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Petroleum (crude oil) is a naturally occurring liquid fossil fuel found in rock formations in the Earth, consisting of a complex mixture of hydrocarbons of various lengths. Due to its high energy density, easy transportability and relative abundance, petroleum has become the world's most important source of energy since the mid 1950s. Most petroleum is used for producing gasoline (petrol) and fuel oil, both important primary energy sources. Petroleum is also the raw material for many chemical products.
Most petroleum is extracted from oil wells found in oil fields. Although it is also possible to extract petroleum from oil shale or tar sands, both of which are known to exist in large quantities, doing so at low cost and without impacting the environment remains a challenge. The first modern oil well was drilled in 1848 in Azerbaijan, but it was the introduction of the internal combustion engine in the early 20th century that provided the demand that has largely sustained the industry to this day. The top three oil producing countries are Saudi Arabia, Russia, and the United States.
Burning oil releases carbon dioxide into the atmosphere, one of the major greenhouse gases contributing to global warming. Bioethanol and biodiesel are already used to some extent as alternatives, notably in Brazil, as are hybrid electric vehicles. Trials using hydrogen fuel are also in progress. There are further concerns about security of supply, oil price rises and whether world oil production may reach a peak, before declining, as predicted by Hubbert peak theory.