Microbes in production of biogas

Gobar gas (biogas) is produced when methanogens such as $Methanobacterium$ act anaerobically on cattle dung.
Biogas is a mixture of gases: methane (about 50–70%), carbon dioxide (about 30–40%) and traces of $H_2$ and $H_2S$.
So the gas present in the largest amount is methane, which is also the component that burns.
Butane and propane are constituents of LPG, not of gobar gas.
Hence the maximum amount is that of methane.

During sewage treatment, the activated sludge is pumped into anaerobic sludge digesters.
Here anaerobic bacteria (methanogens) digest the bacteria and fungi of the sludge and produce a mixture of gases: methane, hydrogen sulphide and carbon dioxide (biogas).
Methane is the main component; it is also called marsh gas because it is produced in the same way in marshes.
Laughing gas is nitrous oxide ($N_2O$), propane is a petroleum gas and mustard gas is a synthetic chemical warfare agent; none is formed in this way.
Hence marsh gas is mainly produced.