Natural Gas is a mixture of gasses, but by far the biggest component is Methane, CH₄.

Methane is an odourless, colourless gas which is slightly lighter than air.  When it burns with plenty of air, it produces carbon dioxide and water:

CH₄ + 3O₂ → CO₂ + 2O₂

That is, it produces 1 molecule of carbon dioxide and 2 molecules of water.  This is one of the reasons that gas fired power stations are preferred to coal fired ones.  There is also less pollution with gas.  Natural gas is the cleanest burning of the fossil fuels.

Impurities

Although Methane is the main component of Natural Gas, it is not the only one.  Natural Gas contains a variable amount of the longer chain hydrocarbon molecules such as Ethane; C₂H₆, Propane; C₃H₈, and Butane; C₄H₁₀.   These are frequently removed and sold separately.

There can also be some carbon dioxide, Nitrogen and the noble gasses Helium, Neon, Argon and Xenon.  A tiny amount of Oxygen can be present.

Another impurity sometimes present is Hydrogen Sulphide; H₂S.  This gas is sometimes called rotten egg gas because of its smell.  If it is burned with the rest of the gas it would produce Sulphur Dioxide, SO₂ which is a major pollutant.  The Hydrogen sulphide is generally removed before the gas is used.  The Sulphur is a saleable by product.

Abundance

Natural gas is probably the most abundant of all the fossil fuels.  However, it varies enormously in the ease, and therefore the cost, of extraction.  The cheapest sources tend to be used up first so this fuel, like fossil fuel in general is likely to become progressively more expensive although it is likely to be part of out energy mix for the foreseeable future.

Sources

Energy Safe Australia:

http://www.esv.vic.gov.au/ForConsumers/OverviewofgasindustryinAustralia/NaturalGasinAustralia/tabid/296/Default.aspx

Map of Gas Fields and Pipelines in Australia

http://www.chemlink.com.au/gas.htm

Background

http://www.naturalgas.org/overview/background.asp