Natural gas, like all fossil fuels, is the remains of prehistoric living matter that existed millions of years ago. In the marine environment, dead organisms came to rest on the ocean floor, where they were covered by layer after layer of mud or silt which over time became layers of rock. Buried beneath the sea bed, the hydrocarbons stored inside the once-living matter were preserved, but their composition was altered by the heat and pressure from the rocks above. Millions of years later, deposits of these hydrocarbons can be found either in liquid form as petroleum, or as natural gas.
Natural gas forms in porous layers of rock, with bubbles of gas trapped inside the rock. Gas fields occur when a layer of impervious rock, such as limestone, has formed above the layer of porous rock, preventing the gas from escaping. Deposits of natural gas are often found alongside petroleum deposits. Natural gas is colourless and odourless. It consists mostly of methane, but mixed in with the methane are likely to be other hydrocarbon compounds and a few impurities.
How do we Locate Natural Gas?
Geologists are able to identify locations where natural gas is likely to have formed. Seismic surveys and analysis of rock samples can help to establish whether or not gas is present; a well is drilled, and the gas, if it is there, will simply flow up from the rock into the well.
When natural gas is present in oil fields, it is extracted along with the petroleum and the gas is then separated off.
Composition and Combustion
One deposit of natural gas can differ greatly in composition from another. However, analysis of a sample of natural gas may well reveal up to a dozen different hydrocarbons. Methane will normally account for by far the largest part of the sample, typically between 70% and 90%. Ethane, propane and butane are also likely to be present. There may be small amounts of carbon dioxide, nitrogen, pentane and oxygen. Traces of rare gases are sometimes found.
Methane has a high calorific value, and it burns cleanly, releasing only carbon dioxide and water. Ethane, propane and butane also have a high calorific value. However, carbon dioxide and other impurities are undesirable as their presence will reduce the overall calorific value.
Advantages and Disadvantages of Natural Gas
The fact that natural gas is a clean fuel gives it an advantage over coal and petroleum, our other two main fossil fuels. The combustion of natural gas produces no particulate emissions: no smoke, no ash or residue and no smell. It burns to produce a hot, bright flame. In the past, gas was used to provide light in houses and also for municipal street lighting, but today it is used almost exclusively as a source of thermal energy.
Gas is a popular household energy choice in the UK for both central heating and cooking. Industrially, natural gas is used in manufacturing, and its importance to the electricity generating industry is growing, as the UK government is encouraging the use of natural gas in power stations in preference to other fossil fuels. Globally, it is estimated that natural gas currently accounts for around 20% of our total energy usage.
However, although from the end user’s point of view gas is a very convenient and clean-burning fuel, it does nonetheless affect the environment to some extent. Like all fossil fuels, it produces carbon dioxide, and if there are impurities in the gas it may also produce small amounts of other unwanted emissions. So although natural gas produces significantly lower levels of harmful emissions than other fossil fuels, it is not carbon neutral and therefore, from an environmental point of view, it is not an ideal energy source.
Another issue that applies to all fossil fuels is the effect that extracting them from the ground has on the environment. Inevitably, mining coal and drilling for oil and gas can disturb the ecological balance as well as causing visual pollution.
The Future of Natural Gas
Gas has many advantages as an energy source, but we cannot depend on fossil fuels as a source of energy indefinitely. One day, supplies will be exhausted, or will at least be depleted to the point where the expense of extracting them – and the energy used in doing so – will outweigh the value of the fuel we obtain. However, supplies of gas are believed to be less endangered than those of other fossil fuels. It has been estimated that there is sufficient gas in the gas fields that we know to exist, to last for between 150 and 200 years; but it is likely that there are many more gas fields yet to be discovered.