Petroleum is a liquid found in formations under the earths surface. It can be refined to make fuels. "It consists of hydrocarbons of various molecular weights and other liquid organic compounds"-ask.com Theier are two types of oil, crude and brent crude, both are used and make the basis of the oil price. Oil is a fossil fuel due to the fact it is made from the fossils of organisms, both land and water, that are compressed ove millions of years in rock formations. Due to these rocks formations being so close to the earths core, the heat melts them into a liquid that we call petroleum.