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

Abstract

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.

Low‐temperature thermal history of the McArthur Basin: Influence of the Cambrian Kalkarindji Large Igneous Province on hydrocarbon maturation

“…The Palaeoproterozoic–Mesoproterozoic McArthur Basin in northern Australia has been the subject of considerable study into syn‐depositional basin processes and conditions, from which a confident understanding of the prolonged sedimentary system is emerging (see Cox et al, 2022, for an overview). Overlying Palaeoproterozoic basement of the North Australian Craton, the McArthur Basin is an unmetamorphosed and largely undeformed sequence of sedimentary and volcanic rocks deposited over a period of more than 500 million years (e.g., Ahmad & Munson, 2013; Rawlings, 1999; Yang, Collins, Blades, et al, 2020).…”

Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and its subsequent applications

“…Lithologically, the Roper Group comprises of a series of coarsening-up sequences dominated by marine mudstone and interbedded sandstone with minor successions of intraclastic limestone (Abbott and Sweet, 2000;Ahmad and Munson, 2013;Jackson et al, 1987; 130 Yang et al, 2018). Records of water-column euxinia and redox stratification as well as fluctuating salinity levels suggests that the Roper Group formed in an intermittently restricted marine basin within an epicontinental setting similar to the modern Black Sea, or Baltic Sea (Ahmad and Munson, 2013;Beyer et al, 2016;Cox et al, 2022;Cox et al, 2016;Mukherjee and Large, 2016;Revie and MacDonald, 2017;Yang et al, 2018).…”

“…In this study, we show how in situ Rb-Sr analysis of the Proterozoic Velkerri Formation in the Roper Group, McArthur Basin, northern Australia (Figure 1) dated clay-mineral recrystallisation at temperatures similar to those at 100 which kerogen catagenesis occurs. The Roper Group is a good case study for in situ Rb-Sr shale dating as it has been shown to be dominated by authigenic clays (Rafiei and Kennedy, 2019;Subarkah et al, 2021) and is chronologically well-constrained (Ahmad and Munson, 2013;Bodorkos et al, 2020;Cox et al, 2022;Kendall et al, 2009;Southgate et al, 2000;Subarkah et al, 2021;. Furthermore, the resurgence of interest in the resource potential of the organic-rich Velkerri Formation has also yielded a framework of palaeotemperature data 105 that aim to discern the maturation history of hydrocarbons within the unit (Ahmad and Munson, 2013;Capogreco, 2017;Cox et al, 2016;Crick et al, 1988;George and Ahmed, 2002;Jarrett et al, 2019;Lemiux, 2011;Revie, 2014;Summons et al, 1994;Taylor et al, 1994;Volk et al, 2005).…”

“…Here, we targeted the Velkerri Formation (Figure 1) from the thoroughly investigated well Altree 2 (Capogreco, 2017;Cox et al, 2022;Cox et al, 2016;George and Ahmed, 2002;Jarrett et al, 2019;Lemiux, 2011;Nguyen et al, 110 2019;Nixon et al, 2021;NTGS, 1989NTGS, , 2009NTGS, , 2010NTGS, , 2012Revie, 2014;Sander et al, 2018;Yang et al, 2018). We show that common hydrocarbon maturation proxies such as Tmax data from Rock-Eval pyrolysis and illite crystallinity can help define the temperature sensitivity of the Rb-Sr isotopic system in organic-rich shales.…”

“…In addition, the well terminated an intrusion of the Derim Derim Dolerite and also intersected lavas of the Kalkarindji LIP that directly overlie the Proterozoic sedimentary rocks. Importantly, this well has also been the focus of numerous geochronological, geochemical and geobiological investigations from academia, private explorers as well as the Northern Territory Geological Survey (NTGS) which provide important complementary 170 data to supplement this study (Bodorkos et al, 2020;Cox et al, 2022;Cox et al, 2016;Cox et al, 2019;George and Ahmed, 2002;Jarrett et al, 2019;Lemiux, 2011;Nguyen et al, 2019;Nixon et al, 2021;NTGS, 2009NTGS, , 2010NTGS, , 2012Sander et al, 2018;Warren et al, 1998;Yang et al, 2018).…”