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Petroleum, or crude oil, is a naturally occurring, flammable liquid found in rock formations in the Earth consisting of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds.

Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown. In the reservoir it is usually found in association with natural gas, which being lighter forms a gas cap over the petroleum, and saline water which being heavier generally floats underneath it. Crude oil may also be found in semi-solid form mixed with sand as in the Athabasca oil sands in Canada, where is usually referred to as crude bitumen. 

Due to its high energy density, easy transportability and relative abundance, oil has become the world's most important source of energy since the mid-1950s. Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics; the 16 percent not used for energy production is converted into these other materials.

Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. There is also petroleum in oil sands. Known reserves of petroleum are typically estimated at around 1.2 trillion barrels without oil sands or 3.74 trillion barrels with oil sands. Consumption is currently around 84 million barrels per day. Because the energy return over energy invested ratio of oil is constantly falling, recoverable oil reserves are significantly less than total oil in place. 

At current consumption levels, and assuming that oil will be consumed only from reservoirs, known recoverable reserves would be gone around 2039, potentially leading to a global energy crisis. However, there are factors which may extend or reduce this estimate, including the rapidly increasing demand for petroleum in China, India, and other developing nations; new discoveries; energy conservation and use of alternative energy sources; and new economically viable exploitation of non-conventional oil sources.

Petroleum is used mostly, by volume, for producing fuel oil and gasoline, both important "primary'' energy sources. Eighty four percent of the hydrocarbons present in petroleum is converted into petroleum-based fuels, including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas. The lighter grades of crude oil produce the best yields of these products, but as the world's reserves of light and medium oil are depleted, oil refineries are increasingly having to process heavy oil and bitumen, and use more complex and expensive methods to produce the products required. Because heavier crude oils have too much carbon and not enough hydrogen, these processes generally involve removing carbon from or adding hydrogen to the molecules, and using fluid catalytic cracking to convert the longer, more complex molecules in the oil to the shorter, simpler ones in the fuels.