fossil fuel

fuels formed from the remains of plants and animals that lived in an earlier era (coal, petroleum, natural gas)

Fossil fuels or mineral fuels are hydrocarbons found within the top layer of the earth’s crust. They range from very volatile materials with low carbon:hydrogen ratios like methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. This is known as the biogenic theory and was first introduced by Mikhail Lomonosov in 1757. There is an opposing theory that the more volatile hydrocarbons, especially natural gas, are formed by abiogenic processes, that is no living material was involved in their formation.
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Noun
1. fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content
(hypernym) fuel
(hyponym) coal

Any naturally occurring organic fuel, such as petroleum, coal, and natural gas. (DOE3)

Fuel derived from ancient organic remains; e.g. peat, coal, crude oil, and natural gas.