Petroleum Petroleum (L. petroleum, from Greek: petra (rock) + Latin: oleum (oil)) or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. <a href="http://en.wikipedia.org/wiki/Petroleum">http://en.wikipedia.org/wiki/Petroleum</a> 25.