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 δ13Cneo-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 δ13Cneo-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. The carbon isotopic composition of neo-pentane ranges from δ13C −50.8 to −24.4‰ VPDB and is typically isotopically more depleted in 13C (up to 9.7‰) than iso-pentane and n-pentane. This wide carbon isotopic variability between the pentane isomers most likely results from the interplay between decomposition, isomerisation and methylation mechanisms for neo-pentane formation. In biodegraded gases the iso-pentane and n-pentane become progressively enriched in 13C, resulting in much larger isotopic differences (Δδ13Cneo-pentane–iso-pentane values of up to −16.6‰). In this paper we show how the carbon isotopic composition of neo-pentane is related to source rock type and age, and demonstrate the usefulness of neo-pentane in gas–gas correlation, particularly in biodegraded accumulations.