The molecular mass of oxygen gas is 32 and the molecular mass of methane is 16. The methane will tend to rise and mix, leaving plenty of oxygen near the surface. It's unlikely to asphyxiate humans unless it happens really, really fast.
People and animals have been asphyxiated by massive CO2 releases because they can happen very quickly, and CO2 has a molecular mass of 44, making it stay near the ground for longer, displacing lighter oxygen and nitrogen gases.