Fossil fuels are
fuels formed by natural processes such as
anaerobic decomposition of buried dead
organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.
Fossil fuels contain high percentages of
carbon and include
coal,
petroleum, and
natural gas. They range from
volatile materials with low
carbon:
hydrogen ratios like
methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like
anthracite coal. Methane can be found in
hydrocarbon fields, alone, associated with oil, or in the form of
methane clathrates. 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.
This
biogenic theory was first introduced by
Georg Agricola in 1556 and later by
Mikhail Lomonosov in the 18th century.