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Petroleum is a complex mixture of hydrocarbons (compounds composed of hydrogen and carbon) that occur in the Earth in liquid, solid or gaseous form. The liquid form of petroleum is known as crude oil, which is a naturally-occurring substance found trapped in certain rocks below the earth's crust (lithosphere). It is a dark, sticky liquid which is highly flammable and can be burned to produce energy. Along with a similar hydrocarbon, natural gas, crude oil makes an excellent fuel. The world reserves of crude oil are estimated at more than one trillion barrels (one barrel equals 42 US gallons or 159 litres), of which more than 75% belongs to the 11 OPEC (Organisation of Petroleum Exporting Countries) countries. Currently, OPEC’s members 40 percent of the total world output, which stands at approximately 75 million barrels per day. Crude oils are such a useful foundation for so many different substances because they contain hydrocarbons. Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings. Crude oil contains countless types of different hydrocarbons all mixed together. The hydrocarbons must be separated to produce anythin