Petroleum Definition (Crude Oil)

Gasoline pump
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Petroleum or crude oil is any naturally-occurring flammable mixture of hydrocarbons found in geologic formations, such as rock strata. Most petroleum is a fossil fuel, formed from the action of intense pressure and heat on buried dead zooplankton and algae. Technically, the term petroleum only refers to crude oil, but sometimes it is applied to describe any solid, liquid or gaseous hydrocarbons.

Composition of Petroleum

Petroleum consists primarily of paraffins and naphthenes, with a smaller amount of aromatics and asphaltics. Near the surface, lighter hydrocarbons (methane, ethane, propane, butane) are gases. Heavier compounds are liquids or solids. Trace metals include iron, copper, nickel, and vanadium. The chemical composition of a specimen is a sort of fingerprint for the source of the petroleum.

The chemical composition also determines the color of petroleum. Often, it is black or brown, but it may be reddish, yellow, or green.


  • Norman, J. Hyne (2001). Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production (2nd ed.). Tulsa, OK: Penn Well Corp. ISBN 978-0-87814-823-3. 
  • Speight, James G. (1999). The Chemistry and Technology of Petroleum (3rd ed.). New York: Marcel Dekker. ISBN 978-0-8247-0217-5. 
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Helmenstine, Anne Marie, Ph.D. "Petroleum Definition (Crude Oil)." ThoughtCo, Aug. 27, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 27). Petroleum Definition (Crude Oil). Retrieved from Helmenstine, Anne Marie, Ph.D. "Petroleum Definition (Crude Oil)." ThoughtCo. (accessed January 25, 2021).