Oil shale is a sedimentary rock formed from the remains of ancient organisms that lived millions of years ago. It contains kerogen, which breaks down into hydrocarbons like oil and natural gas when heated. The largest oil shale deposit in the United States is the Green River formation, spanning Colorado, Utah, and Wyoming, which contains over 1.8 trillion barrels of shale oil. Oil shale is classified based on its depositional environment such as lacustrine, marine, or terrestrial, and mineral content including carbonate-rich, siliceous, or cannel shale types.

1. 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).

2. 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

3. 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.

4. 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