World of Earth Science | Natural Gas

Natural gas is a mixture of hydrocarbons (molecules that contain only carbon and hydrogen) and gases (most notably methane, ethane, propane, and butane) that exist naturally in rocks beneath the surface of the earth. It is widely used as a heating source, and in some cases, specific portions of the natural gas are used as starting materials in industrial processes. Natural gas is the product of the decaying of living matter over millions of years (as is also true for petroleum). Specific conditions, including low oxygen levels, are necessary for this to occur. The hydrocarbon gases are trapped in geological formations known as anticlines. Each of the major hydrocarbon components of natural gas is used as a fuel source.

Natural gas has its origins in decayed living matter, most likely as the result of the action of bacteria upon dead animal and plant material. In order for most bacteria to effectively break down organic matter to hydrocarbons, there must be low levels of oxygen present. This would mean that the decaying matter was buried (most likely under water) before it could be completely degraded to carbon dioxide and water. Conditions such as this are likely to have been met in coastal areas where sedimentary rocks and marine bacteria are common. The actions of heat and pressure along with bacteria produced a mixture of hydrocarbons. The smaller molecules which exist as gases were then either trapped in porous rocks or in underground reservoirs where they formed sources of hydrocarbon fuels.

Natural gas, like petroleum, is a mixture of many organic substances. The exact composition of different sources of natural gas varies slightly, but in all cases, methane is by far the most common component, with other hydrocarbons also being very common. Other gases such as oxygen, argon, and carbon dioxide make up the rest of most natural gas sources. The largest sources of natural gas in the United States are found in Alaska, Texas, Oklahoma, western Pennsylvania, and Ohio. It is estimated that the supply of natural gas in this country may be sufficient to last for two centuries—although the more readily accessible sources have been used, meaning that it will be more expensive to obtain natural gas in the future.

Natural gas is believed to have been first discovered and used by the Chinese, perhaps as early as 1000 B.C.Shallow stores of natural gas were released from just beneath the ground and piped short distances to be used as a fuel source. Natural gas could provide a continuous source of energy for flames. These "eternal fires" were found in temples and also used as attractions for visitors. In the 1800s, natural gas began to be piped short distances as a light source. With the discovery of oil in the 1860s, natural gas was largely ignored as a fuel source. One of the early difficulties with natural gas was in transporting it from the source to other sites for use. The combination of electric lights and petroleum meant that containers of natural gas were used as heat sources for cooking in homes but for little else.

As the technology for piping gas from the source began to improve, it became possible to pipe natural gas over thousands of miles. This has meant that natural gas has become as convenient as petroleum and coal to use as a fuel source, and often with far less pollution. Natural gas burns with almost no byproducts except for carbon dioxide and water (as opposed to coal which often has large amounts of sulfur in it), and the heat released from the reaction (combustion of any of the hydrocarbon components of natural gas is an exothermic process). The combustion of methane, the most prevalent component of natural gas, is described by the reaction below:

CH₄ + O₂ → CO₂ + H₂O + heat energy

Ethane is used less as a fuel source than as a starting material for the production of ethylene (acetylene), which is used in welding.

Both butane and propane are relatively easy to liquefy and store. Liquefied propane and butane are used in disposable lighters and as camping fuels.

Because gases take up large amounts of space, they can be inconvenient to transport and store. The ability to liquefy the components of natural gas (either as a mixture or in isolation) has made natural gas much more practical as an energy source. The liquefaction of natural gas takes advantage of the different boiling points of methane, ethane, and other gases as a way of purifying each substance. A combination of refrigeration and increased pressure allows the individual gases to be stored and transported conveniently. At one time, the natural gas that often accompanied petroleum in the ground was simply burned off as a means of getting rid of it. Recently, however, this gas has been collected, liquefied and used along with the petroleum.

See also Fuels and fuel chemistry; Petroleum extraction