Oil shale

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