Petroleum, or crude oil, is a fossil fuel and nonrenewable source of energy.


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Petroleum is used in countless products we use every day. Petroleum is also known as oil. It is a fossil fuel. In other words, it is made of the remains of ancient living organisms.

Petroleum is found underground and brought out with giant drilling machines. When it comes from the ground, oil is usually black or dark brown. It is used to make gasoline for cars and many other products.

Formation of Petroleum

Petroleum began its story millions of years ago. Plants, algae and tiny sea creatures drifted in the oceans. These organisms eventually died and sank to the bottom. Over time, they were buried under millions of tons of sand and rock.

Then, the ancient seas dried up. Deep below these dry areas, the dead organisms were pressed between the millions of tons of rock and Earth's layers. Underground, the material faced extremely high heat. With pressure, the matter began to transform into a substance called kerogen.

With more heat, time and pressure, the kerogen transformed into a mix of hydrogen and carbon. This mix is called a hydrocarbon. Some other examples of hydrocarbon materials are coal and natural gas.

Chemistry and Classification of Crude Oil

The gasoline we use to fuel our cars is made of petroleum. So are the fabrics of our backpacks, shoes and thousands of other useful products. However, after the oil is drilled, it is usually crude, meaning it isn't ready to be used right away. Its chemical makeup can be very different depending on where it comes from.


Crude oil is composed of hydrocarbons. About one-tenth is the chemical hydrogen. Nearly nine-tenths of it is the chemical carbon. There are also tiny bits of other elements such as sulfur, iron, and copper mixed in. Crude oil usually has to be cleaned to get rid of these extra elements and make useful products.


Oil is grouped by three main categories: the place it was drilled, the amount of sulfur in it and its API gravity.

Classification: Geography

There are three primary places oil comes from. The first, Brent Crude, comes from 15 different oil fields between Scotland and Norway in the North Sea. These fields supply oil to most of Europe.

West Texas Intermediate (WTI) is the oil that is produced mostly in the U.S. state of Texas. It is "sweet" and light," which is of high quality. WTI supplies much of North America with oil.

Finally, there is Dubai crude, also known as Fateh or Dubai-Oman crude. This oil is light and sour. It is produced in Dubai, part of the United Arab Emirates. The nearby country of Oman has recently begun producing oil. These crudes are mostly shipped to Asia.

OPEC is a group that helps set the average price of petroleum. The name stands for the Organization of the Petroleum Exporting Countries. It was founded in 1960 by the countries Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela. Other countries have joined or left since then.

Classification: Sulfur Content

Sulfur is often found in petroleum. Sulfur contributes to air pollution. Petroleum with more than 0.5 percent sulfur is called "sour." Petroleum with less than 0.5 percent sulfur is "sweet."

Sweet oil is usually much more valuable than sour. It does not require as much refining, or filtering, and is less harmful to the environment.

Classification: API Gravity

The American Petroleum Institute (API) is a group that works to help oil and natural gas businesses. The API has set rules that many oil- and gas-related products follow. The API has also created some shared units of measurement.

API gravity is a measure of the density of petroleum liquid compared to water. If a petroleum liquid's API gravity is greater than 10, it is "light." It floats on water. If the API gravity is less than 10, it is "heavy," and sinks in water.

Light oils are preferred because they have more hydrocarbons. Heavier oils have more metals and sulfur and need more refining.

Petroleum Reservoirs

Petroleum is found in underground pockets called reservoirs. Deep underground, the pressure is extremely high. Petroleum slowly seeps up toward the surface, where there is less pressure. It continues to move from high to low pressure until it hits a layer of rock that it cannot soak through. The petroleum then collects in pools called reservoirs, which can be several hundred yards below Earth's surface.

The amount of petroleum in a reservoir is measured in barrels or tons. An oil barrel is about 159 liters (42 gallons). This measurement is usually used by U.S. oil companies. Oil producers in Europe and Asia often measure oil in metric tons. There are about six to eight barrels of oil in a metric ton.

Extracting Petroleum

Oil is reached by drilling deep underground. On land, it is drilled with an oil or drilling rig. Offshore, it is drilled from an oil platform.

Underground Drilling

Air rotary drilling rigs are the most common kind of rig. This system uses a huge engine-powered drill bit. A drill bit is a cutting tool that creates a round hole known as a borehole.

As the drill bit cuts through the earth, small pieces of rock are chipped off. A strong flow of air is pumped down the center of the hollow drill and out the bottom of the drill bit. The air then rushes back toward the surface. As it rises, it carries tiny chunks of rock with it. These rock samples are carefully examined for signs of oil.

When the drill hits oil, some of the oil shoots from the ground and high into the air. This sudden release of oil is known as a "gusher."

Once a reservoir has been located, pumps are used to extract the oil. Their up-and-down motion pushes oil up to the surface.

Even after pumping, most of the oil can remain tightly trapped in the underground reservoir. Other steps are necessary to extract leftover oil. This stage is called secondary recovery.

One common approach is water flooding. Oil producers first flood boreholes with water. The weight of the water then forces oil out of the reservoirs and into nearby wells. A well is a shaft drilled into the rock.

Drilling on the Water

Drilling offshore is much more expensive than drilling onshore. The platform needs to be huge and strong enough to stand up to the force of ocean waves.

The platform can be tied to the ocean floor and float. Or, it can be fixed to the bottom of the ocean with concrete or steel legs.

Oil platforms can cause major environmental disasters. Problems with the drilling equipment can cause the oil to explode out of the well and into the ocean. Millions of barrels of oil can be released before the well is plugged.

Effects of Oil on Environment

When oil spills in the ocean, it causes enormous damage to animal populations. Birds, fish and marine mammals such as seals and whales can be killed or sickened.

Oil does not always have to be extracted through deep drilling. It sometimes seeps all the way to the surface and bubbles above ground. Bitumen is a form of petroleum that sometimes rises to Earth's surface. It is black and very sticky.

Bitumen is usually mixed with "oil sands" or "tar sands." This makes it very difficult to extract.

Before it can be used, bitumen needs to be refined or cleaned of its unwanted elements. Unfortunately, this refining is both expensive and harmful to the environment. However, bitumen has special properties that make it very useful. We depend on it to make and fix our roads. It is also used for roofing and other products.

Bitumen for Our Roads

Most of the world's bitumen is found in the eastern part of Alberta, Canada. The North Caspian Basin of Kazakhstan, and Siberia, Russia, also contain a good deal.

However, this bitumen is located beneath part of the boreal forest, also called the taiga. This makes extraction both difficult and environmentally dangerous.

The taiga circles the upper Northern Hemisphere, mostly in Canada, Russia, and Scandinavia. It is sometimes called the "lungs of the planet" because it filters tons of water and oxygen through the leaves of its trees every day. Every spring, the taiga releases immense amounts of oxygen into the atmosphere. The release of this oxygen keeps the world's air clean. The taiga is also home to many kinds of animal life.

Purifying Oil for Products

Before it can be turned into fuel, oil first needs to be purified. Its unwanted elements need to be removed. This is known as refining. Oil that hasn't yet been refined is called crude oil.

Crude oil comes out of the ground with impurities. These unwanted elements range from sulfur to sand. They are separated out through heating the crude oil.

Modern oil production began in the 1850s after the Industrial Revolution created many new uses for petroleum. Factories needed oil to keep their machines running. The invention of the automobile further increased demand for petroleum.

Popularity of Oil Began with Cars

Petroleum production has expanded greatly over the years. In 1859, the United States produced 2,000 barrels of oil. By 1906, that number was 126 million barrels a year. Today, the United States produces about 6.8 billion barrels of oil every year.

More than 70 million barrels are produced worldwide every day. That is almost 49,000 barrels a minute.

The United States uses far more oil than any other country. In 2017, it used more than 19 million barrels of oil every day.

Gasoline is the major product made from crude oil. It is used by people all over the world to power cars, boats, jets, and scooters. We depend on it for transportation to school, work or vacation. A single barrel of petroleum produces about 72 liters (19 gallons) of gasoline.

Petroleum is not just used as a fuel, though. It is also an ingredient in thousands of everyday items, from nail polish to garbage bags.

Carbon Cycle

There are major problems with drilling for fossil fuels. Carbon is a common element on Earth. Most petroleum is made of old carbon material.

Carbon is absorbed by plants and is part of every living thing. Carbon can be released naturally, like through volcanoes or when water evaporates. When carbon is released into the air, it absorbs and stores heat. It balances Earth's temperature and makes our planet livable.

Huge amounts of carbon are stored underground. However, since the 1700s, large amounts of fossil fuel have been drilled and burned for energy. This releases the carbon that has been left underground. It upsets the carbon balance. It affects the quality of our air and water and makes the planet warmer overall.

Drilling for natural resources not only releases the carbon stored in fossil fuels. It also releases the carbon stored in the forest. Burning gasoline is very harmful, too. Every liter (gallon) of gas that is burned in a car's engine releases about nine kilograms (20 pounds) of carbon dioxide into the environment. Gasoline and diesel also pollute the atmosphere with toxic chemicals.

People and Petroleum

That said, oil is a major part of modern life. Cheap energy is especially helpful in poorer countries. Petroleum is not just used for fuel. It is also used to create many chemicals and medicines. It is used for crucial items such as contact lenses and bandages. In many countries, the oil industry also plays a big role in creating jobs and business.

Peak Oil

Oil is nonrenewable. That means we will eventually run out of oil to consume. Peak oil is the point when the maximum possible amount of petroleum has been drilled. After peak oil, we will have less and less petroleum at our disposal. Oil will become rarer and more expensive.

It is impossible to know when we will reach peak oil. Some geologists argue we have already passed it. Many say that peak oil might be reached within 20 years. Others think it is further out.

Petroleum Alternatives

Experts in many groups are worried about petroleum drilling. Governments and environmental groups are asking citizens to change their habits so we do not use so much oil.

Algae could be used to replace petroleum. Algae grow extremely quickly. Its oil can be used for fuel. About 38,850 square kilometers (15,000 square miles) of algae would create enough fuel to replace all of the U.S.'s petroleum needs. That area is less than half the size of the U.S. state of Maine. Algae even absorb pollution and release oxygen.

The country of Sweden wants to greatly reduce the amount of oil it uses by 2020. It is trying to use more renewable energy, like wind farms, wave and solar energy and cleaner fuels. Unlike petroleum, these sources don't disappear once they are used. People's habits also need to change. For example, people could more public buses and trains instead of cars.

Fast Fact

Leading Petroleum Consumers
1. United States
2. China
3. Japan
4. India
5. Saudi Arabia
Source: US Energy Information Administration

Fast Fact

A “petroleum play” is full of drama! A petroleum play is a group of oil fields in a single geographic region, created by the same geologic forces or during the same time period. A petroleum play may be defined by a time period (Paleozoic play), rock type (shale play), or a combination of both.

Fast Fact

Leading Petroleum Producers
1. Saudi Arabia
2. Russia
3. United States
4. Iran
5. China
Source: U.S. Energy Information Administration

Fast Fact

Proven Reserves
These nations have the world’s largest proven oil reserves.
1. Saudi Arabia
2. Venezuela
3. Canada
4. Iran
5. Iraq
Source: U.S. Energy Information Administration

Fast Fact

Tar Pits
In Los Angeles, California, United Stats, bitumen has been seeping to Earth’s surface for thousands of years at what is now called the La Brea Tar Pits. The pits have preserved fossils of saber-toothed cats, mastodons, turtles, dire wolves, horses, and other plants and animals that were trapped in the sticky substance 40,000 years ago. Bitumen continues to bubble up through the ground today.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Andrew Turgeon
Elizabeth Morse
Mary Crooks, National Geographic Society
Jeannie Evers, Emdash Editing, Emdash Editing
National Geographic Society
Last Updated

October 19, 2023

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