What Is Petroleum?

How did petroleum form?

Petroleum formed over millions of years from the remains of ancient marine organisms (mostly plankton and algae) that settled to the seafloor and were buried under sediments. Under heat and pressure deep underground, the organic material gradually transformed into hydrocarbons. Most petroleum formed between 50 and 300 million years ago. The petroleum then migrated upward through porous rocks until it was trapped by impermeable rock layers, forming the underground reservoirs we drill into today. The process is so slow that petroleum is considered a non-renewable resource on human timescales.

What is petroleum made of?

Petroleum is a complex mixture of thousands of different hydrocarbon compounds, plus smaller amounts of sulfur, nitrogen, oxygen, and trace metals. The hydrocarbons range from small molecules like methane (CH4) to large complex molecules with dozens of carbon atoms. Most are alkanes (saturated hydrocarbons), but petroleum also contains aromatic compounds (benzene-like rings) and unsaturated hydrocarbons. The specific composition varies by source: petroleum from different oil fields has different colors, densities, and chemical mixes. Sweet crude has low sulfur; sour crude has higher sulfur and is harder to refine.

How is petroleum refined?

Refining petroleum starts with fractional distillation, where crude oil is heated and the different hydrocarbons separate by their boiling points. The lightest fractions (propane, butane) come off first as gases. Gasoline comes next, then kerosene, jet fuel, diesel, heating oil, and finally heavy residue used for asphalt and lubricants. The fractions can be further processed: cracking breaks large molecules into smaller, more valuable ones; reforming rearranges molecules to make higher-octane gasoline; isomerization converts straight chains to branched ones. Modern refineries produce dozens of different products from the same crude oil.

What is petroleum used for?

Petroleum is the source of most transportation fuels: gasoline, diesel, jet fuel, and marine fuel. It provides heating oil for homes and industrial heating. It's the feedstock for most plastics: polyethylene, polypropylene, PVC, polystyrene, nylon, and polyester all start from petroleum. Petroleum is used to make lubricants, asphalt for roads, waxes, solvents, and thousands of specialty chemicals. Pharmaceuticals, fertilizers, pesticides, paints, dyes, and detergents often have petroleum-derived components. The shift away from petroleum is a major focus of modern materials science due to climate concerns.