Swamps, cows and gas wells have one thing in common – they all produce methane.
Methane is a greenhouse gas with 21 times the effect of carbon dioxide. So it is important to understand how much each of these sources produces and the relative comparisons.
Global methane sources generate around 500 – 600 million tonnes of CO2-equivalent methane every year.
Emissions from natural (or biogenic) sources account for more than 70% of the global total.
These sources include wetlands, rice agriculture, livestock, landfills, forests, oceans and termites. Wetlands are the largest source of methane at around 30% of all methane sources.
Cows produce 16% of the methane, which they generate when digesting grasses. In any given year a cow produces about 1,680 Kg CO2-equivalent of methane,  And there a lot of cattle!
The other (non-biogenic) 30% of all methane sources includes emissions from fossil fuels, biomass burning, waste treatment and geological sources.
The petroleum industry produces about 11% of total methane emissions.
CSIRO recently released a study that had been completed in November 2016. It measured methane emissions at nine well completions and one well workover at two CSG sites in Queensland.
The measurements found total methane emissions from well completions were low, ranging from virtually zero to a maximum of 373kg CH4 for the entire completion. No further emissions were detected on completed wells after they had been fitted with the wellhead. 
This follows a 2014 study in which CSIRO made direct measurements of 43 individual coal seam gas wells in Australia .
Its researchers measured a median methane emission for a well of 0.6 g/min, which is about the same as four cows. These measured emission rates are very much lower than those that have been reported for US unconventional gas production.
So as more science is applied, and direct measurements are made of methane measurements, a picture emerges as to where the gas industry fits relative to other sources such as wetlands and cows.