Oil Shale is organic-rich sedimentary rocks that have contain kerogen (insoluble organic matter in sedimentary rock. When subjected to intense heat, these shales yield oil. Oil shales range from brown to black in color. They are flammable and burn with a sooty flame. Some oil shales are true shales in which clay minerals are predominant. Others are actually limestones and dolomites. Much of the original organic material in oil shales is unrecognizable, but it is believed to be derived from plankton, algae, and microorganisms that live in fresh sediment. In previous centuries, small amounts of oil have been successfully recovered from oil shales. During the past century, oil shales have been mined with rock types varying from shale to marl and other carbonate rocks. Various pilot plants have been built to extract oil from shales, but the commercial results have been modest so far.
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Oil Shale Classification
Classification are generally classified by their mineral content and depositional history. Depositional history of a sedimentary rocks is type of enbironment in which rock developed.The depositional history of an oil shale includes the organisms and sediments that were deposited, as well as how those deposits interacted with pressure and heat.
The van Krevelen Diagram
The van Krevelen Diagram is a method of classifying oil shales based on their depositional history. The diagram divides oil shales according to where they were deposited: in lakes (lacustrine), in the ocean (marine), or on land (terrestrial).
Oil shales from lacustrine environments formed mostly from algae living in freshwater, saltwater, or brackish water. Lamosite and torbanite are types of oil shales associated with lacustrine environments. Lamosite deposits make up some of the largest oil shale formations in the world. Torbanite deposits are found mainly in Scotland, Australia, Canada, and South Africa.
Oil shales from marine environments formed mostly from deposits of algae and plankton. Kukersite, tasmanite, and marinite are types of marine shales. Kukersite is found in the Baltic Oil Shale Basin in Estonia and Russia. Tasmanite is named after the region in which it was discovered, the island of Tasmania, Australia. Marinite, the most abundant of all oil shales, is found in environments that once held wide, shallow seas. Although marinite is abundant, it is often a thin layer and not economically practical to extract. The largest marinite deposits in the world are in the United States, stretching from the states of Indiana and Ohio through Kentucky and Tennessee.
Oil shales from terrestrial environments formed in shallow bogs and swamps with low amounts of oxygen. The deposits were mostly the waxy or corky stems of hardy plants. Cannel shale, also called cannel coal or “candle coal,” is probably the most familiar type of terrestrial oil shale. Cannel coal was used primarily as fuel for streetlights and other illumination in the 19th century.
Classifying Oil Shales by Mineral Content
Thye are classified in three main types based on their mineral content: carbonate-rich shale, siliceous shale, and cannel shale.
Carbonate-rich shale deposits have high amounts of carbonate minerals. Carbonate minerals are made of various forms of the carbonate ion (a unique compound of carbon and oxygen). Calcite, for instance, is a carbonate mineral common in carbonate-rich shales. Calcite is a primary component of many marine organisms. Calcite helps form the shells and hard exteriors of oysters, sea stars, and sand dollars. Plankton, red algae, and sponges are also important sources of calcite.
Siliceous shale is rich in the mineral silica, or silicon dioxide. Siliceous shale formed from organisms such as algae, sponges, and microoganisms called radiolarians. Algae have a cell wall made of silica, while sponges and radiolarians have skeletons or spicules made of silica. Siliceous oil shale is sometimes not as hard as carbonate-rich shale, and can more easily be mined.
Cannel shale has terrestrial origins, and is often classified as coal. It is made up from the remains of resin, spores, and corky materials from woody plants. It can contain the minerals inertinite and vitrinite. Cannel shale is rich in hydrogen, and burns easily.
Oil Shale Composition
The composition of oil shale may vary according to the depositional mechanism and setting. The composition of the original organic matter may impact the chemical composition of the embedded kerogen.
Quality Factors
The quality of oil shale is important in determining its suitability for production. Some of the important determinants of quality include:
- Richness/Grade (litres per ton l/t))
- Organic material content (as a percentage of weight)
- Hydrogen content,
- Moisture content, and
- Concentrations of contaminants including:
- Nitrogen and
- Sulfur and metals.
Richness/Grade
The commercial desirability of any oil shale deposit is dependent on the richness of the shale. Commercially attractive grades of oil shale contain 100 l/t or more. There are some deposits of oil shale that contain 300 l/t (4). The richness may in fact result in greater yields than determined by Fischer Assay due to efficiencies in processing. Oilshales vary considerably in terms of richness or grade, which is determined by the percentage of organic carbon in the ore. Yield is an expression of the volume of shale oil that can be extracted from the oil shale. Richness of oil shale may be assessed by methodologies including Fischer Assay which is the traditional method but may not provide the total potential volume of oil that can be produced from the shale. A newer method, known as Rock-Evaluation, may provide a better measure of true potential yield (5).
Recoverability
The volume of oil shale is often expressed as the oil shale in place. This is an estimate of the total volume of oil shale contained in the ore taking into account the quality of the resource. Some deposits also have estimates of the recoverable resource which takes into consideration additional factors to determine the volume ofshale that may actually be extracted from the ore. There is variation in the degree that individual deposits around the world have been evaluated or characterized; thus, the volume of shale oil is not fully known.
Oil Shale Formation
Oil shale is formed from organic material which may have several different origins. It is often categorized according to the origin of the organic material into three major categories: terrestrial, lacustrine, and marine (2).
- Terrestrial oil shale is formed from organic material, plant and animal matter that once lived on land, similar to the material that produces coal.
- Lacustrine oil shale descends from fresh or brackish water algae remains.
- Marine oil shale deposits are the result of salt water algae, acritarchs, and dinoflagellates.
The origin of the oil shale may impact its quality and/or the other minerals that are found within the deposit
Distribution
Deposits of oil shale may be found at varying depths below the surface. Oil shale occurs in nearly 100 major deposits in 27 countries worldwide (3). It is generally found atshallow depths of lessthan 900 meters, whereas deeper, warmer geologic zones are required to form conventional oil. Some deposits are close to the surface in relatively thin beds of shale. Other deposits may be found deeper beneath the surface (greater than 300 meters) in very thick beds (300 meters or more in thickness).
Characteristics and Properties
Thye are found in different host rock types, but most deposits are either carbonate or silica based. The type of rock that comprises the shale body affects the mining and heating approaches, moisture content, and air and carbon emissions released during processing. Silica and clay based oil shales tend to have higher moisture contents. Carbonate rock may crumble in mining, crushing, and handling creating small particles called fines. Fines require different retorting approaches than lump shale. Fines can also contaminate shale oil with particulates that are difficult to remove. Carbonate rock also decomposes when subjected to high temperatures causing the creation and release of carbon dioxide emissions.
World Oil Shale Resources
World oil shale resources are characterized to varying degrees. The largest resource, the United States deposits, contain approximately 75 percent of the world’s oil shale resources and a great deal is known about the quality and extent of these resources. However, there are many deposits around the world in which little is known about the quality and extent of the resource.
Oil Shale Uses
People have been using oil shale for thousands of years. Ancient Mesopotamians used shale oil to pave roads and caulk ships. Ancient Mongolians dipped the tips of their arrows in shale oil during battles, sending flaming arrows at their enemies. In the Middle East, sticky shale oil was even a component of decorative mosaics.
The modern shale industry began in the 19th century. This industry used industrial processes to heat shale in order to extract oil. Shale oil was used for a variety of products, including paraffin wax.
European countries, and later the United States, began extracting oil shale and shale oil and burning them as sources of fuel. The first U.S. shale mining facilities were established in the Ohio River Valley in the states of Pennsylvania, Ohio, West Virginia, and Kentucky.
Extracting and processing shale oil is an expensive and difficult process. Coal, petroleum, and natural gas are less expensive to extract. Australia, Brazil, Switzerland, Sweden, Spain, and South Africa began mining oil shale in the 19th and 20th centuries, but they all stopped production by the 1960s. The U.S. ceased production in the early 1980s.
Many nations, including Estonia, China, and Brazil, continue to rely on oil shale for fuel. It is burned to generate electricity, is a component in chemical industries, and byproducts are used in cement production.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Knaus, E., Killen, J., Biglarbigi, K., & Crawford, P. (2010). An overview of oil shale resources. In ACS symposium series (Vol. 1032, pp. 3-20). Oxford University Press.
- Society, N. (2019). oil shale. [online] National Geographic Society. Available at: https://www.nationalgeographic.org/encyclopedia/oil-shale/ [Accessed 9 May 2019].
