A very unconventional hydrocarbon play: The Mesoproterozoic Velkerri Formation of Northern Australia

Collins, Alan S.1, Cox, Grant M.1, Jarrett, Amber J.M.2, Blades, Morgan L.1, Shannon, April, V.1, Yang, Bo.1, Farkas, Juraj1, Hall, P. Tony1, O’Hara, Brendan3, Close, David3, Baruch, Elizabeth, T.4, Altmann, Carl4, Evans, David5, Bruce, Alex5

1Tectonics and Earth Systems Group, The University Of Adelaide, Adelaide, Australia, 2MinEx CRC, , , 3Geoscience Australia, Canberra, Australia, 4Santos Limited, Adelaide, Australia, 5Origin Energy Ltd, Brisbane, Australia, 6Empire Energy, Sydney, Australia

The ca. 1.5–1.3 Ga Roper Group of the greater McArthur Basin is a component of one of the most extensive Precambrian hydrocarbon‐bearing basins preserved in the geological record, recently assessed as containing 429 million barrels of oil and eight trillion cubic feet of gas (in place). It was deposited in an intra‐cratonic sea, referred to here as the McArthur‐Yanliao Gulf.

The Velkerri Formation forms the major deep‐water facies of the Roper Group. Trace metal redox proxies from this formation indicate that it was deposited in stratified waters, in which a shallow oxic layer overlay suboxic to anoxic waters. These deep waters became episodically euxinic during periods of high organic carbon export. The Velkerri Formation has organic carbon contents that reach ~10 wt%. Variations in organic carbon isotopes are consistent with organic carbon enrichment being associated with increases in primary productivity and export, rather than flooding surfaces or variations in mineralogy.

Although deposition of the Velkerri Formation in an intracontinental setting has been well established, recent global reconstructions show a broader mid to low latitude gulf, with deposition of the Velkerri Formation being coeval with the widespread deposition of organic rich rocks across northern Australia and North China. The deposition of these organic‐rich rocks may have been accompanied by significant oxygenation associated with such widespread organic carbon burial during the Mesoproterozoic.


Alan Collins is a tectonic geologist with wide ranging interests in basin analysis and plate tectonic controls on the development of the earth system

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