What is unconventional gas
Hydrocarbon resources can be classed as conventional or unconventional.  Unconventional refers oil or gas resource that until recently have not been able to be extracted using conventional recovery technologies, either due to the lack of technology or were not economic to produce.  Improvements of geological and geochemical exploration, and drilling and completion technologies have opened up vast new resources that have been overlooked in the past.  They can be charcterised by the reservoir type, the technologies required or the environment it is found in.  Examples include tight oil and gas formations, coalbed methane, shale gas, heavy oil, oil shale, deepwater plays and gas hydrates.  CBM and shale gas are classed as unconventional gas.

Coal Bed Methane
Coal is created by the decay of organic matter over time. As the heat and underground pressure builds - and the coal begins to form - methane is gradually adsorbed into the surface of the coal. The formation of coal over time from the gradual breakdown of organic matter creates methane, which in turn finds itself trapped under increasing depths of burial.  Unlike conventional natural gas reservoirs found in rock such as sandstone, the methane is not held in a void of space but instead clings onto the micropores of the surface of the coal. 

The jagged coal surface is made up of what are known as 'cleats' and it is through this uneven surface that natural gas circulates.  The pressure of the surround water and rock - known as hydrostatic and lithostatic pressure - holds the gas in place.  By releasing the pressure in the coal seams, the methane is separated from the coal, which makes for possible capture, treatment and use as a source of energy.