Sedimentology and carbon-sulphur geochemistry of the Velkerri Formation, a mid-Proterozoic potential oil source in northern Australia

The Velkerri Formation is a flat-lying, 400 m thick shale unit of mid-Proterozoic age that occurs in the McArthur Basin of northern Australia. It underlies an area approximately equivalent to that of the Netherlands and contains intervals tens of metres thick, highly rich in organic matter. TOC values are commonly 1 to 3% but exceed 5% at three levels: the maximum value measured was 8.7% TOC. The region has not been buried deeper than about 2500 m and the organic matter is mature; two of these intervals represent impressive oil source beds. Jackson et al. reported the discovery of “live” oil from the lower interval, thereby raising considerable interest in the petroleum potential of the sequence. Initial reconnaissance geochemical studies of material from one drillhole indicated deposition in a large lake or barred bay (Donnelly and Crick). Detailed facies analysis, however, indicates deposition in a variety of sub-environments on an epicratonic marine shelf. In most of these sub-environments the depositional interface was beyond the effects of normal wave and tidal current activity. The stratigraphic relationships between the Velkerri Formation and associated wave- and tide-deposited shoreface sandstones of the Bessie Creek and Moroak Sandstones, and the widespread presence of glauconite throughout the Velkerri Formation are incompatible with the lacustrine-barred bay setting suggested by Donnelly and Crick. The results of a subsequent systematic geochemical sampling program of a second drillhole, presented here, provide some support for a marine origin. Although C/S plots (Berner and Raiswell) have been successful in differentiating between marine and freshwater environments in Phanerozoic rocks their use in the Proterozoic should be approached with caution, and preferably only within the constraints of a full consideration of depositional and diagenetic influences on CSFe systematics. Degree of pyritiation of iron (DOP, Raiswell et al.) and C/Fe/S plots (Aller et al.), as used in this study, are additional tools suitable for classifying the depositional environment. Due to the limited regional information it is difficult to classify the Velkerri Formation into one of the published models for anoxic basin sediments. It shows some similarities to several European Mesozoic oil prone beds which developed during major marine transgressions (e.g. Hallam).