Revaluation of Carbon-Isotope Compositions of Natural Methanes

A series of long-term thermal cracking experiments at 400°C on n-octadecane and a comparison with previous experiments at higher temperatures established that temperature effects for 400°C and above are unimportant relative to the nature of the bonds being broken in determining the carbon-isotope composition of methane. The isotopic composition and experimental production-rate data suggest that, in nature, most methane is produced at temperatures higher than 100°C.