Oil shale is a type of sedimentary rock that is rich in kerogen. Kerogen is a part of rock that breaks down and releases hydrocarbons when heated. Hydrocarbons are substances made entirely of hydrogen and carbon. Petroleum and natural gas are probably the most familiar hydrocarbons. The hydrocarbons in oil shale can be used as an alternative to petroleum or natural gas.

Like traditional petroleum, natural gas, and coal, oil shale and kerogen are fossil fuels. Fossil fuels developed from the remains of algae, spores, plants, pollen, and a variety of other organisms that lived millions of years ago in ancient lakes, seas, and wetlands.

When these organisms died and drifted to the seabed, they were buried under new layers of plants and sediment. They encountered intense pressure and heat, decomposed, and slowly transformed into the waxy substance known as kerogen.

There is not a consistent chemical composition of kerogen, because it has a variety of origins. Kerogen that formed from land plants (called humic kerogen) usually has a higher oxygen content than kerogen formed from plankton (called planktonic kerogen). However, all types of kerogen consist mainly of hydrocarbons; smaller amounts of sulfur, oxygen, and nitrogen; and a variety of minerals.

Oil shale can be thought of as a precursor to oil and natural gas. With more pressure and over more geological time, kerogen would heat to its “oil window” or “gas window” (the temperature at which it would release crude oil or natural gas).

A sedimentary rock, oil shale is found all over the world, including China, Israel, and Russia. The United States, however, has the most shale resources.

Spanning the U.S. states of Colorado, Utah, and Wyoming, the Green River formation is an underground oil shale formation that contains as much as 1.8 trillion barrels of shale oil. Although not all of this can be extracted, it is more than three times the proven petroleum reserves of Saudi Arabia. 

Oil Shale, Shale Oil, and Oil-Bearing Shale

Oil shale, shale oil, and oil-bearing shale are three different substances. Oil shale is a sedimentary rock. As it reaches its oil window, oil shale releases a liquid known as shale oil. Oil shale is the rock from which shale oil is extracted.

Shale oil is similar to petroleum, and can be refined into many different substances, including diesel fuel, gasoline, and liquid petroleum gas (LPG). Companies can also refine shale oil to produce other commercial products, such as ammonia and sulfur. The spent rock can be used in cement.

Oil-bearing shales are underground rock formations that contain trapped petroleum. The petroleum trapped within the rocks is known as “tight oil” and is difficult to extract. Companies extracting tight oil often use hydraulic fracturing (fracking), while companies extracting shale oil most often use heat.

The Bakken formation, for example, is made of oil-bearing shale. It is a series of layered shale rocks with a petroleum reservoir trapped between the layers. The Bakken formation stretches from the province of Saskatchewan, Canada, through the U.S. states of Montana and North Dakota. Improved drilling technologies have allowed companies to extract oil from the Bakken formation, creating an economic boom in the region.

Classifying Oil Shales

Oil shales are often classified by their depositional history and mineral content. A sedimentary rock’s depositional history is the history of the type of environment in which the 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

Oil shales 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.

Using Oil Shale

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.

Extracting Shale Oil

Obtaining shale oil from oil shale involves heating kerogen in a process called pyrolysis. Pyrolysis is a form of heating without the use of oxygen. At about 60-160 degrees Celsius (140-320 degrees Fahrenheit), kerogen reaches its natural “oil window.” At 120-225 degrees Celsius (248-437 degrees Fahrenheit), kerogen reaches its natural “gas window.” For production of oil shale, the temperatures are much higher.

Pyrolysis can either be done ex situ (above ground) or in situ (below ground).

Ex Situ

During the ex situ process, oil shale is first extracted from the earth by surface or underground mining. The rock is crushed, and then retorted (heated) to release the shale oil. The shale oil is then refined of impurities, such as sulfur.

In Situ

In situ is a new, experimental method of extracting shale oil.

During the in situ process, oil shale is not mined or crushed. Instead, the rock is heated to its oil window while it is still underground.

One technology used for in situ oil extraction is known as volumetric heating. In this process, the rock is heated directly with an electric current. The heating element is injected either directly in a horizontal well or into a fractured area of the rock, until the oil shale begins producing shale oil. The oil could then be pumped directly from underground.

Combined Technologies

Some methods are designed for both in situ and ex situ extraction.

The internal combustion process uses a combination of gas, steam and spent shale produced by ex situ processing. These compounds are burned for pyrolysis. The hot gas is continually cycled through the oil shale, pyrolyzing the rock and releasing oil.

Unfortunately, substances in the oil shale, such as sulfides, react with water to form toxic compounds that are harmful to the environment and to us. Sulfides can cause effects from eye irritation to suffocation. Water containing toxic substances is unusable, and expensive to decontaminate.

The process also produces heaps of ash. This ash can pollute ground, air, and water sources.

Another method that can be used either in situ or ex situ involves chemically reactive fluids. The fluids are injected directly into the retort zone (where the rock is being heated). High-pressure hydrogen is one of the most common chemically reactive fluids. It simultaneously heats the rock, removes sulfur, and upgrades the quality of the extracted oil.

Environmental Effects

Mining for oil shale can have damaging effects on the environment. 

When shale oil is combusted (heated), it releases carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas; it absorbs and retains heat in Earth’s atmosphere, a process called the “greenhouse effect.” The greenhouse effect is essential to life on Earth because it helps insulate Earth and keep it at a warm, livable temperature.

The greenhouse effect helps maintain Earth’s “carbon budget.” Carbon is constantly being exchanged between the ocean, the atmosphere, and the Earth itself. Carbon on the earth is contained in plants, soil, fossil fuels, and all living things—including us!

The carbon in fossil fuels (including coal, petroleum, natural gas, and oil shale) has been sequestered, or stored, underground for millions of years. By removing this sequestered carbon from the earth and releasing it into the atmosphere, Earth’s carbon budget is put out of balance. Burning fossil fuels releases carbon into the atmosphere at a much quicker rate than the trees, water, and ground can reabsorb it. More carbon retains more heat in Earth’s atmosphere, and contributes to rising temperatures—global warming, the current period of climate change. Sometimes, climates can rise faster than organisms can adapt.

Another environmental disadvantage to extracting shale oil is the enormous amounts of freshwater required. Water is necessary for drilling, mining, refining, and generating power. Some experts estimate that three litres (0.8 gallon) of water are required to produce just one litre (0.3 gallon) of shale oil. Some of this water is contaminated by toxic compounds, and is costly to decontaminate. 

Mining can also contaminate groundwater. During in situ processing, toxic byproducts are left underground. They can leach into other sources of water, making them unsafe for drinking, hygiene, or development.

Oil Shale and the U.S.

The United States has enormous proven deposits of oil shale. A source of oil in the United States would reduce the need for importing oil from other countries. This would put people to work and make the U.S. less dependent on foreign trade and fluctuating oil prices.

However, not all of oil shale is recoverable. Oil shales’ distinct chemical compounds and depositional history directly affect their energy content. This determines whether they are economically worth recovering.

For example, Australia’s oil shales are siliceous (silica-based). They have fewer impurities and are less complex than the carbonate-based oil shales in the Western United States, and thus cost less to extract and process. 

Depositional history also matters: Oil shales that developed in wetlands or small lakes are very rich in energy. However, these formations are usually small. Larger lakes created larger shale formations, although these usually yield less oil. 

The process of extracting shale oil is expensive, much more expensive than the process of extracting crude oil. Due to this expense, the use of shale oil in the U.S. has fluctuated depending on the price of crude oil. Companies have only mined for oil shale when the price of crude oil is high. Today, the price of oil is relatively high and extraction technology is becoming more efficient and less expensive. The possibility of mining oil shale has again become a possibility. 

Communities, governments, oil companies, and environmental organizations must weigh the cost of extraction with the benefits of an oil resource.