Fuel were present at the sites. As a fuel, wood has remained in use up until the present day, although it has been superseded for many purposes by other sources. Wood has an energy density of 10–20 MJ/kg. Recently biofuels have been developed for use in automotive transport (for example Bioethanol and Biodiesel), but there is widespread public debate about how carbon efficient these fuels are. Fossil fuels are hydrocarbons, primarily coal and petroleum (liquid petroleum or natural gas), formed from the fossilized remains of ancient plants and animals by exposure to high heat and pressure in the absence of oxygen
Fossil fuel be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates. The theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the Earth's crust over millions of years was first introduced by Andreas Libavius “in his 1597 Alchemia [Alchymia]” and later by Mikhail Lomonosov “as early as 1757 and certainly by 1763”.. The first use of the term "fossil fuel" was by the German chemist Caspar Neumann, in English translation in 1759. The United States Energy Information Administration estimates that in 2007 the world's