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


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One of Earth's most in-demand resources is petroleum, also known as oil. Today, petroleum is found underground and is extracted, or brought out, with giant drilling machines.

Crude oil is usually black or dark brown. It can also be yellowish, reddish, tan or even greenish. Petroleum is used to make gasoline for cars and other products, like tires or life jackets.

Formation of Petroleum

Petroleum began its life millions of years ago. Organic matter like plants, algae and plankton drifted in the oceans. These organisms eventually died and sank to the seafloor. Over time, this organic material was buried under millions of tons of sand, rock and dead plants.

Eventually, ancient seas dried up, leaving behind what are called sedimentary basins. Deep below these, the organic material was pressed between millions of tons of rock and Earth's mantle. The mantle is the immense layer directly beneath Earth's crust. The organic material faced very high temperatures here. The organic matter began to transform into a waxy substance called kerogen.

With more heat, time and pressure, the kerogen transformed into a combination of hydrogen and carbon. Heat and pressure can create this combination. Some other examples of hydrocarbon materials are coal and natural gas.

Sedimentary basins, where ancient seas used to lie, are sometimes sources of petroleum. In Africa, the Niger Delta sedimentary basin covers land in Nigeria, Cameroon, and Equatorial Guinea.

Chemistry and Classification of Crude Oil

The gasoline we use to fuel our cars, the fabrics of our backpacks and shoes, and many other useful products are made from petroleum. However, after 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, which are mainly hydrogen (about 13 percent by weight) and carbon (about 85 percent). Other elements such as sulfur (about 0.5 percent) and metals such as iron and copper (less than 0.1 percent) can also be mixed in. Since oil is made of many different ancient plants and organisms, its chemical makeup can greatly vary. It is almost always necessary to refine and filter crude oil to make useful products.


Oil is classified according to three main categories: the geographic location where it was drilled, its sulfur content and its API gravity.

Classification: Geography

Oil is drilled all over the world. However, there are three primary sources of crude oil.

The first, Brent Crude, is a mixture that 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 oil that is produced mostly in the U.S. state of Texas. It is "sweet" and "light"—considered very high-quality. WTI supplies much of North America with oil.

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

The OPEC Reference Basket is another important oil source. OPEC is the Organization of the Petroleum Exporting Countries founded in 1960 by Iran, Iraq, Kuwait, Saudi Arabia and Venezuela. Other countries have joined or left the organization since that time. The OPEC Reference Basket is the average price of petroleum from OPEC's member countries.

Classification: Sulfur Content

Sulfur in crude oil can ruin machines in the refining process and contribute 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 and is less harmful to the environment.

Classification: API Gravity

The American Petroleum Institute (API) is a group that represents oil and natural gas businesses. The API has established standards for a variety of oil- and gas-related products. The API has also established several 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," and 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 greater concentrations of metals and sulfur, and require more refining.

Petroleum Reservoirs

Petroleum is found in underground pockets called reservoirs. Deep beneath Earth, pressure is extremely high. Petroleum slowly seeps out toward the surface, where there is lower pressure. It continues this movement from high to low pressure until it encounters a layer of rock that it cannot soak through. The petroleum then collects in 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 oil producers in the United States. Oil producers in Europe and Asia tend to measure in metric tons. There are about six to eight barrels of oil in a metric ton.

Extracting Petroleum

The total amount of petroleum in a reservoir is called oil-in-place. Many petroleum liquids that make up a reservoir's oil-in-place cannot be extracted. They are simply too difficult, dangerous or expensive to drill. The part of a reservoir's oil-in-place that can be extracted is that reservoir's oil reserves.

Drilling For Oil

On land, oil can be drilled with a structure called an oil or drilling rig. Offshore, oil is drilled from an oil platform.

Most modern wells use an air rotary drilling rig. In this process, engines power a drill bit. A drill bit is a cutting tool used to create a circular hole known as a borehole.

As the drill bit rotates and cuts through the earth, small pieces of rock are chipped off. A powerful flow of air is pumped down the center of the hollow drill, and comes out through the bottom of the drill bit. The air then rushes back toward the surface, carrying with it tiny chunks of rock. These rock samples are carefully examined by oil company geologists.

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

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

Extracting Extra Petroleum

Even after pumping, the majority of the oil can remain tightly trapped in the underground reservoir. Other methods are necessary to extract the remaining petroleum. This stage is called secondary recovery.

One common method is water flooding. Oil producers flood boreholes with water because the weight of the water forces oil out of the reservoirs and into nearby wells.

Drilling offshore is much more expensive than drilling onshore. It requires an enormous structure that can withstand the huge force of ocean waves.

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

Environmental Impacts of Oil Spills

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.

When oil spills in the ocean, it floats on the water. Its presence causes enormous damage to local animal populations. Birds, fish and marine mammals are all threatened by oil spills.

Crude 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 is black, extremely sticky and sometimes rises to Earth's surface.

In its natural state, bitumen is typically mixed with "oil sands" or "tar sands." This makes it extremely difficult to extract.

Unfortunately, refining bitumen is both costly and harmful to the environment. However, we depend on bitumen for its unique properties: about 85 percent of the bitumen extracted is used to make asphalt to pave and patch our roads. A small percentage is used for roofing and other products.

"Lungs of the Planet"

Most of the world's bitumen reserves are in the eastern part of Alberta, Canada. Other major reserves are in the North Caspian Basin of Kazakhstan and Siberia, Russia.

These reserves are 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 and keeps our air clean. The taiga is also home to a wide variety of animal life.

Converting Oil into Products

Before it can be converted into fuel or other products, crude oil first needs to be purified. This process is known as refining.

Crude oil comes out of the ground with impurities, from sulfur to sand. These unwanted elements have to be separated out through heating the crude oil.

Oil was not always used by millions of people as it is today. However, it has always been an important part of many cultures. In many parts of the world it was used as a heating fuel, in lamps and for other purposes.

Modern oil production began in the 1850s, after the Industrial Revolution created many new uses for petroleum. Machinery powered by steam engines quickly became too slow, small-scale and expensive. As a result, petroleum-based fuel was soon in great demand. The invention of the mass-produced automobile in the early 20th century further increased demand for petroleum.

Global Oil Use

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 per 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 per 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.

The gasoline we depend on for transportation to school, work or vacation comes from crude oil. A barrel of petroleum produces about 72 liters (19 gallons) of gasoline and is used by people all over the world to power cars, boats, jets, and scooters.

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

Carbon Cycle

There are major disadvantages to extracting fossil fuels. It is a problematic business. Carbon, an essential element on Earth, makes up most of the hydrocarbons in petroleum. Carbon is absorbed by plants and is part of every living organism. It is naturally released through volcanoes and evaporation, for example. When carbon is released into the atmosphere, it absorbs and retains heat. It balances Earth's temperature and makes our planet livable.

Vast amounts of carbon are stored underground. It is necessary for carbon to stay underground in order for temperatures on Earth to remain stable. However, since the 1700s, large amounts of fossil fuel have been drilled and brought up for energy or fuel. This process releases the carbon that has been left underground. This extra carbon affects the quality of our air, water, and overall climate, causing Earth to warm.

Drilling for natural resources not only releases the carbon stored in fossil fuels. It also releases the carbon stored in the forest itself. Burning gasoline is particularly harmful to the environment. Every liter (gallon) of gas that is combusted in a car's engine releases about nine kilograms (20 pounds) of carbon dioxide. Gasoline and diesel also directly pollute the atmosphere. They release toxic chemicals.

People and Petroleum

That said, oil is a major part of modern civilization. In poorer countries, access to cheap energy can help citizens get jobs and give them more power. Petroleum is used for fuel, as well as many chemicals and medicines. It is also used to make crucial items, such as heart valves, contact lenses, and bandages. Oil reserves are important for improving countries' overall level of jobs and business.

Peak Oil

Oil is a nonrenewable resource. Eventually, we will run out of oil to consume. Even before then, the world's oil reserves will become less capable of meeting the world's need for petroleum. Peak oil is the point when the oil companies have drilled the maximum possible amount of petroleum. After peak oil, petroleum production will only decrease, and costs will rise.

It is impossible to know the year for peak oil. Some geologists argue it has already passed. Many estimate that peak oil might be reached within 20 years. Others think it is further out.

Petroleum Alternatives

Experts in many groups are increasingly concerned about petroleum drilling. Governments and organizations are encouraging citizens to change their habits so we do not rely so heavily on oil.

Algae is a potentially enormous source of energy. It grows extremely quickly, and algae oil can be converted into fuel. About 38,850 square kilometers (15,000 square miles) of algae would provide enough biofuel 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, releases oxygen and does not require freshwater.

The country of Sweden wants to greatly reduce its dependence on oil and other fossil fuel energy by 2020. Experts in science, forestry, and energy have come together to develop new sources of energy we can reuse. These include wind farms, wave and solar energy, and fuel from living things, known as biofuel.

Changes in society's habits could also decrease oil use. Some examples are using more public trains and buses, and video meetings for businesses instead of traveling.

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

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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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