Crude oil is the mixture of petroleum liquids and gases (together with associated impurities) pumped out of the ground by oil wells.
It is described by the location of its origin (e.g., "western Texas" or "Brent") and often by its relative weight or viscosity (light, intermediate, or heavy); it may also be referred to as "sweet", which means it contains relatively little sulfur (in the form of the gas H2S) and requires less refining, or "sour", which means it contains substantial sulfur and requires more refining. The presence of H2S also adds considerably to the production costs as this highly toxic gas cannot simply be emitted into the atmosphere. Usually, it is either stored and then disposed of, or pumped back in the top of the oil reservoir where it expands and helps "push" remaining oil towards producing wells (this is referred to as gas reinjection).
The price of oil fluctuates quite widely in response to crises or recessions in major economies, because any economic downturn reduces the demand for oil. On the supply side the OPEC cartel uses its influence to stabilise or raise oil prices. In the early spring of 1999, the average price of around US$14 per barrel (less than US $0.15 per liter), meant crude oil was the second cheapest liquid in the world. Currently (March 2003), Brent crude stands at $33 per barrel.
Crude oil, like coal and natural gas, is generally held to be the product of compression of ancient vegetation over geological timescales. A few scientists, notably Thomas Gold, have suggested other, abiogenic, theories for the origins of crude oil.