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The sub‑basin is of considerable economic interest as a potential host for unconventional and conventional petroleum resources, particularly in the uppermost Roper Group where stacked play opportunities include liquids rich shale, dry gas shale and hybrid/ tight gas plays<ref>Côté, A., B. Richards, C. Altmann, E. Baruch, and D. Close, 2018, Australia’s premier shale basin: five plays, 1,000,000,000 years in the making: The APPEA Journal, vol. 58, no. 2, p. 799-804.</ref><ref>Altmann, C., B. Richards, A. Côté, C. Bein, E. Baruch-Jurado, and L. Jenkinson, 2020, The Hayfield Sandstone Play: the characterisation of a Mesoproterozoic sourced, Proterozoic sandstone reservoired, tight oil and gas play in the Beetaloo Sub-basin: The APPEA Journal, vol. 60, no. 1, p. 242-266.</ref>. Prospective units for gas and liquids resources have been identified in the organic rich [[mudstones]] of the Kyalla Formation and Velkerri Formation of the Mesoproterozoic Roper Group and the latter is the most technically mature resource play in the Beetaloo.

The sub‑basin is of considerable economic interest as a potential host for unconventional and conventional petroleum resources, particularly in the uppermost Roper Group where stacked play opportunities include liquids rich shale, dry gas shale and hybrid/ tight gas plays<ref>Côté, A., B. Richards, C. Altmann, E. Baruch, and D. Close, 2018, Australia’s premier shale basin: five plays, 1,000,000,000 years in the making: The APPEA Journal, vol. 58, no. 2, p. 799-804.</ref><ref>Altmann, C., B. Richards, A. Côté, C. Bein, E. Baruch-Jurado, and L. Jenkinson, 2020, The Hayfield Sandstone Play: the characterisation of a Mesoproterozoic sourced, Proterozoic sandstone reservoired, tight oil and gas play in the Beetaloo Sub-basin: The APPEA Journal, vol. 60, no. 1, p. 242-266.</ref>. Prospective units for gas and liquids resources have been identified in the organic rich [[mudstones]] of the Kyalla Formation and Velkerri Formation of the Mesoproterozoic Roper Group and the latter is the most technically mature resource play in the Beetaloo.

Despite its age, the Velkerri Formation is remarkably well preserved and relatively undisturbed, it is a fine-grained unit mostly constituted of siliceous mudstone, siltstone and minor sandstone lithologies. It is rarely exposed and most of its characterization is based on drill cores of various vintages. The nominated type section has been cored in drillhole Urapunga-4 from 372 m to 42 m of depth and the core is stored in the National Offshore Petroleum Data & Core Repository at Geoscience Australia in Canberra (Munson 2016), maximum known thickness is 1483 m reported in well Tanumbirini-1. Based on core facies association, presence of glauconite and acritarchs, the depositional environment of the formation is interpreted as marine below wave base, in a distal outer shelf or slope setting (Munson 2016).

Despite its age, the Velkerri Formation is remarkably well preserved and relatively undisturbed, it is a fine-grained unit mostly constituted of siliceous mudstone, siltstone and minor sandstone lithologies. It is rarely exposed and most of its characterization is based on drill cores of various vintages. The nominated type section has been cored in drillhole Urapunga-4 from 372 m to 42 m of depth and the core is stored in the National Offshore Petroleum Data & Core Repository at Geoscience Australia in Canberra<ref name=Mnsn2016>Munson, T. J., 2016, Sedimentary characterisation of the Wilton package, greater McArthur Basin, Northern Territory: Northern Territory Geological Survey, Record 2016-003.</ref>, maximum known thickness is 1483 m reported in well Tanumbirini-1. Based on core facies association, presence of glauconite and acritarchs, the depositional environment of the formation is interpreted as marine below wave base, in a distal outer shelf or slope setting<ref name=Mnsn2016 />.

The Velkerri Formation is formally subdivided into three members: Kalala, Amungee and Wyworrie in ascending stratigraphic order, distinguished in the subsurface by their lithology, organic content, geochemistry, and petrophysical characteristics (Munson and Revie, 2018). The Amungee Member is the main target for hydrocarbon exploration within the Velkerri Formation, it represents the deepest and most distal depositional environment and contains the highest proportion of fine-grained sediments. It is further subdivided informally into three organic-rich shale units (A, B and C organofacies, in ascending stratigraphic order) separated by two clay-rich, organically lean intervals. The A-C organofacies are laterally continuous with strong, ubiquitous gas shows (Close et al. 2017).   

The Velkerri Formation is formally subdivided into three members: Kalala, Amungee and Wyworrie in ascending stratigraphic order, distinguished in the subsurface by their lithology, organic content, geochemistry, and petrophysical characteristics<ref>Munson, T. J., and D. Revie, 2018, Stratigraphic subdivision of the Velkerri Formation, Roper Group, McArthur Basin, Northern Territory: Northern Territory Geological Survey, Record 2018-006.</ref>. The Amungee Member is the main target for hydrocarbon exploration within the Velkerri Formation, it represents the deepest and most distal depositional environment and contains the highest proportion of fine-grained sediments. It is further subdivided informally into three organic-rich shale units (A, B and C organofacies, in ascending stratigraphic order) separated by two clay-rich, organically lean intervals. The A-C organofacies are laterally continuous with strong, ubiquitous gas shows (Close et al. 2017).   

Biomarkers studies indicate that the depositional organic matter is derived from biomass dominated by bacteria with minor input from archaea and eukaryotes (Jarrett et al., 2019). Total organic carbon (TOC) content of the Amungee Member varies across the basin: along the northern margin of the basin where the formation is shallowly buried, TOC can be as high as 20 wt% and thermal maturity indicators are consistent with early liquid hydrocarbon generation. In the deeper central parts of the basin TOC of the organic-rich intervals reduced to ca. 2-4 wt% and maturity indicators are consistent with dry gas generation (Faiz et al., 2016; Delle Piane et al., 2020; Hall et al., 2020). Organic geochemistry laboratory results summarized by Hall et al. (2020) indicate excellent source rock potential for the oil-prone organic-rich intervals of the Velkerri Formation with hydrogen index as high as 800 mg HC/g TOC. Peak hydrocarbon generation is estimated to occur at thermal maturity quantified by average equivalent vitrinite reflectance of 0.95 %EqVR (equivalent vitrinite reflectance) and onset of generation occurs at maturities between 0.44 %EqVR and 0.82 %EqVR (Hall et al., 2020).

Biomarkers studies indicate that the depositional organic matter is derived from biomass dominated by bacteria with minor input from archaea and eukaryotes (Jarrett et al., 2019). Total organic carbon (TOC) content of the Amungee Member varies across the basin: along the northern margin of the basin where the formation is shallowly buried, TOC can be as high as 20 wt% and thermal maturity indicators are consistent with early liquid hydrocarbon generation. In the deeper central parts of the basin TOC of the organic-rich intervals reduced to ca. 2-4 wt% and maturity indicators are consistent with dry gas generation (Faiz et al., 2016; Delle Piane et al., 2020; Hall et al., 2020). Organic geochemistry laboratory results summarized by Hall et al. (2020) indicate excellent source rock potential for the oil-prone organic-rich intervals of the Velkerri Formation with hydrogen index as high as 800 mg HC/g TOC. Peak hydrocarbon generation is estimated to occur at thermal maturity quantified by average equivalent vitrinite reflectance of 0.95 %EqVR (equivalent vitrinite reflectance) and onset of generation occurs at maturities between 0.44 %EqVR and 0.82 %EqVR (Hall et al., 2020).

* Hall L.S., Wang L., Bailey A.H.E., Orr M.L., Owens R., Jarrett A.J.M., Lech M.E., Skeers N., Reese, B. and Woods M. (2020) Petroleum prospectivity of the Beetaloo Sub-basin. Technical appendix for the Geological and Bioregional Assessment Program: Stage 2. Department of the Environment and Energy, Bureau of Meteorology, CSIRO and Geoscience Australia, Australia.

* Hall L.S., Wang L., Bailey A.H.E., Orr M.L., Owens R., Jarrett A.J.M., Lech M.E., Skeers N., Reese, B. and Woods M. (2020) Petroleum prospectivity of the Beetaloo Sub-basin. Technical appendix for the Geological and Bioregional Assessment Program: Stage 2. Department of the Environment and Energy, Bureau of Meteorology, CSIRO and Geoscience Australia, Australia.

* Jarrett, A.J., Cox, G.M., Brocks, J.J., Grosjean, E., Boreham, C.J. and Edwards, D.S., 2019. Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia. Geobiology, 17(4), pp.360-380.

* Jarrett, A.J., Cox, G.M., Brocks, J.J., Grosjean, E., Boreham, C.J. and Edwards, D.S., 2019. Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia. Geobiology, 17(4), pp.360-380.

 

 

* Santos Ltd (2017) Submission 168 to the Scientific Inquiry into Hydraulic Fracturing in the Northern Territory. Viewed 25 March 2019, https://frackinginquiry.nt.gov.au/submission-library.

* Santos Ltd (2017) Submission 168 to the Scientific Inquiry into Hydraulic Fracturing in the Northern Territory. Viewed 25 March 2019, https://frackinginquiry.nt.gov.au/submission-library.

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