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


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We use petroleum every day. It is used to make gasoline for cars, for example. It is even used to make fabric in backpacks.

Petroleum is also known as oil. It is a fossil fuel. In other words, it is made of the remains of ancient living organisms. Oil is found underground. It is brought up with giant drilling machines. When oil comes out of the ground, it usually looks black or dark brown.

Petroleum is used for energy. The gasoline we use for cars, for example, is made with petroleum.

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. They 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 change into a new chemical called kerogen. With more heat, time and pressure, kerogen transformed into a mix of hydrogen and carbon. This mix is called hydrocarbon.

Chemistry and Classification of Crude Oil

The chemicals in oil can be very different depending on where it comes from. The oil that comes out right after drilling is called crude. It usually isn't ready to be used right away. It needs certain chemicals taken out.


Crude oil is made of hydrocarbons. In each of these hydrocarbons, nearly one-tenth is the chemical hydrogen. Nearly nine-tenths of it is the chemical carbon. Often, there are also tiny bits of other elements. Sulfur, iron and copper are some examples. To make useful products, these extra chemicals usually have to be taken out.


Oil can be grouped in three ways. It is grouped by the place where it was drilled. It is grouped based on how much sulfur is in it. It is also grouped based on its API gravity.

Classification: Geography

Most oil only comes from three places. One type of oil is Brent Crude. It comes from 15 different oil fields between the European countries Scotland and Norway in the North Sea. These fields supply oil to most of Europe.

West Texas Intermediate (WTI) is a lighter oil. It is produced mostly in Texas. WTI supplies much of North America with oil. This oil is "light" and "sweet."

Finally, there is Dubai crude, also known as Dubai-Oman crude. These crudes are produced in the Middle East. They are mostly shipped to Asia. These oils are "light" and "sour."

OPEC stands for the Organization of the Petroleum Exporting Countries. It is a group that was started in 1960. At the time, its members were the countries of Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela. Other countries have joined or left. All these countries produce oil. OPEC is able to affect the price of petroleum around the world.

Classification: Sulfur Content

Sulfur is often found in petroleum. This chemical pollutes the air. Petroleum with higher levels of sulfur is called "sour." Petroleum with lower levels is "sweet." Sweet oil is usually worth more than sour. It does not need as much cleaning. It 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 also created some shared units of measurement.

API gravity is a measure of how thick petroleum liquid is 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." It sinks in water. Light oils are preferred because they have more hydrocarbons. Heavier oils have more metals and sulfur, and need more cleaning.

Petroleum Reservoirs

Petroleum is found in underground pockets called reservoirs. Deep underground, pressure is very 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 still be hundreds of yards underground.

The amount of petroleum in a reservoir is measured in either barrels or tons. An oil barrel is about 159 liters (42 gallons). This unit is usually used by U.S. oil companies. Oil companies 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.

Drilling for Oil

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. It 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. This air comes out the bottom of the drill bit. It 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 up from the ground. It rises high into the air. This is known as a "gusher."

Once oil is located, pumps are used to extract it. Their up-and-down motion pushes oil up to the surface.

Flushing Out Trapped Oil

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

Water flooding is one common way of getting at leftover oil. 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 or tunnel drilled into rock.

Offshore Drilling

Drilling offshore costs much more than drilling onshore. The platform needs to be huge and very strong.

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

Oil platforms can cause major disasters. Oil sometimes explodes out of the well and into the ocean. Millions of barrels of oil can pour out before the well is plugged.

Oil spills can cause enormous damage. Birds and fish can be killed or sickened. Marine mammals such as seals and whales can also be harmed.

Oil's Harm to Environment

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. The bitumen is cleaned of its unwanted parts. This refining costs a great deal of money. It also can be very harmful to the environment. However, there is a great demand for bitumen. We depend on it to make and fix our roads. It is also used for roofing and other products.

Most of the world's bitumen lies beneath a huge forest known as the taiga. This makes extraction difficult.

The taiga runs through Canada, Russia, and Scandinavia. It is sometimes called the "lungs of the planet." Every spring, the taiga gives off huge amounts of oxygen. This oxygen keeps the world's air clean. The taiga is also home to many kinds of animal life. It would be terrible for the Earth if the taiga was badly damaged by bitumen extraction.

Readying Oil to Become Gasoline

Before it can be turned into fuels like gasoline, oil first needs to be purified. Its unwanted elements need to be taken out. 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 oil.

Cars Need Gas

Modern oil production began in the 1850s. At the time, new factories were springing up everywhere. They needed oil to keep their machines running. The invention of the automobile created even greater demand for petroleum.

Petroleum production has grown greatly over the years. In 1859, the United States produced 2,000 barrels of oil. In 1906, it produced 126 million barrels. Today, the U.S. 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 U.S. uses far more oil than any other country. In 2017, it used more than 19 million barrels of oil every day.

Gas and Everyday Items

Gasoline is the major product made from crude oil. It is used to power cars, boats, and planes. We depend on it to get where we need to go. 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. It is found in everything from nail polish to garbage bags.

Carbon Cycle

There are major problems with drilling for fossil fuels. Carbon is an important element on Earth. Petroleum is mostly made of old carbon material. Carbon is also part of every living thing. It is absorbed by plants. It can be released into the air naturally, like when water evaporates.

When carbon is released into the air, it traps heat. It balances Earth's temperature and makes our planet livable. Huge amounts of carbon are also stored underground. However, since the 1700s, large amounts of fossil fuel have been drilled and burned for fuel. This releases the carbon that has been left underground. It affects the quality of our air and water. It makes the planet warmer.

Burning gasoline is very harmful to the environment. Every liter (gallon) of gas that is burned in a car engine releases about nine kilograms (20 pounds) of carbon dioxide. Gasoline also releases toxic chemicals into the air.

People and Petroleum

Oil is a major part of modern civilization. Cheap energy is especially useful in poorer countries. Petroleum is also used to make many chemicals and medicines. Oil is even used to make items like contact lenses and bandages.

Peak Oil

Oil is a nonrenewable resource. That means we will someday run out of oil in the world. Peak oil is the moment when the greatest possible amount of petroleum has been drilled. After peak oil, there will be less and less petroleum to buy and sell. Oil will become rarer and more expensive.

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

Petroleum Alternatives

Governments and environmental groups are very worried about petroleum drilling. They are asking citizens to change their habits. They want us to use less oil. They are looking for different energy sources.

Algae could be a big new source of energy. Algae grow quickly. Its oil can be used to make fuel. About 15,000 square miles of algae would make 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.

Countries are also trying to use more renewable energy. This is energy that comes from wind, the sun or other sources. Unlike petroleum, these sources don't disappear once they are used.

People can also change their habits. For example, they could use more buses or 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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