Any naturally occurring carbon-containing material which when burned with air (or oxygen) produces (directly) heat or (indirectly) energy. Fossil fuels can be classified according to their respective forms at ambient conditions. Thus, there are solid fuels (coals); liquid fuels (petroleum, heavy oils, bitumens); and gaseous fuels (natural gas, which is usually a mixture of methane, CH₄, with lesser amounts of ethane, C₂H₆, hydrogen sulfide, H₂S, and numerous other constituents in small proportions).

One important aspect of the fossil fuels is the heating value of the fuel, which is measured as the amount of heat energy produced by the complete combustion of a unit quantity of the fuel. For solid fuels and usually for liquid fuels the heating value is quoted for mass, whereas for gaseous fuels the heating value is quoted for volume. The heating values are commonly expressed as British thermal units per pound (Btu/lb). In SI units the heating values are quoted in megajoules per kilogram (MJ/kg). For gases, the heating values are expressed as Btu per cubic foot (Btu/ft³) or as megajoules per cubic meter (MJ/m³). The table gives heating values of representative fuels. Energy sources Natural gas Petroleum

*Representative values are given because of the spread of subgroups with various heating values.