Precambrian inclusion oils in the Roper Group: a review

Solid bitumen and oil inclusions in the Mesoproterozoic Roper Group of the Roper Superbasin indicate extensive oil migration during the Mesoproterozoic. Although most samples contain abundant bitumen, but very few oil-bearing fluid inclusions, one sample of the Bessie Creek Sandstone and one sample of a dolerite sill were found to contain sufficient oil-bearing fluid inclusions to warrant detailed microthermometric and molecular compositional analyses. In the Bessie Creek Sandstone, the oil inclusions are located within extensive transgranular fractures, which cut pervasive quartz overgrowths and a contact metamorphic fabric caused by the ca 1280 Ma intrusion event. Consequently, oil migration is interpreted to have taken place when the sandstone was well cemented, following extensive diagenesis and dolerite emplacement. Microthermometry of coeval oil and brine inclusions indicates 60 ± 5°C as the minimum temperature of entrapment. Oil-bearing fluid inclusions in the dolerite sill occur as dense clusters within albitised zones of labradorite crystals and in transcrystalline fractures cutting vein calcite and rarely vein quartz. Microthermometry of coeval oil-bearing and aqueous fluid inclusions, combined with textural evidence, indicates that the oil was trapped at about 110°C, after the dolerite had cooled significantly towards ambient diagenetic temperatures. Because the location, association, petrography and microthermometric behaviour of these inclusions are strikingly similar to those described for the underlying Bessie Creek Sandstone, they are interpreted to represent the same oil migration event. Gas chromatography-mass spectrometry (GC-MS) of the inclusion oils reveals non-biodegraded oils, which are very mature in the Bessie Creek Sandstone and are an apparently pre-trapping mixture of a high-maturity gas-condensate and a lower-maturity oil in the dolerite sill. Biomarkers (mainly hopanes, monomethylalkanes, alkylcyclohexanes and traces of steranes) suggest a source rock dominated by prokaryotic cyanobacterial organic matter, with a small contribution from eukaryotes in the case of the dolerite sill oil. Based on its organic richness and proximity, the most likely source for the oils is the organic-rich marine Velkerri Formation, deposited at ca 1430 Ma, with a possible component in the mixed oil from the slightly older ca 1600 Ma Barney Creek Formation of the underlying McArthur Group. Although we cannot fully rule out the possibility of migration while the group was buried beneath Palaeozoic cover, the most likely timing of migration is interpreted to be before significant uplift, during structural inversion between 1300–1000 Ma. The case studies illustrate how oil inclusions can provide unique and often pristine compositional data on extant hydrocarbon fluids, the relative timing of their entrapment and the conditions under which they were trapped.