Oil Shale Rock Type, Composition, Classification, Occurrence

Oil Shale Rock
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Oil shale is a fine-grained, organic-rich sedimentary rock that produces oil and combustible gas upon destructive distillation. The rock primarily contains kerogen, a solid mixture containing organic chemical compounds from which liquid hydrocarbons are produced.

The organic matter in oil shale doesn’t dissolve in common organic solvents. The rock must be decomposed by vigorous heating to release its useful materials. Oil shale’s value depends on how much oil, gas, and byproducts can be recovered. Oil shale deposits with economic value are located near the surface and can be developed by open-pit, conventional mining, or in-situ methods. In-situ methods refer to the methods of oil-sand production that use drilling and steam to produce useful materials.

Colaroda Oil Shale
Colaroda Oil Shale

Rock Composition

In addition to kerogen, oil shales also contain inorganic substances and bitumens. Oil shales are classified based on their depositional environment. The three major types are marine, lacustrine, and terrestrial oil shales.

Kerogen’s chemical composition depends on the composition of organic matter. This, in turn, impacts the overall quality of oil shale. Oil shale quality is important in determining its sustainability for oil and gas production. Key factors that determine the quality include.

  • Richness/grade (liters per metric ton l/t)
  • Percentage weight of organic matter
  • Hydrogen content
  • Moisture content
  • The concentration of contaminants like nitrogen, sulfur, and metals

The USGS Geological Survey has used a lower limit of about 40 liters per metric ton (l/t) for classifying Federal-Oil shale reserves. Other standards suggest lower limits of 25 I/t.

Some oil shale specimens have mineral components consisting of carbonates like calcite, dolomite, and siderite with trace amounts of aluminosilicate compounds. On the other hand, some oil shales have quartz, feldspars, and clay minerals in large concentrations with a lower concentration of carbonates. Some oil shale reserves contain small amounts of sulfides like pyrite and marcasite. These elements indicate that oil shale sediments probably accumulated in waters with very low oxygen content.

Oil shale (kerogenite) (Parachute Creek Member, Green River Formation, Middle Eocene; near Rifle, Colorado, USA
Oil shale (kerogenite) (Parachute Creek Member, Green River Formation, Middle Eocene; near Rifle, Colorado, USA

Oil Shale vs. Coal

Oil shale and coal are often found close to each other. However, both of them differ from each other in terms of mineral content. Oil shales typically have a larger concentration of inert (inactive) mineral matter (60-90%) compared to coal (below 40%). The organic matter in oil shale has lower oxygen and higher hydrogen content than coal.

The precursor of organic matter in oil shale is also different. Most organic matter found in oil shale has an algal origin. However, oil shale may have trace amounts of vascular plants like clubmosses, horsetails, ferns, gymnosperms, and angiosperms. These plants usually constitute a large portion of the organic matter in coal.

Oil Shale Classification

Oil shale is the only rock to receive so many different names in recent years. It is known as cannel coal, alum shale, stellarite, albertite, kerosene shale, bituminite, gas coal, algal coal, boghead coal, schistesbitumineux, torbanite, and kukersite. Some of these names are still used. However, recent research and study have helped systemize the classification system.

Its most useful classification is based on the origin of organic matter. A.C. Hutton developed it in 1991. He divided oil shale into three different categories.

  1. Humic coal and carbonaceous shale
  2. Bitumen-impregnated rock
  3. Oil shale

He further divided the oil shale category into 6 different types

  1. Lamosite
  2. Marintie
  3. Tasmanite
  4. Cannel coal
  5. Kukersite
  6. Torbanite

Where is Oil Shale Found?

Oil shale reserves are located in many parts of the world. The deposits can be as old as the Cambrian age and as new as the Tertiary age. The deposits may be found as minor reserves with little economic value or huge reserves, stretching up to thousands of square kilometers and thicknesses up to 700 meters.

Oil shale reserves can be found in various environments like freshwater, saline lakes, marine basins, subtidal shelves, limin, and coastal swamps. Oil shale reserves are closely associated with coal deposits.

Oil shale reserves are still unknown in the world. However, the most notable oil shale discoveries have been made in the USA, Canada, Estonia, Israel, and Australia.

Production of oil shale in millions of metric tons, from 1880 to 2010.
Production of oil shale in millions of metric tons, from 1880 to 2010.

Oil Shale Success Probability

Oil shale’s success and development as an economically viable resource depend on many factors. The primary factors include the geological setting, physical and chemical properties, and accessibility. Other factors like the availability of roads, railways, power, human resources, and financial muscle of the country also impact the reserve’s development.

Oil Shale Uses

Shale oil is produced from oil shale. Shale oil is similar to petroleum and is refined to obtain different useful substances like diesel fuel, gasoline, and LPG. Shale oil is often refined to produce other commercial products like ammonia and sulfur. The remains can be used in cement manufacturing.

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