Abundance and carbon isotopic composition of neo-pentane in Australian natural gases

Here we report the abundance and carbon isotopic composition of neo-pentane (or 2,2-dimethylpropane) in Australian natural gases derived from source rocks of Late Proterozoic to Early Tertiary age. In Australian natural gases, neo-pentane abundance is low, ranging from 6 to 295 ppm. The delta C-13(neo-pentane) shows a strong source control and, together with the other gaseous hydrocarbons, a maturity effect, albeit less pronounced than the source effect. In severely biodegraded natural gases, neo-pentane abundance is similar to that in non-biodegraded gases as a result of a counter balance between preservation of neo-pentane relative to other wet gases and the addition of biogenic methane. Nevertheless, neo-pentane can become the most abundant pentane isomer with neo-pentane(iso- + n-pentane) ratios up to 9.9. Here, the delta C-13(neo-pentane) is identical between biodegraded and non-biodegraded natural gas from the same source, providing strong evidence that neo-pentane is resistant to biodegradation, even within severely biodegraded gases where the carbon isotopic composition of all the other gaseous hydrocarbons has been altered.