Crude oil is a fossil fuel, and it exists in liquid form in underground pools or reservoirs, in tiny spaces within sedimentary rocks, and near the surface in tar (or oil) sands. Petroleum products are fuels made from crude oil and other hydrocarbons contained in natural gas. Petroleum products can also be made from coal, natural gas, and biomass. Crude oil is the base for lots of products. These include transportation fuels such as gasoline, diesel, and jet fuel. They also include fuel oils used for heating and electricity generation. In 2017, the United States consumed 7.3 billion barrels of crude oil. Petroleum or crude oil is a complex mixture of hydrocarbons and other chemicals. The composition varies widely depending on where and how the petroleum was formed. In fact, chemical analysis can be used to fingerprint the source of petroleum.