Geochemistry of oil in fluid inclusions in a middle Proterozoic igneous intrusion: implications for the source of hydrocarbons in crystalline rocks

Abstract

The ca. 1280 Ma dolerite sill within the Mesoproterozoic Roper Group in the Roper Superbasin, Australia, contains evidence for at least two episodes of hydrocarbon migration represented, respectively, by solid bitumen with a ketone-rich extract, and a mixture of a high maturity gas-condensate and a lower maturity oil within oil-bearing fluid inclusions. The ketone isomers are formed by flash pyrolysis of kerogen during the intrusion of the dolerite sill [Org. Geochem. 21 (1994) 829] and represent the first and oldest phase of hydrocarbon migration. The gas condensate and oil were subsequently trapped as a mixture within fluid inclusions at diagenetic temperatures and pressures of around 110 °C and 250 bars, significantly after cooling of the sill and likely during the Neoproterozoic reactivation of the Roper Superbasin. Either (1) these fluids migrated together and mixed in the reservoir or (2) an earlier oil charge was flushed by a later condensate charge and the oil-condensate mixture was trapped within single fluid inclusions. Oil inclusions occur chiefly within albitised zones of labradorite laths within the dolerite matrix, and within transcrystalline microfractures cutting vein calcite and rarely vein quartz. Oil inclusions trapped in the vein calcite are accompanied by hypersaline Ca/Mg brines. Gas chromatography–mass spectrometry of oil extracted from inclusions within the dolerite matrix shows that the oil is non-biodegraded and was therefore trapped relatively quickly within the host minerals. Trace amounts of biomarkers indicate that the inclusion oil is of a biogenic origin and excludes any abiotic processes that are apparent sources of hydrocarbons in many crystalline rocks. Monomethylalkanes, pentacyclic terpanes chiefly comprising hopanes and diahopanes, and very low concentrations of steranes and diasteranes indicate input from cyanobacterial organic matter with a minor contribution from eukaryotes. The hydrocarbons are likely derived from Proterozoic source rocks such as the directly overlying Velkerri Formation and/or the underlying Barney Creek Formation from the McArthur Group. The study has implications for the source of hydrocarbons in non-sedimentary rocks and suggests that fluid inclusions can be used not only to distinguish between biogenic and abiogenic sources of hydrocarbons but also to provide critical information about the composition of the biomass, fluid migration events, and conditions under which hydrocarbons in different geological settings are trapped.