What is natural gas?

Natural gas, which occurs naturally in underground reservoirs (often in association with oil deposits), is a mixture of hydrocarbons. A typical mixture of what is called “natural gas” is 70% to 90% methane (CH₄) and small but varying proportions (a few percent) of ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). Carbon dioxide (C0₂) contributes anywhere from 0 to 8% of the mix. Trace amounts of oxygen, nitrogen, hydrogen sulfide, and rare gases such as Argon, Helium, Neon, and Xenon may be present.

Some components of natural gas may be referred to simply (and confusingly) as "gas." The gas that powers a barbecue grill is propane, a component of natural gas. Many rural households use bottled propane for cooking, heating, and laundry dryers. Butane is in cigarette lighters and bottled fuels for camping and cooking, and can be used as a refrigerant. Ethane, the most abundant gas after methane in the natural gas mixture, is used in industry to make ethylene (or ethene), an extremely widely used organic compound. Ethylene has many industrial uses, including ripening fruit and making polyethylene, which in turn makes such household goods as plastic bags.

Methane is the essential component of natural gas.

After the refining process (in which the other gases in the mixture such as ethane and butane have been separated and collected to be sold separately), natural gas may be almost pure methane. In this condition it is known as "dry" natural gas, while the unrefined mixture with ethane, propane, and butane is known as "wet" gas.

Methane is colorless and odorless. A chemical compound with a noticeable and unpleasant odor is added to natural gas commercially to make it easier for people to detect natural gas leaks by smell.

Methane is highly flammable and makes an explosive mixture with air. It gives off a lot of energy in combustion, as we shall see.

LNG

Natural gas may be cooled to about -163 degrees C (-260 degrees F) to store and transport it in liquid form. This is known as liquefied natural gas or LNG. It may also be compressed and stored at very high pressure in strong containers; this is compressed natural gas or CNG. (Liquefied petroleum gas, or LPG, is not natural gas; it is a mixture of natural gas components propane and butane.)

What's so important about natural gas?

Since the mid-1960s there has been a dramatic increase in the proven reserves of natural gas, so it has become a fast-growing energy resource. One important thing to retain from this topic is that California is increasingly investing in gas-fired power plants. There are plans to import natural gas in liquefied form — a relatively new product, easily transportable — from Mexico to help supply these plants.

LNG is transported on tanker trucks, railway tankers, and LNG carrier ships, like this one. Source: Wikipedia

The origin of natural gas

Natural gas is a fossil fuel, a category that also includes coal and oil. Fossil fuels formed from the organic remains of plants and animals over very long time periods. Natural gas is believed to require thousands or millions of years to form.

Most natural gas formed from the remains of phytoplankton and zooplankton that accumulated in marine sediments beginning some 300 million years ago, in the Carboniferous period of the Paleozoic era. The organic remains partially decomposed or putrefied and formed a slimy black mud called sapropel. Over time, more sediments built up over the slimy sapropel. The buildup of sediments created heat and pressure on the lower layers, adding to the natural heat from the earth’s core. In some cases, a dearth of oxygen prevented the complete decay of the organic material into carbon dioxide and water. Instead, under heat, pressure, and a dearth of oxygen, the organic remains turned into petroleum compounds, rich in carbon and hydrogen. Once formed, the natural gas might be trapped in an underground reservoir, or escape slowly into the atmosphere.

Methane may also form from the action of microorganisms on waste in landfills. This is called landfill gas. Landfill gas is a local renewable energy source, in contrast to natural gas from deep underground, which is not renewable on human timescales. Read more about landfill gas and its conversion to electricity.

The layer of sediment where oil and gas originate is called a source rock or source bed. The source rock is generally a marine shale, although it may be a type of limestone or an oil shale.

For the oil and gas to accumulate in a major deposit, two conditions must be met beyond the presence of a source bed. The first is the existence of a reservoir bed, a layer of relatively porous and permeable rock in which the oil and gas coming up from the source rock can reside. The oil and gas fill in the pores of the reservoir rock the way water fills the spaces in a sponge.

The second condition is a trap or layer of cap rock to confine the oil and gas. The cap rock overlaying the source bed and reservoir bed must be impermeable, just as the material of an umbrella is impermeable to water. Salt or cemented sandstone are possible trap materials. If the upper layer were not impermeable, the oil and gas would slowly percolate upward and eventually evaporate into the atmosphere.

If oil, gas, and water are present together, they will separate according to density such that the gas rises above the oil, while the oil rests on the water. (The oil and gas are also separated by differences in surface tension.)

The anticline — an arch of stratified rock — is an important geological feature that may be associated with reserves of oil. If the rock composing the anticline is impermeable, oil and gas can accumulate under the crest of the anticline, sometimes with a layer of water below. The majority of the world’s oilfields are developed in anticlinal traps.

Geologists speak of two kinds of traps associated with oil and gas deposits: structural traps and stratigraphic traps. Structural traps are traps formed by folding or faulting of rock layers. An anticline is a kind of structural trap. A stratigraphic trap is formed by changes in rock type or sedimentary features that create a space where hydrocarbons are confined by impermeable layers. See a diagram here.

Schematic of a drilling rig tapping into an oil and gas deposit that has formed in an anticlinal trap. The layer depicted with rocks that look like bricks is impermeable. The water, oil, and natural gas have separated according to density. Source: US Department of Energy

The distribution of natural gas fields

In North America, most of the natural gas reserves are in vast basins underlying Texas and the Gulf of Mexico, and Alaska. This map shows the distribution of deposits, with the areas of largest reserves shown in dark blue.

Distribution of natural gas reserves in the United States. Source: Naturalgas.org and EIA Office of oil and gas.

Worldwide, most of the natural gas reserves are in the Middle East and Eurasia (Russia and the Central Asian and Transcaucasian republics), as shown in this graph:

California has some natural gas deposits of its own. California's natural gas fields are concentrated along the Sacramento River. The Rio Vista gas field is the largest in the state. Other fields lie in the San Joaquin River basin northwest of Fresno, between Bakersfield and Kettleman City, and just offshore of Santa Barbara.

Widespread as these gas fields are, however, they account for only 13.5% of the state's consumption of natural gas (2006 data from the Energy Almanac).

Natural gas reserves by geographic region (2008). Source: US Department of Energy

Production and distribution of natural gas

Oil has been extracted and sold commercially since the early decades of the 19th century, but the development of natural gas as an energy resource dates back only to the 1950s and 1960s. In the 1920s, the supply of natural gas in Southern California exceeded demand. Natural gas that was found in conjunction with oil was simply flared — burned off as a waste product — because it was difficult to collect and distribute.

California began importing gas through pipelines in 1947. Since the 1970s, gas has come into its own as an energy resource and has steadily gained in production, known reserves, and demand.

Once production companies have drilled wells and brought them into production, they process or refine the natural gas to remove water and to separate the methane from the other gases, such as propane and butane, which may be sold separately. The processed gas then flows through pipelines to areas of demand.

Natural gas flows into California from natural gas basins in western Canada, the Rocky Mountains, and other southwestern states.

Energy-generating companies may get their gas directly from the pipelines. Residential and commercial users get their gas from utility companies.

Network of pipelines delivering natural gas to California from Canada and other western states. Source: Energy Almanac

Conventional and unconventional natural gas

Conventional natural gas flows to the surface from an underground reservoir when a well is dug. An underground reservoir is a layer of rock that acts like a giant sponge with oil and gas in the pores.

Unconventional gas exists in other forms that make it more difficult to extract. An example of unconventional gas is shale gas. Shale, a type of sedimentary rock made of fine clay particles, is considered impermeable to the motion of oil and gas molecules. However, oil and gas may be tightly bound up in the rock, and there are techniques to extract these hydrocarbons.

Learn more about other types of unconventional gas sources.

Units of measurement

Natural gas volumes may be expressed in cubic feet occupied at normal temperature and pressure. The price of residential natural gas (used for space and water heating) is may be expressed in dollars per volume in cubic feet. For example, in 2006, the average price of natural gas in California was $11 to $12 per 1,000 cubic feet.

Quantities of natural gas may also be expressed in Btu, a unit of energy. A Btu is a British thermal unit. Talking about the energy content of different fuels makes more sense than talking about weights or volumes, since different fuels come in different forms. For example, a barrel (42 gallons) of crude oil has an energy content of almost 6 million Btu, while a short ton of coal has about 21 million Btu.

One Btu of natural gas is the amount of natural gas that, when burned, will produce enough energy to raise the temperature of 1 pound of water by 1 degree (from 60 to 61 degrees F) at normal pressure (one atmosphere).

Utility companies bill residents for "therms" of natural gas. A therm is 100,000 btus, or about 97 cubic feet of natural gas.

1 Btu = 1055 J
1 Btu = 2.931 x 10^-4 kWh

Consumption of natural gas

About 22% of the energy consumed in the United States comes from the burning of natural gas.

Natural gas has many other uses besides its use as a fuel for power plants or for homes. It is a raw material used in the production of paint and fertilizer, steel, glass, paper, and other products.

The US produced 19,278 billion cubic feet of natural gas in 2007. US consumption for that year was 23,056 billion cubic feet per year. Thus most of the gas consumed in the US, about 84% percent, was produced domestically.

See more statistics about natural gas, including world production and consumption.
Learn more about what natural gas is used for.

Importance of natural gas in California

Natural gas is an important resource for California.

About 43% of the natural gas used in California goes into the production of electricity in gas-fired power plants. The EPA has details on the generation of electricity from natural gas.
About 22% of the natural gas used in California is used by the residential sector, mostly for heating homes and water.
Industrial and commercial users account for most of the rest of the natural gas used in California. Some is lost during storage and transport.

Overall, natural gas provides almost one-third of the state’s total energy requirements.

Future demand and the rise of liquefied natural gas

Analysts expect the use of natural gas to grow, worldwide, in the United States, and in California. In many cases, natural gas use may replace oil use. Natural gas may increasingly be transported not by pipeline but in liquefied form.

In California, an increasing number of power plants are gas-fired. To assure a future supply, California plans to import liquefied natural gas (LNG) from Mexico.

See this site for liquid natural gas in California. This site also has links related to new LNG facilities in Baja California, which are expected to become operational in 2008.
Estimates suggest that the worldwide supply of natural gas should last for 400 years. Learn more about liquefied natural gas and its distribution worldwide.

Environmental impacts of natural gas

Extracting, treating, transporting, and burning gas generates nitrogen oxides, carbon dioxide, and other emissions. However, the average gas-fired plant generates only half as much carbon dioxide as the average coal-fired plant and much less nitrogen oxides and sulfur oxides. So, although coal is cheaper than natural gas, gas-fired power plants produce less pollution and greenhouse gases than coal-fired plants.

Gas-fired power plants use cooling water for cooling machinery, and may discharge warm, polluted water into lakes and rivers, according to permits.

See this EPA site for more on environmental impacts.