page_title: Oil shale
section_title: Geology
hierarchical_section_title: Oil shale / Geology
caption_attribution_description: Outcrop of Ordovician oil shale (kukersite), northern Estonia.
context_page_description: Oil shale is an organic-rich fine-grained sedimentary rock containing kerogen from which liquid hydrocarbons can be produced, called shale oil. Shale oil is a substitute for conventional crude oil; however, extracting shale oil from oil shale is more costly than the production of conventional crude oil both financially and in terms of its environmental impact. Deposits of oil shale occur around the world, including major deposits in the United States. A 2016 estimate of global deposits set the total world resources of oil shale equivalent of 6.05 trillion barrels of oil in place.
  Heating oil shale to a sufficiently high temperature causes the chemical process of pyrolysis to yield a vapor. Upon cooling the vapor, the liquid shale oil—an unconventional oil—is separated from combustible oil-shale gas. Oil shale can also be burned directly in furnaces as a low-grade fuel for power generation and district heating or used as a raw material in chemical and construction-materials processing.
  Oil shale gains attention as a potential abundant source of oil whenever the price of crude oil rises.
context_section_description: Oil shale, an organic-rich sedimentary rock, belongs to the group of sapropel fuels.  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 as shales in the strict sense.  According to the petrologist Adrian C. Hutton of the University of Wollongong, oil shales are not "geological nor geochemically distinctive rock but rather 'economic' term."  Their common defining feature is low solubility in low-boiling organic solvents and generation of liquid organic products on thermal decomposition.
  Oil shale differs from bitumen-impregnated rocks (oil sands and petroleum reservoir rocks), humic coals and carbonaceous shale.  While oil sands do originate from the biodegradation of oil, heat and pressure have not (yet) transformed the kerogen in oil shale into petroleum, that means that its maturation does not exceed early mesocatagenetic.
  General composition of oil shales constitutes inorganic matrix, bitumens, and kerogen.  While the bitumen portion of oil shales is soluble in carbon disulfide, kerogen portion is insoluble in carbon disulfide and may contain iron, vanadium, nickel, molybdenum, and uranium.  Oil shale contains a lower percentage of organic matter than coal. 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 an atomic 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.  The organic components of oil shale derive from a variety of organisms, such as the remains of algae, spores, pollen, plant cuticles and corky fragments of herbaceous and woody plants, and cellular debris from other aquatic and land plants.  Some deposits contain significant fossils; Germany's Messel Pit has the status of a Unesco World Heritage Site. The mineral matter in oil shale includes various fine-grained silicates and carbonates.  Inorganic matrix can contain quartz, feldspars, clays (mainly illite and chlorite), carbonates (calcite and dolomites), pyrite and some other minerals.
  Geologists can classify oil shales on the basis of their composition as carbonate-rich shales, siliceous shales, or cannel shales.
  Another classification, known as the van Krevelen diagram, assigns kerogen types, depending on the hydrogen, carbon, and oxygen content of oil shales' original organic matter. The most commonly used classification of oil shales, developed between 1987 and 1991 by Adrian C. Hutton, adapts petrographic 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 initial biomass deposit.  Known oil shales are predominantly aquatic (marine, lacustrine) origin.  Hutton's classification scheme has proven useful in estimating the yield and composition of the extracted oil.
caption: Outcrop of Ordovician oil shale (kukersite), northern Estonia
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