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EARTH SCIENCE > SOLID EARTH > EARTH GASES/LIQUIDS > PETROLEUM

Definition:
Petroleum is a naturally occurring flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. The name Petroleum covers both naturally occurring unprocessed crude oils and petroleum products that are made up of refined crude oil.

Reference:
U.S. Environmental Protection Agency