Science, Tech, Math › Science Petroleum Definition (Crude Oil) Share Flipboard Email Print MATJAZ SLANIC/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated November 07, 2019 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. Sources 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.