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 label blends containing fuel ethanol on the dispenser. Due to ethanol's greater stability under pressure it is used by Shell in their 100 octane fuel. Similarly IFS add 10% ethanol to their 91 octane fuel, label it premium fuel and sell it more cheaply than regular unleaded. This is converse to the general practice of adding ethanol to a lesser quality fuel to bring its octane rating up to 91. Some concern was raised over the use of ethanol blend fuels in petrol vehicles in 2003, yet manufacturers widely claimed that their vehicles were engined for such fuels. Since then