In 2008 an estimated 40 percent of the energy needs of the united states were supplied by oil or petroleum. The rest was provided by natural gas (approximately 25 percent), coal (23percent), hydroelectric power (4percent), nuclear power (8percent), and other sources (0.5percenr). In addition energy, petroleum is the source of numerous organic chemicals used to manufacture drugs, clothing, and many other products
Unrefined petroleum, a viscous, dark-brown liquid, is often called crude oil. A complex mixture of alkanes, alkanes, cycloalkanes, and aromatic compounds, petroleum was formed in Earth's crust over the course of million of year by the anaerobic decomposition of animal and vegetable matter by bacteria