Crude oil is a naturally occurring liquid fossil fuel found trapped in rock formations deep underground. It formed over millions of years from the remains of ancient marine organisms and serves as the raw material for gasoline, diesel, plastics, and thousands of other products. A single 42-gallon barrel of crude oil yields about 45 gallons of petroleum products after refining, thanks to processing gains that expand the volume.
At its core, crude oil is a complex mixture of hydrocarbons. A typical sample is about 84.5% carbon and 13% hydrogen, with 1 to 3% sulfur and less than 1% each of nitrogen, oxygen, and trace metals. The specific blend varies enormously depending on where the oil comes from, which is why not all crude is created equal.
How Crude Oil Forms
Crude oil begins as tiny marine organisms, mostly plankton, that die and settle on the ocean floor. Over time, layers of sediment bury this organic material. In the early stages, mild heat and pressure compact the sediment and transform the organic matter into a waxy substance called kerogen. Minerals present in the sediment act as natural catalysts for the chemical reactions involved.
As the rock gets buried deeper, rising temperatures and pressures break down the kerogen into chains of hydrogen and carbon atoms: hydrocarbons. This process requires a narrow temperature window. Too much heat produces only natural gas (very short hydrocarbon chains). Too little heat, and the kerogen stays locked in rock without converting to liquid oil.
Even after oil forms, it needs the right geological setup to accumulate in useful quantities. Three conditions have to come together: a source rock rich in organic material buried at the right depth, a porous reservoir rock with connected spaces where oil can pool, and a cap rock or seal above it that prevents the oil from migrating to the surface and dissipating. This combination is uncommon enough that commercially viable oil reservoirs are concentrated in specific regions around the world.
Light vs. Heavy, Sweet vs. Sour
The oil industry classifies crude along two main axes: density and sulfur content. These classifications matter because they determine how easy and expensive the oil is to refine.
Density is measured using a scale called API gravity, where higher numbers mean lighter oil. Light crude scores above 38 degrees API, heavy crude falls at 22 degrees or below, and intermediate crude sits between 22 and 38 degrees. Light crude flows more easily and yields a higher percentage of valuable products like gasoline and jet fuel. Heavy crude is thicker, requires more energy to process, and produces more residual products like asphalt.
Sulfur content splits crude into “sweet” and “sour” categories. Sweet crude contains 0.5% sulfur or less by weight, while sour crude contains 1% or more. Sulfur has to be removed during refining because it produces harmful emissions when burned, so sweet crude commands a premium price. Some of the most sought-after crude oils in the world, like West Texas Intermediate, are both light and sweet.
How Oil Gets Out of the Ground
Extracting crude oil happens in stages, each pulling a bit more from the reservoir.
- Primary recovery relies on the natural pressure inside the reservoir, sometimes assisted by pumps, to push oil to the surface. This stage typically recovers only about 10% of the oil in a reservoir.
- Secondary recovery injects water or gas into the reservoir to displace oil and push it toward wells. This raises total recovery to 20 to 40% of the original oil in place.
- Enhanced recovery uses more advanced techniques to coax out an additional share, potentially reaching 30 to 60% total recovery. The three main approaches are injecting steam to thin out heavy oil so it flows more easily, injecting gases like carbon dioxide that expand underground and push oil toward wells, and injecting chemicals that act like detergents to free oil droplets stuck in rock pores.
Even with the most advanced techniques, a significant portion of the oil in any given reservoir stays in the ground. The economics of extraction determine how far producers go: each stage costs more than the last, and at some point the expense of pulling out another barrel exceeds what that barrel is worth.
Who Produces the Most
The United States is the world’s largest producer of crude oil and petroleum liquids. U.S. output is forecast to reach about 13.7 million barrels per day by 2026, with roughly half of that coming from the Permian Basin in West Texas and New Mexico.
Saudi Arabia and Russia are the next largest producers, each averaging around 9 million barrels per day in 2024, though Saudi Arabia has voluntarily cut production in coordination with OPEC+. Russia averaged 9.2 million barrels per day that year, while Saudi Arabia produced 9.0 million. After those three, Iraq (4.4 million barrels per day), Canada (6.0 million barrels per day of total petroleum liquids), and the United Arab Emirates (2.9 million barrels per day) round out the top producers.
What a Barrel of Oil Becomes
A refinery heats crude oil and separates it into components based on their boiling points, a process called distillation. The lightest molecules rise to the top, and the heaviest sink to the bottom. The exact product mix shifts based on market demand and the type of crude being processed, but the major outputs include gasoline, diesel fuel, jet fuel, heating oil, and heavy fuel oil used in shipping.
Beyond fuels, crude oil is the starting material for an enormous range of everyday products. Petrochemicals derived from oil go into plastics, synthetic rubber, fertilizers, pharmaceuticals, cosmetics, detergents, and synthetic fabrics like polyester and nylon. Asphalt for road paving, lubricants for engines, and wax for candles and food packaging all trace back to a barrel of crude. The sheer breadth of products is why petroleum remains so deeply embedded in modern economies, even as energy systems shift toward alternatives.
Environmental Footprint of Extraction
Not all crude oil carries the same environmental cost. The carbon intensity of producing a barrel varies dramatically depending on where and how it’s extracted. A global analysis found the average upstream carbon intensity is about 10.3 grams of CO₂ equivalent per megajoule of crude oil, but country-level figures range from 3.3 (Denmark) to 20.3 (Algeria).
Two practices drive the highest emissions. The first is gas flaring, where natural gas that surfaces alongside oil gets burned off instead of captured. In countries like Algeria, Iraq, and Nigeria, flaring accounts for 36 to 41% of total extraction emissions. The second is thermal extraction of heavy crude, where large amounts of steam are injected underground to make thick oil flow. Venezuela and Canada, the world’s two largest heavy oil producers, have elevated carbon intensities for this reason. Steam flooding also pushes up emissions in Indonesia, Oman, and parts of California.
Saudi Arabia, despite being one of the world’s largest producers, has relatively low per-barrel emissions. Its oil comes from a small number of extremely large, productive reservoirs that require less energy to operate, and the country has low rates of gas flaring and water production. If the global industry eliminated routine flaring and minimized methane leaks, overall extraction emissions could drop from 10.3 to roughly 5.8 grams of CO₂ equivalent per megajoule, a reduction of more than 40%.