Crude oil is the world's primary energy resource. Predominantly composed of molecules of hydrogen and carbon, the mixture formed when microorganisms, such as algae and plankton, were compressed and heated under layers of sediment for millions of years. Although used as early as 3,000 BCE to waterproof boats, crude oil was not widely extracted and refined until the mid-19th century, when it increasingly transformed society as a raw material for fuels, plastics, cosmetics, pharmaceuticals, and more.
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The mixture is primarily composed of molecules of carbon and hydrogen atoms bonded into a variety of shapes and lengths, giving each different chemical properties, such as boiling points. Refining can remove elemental impurities, such as sulfur, after hydrocarbons are separated, ensuring the resulting products can be used effectively in other machines, such as car engines.
Despite the misconception that oil comes from dinosaurs, the nonrenewable resource forms when tiny organisms are subjected to high pressure and high temperature while buried under layers of sediment over tens to hundreds of millions of years. During this time, they become a mixture that is almost entirely hydrocarbons, which can be harnessed and refined.
The ancient civilization located in modern-day Iraq also used crude oil as mortar in construction, while the Egyptians used it to treat human remains during the mummification process. After the invention of the first motor car in 1885 and the introduction of the affordable Model T in 1908, demand for crude oil skyrocketed.
API Gravity is a unit that describes how heavy or light a crude oil sample is based on its density, with higher values indicating lighter, more valuable oil that requires less processing. The sweetness of crude oil refers to the amount of sulfur it contains, where sour crude oil requires more extensive treatment to remove the more abundant sulfur and sweeten it.
An analysis of surface rocks using aerial and satellite imagery can help narrow the search for crude oil by identifying basins where sedimentary layers have compacted—a key process in oil formation. Surveys that look for variations in Earth's gravitational and magnetic fields consistent with specific rock distributions, or that emit sound waves that reflect off rock layers—not unlike sonar underwater—can also help map subsurface geology.
Requirements for traffic plans, access roads, and noise barriers vary by state and local laws, but once the rig site is prepared, the rig is assembled and digs a well that gets encased by steel and cement to protect groundwater reservoirs. After drilling and testing the extraction pipe, fracking fluid is injected into the rock to create cracks where crude oil resides, allowing it to be pumped out.
When crude oil is boiled, its constituent hydrocarbons turn into steam and rise through a distillation tower, where temperature decreases with height. Because different chemical vapors cool back into liquids at distinct temperatures, hydrocarbons naturally sort themselves by boiling point and can then be processed separately into distinct outputs.
Just as an organism's digestive system can break down complex molecules in food into simple sugars that cells can use, cracking uses catalysts and other chemicals to break apart large hydrocarbons in crude oil into smaller molecules. These can then be chemically reshaped, combined, or purified into useful petroleum products.
More than 85% is refined into fuels, such as gasoline and diesel, while about 10% is converted into raw materials for plastics, synthetic rubbers, and other essential products, such as lubricants. Because the majority of these products are less dense than the starting crude oil and additional inputs are introduced during the refining process, there is a processing gain of three gallons.
An analysis of 54 studies published between 1977 and 2024 in 15 international regions found evidence that proximity to refineries increased the risks of leukemia, asthma, bronchitis, and respiratory symptoms. Several studies also identified worse cardiovascular function in children and adults living near refineries, alongside other acute and chronic health impacts.
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