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

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.