What is natural gas?
Natural gas is a naturally occurring hydrocarbon gas that is found in underground deposits, sometimes on its own but more often along with petroleum or coal deposits. Its composition varies
considerably from place to place but its main constituent is methane (CH4). Methane is the lowest density hydrocarbon (55% of the density of air) and has the highest heat of combustion (55.6
kJ/g). It also has the lowest CO2 to energy output (56.9 g/MJ) of all fuels other than pure hydrogen.
Origin of natural gas
Natural gas, along with oil and coal, is formed by biological and thermal action on the organic remains of plants and animals, especially microorganisms. The two processes involved ― biogenic and thermogenic ― may take millions of years. The biogenic process takes place in shallow anaerobic conditions that are relatively cool (< 50°C). Bacteria convert the complex organic matter
into a mixture of the lighter hydrocarbons, mainly methane, along with small amounts of ethane, propane and butane. The gas can seep out of the ground or sediment and may be seen to bubble to the surface in marshy areas. (The gases released in this process can sometimes ignite spontaneously producing a flickering light known as the “will o’ the wisp”.)
The thermogenic process takes place between 160°C and 220°C and under immense pressures ― tens or even hundreds of times atmospheric pressure. The hydrocarbons formed in these
conditions range from gases to oils and tars.
The gases and oils formed in this way may be squeezed out of the rock layers and escape to the surface. Sometimes they are prevented from escaping by impervious layers (or ‘strata’) of rock, especially if the layers have been folded by geological processes to form a dome-shaped ‘trap’ or reservoir.