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


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The world's engines run because of petroleum.

Petroleum, also known simply as oil, is a fossil fuel formed from the remains of ancient marine organisms. Today, petroleum is found in vast underground reservoirs and extracted with giant drilling machines. Crude oil is usually black or dark brown, but can also be yellowish, reddish, tan or even greenish. Petroleum is used to make gasoline, an important product in our everyday lives, and is part of thousands of other items, like tires or life jackets.

Formation of Petroleum

Petroleum began its life millions of years ago when plants, algae, and plankton drifted in oceans. These organisms eventually died and sank to the seafloor. Over time, they were buried and crushed under millions of tons of sediment and plant debris.

Eventually, ancient seas dried up, leaving behind what are called sedimentary basins. Deep under the basin floor, the organic material was pressed between Earth's mantle, facing very high temperatures, and millions of tons of rock and sediment above. There was almost no oxygen in these conditions, and 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 materials with hydrocarbon are coalpeat and natural gas.

Sedimentary basins, where ancient seas used to lie, are often sources of petroleum. In Africa, for example, 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 the oil is drilled, it is typically 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, it's 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 an 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 Dubai crude, also known as Fateh or Dubai-Oman crude, is a light, sour oil that 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.

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 this organization since that time. The OPEC Reference Basket is the average price of petroleum from OPEC's member countries.

Classification: Sulfur Content

Sulfur is considered an "impurity" in petroleum. Sulfur in crude oil can corrode metal in the refining process and contribute to air pollution. Petroleum with more than 0.5 percent sulfur is called "sour," while petroleum with less than 0.5 percent sulfur is "sweet."

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

Classification: API Gravity

The American Petroleum Institute (API) is a trade association for the 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 the Earth, the 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.

Crude oil is frequently found in reservoirs along with natural gas.

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 because they are 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 in an area where oil reserves have already been found is called developmental drilling. Drilling where there are no known reserves is called exploratory drilling or wildcatting. Directional drilling involves drilling vertically to a known source of oil, then veering the drill bit at an angle to reach additional resources.

Drilling Rigs

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."

Once a reservoir has been located, pumps are used to extract the oil. First, a drilling fluid, known as "mud," is used to create boreholes for extracting oil. Mud pumps circulate this drilling fluid.

Many different kinds of pumps are then used to bring the oil up to the surface. One of the most familiar is the large hammer-shaped pumpjack, which moves up and down above ground. Deep underground, the motion of the pumpjack moves a hollow piston up and down, and this motion constantly carries petroleum up to the surface.

Oil Recovery

Even after pumping, the majority of the oil can remain tightly trapped in the underground reservoir. Other methods are necessary to extract this petroleum, a process called secondary recovery.

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

Drilling offshore is much more expensive than drilling onshore. It requires a massive structure that can withstand the huge force of ocean waves in stormy seas.

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, sea or lake 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.

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

Bitumen and the Boreal Forest

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," which 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, while a small percentage is used for roofing and other products.

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 Northern Hemisphere just below the frozen tundra, 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 and needles of its trees every day. Every spring, the boreal forest 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.

Refining for Useful Products

Refining petroleum is the process of converting crude oil or bitumen into more useful products, such as fuel or asphalt.

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 in a distillation tower.

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.

The modern oil industry was established in the 1850s after the Industrial Revolution created a vast new opportunity for the use of 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.

Petroleum production has rapidly increased. 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 consumes far more oil than any other country. In 2017, it consumed 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, and it is a controversial business.

Carbon, an essential element on Earth, makes up about 85 percent of the hydrocarbons in petroleum. Carbon is absorbed by plants and is part of every living organism. It is naturally released through volcanoes, soil erosion, and evaporation. When carbon is released into the atmosphere, it absorbs and retains heat, regulating Earth's temperature and making our planet livable.

Vast quantities of carbon are stored underground in the form of fossil fuels and in the soil. This underground carbon is necessary because it keeps the Earth's "carbon budget" balanced.

However, since the 1700s, fossil fuels have been aggressively extracted and burned for energy or fuel. This releases the carbon that has been left underground and upsets the carbon budget. It affects the quality of our air, water, and overall climate, causing our planet 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.

Combusting gasoline, which is made from petroleum, 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 emit toxic chemicals.

People and Petroleum

That said, oil is a major part of modern civilization. In poorer developing countries, access to affordable energy can empower citizens and lead to a higher quality of life. Petroleum provides fuel and is a part of 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 economy.

Peak Oil

Oil is a non-renewable resource, and the world's oil reserves will not always meet the world's demand for petroleum. Peak oil is the point when the oil companies are extracting the maximum possible amount of petroleum. After peak oil, petroleum production will only decrease. After peak oil, costs will rise for the remaining supply.

It is impossible to know the precise year for peak oil. Some geologists argue it has already passed, while others maintain that extraction technology will delay peak oil for decades. Many geologists 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 with petroleum drilling. Governments and organizations are encouraging citizens to change their habits so we do not rely so heavily on oil.

Algae is also a potentially enormous alternative source of energy. Algae oil can be converted into fuel. Algae grow extremely quickly and don't take up too much space. About 38,850 square kilometers (15,000 square miles) of algae—less than half the size of the U.S. state of Maine—would provide enough biofuel to replace all of the U.S.'s petroleum needs. Algae absorb pollution, releases oxygen and do not require freshwater.

The country of Sweden wants to drastically reduce its dependence on oil and other fossil fuel energy by 2020. Experts in science, forestry, and energy have come together to develop sources of sustainable energy. These include wind farms, wave and solar energy, and biofuel for hybrid vehicles. Changes in society's habits, such as increasing public transportation and video meetings for businesses, are also part of the plan to decrease oil use.

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

February 9, 2023

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