Petroleum is a naturally occurring mixture of hydrocarbons and other organic compounds found within the Earth's subsurface, particularly in sedimentary basins. The word petroleum comes from Latin roots meaning rock and oil, reflecting its geological occurrence. In broad usage, petroleum can refer collectively to naturally occurring liquid and gaseous hydrocarbons, although in everyday language the term is often used specifically for crude oil and its associated products.
Crude oil is a complex liquid mixture containing hydrocarbons of many different sizes and structures, together with smaller quantities of sulfur, nitrogen, oxygen, metals and other compounds. Petroleum is not a single chemical substance with one fixed formula. Its composition varies according to the source rock, geological maturity, migration history, reservoir conditions and subsequent alteration. Petroleum systems can contain crude oil, natural gas and condensates, with the physical phase depending on pressure, temperature and composition. The petroleum industry therefore uses more precise terms such as crude oil, condensate, natural gas, petroleum liquids and petroleum products when distinguishing different materials.
Petroleum forms mainly through the geological transformation of ancient organic matter preserved in sedimentary rocks. Organic rich sediments can become source rocks when deposited under conditions that allow substantial organic matter to survive decomposition. Burial increases pressure and temperature, and the organic material is transformed into kerogen. With sufficient thermal maturation, kerogen generates hydrocarbons. Liquid hydrocarbons generated within the oil window can migrate out of the source rock and travel through permeable geological pathways.
If they encounter a suitable reservoir and seal, they can accumulate in a trap. Petroleum geology therefore distinguishes between source rocks, reservoir rocks, seals, traps and migration pathways. A petroleum accumulation requires these components to occur in an appropriate geological sequence and at appropriate times. The formation of petroleum generally takes millions of years, which is why conventional petroleum is classified as a fossil resource rather than a renewable resource.
Crude oil contains many classes of hydrocarbons, including alkanes, cycloalkanes and aromatic hydrocarbons. It can also contain resins, asphaltenes and compounds containing sulfur, nitrogen and oxygen. The proportions of these constituents determine important properties such as density, viscosity, volatility, acidity and sulfur content. Petroleum engineers and refiners often classify crude oils as light or heavy and sweet or sour.
Light crude generally contains a greater proportion of lower molecular weight hydrocarbons and has lower density, while heavy crude contains more high molecular weight material and is more viscous. Sweet crude contains relatively little sulfur, whereas sour crude contains more sulfur compounds. These classifications affect refinery requirements, transportation, processing costs and the types of products that can be produced. Crude oil can also contain dissolved natural gas, and gas may separate from the liquid as pressure changes during production.
Petroleum is extracted by drilling wells into underground accumulations. A conventional oil reservoir consists of porous and permeable rock containing hydrocarbons within its pore spaces rather than a giant underground cavity. Natural reservoir pressure may push oil towards production wells, particularly during the early stages of production. As pressure falls, operators can use artificial lift systems or inject water or gas to improve recovery.
Primary recovery relies mainly on natural reservoir energy and artificial lift, while secondary recovery commonly involves water or gas injection. Enhanced oil recovery methods can use additional techniques such as thermal methods, gas injection or chemical flooding to improve displacement of remaining oil. Recovery is never complete because some petroleum remains trapped by capillary forces, geological heterogeneity, viscosity and other physical limitations. The proportion ultimately recovered depends on reservoir properties, fluid characteristics, well design, pressure management and recovery technology.
Petroleum is rarely used exactly as it emerges from the ground. Crude oil is transported to refineries where physical separation and chemical conversion processes produce useful products. Distillation separates crude oil into fractions according to differences in boiling range. Refineries then use processes such as catalytic cracking, hydrocracking, reforming, alkylation and other treatments to alter molecular structures and produce products with desired properties. Major petroleum products include petrol, diesel, jet fuel, heating fuels, lubricants, bitumen and petrochemical feedstocks.
Refining therefore does not simply separate pre existing petrol and diesel from crude oil. It also chemically transforms hydrocarbon molecules. Some refinery streams become feedstocks for petrochemical manufacturing, which produces plastics, synthetic fibres, solvents, detergents, coatings, pharmaceuticals and many other materials. Petroleum consequently functions both as an energy resource and as an industrial chemical feedstock.
Petroleum has had enormous economic and historical importance because liquid hydrocarbons are energy dense, transportable and suitable for internal combustion engines. The widespread adoption of petroleum fuels transformed road transport, aviation, shipping and mechanised agriculture. Petroleum infrastructure includes wells, gathering systems, pipelines, storage terminals, tankers, refineries, fuel distribution networks and petrochemical facilities. The industry's development shaped industrial economies, international trade and infrastructure throughout the twentieth and twenty first centuries.
Petroleum is traded internationally in multiple grades, and prices reflect factors including crude quality, production levels, demand, inventories, transportation capacity, geopolitical conditions and refinery requirements. Different crude oils have different market values because their properties affect processing costs and the product yields that can be obtained. Petroleum therefore exists within a complex global system involving geology, engineering, economics, chemistry, logistics and international trade.
Petroleum use has substantial environmental consequences. Burning petroleum products releases carbon dioxide and contributes to atmospheric greenhouse gas concentrations. Combustion can also produce nitrogen oxides, particulate matter, carbon monoxide and other pollutants, depending on fuel and engine conditions. Petroleum extraction can disturb land and ecosystems, while offshore production creates risks associated with drilling and spills. Refining involves energy use and emissions and can produce hazardous or sulfur containing waste streams that require controlled treatment. Transport infrastructure can also affect ecosystems.
At the same time, petroleum remains embedded in products and services beyond combustion, meaning environmental strategies must address both fuel use and material applications. Alternatives include electrification, renewable electricity, recycled materials, bio based feedstocks and other chemical pathways, but the feasibility of substitution differs among applications.
Petroleum is related to, but should not be treated as identical to, crude oil, natural gas and petroleum products. Crude oil is the naturally occurring liquid hydrocarbon mixture extracted from underground reservoirs. Natural gas is predominantly gaseous and consists mainly of methane, although both oil and gas can originate within the same petroleum system. Petroleum products are materials manufactured by refining crude oil or processing other petroleum streams.
The word petroleum is therefore sometimes used broadly and sometimes narrowly depending on technical context. In geology, petroleum can encompass naturally occurring hydrocarbons in liquid and gaseous forms, while everyday usage often treats petroleum as another name for crude oil. Understanding the terminology is important because statements about petroleum may otherwise become ambiguous. The central scientific concept is that petroleum consists of hydrocarbon rich material generated from ancient organic matter through geological processes and accumulated within suitable subsurface environments.
