Natural gas is a fossil fuel. Like other fossil fuels such as coal and oil, natural gas forms from the plants, animals, and microorganisms that lived millions of years ago.

There are several different theories to explain how fossil fuels are formed. The most prevalent theory is that they form underground, under intense conditions. As plants, animals, and microorganisms decompose, they are gradually covered by layers of soil, sediment, and sometimes rock. Over millions of years, the organic matter is compressed. As the organic matter moves deeper into Earth’s crust, it encounters higher and higher temperatures.

The combination of compression and high temperature causes the carbon bonds in the organic matter to break down. This molecular breakdown produces thermogenic methane—natural gas. Methane, probably the most abundant organic compound on Earth, is made of carbon and hydrogen (CH4).

Natural gas deposits are often found near oil deposits. Deposits of natural gas close to Earth’s surface are usually dwarfed by nearby oil deposits. Deeper deposits—formed at higher temperatures and under more pressure—have more natural gas than oil. The deepest deposits can be made up of pure natural gas.

Natural gas does not have to be formed deep underground, however. It can also be formed by tiny microorganisms called methanogens. Methanogens live in the intestines of animals (including humans) and in low-oxygen areas near the surface of Earth. Landfills, for example, are full of decomposing matter that methanogens break down into a type of methane called biogenic methane. The process of methanogens creating natural gas (methane) is called methanogenesis.

Although most biogenic methane escapes into the atmosphere, there are new technologies being created to contain and harvest this potential energy source.

Thermogenic methane—the natural gas formed deep beneath Earth’s surface—can also escape into the atmosphere. Some of the gas is able to rise through permeable matter, such as porous rock, and eventually dissipate into the atmosphere.

However, most thermogenic methane that rises toward the surface encounters geological formations that are too impermeable for it to escape. These rock formations are called sedimentary basins.

Sedimentary basins trap huge reservoirs of natural gas. In order to gain access to these natural gas reservoirs, a hole (sometimes called a well) must be drilled through the rock to allow the gas to escape and be harvested.

Sedimentary basins rich in natural gas are found all over the world. The deserts of Saudi Arabia, the humid tropics of Venezuela, and the freezing Arctic of the U.S. state of Alaska are all sources of natural gas. Outside Alaska, U.S. basins are primarily around the states bordering the Gulf of Mexico, including Texas and Louisiana. Recently, the northern states of North Dakota, South Dakota, and Montana have developed significant drilling facilities in sedimentary basins.

Types of Natural Gas

Natural gas that is economical to extract and easily accessible is considered “conventional.” Conventional gas is trapped in permeable material beneath impermeable rock.

Natural gas found in other geological settings is not always so easy or practical to extract. This gas is called “unconventional.” New technologies and processes are always being developed to make this unconventional gas more accessible and economically viable. Over time, gas that was considered “unconventional” can become conventional.

Biogas is a type of gas that is produced when organic matter decomposes without oxygen being present. This process is called anaerobic decomposition, and it takes place in landfills or where organic material such as animal waste, sewage, or industrial byproducts are decomposing.

Biogas is biological matter that comes from plants or animals, which can be living or not-living. This material, such as forest residues, can be combusted to create a renewable energy source.

Biogas contains less methane than natural gas, but can be refined and used as an energy source.

Deep Natural Gas
Deep natural gas is an unconventional gas. While most conventional gas can be found just a few thousand meters deep, deep natural gas is located in deposits at least 4,500 meters (15,000 feet) below the surface of Earth. Drilling for deep natural gas is not always economically practical, although techniques to extract it have been developed and improved.

Shale
Shale gas is another type of unconventional deposit. Shale is a fine-grained, sedimentary rock that does not disintegrate in water. Some scientists say shale is so impermeable that marble is considered “spongy” in comparison. Thick sheets of this impermeable rock can “sandwich” a layer of natural gas between them.

Shale gas is considered an unconventional source because of the difficult processes necessary to access it: hydraulic fracturing (also known as fracking) and horizontal drilling. Fracking is a procedure that splits open rock with a high-pressure stream of water, and then “props” it open with tiny grains of sand, glass, or silica. This allows gas to flow more freely out of the well. Horizontal drilling is a process of drilling straight down into the ground, then drilling sideways, or parallel, to Earth’s surface.

Tight Gas
Tight gas is an unconventional natural gas trapped underground in an impermeable rock formation that makes it extremely difficult to extract. Extracting gas from “tight” rock formations usually requires expensive and difficult methods, such as fracking and acidizing.

Acidizing is similar to fracking. An acid (usually hydrochloric acid) is injected into the natural gas well. The acid dissolves the tight rock that is blocking the flow of gas.

Coalbed Methane
Coalbed methane is another type of unconventional natural gas. As its name implies, coalbed methane is commonly found along seams of coal that run underground. Historically, when coal was mined, the natural gas was intentionally vented out of the mine and into the atmosphere as a waste product. Today, coalbed methane is collected and is a popular energy source.

Gas in Geopressurized Zones
Another source of unconventional natural gas is geopressurized zones. Geopressurized zones form 3,000-7,600 meters (10,000-25,000 feet) below Earth’s surface.

These zones form when layers of clay rapidly accumulate and compact on top of material that is more porous, such as sand or silt. Because the natural gas is forced out of the compressed clay, it is deposited under very high pressure into the sand, silt, or other absorbent material below.

Geopressurized zones are very difficult to mine, but they may contain a very high amount of natural gas. In the United States, most geopressurized zones have been found in the Gulf Coast region.

Methane Hydrates
Methane hydrates are another type of unconventional natural gas. Methane hydrates were discovered only recently in ocean sediments and permafrost areas of the Arctic. Methane hydrates form at low temperatures (around 0°C, or 32°F) and under high pressure. When environmental conditions change, methane hydrates are released into the atmosphere.

The United States Geological Survey (USGS) estimates that methane hydrates could contain twice the amount of carbon than all of the coal, oil, and conventional natural gas in the world, combined.

In ocean sediments, methane hydrates form on the continental slope as bacteria and other microorganisms sink to the ocean floor and decompose in the silt. Methane, trapped within the sediments, has the ability to “cement” the loose sediments into place and keep the continental shelf stable. However, if the water becomes warmer, the methane hydrates break down. This causes causes underwater landslides, and releases natural gas.

In permafrost ecosystems, methane hydrates form as bodies of water freeze and water molecules create individual “cages” around each methane molecule. The gas, trapped in a frozen lattice of water, is contained at a much higher density than it would be in its gaseous state. As the ice cages thaw, the methane escapes.

Global warming, the current period of climate change, influences the release of methane hydrates from both permafrost and ocean sediment layers.

There is a vast amount of potential energy stored in methane hydrates. However, because they are such fragile geological formations—capable of breaking down and disrupting the environmental conditions around them—methods for extracting them are developed with extreme caution.

Drilling and Transportation

Natural gas is measured in normal cubic meters or standard cubic feet. In 2009, the United States Energy Information Administration (EIA) estimated that the world’s proven natural gas reserves are around 6,289 trillion cubic feet (tcf).

Most of the reserves are in the Middle East, with 2,686 tcf in 2011, or 40 percent of total world reserves. Russia has the second-highest amount of proven reserves, with 1,680 tcf in 2011. The United States contains just over four percent of the world’s natural gas reserves.

According to the EIA, total world consumption of dry natural gas in 2010 was 112,920 billion cubic feet (bcf). That year, the United States consumed a little more than 24,000 bcf, the most of any nation.

Natural gas is most commonly extracted by drilling vertically from Earth’s surface. From a single vertical drill, the well is limited to the gas reserves it encounters.

Hydraulic fracturing, horizontal drilling, and acidizing are processes to expand the amount of gas that a well can access, and thus increase its productivity. However, these practices can have negative environmental consequences.

Hydraulic fracturing, or fracking, is a process that splits open rock formations with high-pressure streams of water, chemicals, and sand. The sand props open the rocks, which allows gas to escape and be stored or transported. However, fracking requires huge quantities of water, which can radically reduce an area’s water table and negatively impact aquatic habitats. The process produces highly toxic and frequently radioactive wastewater that, if mismanaged, can leak and contaminate underground water sources used for drinking, hygiene, and industrial and agricultural use.

In addition, fracking can cause microearthquakes. Most of these temblors are far too tiny to be felt on the surface, but some geologists and environmentalists warn that the quakes may cause structural damage to buildings or underground networks of pipes and cables.

Due to these negative environmental effects, fracking has been criticized and banned in some areas. In other areas, fracking is a lucrative economic opportunity and providing a reliable source of energy.

Horizontal drilling is a way of increasing the area of a well without creating multiple expensive and environmentally sensitive drilling sites. After drilling straight down from Earth’s surface, drilling can be directed to go sideways—horizontally. This broadens the well’s productivity without requiring multiple drilling sites on the surface.

Acidizing is a process of dissolving acidic components and inserting them into the natural gas well, which dissolves rock that may be blocking the flow of gas.

After natural gas is extracted, it is most frequently transported through pipelines that can be from two to 60 inches in diameter.

The continental United States has more than 210 pipeline systems that are made up of 490,850 kilometers (305,000 miles) of transmission pipelines that transfer gas to all 48 states. This system requires more than 1,400 compressor stations to ensure that the gas continues on its path, 400 underground storage facilities, 11,000 locations to deliver the gas, and 5,000 locations to receive the gas.

Natural gas can also be cooled to about -162°C (-260°F) and converted into liquified natural gas, or LNG. In liquid form, natural gas takes up only 1/600 of the volume of its gaseous state. It can easily be stored and transported places that do not have pipelines.

LNG is transported by a specialized insulated tanker, which keeps the LNG at its boiling point. If any of the LNG vaporizes, it is vented out of the storage area and used to power the transport vessel. The United States imports LNG from other countries, including Trinidad and Tobago and Qatar. However, the U.S. is currently increasing its domestic LNG production.

Consuming Natural Gas

Although natural gas takes millions of years to develop, its energy has only been harnessed during the past few thousand years. Around 500 B.C.E., Chinese engineers made use of natural gas seeping out of the earth by building bamboo pipelines. These pipes transported gas to heat water. In the late 1700s, British companies provided natural gas to light streetlamps and homes.

Today, natural gas is used in countless ways for industrial, commercial, residential, and transportation purposes. The United States Department of Energy (DOE) estimates that natural gas can be up to 68 percent less expensive than electricity.

In residential homes, the most popular use for natural gas is heating and cooking. It is used to power home appliances such as stoves, air conditioners, space heaters, outdoor lights, garage heaters, and clothes dryers.

Natural gas is also used on a larger scale. In commercial settings, such as restaurants and shopping malls, it is an extremely efficient and economical way to power water heaters, space heaters, dryers, and stoves.

Natural gas is used to heat, cool, and cook in industrial settings, as well. However, it is also used in a variety of processes such as waste treatment, food processing, and refining metals, stone, clay, and petroleum.

Natural gas can also be used as an alternative fuel for cars, buses, trucks, and other vehicles. Currently, there are more than five million natural gas vehicles (NGV) worldwide, and more than 150,000 in the United States.

Although NGVs initially cost more than gas-powered vehicles, they are cheaper to refuel and are the cleanest-running vehicles in the world. Gasoline- and diesel-powered vehicles emit harmful and toxic substances including arsenic, nickel, and nitrogen oxides. In contrast, NGVs may emit minute amounts of propane or butane, but release 70 percent less carbon monoxide into the atmosphere.

Using the new technology of fuel cells, the energy from natural gas is also used to generate electricity. Instead of burning natural gas for energy, fuel cells generate electricity with electrochemical reactions. These reactions produce water, heat, and electricity without any other byproducts or emissions. Scientists are still researching this method of producing electricity in order to affordably apply it to electric products.

Natural Gas and the Environment

Natural gas usually needs to be processed before it can be used. When it is extracted, natural gas can contain a variety of elements and compounds other than methane. Water, ethane, butane, propane, pentanes, hydrogen sulphide, carbon dioxide, water vapor, and occasionally helium and nitrogen may be present in a natural gas well. In order to be used for energy, the methane is processed and separated from the other components. The gas that is used for energy in our homes is almost pure methane.

Like other fossil fuels, natural gas can be burned for energy. In fact, it is the cleanest-burning fuel, meaning it releases very few byproducts.

When fossil fuels are burned, they can release (or emit) different elements, compounds, and solid particles. Coal and oil are fossil fuels with very complex molecular formations, and contain a high amount of carbon, nitrogen, and sulfur. When they are burned, they release high amounts of harmful emissions, including nitrogen oxides, sulfur dioxide, and particles that drift into the atmosphere and contribute to air pollution.

In contrast, the methane in natural gas has a simple molecular makeup: CH4. When it is burned, it emits only carbon dioxide and water vapor. Humans exhale the same two components when we breathe.

Carbon dioxide and water vapor, along with other gases such as ozone and nitrous oxide, are known as greenhouse gases. The increasing amounts of greenhouse gases in the atmosphere are linked to global warming and could have disastrous environmental consequences.

Although burning natural gas still emits greenhouse gases, it emits almost 30 percent less CO2 than oil, and 45 percent less CO2 than coal.

Safety

As with any extractive activity, drilling for natural gas can lead to leaks. If the drill hits an unexpected high-pressure pocket of natural gas, or the well is damaged or ruptures, the leak can be immediately hazardous.

Because natural gas dissipates so quickly into the air, it does not always cause an explosion or burn. However, the leaks are an environmental hazard that also leak mud and oil into the surrounding areas.

If hydraulic fracturing was used to expand a well, the chemicals from that process can contaminate local aquatic habitats and drinking water with highly radioactive materials. The uncontained methane released in the air can also force people to temporary evacuate the area.

Leaks can also occur slowly over time. Until the 1950s, cast iron was a popular choice for distribution pipelines, but it allows a high amount of natural gas to escape. The cast iron pipes become leaky after years of freeze-thaw cycles, heavy overhead traffic, and strains from the naturally shifting soil. Methane leaks from these distribution pipelines make up more than 30 percent of the methane emmissions in the U.S. natural gas distribution sector. Today, pipelines are made out of a variety of metals and plastics to reduce leakage.