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Petroleum (from Greek petra – rock and elaion – oil or Latin oleum – oil ) or crude oil is a black, dark brown or greenish liquid found in porous rock formations in the earth. The American Petroleum Institute, in its Manual of Petroleum Measurement Standards (MPMS), defines it as "a substance, generally liquid, occurring naturally in the earth and composed mainly of mixtures of chemical compounds of carbon and hydrogen with or without other nonmetallic elements such as sulfur, oxygen, and nitrogen." Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. It consists of a complex mixture of hydrocarbons, mostly alkanes, but may vary greatly in appearance and composition. Petroleum is used mostly, by volume, for producing fuel oil and petrol (gasoline), both important "primary energy" sources (IEA Key World Energy Statistics). Petroleum is also the raw material for many chemical products, including solvents, fertilizers, pesticides, and plastics. 84% (37 of 44 gallons in a typical barrel) of all petroleum extracted is processed as fuels, including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas; the other 16% is converted into other materials such as plastic. Known reserves of petroleum are estimated at around 1000 gigabarrels, (that is, one thousand billion), and consumption is currently around 84 million barrels per day, or 31 gigabarrels per year. At current consumption levels, world oil supply will be gone in about 33 years. As the supply of petroleum becomes more scarse, consumers will look to renewable fuel sources such as ethanol fuel, solar power, wind power, and fuel cells. Since petroleum is a non-renewable resource, many people are worried about peak oil and eventual depletion in the near future. Due to its continual demand and consequent value, oil has been dubbed black gold. The combining form of the word petroleum is petro-, as in petrodiesel (petroleum diesel). Data gathered from Wikipedia, the free encyclopedia.