Petroleum, “rock oil” or “oil from the earth” (from the Latin petra, “rock” or “stone,” and oleum, “oil”), is a complex mixture of hydrocarbons and other organic compounds. As a technical term, petroleum refers to liquid crude oil as well as natural gas, viscous and solid forms (bitumen), and other petroleum products and possesses a large range of colors and consistencies. In ordinary language, petroleum refers specifically to crude oil. Scientists believe that crude oil (measured in barrels, abbreviated bbls) formed during the past 600 million years in the Earth from the remains of single-celled planktonic animals and plants in ancient seas and lakes. Over time, layers of mud, silt, and sand covered these remains, forming sedimentary rock over them. Geologic heat and pressure from ...

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