Petroleum is a liquid fossil fuel found in underground reservoirs. It formed from ancient marine organisms buried under sediment and transformed by heat and pressure over millions of years. Crude oil is the raw form. Refineries turn it into gasoline, diesel, jet fuel, and hundreds of other products.

Its energy density is hard to beat. A kilogram of petroleum holds about 44 megajoules, far more than coal or wood. It is easy to transport and store. That combination made it the dominant fuel of the 20th century. It powers cars, trucks, ships, and planes. It is also the feedstock for plastics, fertilizers, and pharmaceuticals.

Petroleum has costs. Burning it releases CO2, the main driver of climate change. Drilling and transport can spill, polluting coastlines and ecosystems. Air pollution from vehicles harms health in cities. Demand is shifting as electric vehicles spread, but petroleum remains the world's largest energy source.

Key products

• Gasoline
• Diesel
• Jet fuel
• Heating oil
• Petrochemical feedstocks