What Are Fossil Fuels?

Fossil fuels are carbon-rich energy resources formed over millions of years from the remains of ancient organisms compressed and heated beneath layers of sediment. The three principal types are coal, crude oil, and natural gas, each differing in carbon content, energy density, and physical state. Together they supply approximately 80 percent of global primary energy, underpinning electricity generation, transportation, industrial processes, and residential heating. The chemical energy stored in their carbon-hydrogen bonds is released through combustion, producing heat that can be converted to mechanical work or electricity.

The study of fossil fuels intersects geology, chemical engineering, combustion science, and environmental policy. Their extraction, processing, and end-use are governed by standards from bodies including the U.S. Energy Information Administration, which tracks production and consumption data across all fossil fuel categories, and by research published through the Environmental and Energy Study Institute's fossil fuel coverage.

Formation and Resource Types

Coal is a solid fuel formed from the compression of plant matter in swamp environments over tens to hundreds of millions of years. Its energy content and carbon density vary with rank, from low-rank lignite to high-rank anthracite. Crude oil, or petroleum, is a liquid mixture of hydrocarbons formed from marine microorganisms, refined into gasoline, diesel, jet fuel, and petrochemical feedstocks. Natural gas consists primarily of methane and is found either in dedicated reservoirs or dissolved in crude oil deposits; its lower carbon-to-hydrogen ratio produces fewer carbon dioxide emissions per unit of energy than coal or oil. Each resource type requires different extraction technologies: surface and underground mining for coal, rotary drilling for oil and conventional gas, and hydraulic fracturing for tight shale gas and shale oil formations.

Combustion and Energy Conversion

The energy in fossil fuels is recovered by combustion, which oxidizes carbon to carbon dioxide and hydrogen to water vapor while releasing heat. In power plants, this heat raises steam that drives turbines connected to generators, converting thermal energy to electricity. The efficiency of this thermodynamic cycle is limited by the Carnot efficiency relationship, and modern combined-cycle natural gas plants achieve thermal efficiencies above 60 percent by using both a gas turbine and a steam turbine in series. Internal combustion engines in vehicles operate on a similar principle using gasoline or diesel, while industrial furnaces apply combustion heat directly to processes such as cement calcination and steel production. Combustion byproducts include carbon dioxide, nitrogen oxides, sulfur dioxide, and particulate matter, all of which contribute to air pollution and human health impacts as documented by the U.S. Energy Information Administration's analysis of natural gas and the environment.

Environmental and Climate Impacts

Burning fossil fuels releases carbon that was sequestered underground over geological timescales, adding to atmospheric carbon dioxide concentrations that drive anthropogenic climate change. In 2019, fossil fuel combustion accounted for approximately 74 percent of U.S. greenhouse gas emissions. Oil combustion alone contributed 45 percent of energy-related carbon dioxide emissions in 2020. Beyond greenhouse gas emissions, fossil fuel extraction and use produce a range of local environmental hazards: coal mining generates acid mine drainage and large volumes of combustion ash, oil extraction and transport carry spill risks, and hydraulic fracturing has been linked to induced seismicity and groundwater quality concerns. The U.S. EIA's Frequently Asked Questions on carbon dioxide emissions provides detailed data on the emissions intensity of each fuel type.

Applications

Fossil fuels have applications in a range of fields, including:

• Electricity generation in coal, oil, and natural gas power plants supplying grid power
• Transportation, as gasoline and diesel in passenger vehicles, trucks, ships, and aircraft
• Industrial process heat for steel, cement, glass, and chemical manufacturing
• Residential and commercial space heating using natural gas furnaces and boilers
• Petrochemical feedstocks for plastics, fertilizers, and pharmaceuticals derived from crude oil fractions