Natural or marsh gas is a product of the decay of organic material. It is usually found in petroleum fields, but also occurs anywhere that organic material is left to decay, such as landfill sites and swamps, even during digestion in animals (see flatulence).
The primary component of natural gas is methane, the shortest and lightest hydrocarbon molecule. Due to the heat and attack by the active species, the methane reacts to a methyl radical (CH3), which reacts to formaldehyde (HCHO or H2CO). The formaldehyde reacts to a formal radical (HCO), which then forms carbon monoxide (CO). The process is called oxidative pyrolysis:
- CH4 + O2 CO + H2 + H2O
- H2 + ½ O2 H2O
- CO + ½ O2 CO2
The major difficulty in the use of natural gas is transport. Natural gas pipelines are the preferred means of transport, but this is impractical across oceans. Liquefied natural gas tankers have also been used, but there are some concerns about safety and economics. In many cases, as with oil fields in Saudi Arabia, the natural gas which is recovered in the course of recovering petroleum cannot be profitably sold, and is simply burned at the oil field (known as flaring). This wasteful practice is now illegal in many countries, especially since it adds greenhouse gas pollution to the atmosphere, and since a profitable method may be found in the future. The gas is instead re-injected back into the ground for possible later recovery, and to assist oil pumping by keeping underground pressures higher.
In any form, a strong bad scent is deliberately added to the otherwise colorless and odorless gas, so that leaks can be detected by the smell before an explosion occurs. In mines, sensors are used instead, replacing the previous use of the canary.