Characterisation of fluid inclusion oil in a Mid-Proterozoic sandstone and dolerite (Roper Superbasin, Australia)

Detailed geochemical analysis has been carried out on fluid inclusion oils trapped for over 1 billion years in the Roper Superbasin (northern Australia) in two different geological settings: in healed microfractures in a quartz arenite, and within partially altered plagioclase laths in a dolerite sill. The oil inclusions and co-occurring solid bitumens record multiple oil migration events postdating an intrusive episode (ca. 1300 Ma), most likely during Neoproterozoic basin inversion. Solid bitumen represents the oldest migration event, and occurs in secondary pore space within the Bessie Creek Sandstone and in veins cross-cutting the dolerite sill. It is interpreted to represent thermally altered oil that may have at least partly been generated by contact metamorphism near the igneous intrusion. Inclusion oils in both the dolerite feldspar and in the transgranular fractures in the Bessie Creek Sandstone are pristine, nonbiodegraded fluids with a great similarity to other bitumens, oils and sediment extracts from the Roper Superbasin. Biomarkers indicate that the oils were mainly sourced from prokaryotic cyanobacterial organic matter, with a small contribution from eukaryotes. Based on migration distance, organic richness, geochemistry and similarity of source rock maturity, the Velkerri Formation, which directly overlies the dolerite sill and the Bessie Creek Sandstone, is the most likely source rock for the higher molecular weight hydrocarbons preserved in the fluid inclusion oils. Geochemical data indicate that the oil inclusions in clusters in albitised zones of labradorite crystals in the dolerite matrix represent a different migration event from oil inclusions in transcrystalline fractures in vein calcite in the dolerite sill and in transgranular fractures in quartz in the Bessie Creek Sandstone. The Bessie Creek Sandstone fluid inclusion oil was generated in the peak to late oil window (∼0.9-1.0% VRE). The higher molecular weight (C₁₃₊) proportion of the dolerite feldspar fluid inclusion oil is of lower maturity (∼0.8% VRE) than the same molecular weigh range fluid in the Bessie Creek Sandstone fluid inclusion oil. Both of these oils were most likely generated in the overlying Velkerri Formation, however, the volumetrically dominant low molecular weight hydrocarbons in the dolerite feldspar fluid inclusion oils have a higher maturity (∼1.3% VRE), and may have been generated from the deeper, ca. 1600 Ma Barney Creek Formation in the underlying McArthur Group. This study shows that oil trapped in fluid inclusions has a greater potential to preserve pristine organic geochemical information than porosity-bound oils and dispersed kerogen. If oil inclusions occur in sufficient abundance the analysis of these tiny oil encapsulations is technically feasible, and can give valuable information on the composition of the Earth's early biosphere and Precambrian petroleum systems. Crown