Oil shales and kerogens
Oil shales are sedimentary rocks rich in kerogen, which has not yet transformed into crude oil due to insufficient geological conditions. Kerogen, a complex mixture of organic compounds, can be converted into synthetic oil and gas when subjected to high heat and pressure. The composition of oil shales varies based on the organic material and minerals present, and they can be classified into terrestrial, lacustrine, or marine types, depending on their organic content. Extraction methods for oil shale include surface mining and underground methods, depending on the depth of the deposits.
Once extracted, oil shale undergoes pyrolysis, a thermal decomposition process, to produce shale oil and shale gas. While shale oil can be refined into various fuels, such as kerosene and diesel, its environmental impact raises concerns due to potential pollution and greenhouse gas emissions during mining and processing. Oil shales are widespread globally, with significant resources found in the United States, Russia, and China. Recent interest in oil shales has been prompted by fluctuating petroleum prices and the potential role of kerogens in innovative energy solutions, such as hydrogen storage, which may contribute to addressing climate change challenges.
Subject Terms
Oil shales and kerogens
Summary: Oil shales are sedimentary rocks embedded with kerogen, which has not matured into petroleum, or crude oil, due to insufficient heat, pressure, depth, or time. These kerogens can be used to produce synthetic oil and gas.
Oil shales form from organic material that accumulates in sedimentary rock layers and becomes kerogen, a dense mixture of organic chemical compounds. Under sufficient conditions of heat, pressure, and time, kerogen can develop into crude oil and natural gas. Particular oil shales vary in the richness of their organic matter and composition, including the organic matter that forms the kerogen as well as other compounds and minerals found in the deposit. These variations are due to both the particularities of the organic material that forms the kerogen and the specific composition of the sedimentary rock where it is deposited. Oil shales can be categorized as terrestrial, lacustrine, or marine oil shales, based on the kerogen content. Terrestrial oil shales comprise organic material from land plants and animals. Lacustrine oil shales contain remains of freshwater or brackish water algae. Marine oil shales derive from mostly saltwater algae and plankton.
Oil shales can also be categorized by sedimentary deposition, including siliceous shales and carbonate-rich shales. Siliceous shales are predominantly silica-based shales and tend to have higher moisture content, whereas carbonate-rich shales are more likely to crumble through extraction and handling. The origins of the oil shale have an impact on its quality, method of extraction, and use. Oil shales tend to be found within 0.5 mile (900 meters) of the Earth’s surface, closer than petroleum deposits, which require geologic zones at greater depth and temperature to form. Oil shales are distinct from oil sands in that the latter result from the biodegradation of crude oil reserves. Oil shales, in contrast, are kerogen deposits that have not undergone sufficient conditions to become crude oil and thus remain in an immature state.
Extraction and Processing
Oil shale can be used by humans as solid fuel, since it can be ignited without processing. It is most commonly obtained through surface mining procedures, such as open-pit mining and strip mining, when the oil shale deposit is near the surface. If the oil shale resource is farther from the surface, it is extracted via the room-and-pillar method, similar to the underground mining of coal and iron. Once the oil shale has been extracted, it is processed to remove sulfuric and nitrogenous impurities.
Subjected to sufficiently high temperatures, the kerogen derived from oil shales can be distilled by retorting the kerogen through pyrolysis to produce shale oil and oil-shale gas. Pyrolysis is the process of inducing decomposition in organic materials by applying high heat without oxygen. Char, a solid residue that can be used as fuel, may result from pyrolysis. Shale oil is a kind of synthetic unconventional oil, a petroleum product that can be used like conventional oil but extracted or generated through different means from those used to obtain traditional crude oil. Oil-shale gas, which is different from the natural gas produced from shale, called shale gas, can be used in place of natural gas.
Byproducts and Environmental Impact
The byproducts of converting oil shale into shale oil and oil-shale gas include sulfur, ammonia, asphalt, and waxes. Because its composition is different from that of crude oil, shale oil is not directly substituted for crude oil for all uses. However, some conventional fuels, such as kerosene and diesel fuel, can be produced effectively from shale oil. Additionally, it can be used to fuel power plants and to manufacture carbon materials, resins, glues, tanning agents, cements, fertilizers, and glass.
Much of the environmental concern surrounding oil shale centers on its mining, processing, and use. Like other surface mining activities, oil shale mining results in increased erosion, particulate and acid pollution of surface water and groundwater, and sulfur gas emissions. Particulate air pollution is perhaps the most significant concern during processing. Moreover, during its use as fuel, which requires combustion, oil shale and its derivative products release carbon dioxide, a greenhouse gas, into the atmosphere.
Locations
Unlike petroleum deposits, which are concentrated in certain geographic regions, oil shales are widely distributed throughout the world. As of 2024, the ten countries with the largest known technically recoverable oil shale resources, from largest to smallest, were the United States, Russia, China, Argentina, Libya, United Arab Emirates, Chad, Australia, Venezuela, and Mexico. The United States contains more than half of the world’s known oil shale reserves. There are many deposits around the world, although little is known about either the quality or the extent of these oil shale resources.
By the late 1830s, modern commercial mining of oil shale reserves was under way in Autun, France. The United States began promoting shale oil as a replacement for wood as fuel in the 1850s, establishing the oil shale industry as a key factor in the US economy until the discovery and adoption of crude oil by 1859; it was then found that the processing of oil shales to produce shale oil was costlier than obtaining crude oil. Similarly, from 1850 to 1864, Scotland had a stable shale oil industry until imported crude oil became a cheaper alternative. After a brief revival of interest in shale oil in the 1920s, the United States did not show serious interest in oil shale production until the 1970s and the 1980s, as a result of the Arab Oil Embargo, and then the interest was only temporary.
The early twenty-first century has seen renewed interest in developing an oil shale industry in the United States, a consequence of unstable and high costs for natural gas and petroleum products. Oil shale resources are commercially exploited in Brazil, Australia, Estonia, and China, among other nations. Technically recoverable shale oil resources in the United States were estimated at 78.2 billion barrels in 2024, followed closely by Russia (74.6 billion barrels) and, more distantly, China (32.2 billion barrels).
Kerogens hold potential to help solve the global climate change crisis. Researchers have been looking at various ways to store hydrogen, which is a clean fuel but presents complications because of the difficulty in storing the gas. One possibility is subsurface storage of hydrogen in kerogen.
Bibliography
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