What Is Petroleum?

Petroleum is a naturally occurring liquid hydrocarbon mixture found in underground geological formations, extracted as crude oil and processed into fuels, lubricants, and chemical feedstocks. The word derives from the Latin petra (rock) and oleum (oil). Crude oil consists primarily of hydrocarbons ranging from light gases to heavy waxy solids, along with smaller quantities of sulfur, nitrogen, oxygen, and trace metals. Its composition varies significantly by source: light crude oils from formations such as West Texas Intermediate (WTI) or Brent contain a higher proportion of low-boiling hydrocarbons and yield more gasoline and distillates per barrel, while heavy crudes from Canada's oil sands or Venezuela's Orinoco Belt require more intensive processing to produce the same product slate.

Petroleum underlies modern energy systems and the chemical industry. It is classified by the US Energy Information Administration among the primary fossil fuels and is the source of the largest share of liquid transportation fuels consumed globally.

Geological Formation and Composition

Petroleum forms from organic matter, primarily marine microorganisms, deposited in sedimentary basins over millions of years. Burial under heat and pressure initiates diagenesis and then catagenesis, the process by which biological molecules are converted first to kerogen and then to hydrocarbons. Reservoirs are porous sedimentary rock formations, typically sandstone or carbonate, capped by impermeable layers that prevent upward migration. Crude oil is characterized by API gravity (a measure of density relative to water) and sulfur content: oils above 40 degrees API are considered light, and those with sulfur below 0.5 percent are termed sweet. The US Energy Information Administration's overview of oil and petroleum products describes these classification systems and the distribution of crude oil types in global production.

Refining and Fuel Processing

Converting crude oil into usable products requires a series of physical and chemical separation steps. Atmospheric distillation, the first step in any refinery, heats the crude in a distillation column and separates fractions by boiling point: refinery gases and naphtha at the top, then kerosene and jet fuel, then diesel and gas oils, with heavy residuum at the bottom. Vacuum distillation processes the heavy residuum further under reduced pressure. Conversion units including fluid catalytic cracking (FCC) and hydrocracking break heavier molecules into lighter, more valuable products such as gasoline and diesel. Hydrotreating removes sulfur, nitrogen, and other contaminants to meet fuel quality specifications. A detailed walkthrough of these steps appears in the EIA's explanation of the crude oil refining process, including the role of reforming in producing high-octane gasoline blendstock.

Properties and Quality Metrics

Key properties that govern petroleum's suitability for different applications include viscosity, pour point, flash point, sulfur content, and distillation curve. The Penn State course material on introduction to petroleum refining and crude oil composition covers how these parameters determine which processing configuration a refinery must deploy. Refineries perform continuous feedstock analysis to optimize unit operations as crude blends change.

Applications

Petroleum has applications in a range of fields, including:

• Transportation fuels: gasoline, diesel, jet fuel, and marine bunker fuel
• Heating oil and residential energy supply
• Lubricants for engines, industrial machinery, and hydraulic systems
• Petrochemical feedstocks for plastics, synthetic fibers, and specialty chemicals
• Asphalt and bitumen for road construction and roofing