Recognising biodegradation in gas/oil accumulations through the dC compositions of gas components

A large suite of natural gases (93) from the North West Shelf and Gippsland and Otway Basins in Australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases. About 26% of these gases have anomalous stable carbon isotope compositions in the C1±C4 hydrocarbons and CO2 components, and are interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations between successive n-alkane homologues (up to +29% PDB) and isotopically heavy CO2 (up to +19.5% PDB). Irrespective of geographic location, these anomalous gases are from the shallower accumulations (600±1700 m) where temperatures are lower than 75 C. The secondary biogenic gases are readily distinguishable from thermogenic gases (74% of this sample suite), which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m. While dissolution e€ects may have contributed to the high C enrichment of the CO2 component in the secondary biogenic gases, the primary signature of this CO2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis. Correlation between biodegraded oils and biodegraded ``dry'' gas supports the concept that gas is formed from the bacterial destruction of oil, resulting in ``secondary biogenic gas''. Furthermore, the prominence of methanogenic CO2 in these types of accumulations along with some isotopically-depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked. It is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis. # 2000 Elsevier Science Ltd. All rights reserved.