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Petroleum (Latin Petroleum derived from Greek πετρα - rock + ολιον - oil) or crude oil is a naturally occurring liquid found in formations in the Earth consisting of a complex mixture of hydrocarbons (mostly alkanes) of various lengths. The approximate length range is C<sub>5</sub>H<sub>12</sub> to C<sub>18</sub>H<sub>38</sub>. Any shorter hydrocarbons are considered natural gas or natural gas liquids, while longer hydrocarbon chains are more solid, and the longest chains are coal. In its naturally occurring form, it may contain other nonmetallic elements such as sulfur, oxygen, and nitrogen.[1] It is usually black or dark brown (although it may be yellowish or even greenish) but varies greatly in appearance, depending on its composition. Crude oil may also be found in semi-solid form mixed with sand, as in the Athabasca oil sands in Canada, where it may be referred to as crude bitumen.