around 1913, operating 120 oil shale works,[44] producing 3,332,000 tonnes of oil shale, generating around 2% of the global production of petroleum.[45] The Scottish oil-shale industry expanded immediately beforeWorld War I partly because of limited access to conventional petroleum resources and the mass production of automobiles and trucks, which accompanied an increase in gasoline consumption; but mostly because theBritish Admiralty required a reliable fuel source for their fleet as war in Europe loomed.
Although the Estonian and Chinese oil-shale industries continued to grow afterWorld War II, most other countries abandoned their projects because of high processing costs and the availability of cheaper petroleum.[1][41][46][47] Following the1973 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, because of competition fromcheap conventional petroleum in the 1980s.[11][28]
The global oil-shale industry began to revive at the beginning of the 21st century. In 2003, an oil-shale development program restarted in the United States. Authorities introduced a commercial leasing program permitting the extraction of oil shale and oil sands on federal lands in 2005, in accordance with theEnergy Policy Act of 2005.[50][51]
Shell's experimentalin-situ oil-shale facility, Piceance Basin, Colorado, US
As of 2008[update], oil shale is utilized primarily in Brazil, China, Estonia and to some extent in Germany, and Russia. Several additional countries started assessing their reserves or had built experimental production plants, while others had phased out their oil shale industry.[9] Oil shale serves for oil production in Estonia, Brazil, and China; for power generation in Estonia, China, and Germany; for cement production in Estonia, Germany, and China; and for use in chemical industries in China, Estonia, and Russia.[9][47][52][53]
As of 2009[update], 80% of oil shale used globally isextracted in Estonia, mainly because Estonia uses severaloil-shale-fired power plants,[52][54] which has an installed capacity of 2,967 megawatts (MW). By comparison, China's oil shale power plants have an installed capacity of 12 MW, and Germany's have 9.9 MW.[28][55] A 470 MWoil shale power plant in Jordan is under construction as of 2020.[56] Israel, Romania and Russia have in the past run power plants fired by oil shale but have shut them down or switched to other fuel sources such asnatural gas.[9][28][57] Other countries, such as Egypt, have had plans to construct power plants fired by oil shale, while Canada and Turkey had plans to burn oil shale along with coal for power generation.[28][58] Oil shale serves as the main fuel for power generation only in Estonia, where 90.3% of country's electrical generation in 2016 was produced from oil shale.[59]
According to theWorld Energy Council, in 2008 the total production of shale oil from oil shale was 930,000 tonnes, equal to 17,700 barrels per day (2,810 m3/d), of which China produced 375,000 tonnes, Estonia 355,000 tonnes, and Brazil 200,000 tonnes.[60] In comparison, production of the conventional oil andnatural gas liquids in 2008 amounted 3.95 billion tonnes or 82.1 million barrels per day (13.1×10^6 m3/d).[61]
Overview of shale oil extractionMining of oil shale.VKGOjamaa.
Most exploitation of oil shale involves mining followed by shipping elsewhere, after which the shale is burned directly to generate electricity or undertakes further processing. The most common methods of mining involveopen-pit mining andstrip mining. These procedures remove most of the overlying material to expose the deposits of oil shale and become practical when the deposits occur near the surface.Underground mining of oil shale, which removes less of the overlying material, employs theroom-and-pillar method.[62]
The extraction of the useful components of oil shale usually takes place above ground (ex-situ processing), although several newer technologies perform this underground (on-site orin-situ processing).[63] In either case, the chemical process ofpyrolysis converts the kerogen in the oil shale to shale oil (synthetic crude oil) and oil shale gas. Most conversion technologies involve heating shale in the absence of oxygen to a temperature at which kerogen decomposes (pyrolyses) into gas, condensable oil, and a solid residue. This usually takes place between 450 °C (842 °F) and 500 °C (932 °F).[10] The process of decomposition begins at relatively low temperatures (300 °C or 572 °F) but proceeds more rapidly and more completely at higher temperatures.[64]
In-situ processing involves heating the oil shale underground. Such technologies can potentially extract more oil from a given area of land thanex-situ processes, since they can access the material at greater depths than surface mines can. Several companies have patented methods forin-situretorting. However, most of these methods remain in the experimental phase. Twoin-situ processes could be used: truein-situ processing does not involve mining the oil shale, while modifiedin-situ processing involves removing part of the oil shale and bringing it to the surface for modifiedin-situ retorting in order to create permeability for gas flow in a rubble chimney. Explosives rubblize the oil-shale deposit.[65]
Hundreds of patents for oil shale retorting technologies exist;[66] however, only a few dozen have undergone testing. By 2006, only four technologies remained in commercial use:Kiviter,Galoter,Fushun, andPetrosix.[67]
Oil shale is utilized as a fuel for thermal power-plants, burning it (like coal) to drivesteam turbines; some of these plantsemploy the resulting heat fordistrict heating of homes and businesses. In addition to its use as a fuel, oil shale may also serve in the production of specialtycarbon 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.[52] However, oil shale use for production of these items remains small or only in experimental development.[1][68] Some oil shales yieldsulfur,ammonia,alumina,soda ash, uranium, andnahcolite as shale-oil extraction byproducts. Between 1946 and 1952, a marine type ofDictyonema shale served for uranium production inSillamäe, Estonia, and between 1950 and 1989 Sweden usedalum shale for the same purposes.[1] Oil shale gas has served as a substitute fornatural gas, but as of 2009[update], producing oil shale gas as a natural-gas substitute remained economically infeasible.[69][70]
The shale oil derived from oil shale does not directly substitute for crude oil in all applications. It may contain higher concentrations ofolefins, oxygen, and nitrogen than conventional crude oil.[49] Some shale oils may have higher sulfur orarsenic content. By comparison withWest Texas Intermediate, the benchmark standard for crude oil in thefutures-contract market, the Green River shale oil sulfur content ranges from near 0% to 4.9% (in average 0.76%), where West Texas Intermediate's sulfur content has a maximum of 0.42%.[71] The sulfur content in shale oil from Jordan's oil shales may be as high as 9.5%.[72] The arsenic content, for example, becomes an issue for Green River formation oil shale. The higher concentrations of these materials means that the oil must undergo considerable upgrading (hydrotreating) before serving asoil-refinery feedstock.[73] Above-ground retorting processes tended to yield a lowerAPI gravity shale oil than thein situ processes. Shale oil serves best for producing middle-distillates such askerosene,jet fuel, anddiesel fuel. Worldwide demand for these middle distillates, particularly for diesel fuels, increased rapidly in the 1990s and 2000s.[49][74] However, appropriate refining processes equivalent tohydrocracking can transform shale oil into a lighter-range hydrocarbon (gasoline).[49]
The various attempts to develop oil shale deposits have succeeded only when the cost of shale-oil production in a given region comes in below the price of crude oil or its other substitutes (break-even price). According to a 2005 survey, conducted by theRAND Corporation, the cost of producing a barrel of oil at a surface retorting complex in the United States (comprising a mine, retorting plant,upgrading plant, supporting utilities, and spent shale reclamation), would range betweenUS$70–95 ($440–600/m3, adjusted to 2005 values). This estimate considers varying levels of kerogen quality and extraction efficiency. In order to run a profitable operation, the price of crude oil would need to remain above these levels. The analysis also discussed the expectation that processing costs would drop after the establishment of the complex. The hypothetical unit would see a cost reduction of 35–70% after producing its first 500 million barrels (79 million cubic metres). Assuming an increase in output of 25 thousand barrels per day (4.0×10^3 m3/d) during each year after the start of commercial production, RAND predicted the costs would decline to $35–48 per barrel ($220–300/m3) within 12 years. After achieving the milestone of 1 billion barrels (160 million cubic metres), its costs would decline further to $30–40 per barrel ($190–250/m3).[52][62] In 2010, theInternational Energy Agency estimated, based on the various pilot projects, that investment and operating costs would be similar to those ofCanadian oil sands, that means would be economic at prices above $60 per barrel at current costs. This figure does not accountcarbon pricing, which will add additional cost.[27] According to the New Policies Scenario introduced in itsWorld Energy Outlook 2010, a price of $50 per tonne of emitted CO2 adds additional $7.50 cost per barrel of shale oil.[27] As of November 2021, the price of tonne of CO2 exceeded $60.
A 1972 publication in the journalPétrole Informations (ISSN0755-561X) compared shale-based oil production unfavorably withcoal liquefaction. The article portrayed coal liquefaction as less expensive, generating more oil, and creating fewer environmental impacts than extraction from oil shale. It cited a conversion ratio of 650 liters (170 U.S. gal; 140 imp gal) of oil per oneton of coal, as against 150 liters (40 U.S. gal; 33 imp gal) of shale oil per one ton of oil shale.[41]
A critical measure of the viability of oil shale as an energy source lies in the ratio of the energy produced by the shale to the energy used in its mining and processing, a ratio known as "energy return on investment" (EROI). A 1984 study estimated the EROI of the various known oil-shale deposits as varying between 0.7–13.3,[75] although known oil-shale extraction development projects assert an EROI between 3 and 10. According to the World Energy Outlook 2010, the EROI ofex-situ processing is typically 4 to 5 while ofin-situ processing it may be even as low as 2. However, according to the IEA most of used energy can be provided by burning the spent shale or oil-shale gas.[27] To increase efficiency when retorting oil shale, researchers have proposed and tested several co-pyrolysis processes.[76][77][78]
Mining oil shale involves numerous environmental impacts, more pronounced in surface mining than in underground mining.[79] These include acid drainage induced by the sudden rapid exposure and subsequent oxidation of formerly buried materials; the introduction of metals includingmercury[80] into surface-water and groundwater; increasederosion, sulfur-gas emissions; and air pollution caused by the production ofparticulates during processing, transport, and support activities.[11][12]
Oil-shale extraction can damage the biological and recreational value of land and the ecosystem in the mining area. Combustion and thermal processing generate waste material. In addition, the atmospheric emissions from oil shale processing and combustion includecarbon dioxide, agreenhouse gas. Environmentalists oppose production and usage of oil shale, as it creates even more greenhouse gases than conventional fossil fuels.[81] Experimentalin situ conversion processes andcarbon capture and storage technologies may reduce some of these concerns in the future, but at the same time they may cause other problems, includinggroundwater pollution.[82] Among the water contaminants commonly associated with oil shale processing are oxygen and nitrogen heterocyclic hydrocarbons. Commonly detected examples includequinoline derivatives,pyridine, and various alkyl homologues of pyridine, such aspicoline andlutidine.[83]
Water concerns are sensitive issues in arid regions, such as the western U.S. and Israel'sNegev Desert, where plans exist to expand oil-shale extraction despite a water shortage.[84] Depending on technology, above-ground retorting uses between one and five barrels of water per barrel of produced shale-oil.[62][85][86][87] A 2008 programmaticenvironmental impact statement issued by the U.S.Bureau of Land Management stated that surface mining and retort operations produce 2 to 10 U.S. gallons (7.6 to 37.9 L; 1.7 to 8.3 imp gal) of waste water per 1 short ton (0.91 t) of processed oil shale.[85]In situ processing, according to one estimate, uses about one-tenth as much water.[88]
Some comets contain massive amounts of an organic material almost identical to high grade oil shale, the equivalent of cubic kilometers of such mixed with other material;[90] for instance, corresponding hydrocarbons were detected in a probe fly-by through the tail ofHalley's Comet in 1986.[91]
Core Research Center – U.S. Geological Survey facility in Colorado – a United States Geological Survey facility dedicated to preserving valuable rock-samples threatened with disposal or destruction – including oil shales
Mitigation of peak oil – Gradual reduction of the use and production of fossil fuelsPages displaying short descriptions of redirect targets – discussion of attempts to delay and minimize the impact of "peak oil" (the point in time of maximum global petroleum production), including the development of unconventional oil resources
Oil reserves – Industry concept of crude oil and natural gas reserves and resourcesPages displaying short descriptions of redirect targets – discussion of global crude-oil supplies
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