Natural gas is a naturally occurring mixture of gaseous hydrocarbons, consisting mainly of methane (CH₄), that forms in underground rock formations and sediments. It is both a fossil fuel and an important chemical feedstock. People use natural gas for heating, cooking, electricity generation, transportation, and the manufacture of chemicals, fertilizers, hydrogen, plastics, and other materials.
Most conventional natural gas formed from ancient organic matter altered by heat and pressure over millions of years. However, not all methane called “natural gas” has the same origin. Microorganisms produce biogenic methane at relatively low temperatures, while thermogenic methane forms deeper underground at higher temperatures. Methane also occurs in coal seams, organic-rich shale, landfills, wetlands, and ice-like gas hydrates beneath the seafloor and permafrost.
Key Takeaways: Natural Gas
- Natural gas is a mixture of gases that consists primarily of methane (CH₄), with smaller amounts of other hydrocarbons and nonhydrocarbon gases.
- Most fossil natural gas forms from buried organic matter through biological and geological processes.
- Natural gas occurs in conventional reservoirs, shale, tight rock, coal seams, and gas hydrates.
- Burning natural gas generally produces less carbon dioxide and fewer air pollutants per unit of energy than burning coal or petroleum, but it still releases CO₂.
- Methane itself is a powerful greenhouse gas, so leakage during production, processing, transportation, and use reduces the climate advantage of natural gas.
- Natural gas is colorless and odorless. Suppliers add an odorant to distributed gas so leaks are easier to detect.
- Shale gas, renewable natural gas, synthetic natural gas, coal gas, and methane hydrate differ in their sources and methods of production even though several consist largely of methane.
What Is Natural Gas?
Natural gas is a combustible mixture of gases that occurs naturally in rocks and sediments. Its main component is methane (CH₄), the simplest hydrocarbon.
Raw natural gas may also contain ethane (C₂H₆), propane (C₃H₈), butanes (C₄H₁₀), pentanes and heavier hydrocarbons, along with nitrogen (N₂), carbon dioxide (CO₂), water vapor, hydrogen sulfide (H₂S), helium (He), and other gases. The exact composition varies considerably among deposits.
Natural gas taken directly from a well is therefore not necessarily the same material delivered to a home. Processing removes water and undesirable contaminants and separates valuable natural gas liquids (NGLs), such as ethane, propane, and butanes. Pipeline-quality or “dry” natural gas consists predominantly of methane.
Under ordinary temperature and pressure, natural gas is a gas. It is colorless and naturally odorless. The characteristic “gas leak” smell does not come from methane itself. Gas suppliers add strongly scented sulfur-containing odorants so people can detect leaks.
Why Natural Gas Is Important
Natural gas occupies an unusual position because it is both an energy resource and a chemical raw material.
It provides heat directly for buildings and industrial processes and fuels turbines that generate electricity. Methane is also an important starting material for producing hydrogen and synthesis gas, which in turn are used to manufacture ammonia, methanol, fertilizers, and many other chemicals.
Natural gas also connects chemistry with geology. Its occurrence and composition provide information about organic-rich sediments, burial history, temperature, microbial activity, migration of fluids through rocks, and the formation of petroleum reservoirs.
History and Origin of the Name
Humans encountered natural gas long before understanding its chemistry. Gas escaping naturally from the ground sometimes ignited, producing persistent flames that acquired religious or cultural significance.
Ancient peoples knew of such natural gas seeps. In China, natural gas was collected and transported through bamboo pipes centuries before the development of the modern petroleum industry. Early gas was used as a source of heat, including for evaporating brines to produce salt.
The word gas is much newer. Flemish chemist Jan Baptist van Helmont introduced the term in the 17th century while studying substances that behaved differently from ordinary air. The word is commonly thought to have been inspired by the Greek word chaos.
The term natural gas distinguishes gas occurring naturally underground from manufactured fuel gases. During the 18th and 19th centuries, cities commonly used manufactured coal gas, produced by heating coal. This was also called town gas or illuminating gas.
Commercial natural gas development expanded during the 19th century, particularly after drilling technology made underground deposits accessible. Pipelines eventually allowed natural gas to be transported much farther from producing fields. During the 20th century, extensive pipeline networks helped natural gas replace manufactured coal gas in many countries.
How Natural Gas Forms
There are two major geological pathways for forming most natural gas: biogenic formation and thermogenic formation.
Biogenic Natural Gas
Biogenic, or microbial, natural gas forms when microorganisms break down organic matter under oxygen-poor conditions.
Methanogenic microorganisms produce methane as a metabolic product. This process occurs at relatively low temperatures and shallow depths and also occurs today in environments such as wetlands, lake sediments, landfills, sewage digesters, and the digestive systems of some animals.
Geologically produced microbial methane can accumulate underground and form commercially significant deposits.
Thermogenic Natural Gas
Most conventional fossil natural gas is associated with the burial and heating of organic-rich sediment.
Millions to hundreds of millions of years ago, organic remains accumulated with sediment. Burial subjected this material to increasing temperature and pressure. Chemical reactions gradually converted organic matter into hydrocarbons.
At moderate temperatures, this process can generate petroleum along with natural gas. At greater burial depths and temperatures, larger hydrocarbon molecules break down further, generating methane-rich gas.
This is why oil and natural gas frequently occur together.
How Natural Gas Accumulates Underground
Producing gas is only part of the geological story. For a conventional natural gas deposit to form, gas also has to migrate and become trapped.
Hydrocarbons commonly originate in an organic-rich source rock, such as shale. Gas and petroleum migrate through fractures and connected pore spaces because they are less dense than surrounding formation water.
A conventional reservoir generally includes:
Source rock: Organic-rich rock where hydrocarbons form.
Reservoir rock: Permeable rock, such as sandstone or some limestones, with interconnected pores capable of holding and transmitting gas.
Seal or cap rock: Relatively impermeable rock that prevents hydrocarbons from continuing upward.
Trap: A geological structure or arrangement of rocks that concentrates hydrocarbons.
Gas does not always migrate into a conventional reservoir. In shale formations, for example, much of it remains within the fine-grained source rock itself.
Sources of Natural Gas
Natural gas comes from several geological and biological sources.
Conventional natural gas accumulates in porous and permeable reservoir rocks beneath an impermeable seal.
Associated gas occurs together with crude oil in petroleum reservoirs.
Non-associated gas occurs in reservoirs containing little or no crude oil.
Shale gas remains trapped within low-permeability shale formations.
Tight gas occurs in very low-permeability sandstone, limestone, or other reservoir rocks.
Coalbed methane occurs within coal seams, where methane commonly adsorbs onto the surface of coal.
Biogenic gas results from microbial methane production.
Gas hydrates contain methane and other gases trapped within crystalline cages of water molecules.
Landfills, wastewater treatment facilities, manure digesters, and other decomposing organic materials also produce methane-rich biogas. When purified to appropriate specifications, this material becomes renewable natural gas.
Chemical Composition of Natural Gas
There is no single chemical formula for natural gas because it is a mixture rather than a pure substance.
Its most important component is methane:
CH₄
Other possible components include:
| Component | Formula | Role in Natural Gas |
|---|---|---|
| Methane | CH₄ | Main component |
| Ethane | C₂H₆ | Common hydrocarbon component |
| Propane | C₃H₈ | Natural gas liquid when separated |
| Butane | C₄H₁₀ | Natural gas liquid when separated |
| Carbon dioxide | CO₂ | Noncombustible impurity |
| Nitrogen | N₂ | Noncombustible component |
| Hydrogen sulfide | H₂S | Toxic, corrosive impurity |
| Water | H₂O | Usually removed during processing |
| Helium | He | Valuable component of some deposits |
Raw gas containing appreciable quantities of ethane and heavier hydrocarbons is often called wet gas. Gas from which most of these compounds have been removed is called dry gas.
Gas containing significant hydrogen sulfide is called sour gas, while gas containing little or no H₂S is called sweet gas.
Combustion of Natural Gas
Because methane is the main component, methane combustion approximates the overall combustion chemistry of natural gas.
Complete combustion is:
CH₄ + 2O₂ → CO₂ + 2H₂O + energy
With sufficient oxygen, the major products are carbon dioxide and water.
Incomplete combustion occurs when there is insufficient oxygen. It can produce carbon monoxide (CO) as well as soot and other products.
Carbon monoxide is especially dangerous because it is colorless and odorless and interferes with the blood’s ability to transport oxygen.
How Natural Gas Is Processed
Gas emerging from a well can contain water, liquid hydrocarbons, solids, sulfur compounds, carbon dioxide, nitrogen, and other substances.
Processing varies with the source but commonly includes:
- Separating oil and water from the gas.
- Removing water vapor to reduce corrosion and hydrate formation in pipelines.
- Removing acid gases, particularly H₂S and sometimes CO₂.
- Recovering natural gas liquids, such as ethane, propane, and butanes.
- Removing other impurities as necessary to meet pipeline specifications.
The resulting dry natural gas is transported through pipelines or stored for later use.
Natural gas can also be cooled to approximately −162 °C (−260 °F) to form liquefied natural gas (LNG). Liquefaction greatly reduces its volume, making long-distance transportation by specialized ships practical.
LNG is not a different type of natural gas chemically. It is natural gas in a different physical state.
Uses of Natural Gas
Natural gas has domestic, commercial, industrial, transportation, and chemical uses.
Heating and Cooking
Homes and commercial buildings use natural gas for:
- Space heating
- Water heating
- Cooking
- Clothes drying
- Fireplaces
Electricity Generation
Natural gas fuels gas turbines and combined-cycle power plants. Combined-cycle plants use both a gas turbine and steam generated from the turbine’s hot exhaust, increasing overall efficiency.
Industrial Heating
Industries use natural gas in furnaces, kilns, boilers, dryers, and other equipment requiring controlled high-temperature heat.
Chemical Manufacturing
Natural gas is an important feedstock for the chemical industry.
Steam reforming of methane produces synthesis gas and hydrogen. Hydrogen is essential for producing ammonia (NH₃), which is the starting material for many nitrogen fertilizers.
Natural gas also contributes to the manufacture of methanol, plastics, solvents, and numerous other chemicals.
Transportation
Compressed natural gas (CNG) and liquefied natural gas (LNG) can fuel vehicles. CNG is stored as highly compressed gas, while LNG is stored as a cryogenic liquid.
Benefits of Natural Gas as a Fuel
Natural gas has several practical advantages.
High energy content: Methane releases substantial heat during combustion.
Efficient electricity generation: Modern combined-cycle plants convert a relatively high fraction of fuel energy into electricity.
Lower carbon dioxide emissions than other fossil fuels: For the same amount of energy produced, natural gas combustion generally releases less CO₂ than coal or petroleum products.
Relatively clean combustion: Natural gas combustion generally produces less sulfur dioxide, particulate matter, and some other pollutants than coal combustion.
Flexible power generation: Gas turbines can change output relatively quickly, making them useful for responding to changes in electricity demand and variable renewable electricity production.
Extensive infrastructure: Many regions already have pipelines, storage facilities, processing plants, and distribution systems.
Drawbacks of Natural Gas
Natural gas also has significant disadvantages.
It is usually a fossil fuel. Burning fossil natural gas adds carbon dioxide to the atmosphere.
Methane is a potent greenhouse gas. Methane can escape during drilling, production, processing, storage, transmission, distribution, and use. These emissions reduce the climate advantage gained from lower CO₂ emissions during combustion.
Extraction affects the environment. Wells, roads, pipelines, water use, wastewater, and land disturbance can affect ecosystems and communities.
Hydraulic fracturing has environmental concerns. Producing gas from shale and other low-permeability rocks often involves horizontal drilling and hydraulic fracturing. Potential concerns include water consumption, wastewater disposal, air emissions, habitat disturbance, induced seismicity associated particularly with wastewater injection, and risks from improperly constructed wells.
Natural gas is flammable. Escaping gas can cause fires or explosions under suitable conditions.
Incomplete combustion produces carbon monoxide. Malfunctioning or poorly ventilated appliances pose a serious health hazard.
Is Natural Gas Safe?
Natural gas is safe when equipment is properly installed, maintained, and ventilated, but it is not harmless.
Methane is not highly toxic in the way carbon monoxide or hydrogen sulfide is. However, concentrated methane can displace oxygen in an enclosed space, creating an asphyxiation hazard.
More commonly, leaked natural gas presents a fire and explosion hazard. Methane mixed with air burns only over a particular range of concentrations, but an enclosed accumulation within this range can ignite explosively.
Natural gas is naturally odorless. Utilities therefore add small quantities of powerful odorants, often sulfur-containing compounds, to distributed gas. The familiar rotten-egg-like smell warns of a possible leak.
Carbon monoxide represents a different hazard. It forms through incomplete combustion rather than simply leaking from a natural gas supply. Proper ventilation, appliance maintenance, and working carbon monoxide detectors reduce this risk.
Types of Natural Gas and Related Fuel Gases
Several substances have names containing “natural gas” or are used as substitutes for it. They are not all the same material.
| Type | Source | Main Component(s) | Key Feature |
|---|---|---|---|
| Conventional natural gas | Underground reservoirs | Mainly CH₄ | Migrated into permeable reservoir rock |
| Shale gas | Shale formations | Mainly CH₄ | Unconventional geological source |
| Coal gas | Manufactured from coal | H₂, CH₄, CO and others | Manufactured fuel gas |
| Synthetic natural gas | Manufactured | Mainly CH₄ | Made to resemble pipeline natural gas |
| Renewable natural gas | Purified biogas | Mainly CH₄ | Derived from recently living organic material |
| Natural gas hydrate | Marine/permafrost sediments | Usually CH₄ + H₂O lattice | Solid crystalline clathrate |
Natural Gas
In ordinary usage, natural gas means the methane-rich fossil gas extracted from geological deposits.
It may come from conventional reservoirs or unconventional sources such as shale, tight formations, or coal seams.
Shale Gas
Shale gas is natural gas trapped within fine-grained shale formations.
Chemically, processed shale gas can be very similar to natural gas from a conventional reservoir. The distinction is primarily geological, not chemical.
Shale has very low permeability, so gas does not readily flow through it. Commercial production commonly combines horizontal drilling with hydraulic fracturing, which creates or enlarges fractures through which gas can reach the well.
Coal Gas
Coal gas is a manufactured fuel gas produced from coal rather than naturally occurring methane extracted from a reservoir.
Historically, coal was heated in the absence of air in a process called destructive distillation. The resulting gas mixture could contain hydrogen, methane, carbon monoxide, and other compounds.
Coal gas was widely used for lighting, heating, and cooking before natural gas distribution became widespread.
It is not the same as coalbed methane, which is naturally occurring methane found in coal seams.
Synthetic Natural Gas
Synthetic natural gas (SNG) is a manufactured gas designed to have properties similar to conventional natural gas.
It is generally methane-rich and can be made from coal, biomass, or other carbon-containing feedstocks.
One route first produces synthesis gas, or syngas, containing primarily carbon monoxide and hydrogen. Methanation then converts these gases into methane.
Synthetic natural gas and synthesis gas are therefore not synonyms.
Renewable Natural Gas
Renewable natural gas (RNG), also called biomethane, is purified biogas that can substitute for conventional natural gas.
Microorganisms produce biogas through anaerobic decomposition of organic matter. Sources include:
- Landfills
- Sewage treatment facilities
- Livestock manure
- Food waste
- Agricultural residues
- Anaerobic digesters
Raw biogas typically contains roughly 45–65% methane together with considerable CO₂ and smaller quantities of other substances. Processing removes CO₂, water, hydrogen sulfide, and other contaminants, producing methane-rich RNG. The EPA describes upgraded RNG as containing at least about 90% methane, while pipeline-injected RNG is typically 96–98% methane.
The methane molecule in RNG is chemically identical to methane in fossil natural gas. The distinction is its source, not its molecular structure.
Crystallized Natural Gas: Gas Hydrates and Methane Clathrates
Natural gas can also occur in an extraordinary solid form called a gas hydrate or clathrate hydrate.
A gas hydrate is an ice-like crystalline solid in which water molecules form cages around gas molecules. Methane is the most common gas trapped in naturally occurring hydrates, so the terms methane hydrate and gas hydrate are often used almost interchangeably.
Gas hydrates form under conditions of sufficiently low temperature and high pressure. They occur primarily beneath deep ocean sediments and within or beneath permafrost.
The methane is not chemically bonded to the water as it would be in an ordinary chemical compound. Instead, water molecules form a crystalline cage that physically encloses methane molecules.
Gas hydrates can store remarkable quantities of methane. When pressure decreases or temperature rises sufficiently, the hydrate destabilizes into water and gas.
Gas hydrates are scientifically important as a potentially enormous methane reservoir, a possible future energy resource, a component of the global carbon cycle, and a possible geological hazard. Commercial extraction remains technically challenging and is an active area of research.
Natural Gas vs Propane
While natural gas and propane are related, they are not the same fuel.
Natural gas consists mainly of methane (CH₄). Commercial propane consists mainly of propane (C₃H₈).
Propane is one of the hydrocarbons that may be separated from raw natural gas during processing. Unlike methane, propane is easily liquefied under moderate pressure at ordinary temperatures. This makes liquefied petroleum gas (LPG) practical for storage in portable tanks.
LPG and LNG are also different. LPG consists mainly of propane and butane, while LNG is primarily methane cooled to a cryogenic liquid.
Natural Gas vs Biogas
Biogas and natural gas both contain methane, but their compositions and origins differ.
Raw biogas forms through microbial decomposition of organic material without oxygen and usually contains substantial carbon dioxide in addition to methane.
Conventional natural gas generally comes from geological deposits and, after processing, contains a much higher proportion of methane.
Purifying biogas until it meets appropriate fuel specifications produces renewable natural gas or biomethane.
Natural Gas and Climate Change
Natural gas affects climate in two important ways.
First, combustion produces carbon dioxide:
CH₄ + 2O₂ → CO₂ + 2H₂O
Natural gas generally releases less CO₂ per unit of useful energy than coal, but its combustion still transfers fossil carbon into the atmosphere.
Second, unburned methane can escape directly into the atmosphere. Methane is itself an important greenhouse gas. Consequently, evaluating the climate impact of natural gas requires considering the entire supply chain rather than combustion alone.
Renewable natural gas presents a somewhat different carbon balance because its carbon comes from recently living material rather than ancient geological carbon. Capturing methane that would otherwise escape from landfills or manure systems can also prevent direct methane emissions. However, RNG is not automatically emission-free; its overall impact depends on the feedstock, production system, leakage, processing, transportation, and what would have happened to the organic waste otherwise.
Common Misconceptions About Natural Gas
Misconception: Natural gas is pure methane.
Methane is usually the largest component, but raw natural gas is a mixture. It can contain ethane, propane, butanes, CO₂, nitrogen, water, hydrogen sulfide, helium, and other substances.
Misconception: Natural gas has a rotten-egg smell.
Pure methane is odorless. Gas suppliers deliberately add odorants to make leaks noticeable.
Misconception: Natural gas is not a fossil fuel.
Conventional natural gas is a fossil fuel. Renewable natural gas is different because it comes from recently produced biological material.
Misconception: Natural gas does not produce carbon dioxide.
Burning methane produces both carbon dioxide and water. It generally produces less CO₂ per unit of energy than coal, but its CO₂ emissions are not zero.
Misconception: Shale gas is a different chemical from natural gas.
Shale gas describes the geological source of the gas. After processing, it is predominantly methane just like conventional natural gas.
Misconception: Coal gas and coalbed methane are the same thing.
Coal gas results from processing coal. Coalbed methane is naturally occurring methane found within coal seams.
Misconception: Synthetic natural gas and syngas are the same substance.
Syngas is primarily a mixture of carbon monoxide and hydrogen. Synthetic natural gas is methane-rich fuel that may be manufactured from syngas.
Misconception: LNG and LPG are the same.
LNG is liquefied natural gas and consists mainly of methane. LPG is liquefied petroleum gas and consists mainly of propane and butane.
Misconception: Methane hydrate is frozen methane.
Methane hydrate is not simply solid methane. It is a crystalline structure of water molecules containing methane molecules within molecular cages.
Frequently Asked Questions
What is natural gas made of?
Natural gas consists mainly of methane (CH₄). Depending on its source and degree of processing, it also contains varying quantities of ethane, propane, butanes, heavier hydrocarbons, nitrogen, carbon dioxide, water, hydrogen sulfide, helium, and other substances.
Is natural gas a fossil fuel?
Conventional natural gas is a fossil fuel because it largely forms from ancient organic matter over geological time. Renewable natural gas is methane-rich gas produced from modern biological material.
Is natural gas renewable or nonrenewable?
Fossil natural gas is nonrenewable on human timescales. Renewable natural gas produced from biogas is considered renewable when derived from replenishable organic waste or biomass sources.
Is natural gas methane?
Natural gas is mostly methane, but the terms are not exact synonyms. Methane is a pure chemical compound, CH₄, while natural gas is a mixture.
What color is natural gas?
Natural gas is colorless.
What does natural gas smell like?
Pure natural gas has essentially no odor. Utilities add strong-smelling odorants to distributed natural gas to make leaks easier to detect.
Is natural gas lighter than air?
Methane is less dense than air, so methane-rich natural gas generally rises when released into open air. Air currents, enclosed spaces, gas composition, and other conditions affect how a real leak disperses.
Does natural gas produce carbon monoxide?
Complete combustion primarily produces carbon dioxide and water. Incomplete combustion can produce dangerous carbon monoxide.
Is natural gas the same as propane?
No. Natural gas consists mainly of methane, while propane fuel consists mainly of C₃H₈. Propane may be recovered during natural gas processing.
Is shale gas natural gas?
Yes. Shale gas is natural gas obtained from shale formations. The term describes where and how the gas occurs rather than a fundamentally different fuel.
What is the difference between CNG and LNG?
Compressed natural gas (CNG) is methane-rich gas stored at high pressure. Liquefied natural gas (LNG) is natural gas cooled until it becomes a liquid. Both are predominantly the same chemical material stored in different physical forms.
Can natural gas occur as a solid?
Yes. Methane and other natural gases occur naturally in solid gas hydrates, where water molecules form crystalline cages around gas molecules. These deposits occur mainly in marine sediments and permafrost regions.
Glossary
Associated gas: Natural gas occurring in a reservoir together with crude oil.
Biogas: Gas produced by anaerobic microbial decomposition of organic matter, consisting mainly of methane and carbon dioxide.
Biogenic gas: Natural gas, particularly methane, produced by microorganisms.
Coalbed methane: Naturally occurring methane associated with coal seams.
Coal gas: Manufactured fuel gas produced by processing coal.
Compressed natural gas (CNG): Natural gas stored under high pressure.
Dry gas: Natural gas that consists predominantly of methane after heavier hydrocarbons and water have been removed.
Gas hydrate: Ice-like crystalline material in which water molecules form cages containing gas molecules.
Liquefied natural gas (LNG): Natural gas cooled to approximately −162 °C (−260 °F), causing it to become liquid.
Methane (CH₄): The simplest hydrocarbon and principal component of natural gas.
Natural gas liquids (NGLs): Hydrocarbons such as ethane, propane, and butanes separated from raw natural gas.
Renewable natural gas (RNG): Biogas purified to produce methane-rich gas suitable as a substitute for fossil natural gas.
Shale gas: Natural gas occurring in low-permeability shale.
Sour gas: Natural gas containing significant hydrogen sulfide.
Sweet gas: Natural gas containing little or no hydrogen sulfide.
Synthetic natural gas (SNG): Manufactured methane-rich gas designed to substitute for conventional natural gas.
Syngas: Synthesis gas, a mixture composed primarily of hydrogen and carbon monoxide and used as an intermediate for producing fuels and chemicals.
Thermogenic gas: Natural gas formed through thermal alteration of buried organic matter.
Tight gas: Natural gas occurring in rock with very low permeability.
Wet gas: Raw natural gas containing appreciable quantities of ethane and heavier hydrocarbons.
References and Further Reading
- Blanchard, Charles (2021). The Extraction State: A History of Natural Gas in America. University of Pittsburgh Press. ISBN 978-0-8229-8777-2.
- Hammer, Georg; Lübcke, Torsten; et al. (2006). “Natural Gas”. Ullmann’s Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a17_073.pub2. ISBN 978-3-527-30385-4.
- Kvenvolden, Keith A. (1993). “Gas hydrates—geological perspective and global change”. Reviews of Geophysics. 31 (2): 173–187. doi:10.1029/93RG00268
- Wang, Qiang; Chen, Xi; Jha, Awadhesh N.; Rogers, Howard 2014). “Natural gas from shale formation – The evolution, evidences and challenges of shale gas revolution in United States”. Renewable and Sustainable Energy Reviews. 30: 1–28. doi:10.1016/j.rser.2013.08.065