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Petroleum occurs naturally near the earth's surface in liquid, gas, and solid forms. The word petroleum in Greek literally means rock oil. Petroluem is a naturally-occurring energy sources known as a fossil fuel, meaning it derives from previously living carbon-based organic lifeforms that have been transformed, via a variety of chemical processes over time, into carbon-based inorganic substances from which humans have learned to extract stored energy.
"Recirculating petroleum from the McClintock Well #1, the oldest continuously producing oil well, July 30, 2011" by Niagara is licensed under CC BY-SA.
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Encyclopædia Britannica Online
N.d. Petroleum. http://www.britannica.com/Ebchecked/topic/454269/petroleum,
accessed May 6, 2014.