Fossil fuel embedded energy is still photosynthetic in origin. Terrestrial plants, on the other hand, tended to form coal and methane. Many of the coal fields date to the Carboniferous period of Earth's history. Terrestrial plants also form type III kerogen, a source of natural gas. There is a wide range of organic, or hydrocarbon, compounds in any given fuel mixture. The specific mixture of hydrocarbons gives a fuel its characteristic properties, such as boiling point, melting point, density, viscosity, etc. Some fuels like natural gas, for instance, contain only very low boiling, gaseous components. Others such as gasoline or diesel contain
Ethanol fuel by country and Hawaii — require ethanol to be blended with gasoline motor fuel. Many cities also require ethanol blends due to non-attainment of federal air quality goals. Several motor vehicle manufacturers, including Ford, Chrysler, and GM, sell flexible-fuel vehicles that can use gasoline and ethanol blends ranging from pure gasoline all the way up to 85% ethanol (E85). As of early 2013, there were about 11 million E85-compatible vehicles on U.S. roads, though actual use of E85 fuel is limited, not only because the ethanol fueling infrastructures is limited, but also because many owners are not aware their vehicle is flex-fuel