Petroleum is a naturally occurring liquid found deep underground, made up of a complex mixture of hydrogen and carbon molecules along with small amounts of other elements. Often called crude oil, it forms over millions of years from the remains of ancient marine organisms buried under layers of sediment. It is the raw material behind gasoline, diesel, plastics, and thousands of everyday products, making it one of the most economically significant substances on Earth.
What Petroleum Is Made Of
At its most basic, petroleum is a blend of hydrocarbons, molecules built entirely from carbon and hydrogen atoms. The number of carbon atoms per molecule ranges from 4 to more than 40, and that range is what gives crude oil its diversity. Lighter molecules with fewer carbons behave like gases or thin liquids. Heavier molecules with 16 to 40 or more carbons are thick and viscous, eventually forming tar-like substances.
The hydrocarbons in crude oil fall into a few major families. Paraffinic hydrocarbons (the alkane series) are straight or branched chains of carbon atoms linked by single bonds. Naphthenic hydrocarbons also use single bonds but arrange themselves in ring structures instead of chains. Aromatic hydrocarbons contain at least one ring with a special type of shared bonding that makes them chemically stable. Finally, asphaltenes are the heaviest, most complex molecules in the mix, responsible for the dark color and sticky texture of heavier crudes. Small amounts of impurities like carbon dioxide, hydrogen sulfide, and nitrogen gas can also be present.
How Petroleum Forms Underground
Petroleum starts as organic matter, mainly the remains of tiny marine plants and animals that settled on ancient ocean floors. Over time, layers of sediment buried this material deeper and deeper. The transformation from biological debris to crude oil happens in three broad stages driven first by microbial activity and later by heat and pressure.
In the earliest stage, called diagenesis, microbes break down much of the organic material into simple molecules like carbon dioxide, water, and methane. What survives this biological processing gradually converts into a waxy, insoluble substance known as kerogen. This stage takes place at relatively shallow depths where temperatures stay below about 50°C (122°F).
As burial continues and temperatures rise, kerogen enters the main oil-generating phase, called catagenesis. Here, heat cracks the large kerogen molecules apart, releasing liquid petroleum. This “oil window” typically occurs at temperatures that correspond to moderate burial depths over geologic timescales of tens of millions of years. At even greater depths and higher temperatures, the remaining organic material generates mostly natural gas rather than oil, a phase known as metagenesis. The entire process, from living organism to extractable crude, spans roughly 10 to hundreds of millions of years.
Physical Properties and Classification
Crude oil straight from the ground varies enormously in appearance and behavior. Some crudes are pale yellow and flow almost like water. Others are nearly black and so thick they barely pour. The density of a typical crude oil hovers around 0.87 grams per milliliter, but this changes quickly once lighter components begin to evaporate. Kuwait crude, for example, starts at 0.869 g/ml and climbs to 0.955 g/ml after losing about 38% of its weight to evaporation.
The petroleum industry classifies crude oils using two key characteristics: density and sulfur content. Density is measured on the API gravity scale, where higher numbers mean lighter oil. Sulfur content divides crudes into “sweet” (low sulfur) and “sour” (high sulfur) categories. Light, sweet crudes are the most valuable because they yield more gasoline and diesel with less refining effort, and they produce fewer pollutants. Heavy, sour crudes require more processing and typically sell at a discount.
How Crude Oil Becomes Usable Products
Raw petroleum is not useful on its own. It must be separated into its component parts through fractional distillation, a process that exploits the fact that different hydrocarbons boil at different temperatures. In a refinery, crude oil is heated in a tall column. The lightest molecules rise to the top and condense first, while heavier molecules settle toward the bottom.
This process yields a range of products in order of increasing boiling point: petroleum gases, gasoline, naphtha, kerosene (also used as jet fuel), diesel fuel, heavy fuel oils, and finally residual material. About 6% of crude oil consists of hydrocarbons with more than 40 carbon atoms, a fraction that ends up as asphalt for roads and roofing. The heaviest residual fraction, sometimes labeled Fuel Oil #6, is extremely viscous and composed mainly of high-molecular-weight hydrocarbons. Further chemical processing, such as cracking and reforming, converts less desirable fractions into higher-value fuels and raw materials for manufacturing.
Products Beyond Fuel
Fuel gets most of the attention, but petroleum is also the starting material for an enormous range of non-fuel products. The U.S. Department of Energy lists hundreds of everyday items derived from oil and natural gas. Plastics are the most obvious, showing up in everything from credit cards and cell phones to artificial limbs and heart valves. But petroleum derivatives also appear in places most people wouldn’t expect: aspirin, antihistamines, cortisone, vitamin capsules, food preservatives, and fertilizers.
Personal care products rely heavily on petroleum chemistry too. Shampoo, hand lotion, lipstick, nail polish, shaving cream, hair coloring, and perfumes all contain petroleum-derived ingredients. Around the house, you’ll find petroleum in detergent, dishwashing liquid, house paint, insulation, trash bags, candles, crayons, and ink. Even solar panels and soft contact lenses depend on petroleum-based materials during manufacturing. Petroleum jelly itself, refined to medical grade, is one of the most widely recommended skin protectants. It works by sitting on top of the skin like a greenhouse roof, preventing moisture loss. Because it is chemically similar to proteins in human skin, it is used to treat everything from chapped lips to dry hands, and one study found it to be the best way to reduce eczema risk in newborns.
Where the World’s Oil Is Located
Global proven oil reserves total roughly 1.7 to 1.8 trillion barrels, depending on the source. “Proven reserves” means the oil that geologists and engineers are highly confident can be extracted commercially with current technology and prices.
Venezuela and Saudi Arabia hold the largest shares. According to BP’s figures, Venezuela tops the list at about 303.8 billion barrels, followed by Saudi Arabia at 297.5 billion barrels. Canada ranks third with roughly 168 to 172 billion barrels, most of it locked in oil sands in Alberta. Iran (around 157.8 billion barrels) and Iraq (around 145 billion barrels) round out the top five. The United States holds an estimated 68.8 billion barrels. Estimates vary between organizations like the U.S. Energy Information Administration, OPEC, and BP because they use slightly different methodologies and reporting years.
Environmental Cost of Burning Petroleum
Burning petroleum products releases carbon dioxide, the primary greenhouse gas driving climate change. The EPA’s emission factors put the combustion of crude oil at about 10.29 kilograms of CO₂ per gallon. Motor gasoline is slightly lower, at 8.78 kilograms of CO₂ per gallon. These figures capture only what comes out of the tailpipe or smokestack. They do not include the additional emissions from extracting, transporting, and refining the oil, which add meaningfully to the total carbon footprint.
Beyond greenhouse gases, petroleum combustion releases smaller quantities of methane and nitrous oxide, both of which trap heat in the atmosphere far more effectively than CO₂ on a per-molecule basis. Oil spills during extraction and transport also pose serious risks to marine and coastal ecosystems. When lighter components evaporate from spilled crude, the remaining heavy hydrocarbons form tar balls that can persist in the environment for years, affecting wildlife and shorelines long after the initial spill.