Crude oil

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 as it is strongly controlled by the levels of supply from OPEC. 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.

Some crude oils include

• Nymex[?] (New York)
• Brent[?] (London from the North Sea

See also:

• Fossil fuel
• OPEC
• North Sea oil[?]
• List of oil companies
• List of oil-producing states
• Hubbert curve
• Offshore construction