Methane is CH4, ie. one atom of carbon bonded to four atoms of hydrogen, so if burned perfectly with oxygen you’d get one molecule of carbon dioxide and two molecules of water vapour:
CH4 + 2O2 → CO2 + 2H2O
So even without leakage, burning methane still produces carbon dioxide. Methane is the simplest of the hydrocarbons. It has the highest hydrogen-to-carbon ratio of any hydrocarbon, but it still has carbon.
