Yes, I know it's not technically just methane, but methane is its primary constituent.
"Fossil gas" is also probably a suitable term, but I like "fossil methane" because it gets that spooky chemical name benefit.
If you look at global warming potential over a 100 year tome period, methane emission is 36 times that of an equivalent mass of CO2. Methane comes out of the atmosphere by reacting with hydroxyl radicals, oxidising to form CO2. 88% of the methane reacts this way, meaning that one gram of methane will form 2.4 grams of CO2.