oil shale

Geol.

a black or dark-brown shale or siltstone rich in bitumens, from which shale oil is obtained by destructive distillation.

[1870-75]

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Any fine-grained sedimentary rock that contains solid organic matter (kerogen) and yields significant quantities of oil when heated.

This shale oil is a potentially valuable fossil fuel, but the present methods of mining and refining it are expensive, damage the land, pollute the water, and produce carcinogenic wastes. Thus, oil shale will probably not be exploited on a wide scale until other petroleum resources have been nearly depleted. Estonia, China, and Brazil have facilities for producing relatively limited quantities, and the U.S. government operates an experimental plant in Colorado.

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▪ geology

Introduction

      any sedimentary rock containing various amounts of solid organic material that yields hydrocarbons, along with a variety of solid products, when subjected to pyrolysis—a 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).

History of use

Discovery and early application

      The first notable reference to oil from shale was in 1596, when the personal physician of Duke Frederick of Württemberg mentioned that mineral oil distilled from oil shale could be used for healing. In 1694, during the reign of William and Mary, British Crown Patent No. 330 was granted to three subjects who had found “a way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone.” Also about this time, enough oil was actually produced by the distillation of oil shale to light the streets of Modena, Italy.

      A commercial shale oil industry was founded in 1838 in Autun, Fr., to produce lamp fuel. By the middle of the 19th century the demand for oil was much greater than could be supplied by the whaling industry. As oil prices rose, numerous oil shale retorts were constructed along the Ohio River in the United States. The first one was built in 1855, but all of them had disappeared by 1860 because E.L. Drake's discovery of crude oil in Pennsylvania in 1859 (see above) opened the market to a cheaper source of oil. Oil shale was retorted in Canada from 1859 to 1861 on the shores of Lake Huron in southwestern Ontario but became economically unattractive with the discovery of crude oil nearby. In Scotland, however, a commercial shale oil industry began in 1862 and operated for about 100 years until the resource was depleted.

      A number of other countries also developed oil shale processing facilities: Australia in 1865, Brazil in 1881, New Zealand in 1900, Switzerland in 1915, Sweden in 1921, Spain in 1922, and South Africa in 1935. By 1966, however, all these shale oil plants had closed.

      In eastern Europe, oil shale retorting was initiated in Estonia in 1921. The process has been continued to the present, with a daily production of approximately 32,000 barrels of oil. An oil shale processing operation that opened in 1929 in Manchuria in northeastern China also is still producing. It yields an estimated 5,000 barrels of oil per day.

      The western oil shales of the United States have been considered economically valuable for more than 70 years. During the mid-1800s, oil was burned and distilled locally from shale in Utah. In Colorado, shale oil was used as smudge in peach orchards about the end of the 19th century. No appreciable output of shale oil, however, was realized until the 1920s, when some 3,600 barrels were produced at a U.S. government plant at Rulison, Colo., and more than 12,000 barrels from a private industrial operation in Nevada. These facilities were closed by 1930 in the wake of the discovery of major conventional oil fields in Texas, Oklahoma, and California.

Modern importance

      The shale oil industry reached its highest point of development immediately after World War II. At that time, however, plants were small, with capacities of only about 350 to 1,500 barrels of oil per day. Cost of production was high because of the labour necessary for mining and crushing the rock. During the 1950s the low cost of oil shipped from the Middle East made shale oil uneconomic, and the greatly increased consumption of oil made the amount of shale oil produced insignificant. Therefore, all shale oil plants throughout the world closed down between 1952 and 1966, with the exception of those in Estonia and Manchuria, where mining and economic conditions justified continued exploitation.

      The cycles of competition between synthetic and conventional oil are related to discoveries of major conventional oil fields. The giant oil fields found in the Middle East after World War II virtually eliminated any remaining commercial interest in the production of shale oil until the energy shortages of the late 1960s and '70s. These shortages prompted several countries to survey their oil shale deposits in order to determine whether sufficient reserves were present to justify the large investments that would be needed to turn shale oil into a practical energy source.

       Brazil and the United States developed pilot plants with which to exploit their deposits of oil shale: the Irati shales of Permian age in Brazil and the Green River shales of Eocene age (about 36,600,000 to 57,800,000 years old) in Colorado, Utah, and Wyoming. These two oil shale deposits are the largest in the world. The Brazilian national oil company, Petróleo Brasileiro (commonly called Petrobrás), developed a commercial shale oil extraction technology on the basis of work at a pilot plant that began operation since 1972. The operation was located at São Mateus do Sul in Paraná in southern Brazil. More than 1,500,000 barrels of shale oil and 20,000 tons of sulfur were extracted from over 3,500,000 tons of Irati oil shale before the project was canceled owing to the relatively low costs of conventional oil after the mid-1980s.

      Since the early 1980s, shale oil development in the United States has also been limited in large part because of falling oil prices resulting from increased world crude oil production. The only completed plant capable of producing shale oil is the Unishale B retort in Parachute Creek, Colo. The plant is still in the testing phase, and only experimental amounts of shale oil have been produced. It has a projected capacity of about 10,400 barrels per day, however. In the United States, as in most other countries, shale oil development faces an uncertain future, at least until conventional oil resources are more nearly depleted.

Joseph P. Riva, Jr. Gordon I. Atwater

Composition, formation, and occurrence of oil shales

Mineral and organic constituents

      The mineral constituents of oil shales vary according to sediment type. Some are true shales in which clay minerals are predominant. Others, such as the Green River shales of the western United States, are carbonates (e.g., dolomites and limestones) with subordinate amounts of other minerals. The various oil shales that have been mined during the past century have ranged from shale to marls and carbonates. The excluded sedimentary variety is sandstone, because the environment in which sandstones are deposited is not compatible with the accumulation and preservation of organic matter.

      The organic matter contained in oil shales is principally kerogen, an insoluble solid material. The shales range from brown to black in colour. They have low specific weight and are flammable, burning with a sooty flame. Their external structure is laminar, and, in a stratigraphic section, alternating darker and lighter strata correspond to the periodic changes of organic content. Oil shales are quite resistant to the oxidizing effects of air. In terms of chemical composition, they consist primarily of silica, iron, aluminum, calcium, magnesium, sodium carbonates, silicates, oxides, and sulfides. The chemical composition of the organic matter in oil shales is variable, but the hydrogen content is always high. The oxygen content varies, as does the amount of nitrogen, which is much less abundant.

Formation

      Some oil shale kerogens are composed almost entirely of algal (algae) remains, whereas others are a mixture of amorphous organic matter with a variable content of identifiable organic remnants. The main algal types are Botryococcus and Tasmanites.

      Botryococcus is a fresh- or brackish-water alga that forms colonies. Permian kerogens from Autun, Fr., and Carboniferous and Permian torbanite from Scotland, Australia, and South Africa appear to consist almost exclusively of Botryococcus colonies, as does Recent (post-Pleistocene) coorongite from Australia.

      Tasmanites is a marine alga the remains of which make up nearly all the kerogen of such oil shales as the Permian tasmanite of Australia and the Jurassic-Cretaceous tasmanite of Alaska. The remains of Tasmanites also are present in many other shales, such as the Lower Toarcian shales (those about 190,000,000 years in age) of the Paris Basin in France and the Lower Silurian shales (those about 423,000,000 years in age) of Algeria.

      Often only a minor part of the kerogen in oil shales is made of recognizable organic remnants. The rest is amorphous, probably because of microbial alteration during sedimentation. Amorphous organic material (sapropelic matter) associated with minerals constitutes thick accumulations of oil shale, such as the Permian Irati shales of Brazil and the Eocene Green River shales of the western United States. The organic material may have been derived from planktonic organisms (e.g., algae, copepods, and ostracods) and from microorganisms (e.g., bacteria and algae) that normally live in fresh sediment.

      A characteristic typical of the various types of oil shale is a very fine lamination of thin alternating layers of minerals and organic matter. This lamination results from sedimentation in quiet waters in which either carbonates are precipitated from solution or clay minerals are transported as extremely fine detritus. Also, a succession of seasonal or other periodic events is suggested by the layering.

The geologic environment

      A common geologic environment in which oil shales, often of considerable thickness, are deposited is large lake basins, particularly those of tectonic origin. Mineralogically, these oil shales are marls or argillaceous limestones, which may be associated with volcanic tuffs and evaporites. The major oil shale deposits of this type are the Green River shales of Eocene age in the western United States, along with the oil shales of Triassic age (about 208,000,000 to 245,000,000 years old) in Congo (Kinshasa) and the Albert shales of Mississippian origin (roughly 320,000,000 to 360,000,000 years old) in New Brunswick, Can.

      Oil shales deposited in shallow marine environments are thinner but of greater areal extent. The mineral phase is mostly clay and silica minerals, though carbonates also may occur. Extensive deposits of black shales of this variety were formed during the Cambrian Period (from about 505,000,000 to 540,000,000 years ago) in northern Europe and Siberia; the Silurian (about 408,000,000 to 438,000,000 years ago) in North America; the Permian (about 245,000,000 to 286,000,000 years ago) in southern Brazil, Uruguay, and Argentina; the Jurassic (about 144,000,000 to 208,000,000 years ago) in western Europe; and the Miocene Epoch of the Tertiary (about 5,300,000 to 23,700,000 years ago) in Italy, Sicily, and California.

      Oil shales also have been deposited in small lakes, bogs, and lagoons where they are associated with coal seams. Deposits of this type occur in the Permian sequence of western Europe and in the Tertiary beds of Manchuria (Northeast), China.

World oil shale resources

      Oil shales are found in many places throughout the world, yet worldwide shale oil development has been economically more attractive than conventional oil development for only a few brief periods in the 20th century. In earlier centuries oil shales were successfully exploited in a number of locations (see above History of use (oil shale)).

The United States

       Recoverable Shale Oil Resources of the United States*, TableOil shale deposits that are commercially viable at present are the Estonian shales and the Manchurian shales in China. Nearly 60 percent of the world's potentially recoverable shale oil resource is concentrated, however, in the United States (see Table (Recoverable Shale Oil Resources of the United States*, Table)). The aforementioned western and minable eastern oil shales of the United States have been estimated to contain an in-place oil resource of some 1,670,000,000,000 barrels. Using a 50 percent allowance for unrecoverable shale and a 25 percent allowance for conversion to synthetic fuel, the production potential for shale oil in the United States is estimated to be 626,000,000,000 barrels. The Mahogany Zone of the Parachute Creek Member of the Eocene Green River formation in the Piceance Basin of northwestern Colorado is a major target for future shale oil production. Estimated to contain some 59,000,000,000 barrels of recoverable shale oil, it is a thick, rich, consistent, saucer-shaped bed that outcrops around the edges of the basin, offering opportunities for mining by adits (nearly horizontal passages from the surface). At the centre of the basin the zone is more than 150 metres deep and accessible only by vertical or inclined shafts.

      The western oil shales of the United States are richer than its eastern shales, yielding from 84 to 168 litres of raw shale oil for each metric ton of oil shale processed (from 20 to 40 U.S. gallons per short ton). The oil is relatively high in paraffins (paraffin hydrocarbon). Thus, with upgrading, it becomes an excellent refinery feedstock that is well suited to large yields of diesel and jet fuel.

      Although the organic content of western and eastern oil shales is the same, the eastern shales yield only 34 to 63 litres of raw oil per metric ton of oil shale. Eastern shale oils are more aromatic and, when upgraded, are better suited as a feed for catalytic crackers in the production of gasoline.

Worldwide

       Recoverable Shale Oil Resources of the World*, TableThe Table (Recoverable Shale Oil Resources of the World*, Table) provides estimates of the world's recoverable shale oil as reported in the technical literature. Allowance is made for unrecoverable shale and for conversion to synthetic fuel. Brazil's oil shale resources are the world's second largest (28 percent of the total). Estonia, Russia, Congo (Kinshasa), Australia, Canada, Italy, and China also have significant oil shale resources. The world's total recoverable shale oil resource is estimated at some 1,067,000,000,000 barrels.

Recovery and exploitation of oil shales

Minimum organic requirement

      As noted above, the organic matter in oil shales is kerogen, with no oil and little extractable bitumen naturally present. The kerogen of oil shale is not distinct from the kerogen of petroleum source rocks, and to some extent the pyrolysis process for extracting oil from oil shales is comparable to the burial of source rocks at depth and the subsequent formation of oil by the resulting elevation of temperature.

      Nonetheless, oil shale must have a large amount of organic matter to be of commercial interest, larger than the 0.5 percent of organic carbon in a source rock from which commercial accumulations of oil or gas may be generated, provided that depth of burial, migration paths, and trapping mechanisms are favourable. The organic matter in a commercial oil shale must provide more energy than is required to process the shale. If the kerogen content of the shale is 2.5 percent by weight, its total calorific value is needed for processing. This is because at an average pyrolysis temperature of 500° C the energy required for heating is about 250 calories per gram of rock and the calorific value of kerogen is 10,000 calories per gram. Oil shale with a kerogen content below the threshold of 2.5 percent therefore cannot be employed as a source of energy. Frequently used is a lower limit of 5 percent organic content, which corresponds to an oil yield of approximately 25 litres per metric ton (6 U.S. gallons per short ton) of rock. Even this amount is not considered of potentially commercial grade in the United States, where 10 U.S. gallons per short ton (42 litres per metric ton) is often cited as a lower limit for oil shales.

Recovery processes

Pyrolysis

      The technology for producing oil from oil shale is based on pyrolysis of the rock. The heat breaks the various chemical bonds of the kerogen macromolecule, liberating small molecules of liquid and gaseous hydrocarbons, as well as nitrogen, sulfur, and oxygen compounds. However, since the shale quickly reaches a high temperature, industrial reactions are somewhat different from those that occur under natural subsurface conditions. Thus, liquid industrial products usually include a large proportion of olefins and sulfur and nitrogen compounds. Also, industrial gases may contain large amounts of hydrogen sulfide and ammonia.

Aboveground processing

      Three basic steps are involved in the aboveground processing of oil shales—mining, crushing, and retorting (retort) (heating). Various retorting processes have been used over the years. The Pumpherston process, which involves external heating through the wall of the retort, was used in Scotland beginning in 1862. This process was widely employed with various refinements introduced later in continental Europe. The capacities of the retorting units, however, were low and energetic balances poor.

      Combustion inside the retorting unit results in better energetic balances, but low-calorific gas diluted by nitrogen and combustion products results. This technique is used in Russia and China and is being tested in the United States. Still another method involves the circulation of externally heated gas through the shale. The resulting energetic balance is satisfactory, and the gas produced is of high calorific value. Used in France, this approach was adopted by Brazil and is the subject of experimentation in the United States.

      An entirely new experimental process involves heating the shale with hot solids, which ensures a good energetic balance and a high calorific value of the gas produced. In certain parts of the world hot shale ashes are used as a calorific vehicle, while in the United States externally heated ceramic balls are employed. This technology is more complex than any of the others.

Subsurface processing

      Subsurface processing differs from aboveground processing in that retorting to produce oil and gas takes place underground, or in situ. The oil shale is fractured underground by explosives and then heated by a controlled underground fire. Fuels produced from the heated oil shale are pumped to the surface and collected. Several in situ processes have been tested in the United States; they have resulted in both high and low rates of recovery efficiency.

Economic and technical constraints

      An emerging shale oil industry faces many economic uncertainties, the most significant of which is the comparatively lower price of conventional oil development. The capital requirements for a commercial shale oil project range up to several billion U.S. dollars, making almost any conventional oil development, with comparable production, less expensive. Thus, it is unlikely that wide-scale exploitation of shale oil will occur until conventional and heavy oils are more nearly depleted.

      Furthermore, technical difficulties remain in retorting. In the United States, for example, component design problems in the retorting process have hampered the operation of a commercial-scale, aboveground facility. Also, experiments with in situ retorting have resulted in quite varied recovery efficiencies.

      In addition, the shale oil industry may be constrained by environmental factors. In semiarid regions, such as the western United States, limited water resources pose a problem because large amounts of water are required for the extraction process. Moreover, mining and processing may have adverse effects on groundwater and air quality. The vast amounts of rock material that have to be moved in a shale oil recovery operation also may adversely affect the integrity of the land, grazing and agricultural activities, and local fauna and flora.

      There is little prospect of widespread oil shale exploitation in the near term. Such broad development must await more favourable economic conditions that would most likely be brought about by the depletion of conventional and heavy oil resources. Even then technological and environmental constraints would have to be dealt with. Oil shale could possibly become a major supplier of the world's energy but probably not until well into the 21st century.

Joseph P. Riva, Jr.

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 Erdöl und Erdgas, 2nd ed. (1976); and T.F. Yen (ed.), Science and Technology of Oil Shale (1976). Information also is presented in Zeitschrift für Angewandte Geologie (monthly); Erdöl-Erdgas (monthly); and Oil and Gas Journal (weekly).

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