Fossil fuels are combustible materials formed through the transformation of buried organic matter over geological time. The principal types are coal, petroleum, and natural gas. They supply energy for heating, transportation, industrial processes, and electricity generation, and also provide chemical raw materials. Fossil fuels are classified as nonrenewable because their formation is far slower than their extraction and consumption. Their defining characteristic is their geological origin, rather than simply their chemical composition or ability to burn. (eia.gov)
Formation and geological occurrence
The organic material from which fossil fuels formed ultimately obtained much of its chemical energy through photosynthesis. Burial preserved some of this material, which heat and pressure subsequently transformed. Coal developed principally from terrestrial vegetation accumulated in swampy environments, whereas petroleum originated largely from organisms deposited in aquatic environments. These processes operated over millions of years. (eia.gov)
Coal formation, or coalification, progressively changes buried plant matter into denser, drier, and more carbon-rich material. The major coal ranks are lignite, subbituminous coal, bituminous coal, and anthracite. Rank reflects the extent of transformation and is associated with differences in carbon content and heating value; coal is therefore not a chemically uniform fuel. (usgs.gov)
Oil and gas occur in subsurface rock formations rather than necessarily in open underground cavities. They are extracted from onshore and offshore wells, including wells accessing relatively impermeable formations. Natural gas also occurs within coal seams as coalbed gas. The distribution and accessibility of deposits depend on their geology, which helps explain why extraction methods and costs differ among locations. (eia.gov)
Principal types and properties
Coal is a combustible sedimentary rock containing carbon-rich organic material and mineral constituents. It is used for power generation and industrial heat; suitable bituminous coals also supply material for coke production in iron and steel manufacturing. Moisture and mineral content influence its usable heat output and the residues produced during combustion. (eia.gov)
Petroleum, commonly called crude oil before refining, is a mixture of hydrocarbons. Refineries separate and transform it into gasoline, diesel, kerosene, and other products. Petroleum-derived materials also have nonfuel uses, including lubricants, waxes, construction materials, and chemical feedstocks. Consequently, petroleum consumption does not always mean immediate combustion. (eia.gov)
Natural gas consists mainly of methane. Burning it generally releases less carbon dioxide and fewer conventional air pollutants per unit of energy than burning coal or petroleum products. However, methane escaping during production, processing, storage, or transport is itself a potent greenhouse gas. Methane produced from recently formed biological material can have similar chemistry without being fossil-derived, as illustrated by biogas. (eia.gov)
Extraction, processing, and use
Coal is obtained through surface or underground mining. Surface extraction removes material above coal seams, while deeper deposits require underground workings. Oil and gas production uses drilled wells; horizontal drilling and hydraulic fracturing have expanded production from shale and other tight formations. These techniques improve access to resources but also require equipment, land, and management of production wastes. (eia.gov)
Petroleum refining begins with distillation, which separates fractions according to boiling point. Subsequent conversion processes alter the composition of those fractions. Cracking breaks larger hydrocarbon molecules into smaller ones, often using catalysis, while treatment and blending produce finished fuels with specified properties. Refining thus combines physical separation with chemical transformation rather than merely filtering crude oil. (eia.gov)
Fossil fuels are primary energy sources, whereas electricity is a secondary energy form generated from primary sources. This distinction matters when comparing energy statistics: fuel consumed by a generating plant and electricity delivered to a customer measure different stages of the energy system. Energy quantities can be converted into common heat units to compare fuels otherwise measured in barrels, cubic feet, or tonnes. (eia.gov)
Historical and economic significance
Coal-fired steam engines helped power the Industrial Revolution, supplying mechanical power to factories, mills, and mines. Improvements associated with James Watt reduced coal consumption and broadened the applications of steam power. Fossil fuels subsequently became central to energy systems; petroleum, natural gas, and coal have dominated the United States energy mix for more than a century, although their relative contributions have changed. (blog.sciencemuseum.org.uk)
Resource estimates must be distinguished from reserves. In oil and gas assessment, resources include material whose existence or recoverability is estimated from geological evidence. Reserves are discovered quantities expected to be commercially recoverable under specified conditions. Their size therefore depends not only on physical deposits but also on technology, economic conditions, operating practices, and regulations. A reserve estimate is not a fixed measure of all fuel remaining underground. (usgs.gov)
Environmental effects and emissions reduction
Extraction and use affect land, air, and water. Coal mining can alter landscapes and streams, while coal ash can contaminate groundwater. Combustion can emit sulfur dioxide, nitrogen oxides, particulates, and heavy metals. Gas production can disturb vegetation and generate contaminated wastewater, and leaks create both emissions and safety hazards. The scale and type of impact vary with the fuel and production method. (eia.gov)
Burning fossil fuels transfers stored geological carbon into the active carbon cycle. Carbon dioxide emissions from fossil-fuel combustion and industrial processes are a major contributor to human-caused climate change. Comparisons using life-cycle assessment must consider production and transport as well as combustion, particularly methane leakage from gas systems. (ipcc.ch)
Emissions-control measures address different problems. Scrubbers and other equipment reduce conventional pollutants, while carbon capture and storage separates carbon dioxide for underground storage. Climate-mitigation pathways assessed by the IPCC also involve improved efficiency, electrification, and greater use of renewable energy and other low-carbon sources. Pollution controls alone do not eliminate the carbon dioxide released by fossil-fuel combustion. (eia.gov)