Fossil Fuels
Some hydrocarbons were formed from plants and animals that lived in Earth's oceans and swamps millions of years ago. After those plants and animals died, they were buried under layers of rock and soil. High temperature and pressure deep in Earth's crust changed those remains into deposits of hydrocarbons called fossil fuels. Fossil fuels are mixtures of hydrocarbons that formed from the remains of plants or animals. 
Three types of fossil fuels are coal, natural gas, and petroleum. The type of fossil fuel produced depends on the origin of the organic material and the conditions under which it decays.
Coal
The ferns in Figure 7A are similar to those that produced the coal in Figure 7B. Coal is a solid fossil fuel that began to form about 300 million years ago in ancient swamps. Giant tree ferns and other plants were buried in those swamps. After millions of years of pressure, the plant remains produced a mixture of hydrocarbons. Most of the hydrocarbons in coal are aromatic hydrocarbons with high molar masses. These compounds have a high ratio of carbon to hydrogen. So burning coal produces more soot than burning other fossil fuels does.
Natural Gas
The second main fossil fuel, natural gas, formed from the remains of marine organisms. Natural gas is mostly methane. It also contains ethane, propane, and isomers of butane. Natural gas is distributed through a network of underground pipes. It is used for heating and cooking, and to generate some electricity. Deposits of natural gas are found with deposits of coal and petroleum. Enormous amounts of unused natural gas resources also exist as methane frozen together with water, called methane hydrate. Methane hydrate is found in Arctic soils and in offshore sediments.
Petroleum
The third main fossil fuel, petroleum, also formed from the remains of marine organisms. Petroleum, often known as crude oil, is pumped from deep beneath the Earth's surface. It is a complex liquid mixture of hydrocarbons, comprised mainly of long-branched alkanes and alkenes. For petroleum to be useful, it must be separated into simpler mixtures, or fractions, such as gasoline and heating oil.
Figure 7 Fossil fuels form from the remains of plants and animals. A The ferns shown are similar to ferns that lived millions of years ago. B The imprints of ferns left on the lump of coal are evidence that the coal formed when plant remains were compressed under layers of rock and soil.
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