Australia, Northern TerritoryVelkerri Formation, Beetaloo Sub-basin

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By Claudio Delle Piane, CSIRO Energy, Australia

The Beetaloo Sub-basin is a concealed, composite depocenter and a component of a group of Paleoproterozoic to Mesoproterozoic sedimentary basins collectively described as the Greater McArthur Basin, an intra-cratonic basin developed on crystalline basement rocks of the North Australian Craton (Fig. 1). Sedimentary infill is interpreted to be as thick as 9000 m mostly constituted of siliciclastic sedimentary and minor volcanic rocks of the Palaeoproterozoic Tawallah and McArthur groups, and the Mesoproterozoic Nathan and Roper groups (Fig. 2).

The limits of the Beetaloo Sub-basin have been recently updated and formalized by the Northern Territory Geological Survey and have been constrained using lithostratigraphic data from 26 wells tied to stratigraphic interpretations of 96 seismic surveys. Accordingly, the sub‑basin boundary is currently defined using the top of the Kyalla Formation (Roper Group), constrained by a cut-off depth of 400 m below surface[1]. It is important to note that the current geophysical definition does not correspond to the extent of sediments of interest for hydrocarbon exploration and hydrocarbon plays have been identified beyond the extents of the geophysically defined boundary[2].

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[3][4]. 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[5], 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[5].

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[6]. 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[7].

Biomarkers studies indicate that the depositional organic matter is derived from biomass dominated by bacteria with minor input from archaea and eukaryotes[8]. 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[9][10][11]. Organic geochemistry laboratory results summarized by Hall et al.[11] 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[11].

Mineralogical analyses, geo-mechanical laboratory tests and down-hole fracture tests conducted so far indicate that the organic rich mudstones of the Amungee Member have good reservoir and completion qualities that should result in brittle behavior conductive to fracture propagation[12].

Hydrocarbon exploration in the McArthur Basin began in the 1980s targeting conventional oil accumulations in structural and stratigraphic traps. Although several shows were identified, no commercial discoveries were made. Following the unconventional oil/gas success in North America, exploration in the Beetaloo Sub-basin has focused on unconventional hydrocarbon resources in the Mesoproterozoic Roper Group. More than 30 petroleum wells have been drilled in the Beetaloo Sub-basin and surrounding areas of the greater McArthur Basin (Fig. 1) and approximately half of these wells have been drilled in the last ten years.

The first multi-stage hydraulic fracture stimulation in the Beetaloo was completed in 2016 on the Amungee NW-1H horizontal well. This operation represents the longest ‘plug and perf’ type horizontal completion in Australia[7]. Production test run for 57 days, producing a total of 66 TJ of dry gas (CH4 >90% with CO2 ~3%) at an average rate of 1.15 TJ/d, with a final rate of 1.12 TJ/d[7].

Further activity in the basin was delayed due to a moratorium on exploration in the Northern Territory and an independent inquiry into hydraulic fracturing that resulted in a number of recommendations to mitigate environmental, social and economic risks and impacts of hydraulic fracturing . The moratorium was lifted in 2018 and subsequently more vertical wells targeting the Velkerri shales have been drilled and tested. The vertical Tanumbirini-1 well has been tested for 130 days with resulting gas flow exceeding 1.2 million cubic feet per day and settling at 400 thousand cubic feet per day with minimal decline . At the time of writing vertical well Carpentaria 1 is undergoing fracture stimulation to test the liquid-rich potential of several intervals within the Velkerri Formation and flowing gas to the surface at encouraging rates . Additionally, in May 2021 drilling of horizontal well Tanumbirini-2H has begun, targeting a total depth of 4,800 m and hydraulic fracture-stimulation of the Velkerri Formation with 10-20 stages.

At the time of writing the Beetaloo Sub-basin is considered Australia’s most promising shale gas opportunity and falls under the Strategic Basin Plans Program announced in the 2020-21 Australian Federal Budget to accelerates its development and industry investment . According to Geoscience Australia, shale gas resources in the Beetaloo Sub-basin amount to 178,200 PJ suggesting it is a world-class resource comparable to several of the major US shale gas basins. There is active exploration for petroleum resources in the Beetaloo Sub-basin, and key holders of exploration permits in the area are Santos and Tamboran JV, Origin Energy and Falcon Oil Gas JV and Empire Energy.

References

  1. ↑ Williams, B., 2019, Definition of the Beetaloo Sub basin: Northern Territory Geological Survey, Record 2019 01
  2. ↑ Bruce, A., and D. Garrad, 2021, Unlocking the eastern extension of the Beetaloo Sub-basin middle Velkerri shales: Annual Geoscience Exploration Seminar (AGES) Proceedings, Alice Springs, Northern Territory 20-21 April 2021, p. 37.
  3. ↑ 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.
  4. ↑ 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.
  5. ↑ 5.0 5.1 Munson, T. J., 2016, Sedimentary characterisation of the Wilton package, greater McArthur Basin, Northern Territory: Northern Territory Geological Survey, Record 2016-003.
  6. ↑ 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.
  7. ↑ 7.0 7.1 7.2 Close, D., A. Côté, E. Baruch, C. Altmann, F. Mohinudeen, B. Richards, R. Ilett, R. Evans, and S. Stonier, 2017, Exploring the Beetaloo: will Australia’s first viable shale play be sourced by billion year old gas?: The APPEA Journal, vol. 57, no. 2, p. 716-721.
  8. ↑ Jarrett, A. J., G. M. Cox, J. J. Brocks, E. Grosjean, C. J. Boreham, and D. S. Edwards, 2019, Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia: Geobiology, vol. 17, no. 4, p. 360-380.
  9. ↑ Faiz, M., C. Altmann, M. Dunne, E. Baruch, D. Close, A. Cote, B. Richards, and P. Ranasinghe, 2016, Precambrian organic matter and thermal maturity of the Beetaloo Basin, Northern Territory, Australia: Australian Earth Sciences Convention, Adelaide, South Australia, p. 26-30.
  10. ↑ Delle Piane, C., I. T. Uysal, M. Faiz, Z. Pan, J. Bourdet, Z. Li, M. D. Raven, and D. N. Dewhurst, 2020, Thermal maturity and reservoir quality of the Velkerri Formation, Beetaloo Sub-basin, Northern Territory: The APPEA Journal, vol. 60, no. 2, p. 697-702.
  11. ↑ 11.0 11.1 11.2 Hall L. S., L. Wang, A. H. E. Bailey, M. L. Orr, R. Owens, A. J. M. Jarrett, M. E. Lech, N. Skeers, B. Reese, and M. Woods, 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.
  12. ↑ Santos Ltd., 2017, Submission 168 to the Scientific Inquiry into Hydraulic Fracturing in the Northern Territory.

Other sources

  • Bodorkos, S., Crowley, J.L., Claoué-Long, J.C., Anderson, J.R. and Magee Jr, C.W., 2020. Precise U–Pb baddeleyite dating of the Derim Derim Dolerite, McArthur Basin, Northern Territory: old and new SHRIMP and ID-TIMS constraints. Australian Journal of Earth Sciences, pp.1-15.

See also

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