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Fossil Fuels



Fossil Fuels, energy-rich substances that have formed from long-buried plants and microorganisms. Fossil fuels, which include petroleum, coal, and natural gas, provide most of the energy that powers modern industrial society. The gasoline that fuels our cars, the coal that powers many electrical plants, and the natural gas that heats our homes are all fossil fuels.

Chemically, fossil fuels consist largely of hydrocarbons, which are compounds composed of hydrogen and carbon. Some fossil fuels also contain smaller amounts of other compounds. Hydrocarbons form from ancient living organisms that were buried under layers of sediment millions of years ago. As accumulating sediment layers exerted increasing heat and pressure, the remains of the organisms gradually transformed into hydrocarbons. The most commonly used fossil fuels are petroleum, coal, and natural gas. These substances are extracted from the earth’s crust and, if necessary, refined into suitable fuel products, such as gasoline, heating oil, and kerosene. Some of these hydrocarbons may also be processed into plastics, chemicals, lubricants, and other nonfuel products. Geologists have identified other types of hydrocarbon-rich deposits that can serve as fuels. Such deposits, which include oil shale, tar sands, and gas hydrates, are not widely used because they are too costly to extract and refine. See also Energy Supply, World.

The majority of fossil fuels are used in the transportation, manufacturing, residential heating, and electric-power generation industries. Crude petroleum is refined into gasoline, diesel fuel, and jet fuel, which power the world’s transportation system. Coal is the fuel most commonly burned to generate electric power, and natural gas is used primarily in commercial and residential buildings for heating water and air, for air conditioning, and as fuel for stoves and other heating appliances.



Fossil fuels formed from ancient organisms that died and were buried under layers of accumulating sediment. As additional sediment layers built up over these organic deposits, the material was subjected to increasing temperatures and pressures. Over millions of years, these physical conditions chemically transformed the organic material into hydrocarbons.

Most organic debris is destroyed at the earth's surface by oxidation or by consumption by microorganisms. Organic material that survives to become buried under sediments or deposited in other oxygen-poor environments begins a series of chemical and biological transformations that may ultimately result in petroleum, natural gas, or coal. Many such deposits occur in sedimentary basins (depressed areas in the earth’s crust where sediments accumulate), and along continental shelves. Sediments may accumulate to depths of several thousand feet in a basin, exerting pressures up to one hundred million pascals (tens of thousands of pounds per square inch) and temperatures of several hundred degrees on the organic material. Over millions of years, these conditions can chemically transform the organic material into petroleum, natural gas, coal, or other types of fossil fuels.


Petroleum Formation

Petroleum formed chiefly from ancient, microscopic plants and bacteria that lived in the ocean and saltwater seas. When these microorganisms died and settled to the seafloor, they mixed with sand and silt to form organic-rich mud. As layers of sediment accumulated over this organic ooze, the mud was gradually heated and slowly compressed into shale or mudstone, chemically transforming the organic material into petroleum and natural gas.

Sometimes, the petroleum and natural gas would slowly fill the tiny holes within nearby porous rocks, which geologists call reservoir rocks. Because these porous rocks were usually filled with water, the liquid and gaseous hydrocarbons (which are less dense and lighter than water) migrated upward, through the earth’s crust, sometimes for long distances. A portion of these hydrocarbons would eventually encounter an impermeable (nonporous) layer of rock in an anticline, salt dome, fault trap, or stratigraphic trap. The impermeable rock would trap the hydrocarbons, creating a reservoir of petroleum and natural gas. Exploration geologists seek these underground formations because they often contain recoverable petroleum deposits. The fluids and gases caught in these geologic traps typically separate into three layers: water (highest density, bottom layer), petroleum (middle layer), and natural gas (low density, top layer).


Coal Formation

How Coal Forms

The coal we find today formed from generations of plants that died in ancient tropical swamps and accumulated on the swamp bottoms. The plant material first formed a compact organic material called peat. As layers of sediment gradually accumulated over the peat, the pressure and heat exerted by the thickening layers gradually drove out the moisture and increased the carbon content of the peat, forming coal.

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Coal is a solid fossil fuel formed from ancient plants—including trees, ferns, and mosses—that grew in swamps and bogs or along coastal shorelines. Generations of these plants died and were gradually buried under layers of sediment. As the sedimentary overburden increased, the organic material was subjected to increasing heat and pressure that caused the organic material to undergo a number of transitional states to form coal. The mounting pressure and temperature caused the original organic material, which was rich in carbon, hydrogen, and oxygen, to become increasingly carbon-rich and hydrogen- and oxygen-poor. The successive stages of coal formation are peat (partially carbonized plant matter), lignite (soft brownish-black coal with low carbon content), subbituminous coal (soft coal with intermediate carbon content), bituminous coal (soft coal with higher carbon and lower moisture content than subbituminous coal), and anthracite (hard coal with highest carbon content and lowest moisture content). Because anthracite is the most carbon-rich, moisture-deficient form of coal, it has the highest heating value.

Cutting Peat

A worker cuts peat from lush peatland in Ireland. Peat is the first stage in the transformation of vegetation into coal. For hundreds of years, people have cut, dried, and burned it for heating and cooking. This compact, dark-brown material contains about one third less heating value than coal.

Farrell Grenan/Photo Researchers, Inc.

Two regions in the United States produce nearly all the coal mined in the United States. One region covers the Appalachian region (encompassed by Alabama, Georgia, Kentucky, Maryland, Ohio, Pennsylvania, Tennessee, Virginia, and West Virginia) plus the Illinois Basin (encompassed by Illinois, Indiana, and western Kentucky). The other area covers parts of Montana and Wyoming. The extensive coal deposits in these two regions were formed during the Carboniferous Period (360 million to 290 million years before present) and the Tertiary Period (65 million to 1.6 million years before present), respectively.


Natural Gas Formation

Most natural gas is formed from plankton—tiny water-dwelling organisms, including algae and protozoans—that accumulated on the ocean floor as they died. These organisms were slowly buried and compressed under layers of sediment. Over millions of years, the pressure and heat generated by overlying sediments converted this organic material into natural gas. Natural gas is composed primarily of methane and other light hydrocarbons. As discussed previously, natural gas frequently migrates through porous and fractured reservoir rock with petroleum and subsequently accumulates in underground reservoirs. Because of its light density relative to petroleum, natural gas forms a layer over the petroleum. Natural gas may also form in coal deposits, where it is often found dispersed throughout the pores and fractures of the coal bed.




Other Fossil Fuels

Geologists have identified immense deposits of other hydrocarbons, including gas hydrates (methane and water), tar sands, and oil shale. Vast deposits of gas hydrates are contained in ocean sediments and in shallow polar soils. In these marine and polar environments, methane molecules are encased in a crystalline structure with water molecules. This crystalline solid is known as gas hydrate. Because technology for the commercial extraction of gas hydrates has not yet been developed, this type of fossil fuel is not included in most world energy resource estimates. However, in February 2007 the U.S. Department of Energy completed the drilling of a well to retrieve core samples of gas hydrates found in the Prudhoe Bay region of Alaska’s North Slope. By identifying the nature of these gas hydrates, the Energy Department hoped to evaluate the potential of natural gas production from this region.

Tar sands are heavy, asphaltlike hydrocarbons found in sandstone. Tar sands form where petroleum migrates upward into deposits of sand or consolidated sandstone. When the petroleum is exposed to water and bacteria present in the sandstone, the hydrocarbons often degrade over time into heavier, asphaltlike bitumen. Oil shale is a fine-grained rock containing high concentrations of a waxy organic material known as kerogen. Oil shale forms on lake and ocean bottoms where dead algae, spores, and other microorganisms died millions of years ago and accumulated in mud and silt. The increasing pressure and temperature from the buildup of overlying sediments transformed the organic material into kerogen and compacted the mud and silt into oil shale. However, this pressure and heat was insufficient to chemically break down the kerogen into petroleum. Because the hydrocarbons contained in tar sand and oil shale are not fluids, these hydrocarbons are more difficult and costly to recover than liquid petroleum.



Geologists use a variety of sophisticated instruments to locate underground petroleum, natural gas, and coal deposits. These instruments allow scientists to interpret the geologic composition, history, and structure of sedimentary basins in the earth’s crust. Once located, petroleum and natural gas deposits are removed by wells drilled down into the deposit, while coal is removed by excavation.


Petroleum and Natural Gas

To locate deposits of petroleum and natural gas, exploration geologists search for geologic regions containing the ingredients necessary for petroleum formation: organic-rich source rock, burial temperatures sufficiently high to generate petroleum from organic material, and petroleum-trapping rock formations.

When potentially petroleum-rich geologic formations are identified, wells are drilled into the sedimentary basin. If a well intersects porous reservoir rock containing significant petroleum and natural gas deposits, pressure inside the trap may force the liquid hydrocarbons spontaneously to the surface. However, pressure inside the trap typically declines to the point where the petroleum must be pumped to the surface.

Between 1949 and 2002, approximately 2.5 million exploration and development wells were drilled in the United States. While domestic petroleum exploration continues on land, oil companies are also exploring promising offshore locations along the continental margins.

Once petroleum has been extracted from the ground, it is transported by pipeline, truck, or tanker to a refinery to be separated into liquid and gas components. Raw petroleum is heated to distill hydrocarbons by molecular weight. Lighter molecules are separated and refined into gasoline and other fuels, while heavier molecules are processed into engine lubricants, asphalt, waxes, and other products. Because demand for fuel far exceeds demand for the products made from the heavier hydrocarbons, refiners often break apart the heavy molecules into lighter ones that can be refined into gasoline. They do so by means of processes called thermal cracking and catalytic cracking.



Big Pit Mining Museum, Blaenafon

Coal mining was once the main industry of the Rhondda Valley in Great Britain. The Big Pit ("Pwll Mawr") coal mine ceased production in 1980 but its buildings and machinery were preserved and converted into a museum of mining. Visitors descend into the mine via the 90-m (300-ft) lift shaft to walk through underground roadways, air doors, stables (for pit ponies), and engine houses constructed by past generations of miners.

Spectrum Colour Library

Because of their enormity, the world’s most extensive coal beds have already been identified. Currently, scientists and engineers are working on finding the most economically efficient means of removing the coal. Coal mining underwent a complete transformation in the United States during the 20th century, evolving from a labor-intensive, hand-mining industry to a modern mechanized industry. The modern coal mining industry uses some of the largest, most sophisticated excavating equipment ever developed.

Modern underground mining commonly employs machines called longwall miners to remove coal. These machines use rotating drums studded with picks to rip coal from seams in large chunks.

Surface-mine operators use mammoth earth-moving shovels to mine coal. These shovels first remove overlying soil and rock so the coal beds can be blasted apart. The blasted coal is scooped up and loaded into the beds of huge trucks for transport.







Annual Consumption of Fossil Fuels

In 2003, 362 quadrillion British thermal units (Btu) of fossil fuels were consumed worldwide. These fuels were in the form of oil, gas, and coal. The United States, with less than 5 percent of the world’s population, consumed 24 percent of the oil, 23 percent of the gas, and 23 percent of the coal.

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In 2003, the most recent year for which data are available, the world consumed 29 billion barrels of petroleum, 5 billion metric tons of coal, and 2.7 trillion cubic meters (96 trillion cubic feet) of natural gas. Overall, the United States accounts for approximately 25 percent of worldwide energy consumption, while it has less than 5 percent of the world’s population.



Once fossil fuel has been extracted and processed, it can be burned for direct uses, such as to power cars or heat homes, or it can be combusted for the generation of electrical power.


Direct Combustion

Fossil fuels are primarily burned to produce energy. This energy is used to power automobiles, trucks, airplanes, trains, and ships around the world; to fuel industrial manufacturing processes; and to provide heat, light, air conditioning, and energy for homes and businesses. About two-fifths of all energy consumed in the United States is used by industry, one-third by homes and businesses, and about one-fourth by transportation.

To provide fuel for transportation, petroleum is refined into gasoline, diesel fuel, jet fuel, and other derivatives used in most of the world’s automobiles, trucks, trains, aircraft, and ships. In the United States, transportation accounts for about two-thirds of total petroleum consumption—more than two-thirds of which is burned as automobile gasoline.

Demand for natural gas, historically considered a waste by-product of petroleum and coal mining, is growing in business and industry because it is a cleaner-burning fuel than petroleum or coal. Natural gas, which can be piped directly to commercial plants or individual residences and used on demand, is used for heating and for air conditioning. Residential uses of natural gas also include fuel for stoves and other heating appliances.


Electricity Generation

In addition to direct combustion for commercial uses, fossil fuels are also burned to generate most of the world’s electric power. In 2003 fossil fuel–fired power plants produced 65 percent of the world’s electrical power, down from 71 percent in the late 1970s. In 2003 the world’s remaining electricity supply was generated primarily by hydroelectric power (17 percent) and nuclear fission (16 percent), with solar, geothermal, and other sources accounting for a relatively small amount.



Acid rain and global warming are two of the most serious environmental issues related to large-scale fossil fuel combustion. Other environmental problems, such as land reclamation and oil spills, are also associated with the mining and transporting of fossil fuels.


Acid Rain

Anatomy of an Air Scrubber

A venturi air scrubber removes polluting particles from gas emissions by spraying a scrubber liquid directly into the emissions. The scrubber liquid surrounds the dirty particles, which are carried with the gas emissions into the separator cylinder. As the gas cycles upward through the cylinder, the liquid-covered particles drop from the gas into the contaminated liquid reservoir.

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When fossil fuels are burned, sulfur, nitrogen, and carbon combine with oxygen to form compounds known as oxides. When these oxides are released into the air, they react chemically with atmospheric water vapor, forming sulfuric acid, nitric acid, and carbonic acid, respectively. These acid-containing water vapors—commonly known as acid rain—enter the water cycle and can subsequently harm the biological quality of forests, soils, lakes, and streams.

Clean Air Scrubbers

The coal burned to power this wood processing plant emits gas waste laden with polluting particles. Anti-pollution devices called scrubbers (visible as metal pipes) are used to trap the particles from the gaseous emissions. The scrubbers prevent most of these particles from entering the atmosphere, where they can change into environmentally damaging compounds such as acid rain.

Charles Gupton/Stock, Boston/PNI

The U.S. Clean Air Act has significantly reduced emissions of acid-producing oxides. For example, the legally mandated removal of sulfur-bearing compounds from coal prior to burning has significantly reduced atmospheric levels of sulfur dioxide. Environmental laws also require use of pollution-trapping equipment, such as air scrubbers. Scrubbers—devices installed inside the smokestacks at a coal-burning plant—filter out sulfur-dioxide vapors and other compounds before these pollutants enter the atmosphere.


Ash Particles

Combustion of fossil fuels produces unburned fuel particles, known as ash. In the past, coal-fired power plants have emitted large amounts of ash into the atmosphere. However, government regulations also require that emissions containing ash be scrubbed or that particles otherwise be trapped to reduce this source of air pollution. While petroleum and natural gas generate less ash than coal, air pollution from fuel ash produced by automobiles may be a problem in cities where diesel and gasoline vehicles are concentrated.


Global Warming

Carbon dioxide is a major by-product of fossil fuel combustion, and it is what scientists call a greenhouse gas. Greenhouse gases absorb solar heat radiated from the earth’s surface and retain this heat, keeping the earth warm and habitable for living organisms. Rapid industrialization through the 19th and 20th centuries, however, has resulted in increasing fossil fuel emissions, raising the percentage of carbon dioxide in the atmosphere by about 28 percent. This dramatic increase in carbon dioxide has led some scientists to predict a global warming scenario that could cause numerous environmental problems, including disrupted weather patterns and polar ice cap melting. See also Global Warming.

Although it is extremely difficult to attribute observed global temperature changes directly to fossil fuel combustion, some countries are working together to lower emissions of carbon dioxide from fossil fuels. One proposal is to establish a system requiring companies to pay to emit carbon dioxide above a specified level. This payment could take several forms, including: (1) purchasing the rights to pollute from a company whose carbon dioxide emissions fall below the specified level; (2) purchasing and then preserving forests, which absorb carbon dioxide; and (3) paying to upgrade a carbon dioxide emitting plant in a lesser-developed country, lowering the upgraded plant’s carbon dioxide emissions.


Petroleum Recovery and Transportation

Environmental problems are created by drilling oil wells and extracting fluids because the petroleum pumped up from deep reservoir rocks is often accompanied by large volumes of salt water. This brine contains numerous impurities, so it must either be injected back into the reservoir rocks or treated for safe surface disposal.

Petroleum usually must also be transported long distances by tanker or pipeline to reach a refinery. Transport of petroleum occasionally leads to accidental spills. Oil spills, especially in large volumes, can be detrimental to wildlife and habitat.


Coal Mining

Coal Strip Mine

This excavated area, which borders farmland, has been strip mined for coal. Unless the strip mine is filled and revegetated, surface water runoff from the mined area can flush sediments and sulfur-bearing compounds (acid drainage) into nearby streams and rivers, endangering plant and wildlife communities.

Charles E. Rotkin/Corbis

Surface coal mining operations, often called strip mines, use massive shovels to remove soil and rock overlying the coal, disrupting the natural landscape. However, new land reclamation methods, driven by stringent laws and regulations, now require mining companies to restore strip-mined landscapes to nearly premined conditions.

Another environmental problem associated with coal mining occurs when freshly excavated coal beds are exposed to air. Sulfur-bearing compounds in the coal oxidize in the presence of water to form sulfuric acid. When this sulfuric acid solution, known as acid mine drainage, enters surface water and groundwater, it can be detrimental to water quality and aquatic life. Efforts are currently underway to remove sulfuric acid from mine drainage before it reaches rivers, lakes, and streams. For example, scientists are studying whether artificial wetlands have the ability to neutralize acid mine drainage.



Because the global economy is powered by fossil fuels, it is critical to know how long world reserves will last. However, estimating the world’s remaining fossil fuel reserves requires extensive information, including comprehensive geological maps of the world’s sedimentary basins, models of energy production systems, and data showing world energy consumption patterns and trends.


Reserves and Resources

When estimating the world’s fossil fuel supply, experts distinguish between reserves and resources. Reserves are fossil fuel deposits that have already been discovered and are immediately available. Resources are fossil fuel deposits that geologists believe are located in certain sedimentary basins, but have not yet been discovered. Because geologists base fossil fuel resource estimates on the location, extent, and formation of deposits recovered in geologically similar basins, resource estimates are less certain than reserve estimates. Both reserve and resource estimates are revised as data about new and existing deposits become available.

Fossil fuel reserves can be further divided into proved reserves and inferred reserves. Proved reserves are deposits that have been measured, sampled, and evaluated for production quality. Inferred reserves have been discovered but have not been measured or evaluated.

The definition of fossil fuel resources can be narrowed to technically recoverable resources. This definition does not consider whether a deposit can be extracted economically, but only whether the fossil fuel can be recovered using existing technology. By definition, the world fossil fuel supply increases as technological advances are made allowing previously unrecoverable resources to be extracted and processed.










World Energy Data

World Energy Production by Source

Although there is increasing interest in alternate energy sources such as solar power, almost two-thirds of the world’s energy comes from oil and natural gas. Other nonrenewable resources such as coal are still heavily used in countries such as China.

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Worldwide deposits of fossil fuels are finite. Some experts use data on world energy deposits to estimate how many years world energy supplies will last at current and projected consumption rates.

Natural Gas Production and Consumption

These charts show the countries that produce the most natural gas and the countries that consume the most natural gas. Natural gas is used as a residential and industrial fuel, and also as a raw material in manufacturing. The United States consumes more natural gas than any other country in the world.

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At the beginning of the 21st century, the world reserves of petroleum were estimated to be roughly 1.3 trillion barrels. By 2003 worldwide consumption of petroleum totaled 29 billion barrels a year. The world’s natural gas reserves were estimated to be roughly 170 trillion cubic meters (6,000 trillion cubic feet). Worldwide consumption of natural gas by 2003 totaled 2.7 trillion cubic meters (96 trillion cubic feet) a year. At the beginning of the 21st century the world coal reserves were estimated to be roughly 1 trillion metric tons, and by 2003 worldwide consumption of coal totaled 5 billion metric tons a year. Total worldwide energy consumption is expected to grow at about 2.2 percent per year until 2015.

Coal Production and Consumption

A comparison of the top ten coal-producing countries and the top ten coal-consuming countries shows that China is both the leading producer and the leading consumer of coal. Coal is burned in power plants to produce electricity and in steel mills to make coke for the production of steel.

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Theoretical models can be developed to estimate how many years the world fossil fuel supply will last. However, these models are complicated by technological advances in the energy production industry, unexpected discoveries of new fossil fuel deposits, and political, social, and economic factors that influence energy production and consumption.

Fossil Fuels Production and Consumption

These charts show the countries that produce the most fossil fuels and the countries that consume the most fossil fuels. Fossil fuels include petroleum, coal, and natural gas. The United States produces and consumes more total fossil fuels than any other country in the world.

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Because fossil fuels are being consumed at much faster rates than they are produced in the earth’s crust, humankind will eventually deplete these nonrenewable resources. While it is unclear how far in the future this will happen, there is evidence that some regions are becoming depleted in certain types of fossil fuels. For example, production of crude petroleum in the United States peaked in 1970. Today the United States imports significantly higher proportions of its petroleum needs.


Alternative Energy Sources

The prospect of reducing the world’s dependence on fossil fuels is problematic. Alternative energy industries, such as nuclear energy, hydroelectric energy, solar energy, wind energy, and geothermal energy exist, but these energy sources currently only account for a combined 14 percent of energy consumed worldwide. To date, alternative energy sources have been hindered by technological and environmental difficulties. For instance, although the uranium that fuels nuclear power is abundant, the risk of nuclear accidents and the difficulty associated with safe disposal of radioactive waste have led to the decline of the nuclear power industry (see Chernobyl’). Conversely, solar and wind power seem environmentally safe, but they are unreliable as steady sources of energy. As global energy consumption grows each year, development of certain alternative energy sources becomes increasingly important.

Contributed By:
Gene Whitney

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserv

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