The northern Lawn Hill Platform (nLHP) is considered an emerging region with less than 15 wells drilled to date. With renewed interest in unconventional gas, new exploration opportunities exist in this early Proterozoic region. Petroleum systems analysis is presented here to improve the understanding of burial history, source rock richness and maturity of the nLHP of the Isa Superbasin, far NW Queensland. A pseudo-3D geological model was built and calibrated, in combination with 1-D burial and thermal history modelling of Desert Creek 1 and Egilabria 1. These were combined with source rock characteristics (e.g., Rock Eval and kerogen kinetics) which helped assess the hydrocarbon generation potential by source rock, allowing a broader assessment of petroleum prospectivity of the nLHP. The study focussed on two potential source rocks; the Lawn 4 Sequence and the River Supersequence. Maturity modelling of the Lawn 4 Sequence at Desert Creek 1 and Egilabria 1 predicted equivalent vitrinite reflectance (EqVR) of over 1.2% and 2%, respectively. The River Supersequence was modelled as overmature at both wells. Combining these results with the pseudo-3D model and source rock characteristics demonstrates that the highest maturities are encountered in the deepest depocentres to the east and gradually decrease in maturity to the west, indicating some potential for wet gas. Modelling results show generation of varying amounts of gas and oil from each potential source rock. Overall, due to the age of the sediments, maximum depth of burial and high paleotemperatures, the most likely hydrocarbon phase is gas from primary generation and supplemented by secondary gas from oil cracking. In spite of high maturities, encouraging gas shows from the Egilabria prospect support continued exploration interest in this region for unconventional hydrocarbons.

This report highlights results of petroleum systems analysis undertaken on the northern Lawn Hill Platform area of the Isa Superbasin, specifically focusing on burial and thermal history modelling. A second report will highlight the results of the source rock analysis and maturity modelling.

The Geological and Bioregional Assessments (GBA) Program is a series of independent scientific studies undertaken by Geoscience Australia and the CSIRO, supported by the Bureau of Meteorology, and managed by the Department of Agriculture, Water and the Environment. The Program consists of three stages across three regions with potential to deliver gas to the East Coast Gas Market. Stage 1 was a rapid regional prioritisation conducted by Geoscience Australia, to identify those sedimentary basins with the greatest potential to deliver shale and/or tight gas to the East Coast Gas Market within the next five to ten years. This prioritisation process assessed 27 onshore eastern and northern Australian basins with shale and/or tight gas potential. Further screening reduced this to a shortlist of nine basins where exploration was underway. The shortlisted basins were ranked on a number of criteria. The Cooper Basin, the Beetaloo Sub-basin and the Isa Superbasin were selected for more detailed assessment. Stage 2 of the program involved establishing a baseline understanding of the identified regions. Geoscience Australia produced regional geological evaluations and conceptualisations that inform the assessment of shale and/or tight gas prospectivity, ground- and surface-water impacts, and hydraulic fracturing models. Geoscience Australia’s relative prospectivity assessments provide an indication of where viable petroleum plays are most likely to be present. These data indicate areal and stratigraphic constraints that support the program’s further work in Stage 3, on understanding likely development scenarios, impact assessments, and causal pathways. <b>Citation:</b> Hall Lisa S., Orr Meredith L., Lech Megan E., Lewis Steven, Bailey Adam H. E., Owens Ryan, Bradshaw Barry E., Bernardel George (2021) Geological and Bioregional Assessments: assessing the prospectivity for tight, shale and deep-coal resources in the Cooper Basin, Beetaloo Subbasin and Isa Superbasin. <i>The APPEA Journal</i><b> 61</b>, 477-484. https://doi.org/10.1071/AJ20035

The South Nicholson region, which includes the Paleoproterozoic Isa Superbasin, the Mesoproterozoic South Nicholson Group and overlying younger sediments, is sparsely explored and has recently come into increased focus as a result of the Australian Government’s Exploring for the Future program. Previous exploration has identified potential shale gas plays within the River and Lawn supersequences of the Isa Superbasin in northwest Queensland’s northern Lawn Hill Platform region. Understanding mineralogy is important for characterising shale reservoirs, as mechanical properties such as shale brittleness are influenced by mineral composition. Mineralogy can, therefore, be utilised as a proxy for mechanical properties that are crucial to minimising risks associated with exploring for and developing shale reservoirs. This study utilises three different methods for calculating brittleness; XRD mineralogy, XRF major element geochemistry, and geomechanical properties. Results indicate highly variable mineralogy within the analysed samples, demonstrating heterogeneity in shale brittleness throughout the studied supersequences. Brittleness calculated from XRD analysis ranges from ductile to brittle with zones of brittle shales present in all supersequences. Increasing quartz and decreasing clay content is the dominant control on shale brittleness in the studied samples. Correlation between XRF major element geochemistry and XRD mineralogy is demonstrated to be moderate to poor, with brittleness derived from XRF major element geochemistry observed to be significantly higher than brittleness derived from XRD mineralogy. Conversely, brittleness derived from geomechanical properties agrees closely with XRD mineralogy derived brittleness. Hence, XRF major element geochemistry data are not recommended in the South Nicholson region to calculate brittleness. Analysis of brittleness indices from this study, in combination with total organic carbon content drawn from regional geochemical analysis in the South Nicholson region, identifies potential shale gas target intervals in the River, Term, and Lawn supersequences. Data presented on correlated well sections highlights intervals of exploration interest within these supersequences, being those depths where high organic content, brittle rocks are identified. The rocks that meet this criteria are primarily constrained to the already known potential shale gas plays of the River and Lawn supersequences. Recent data from Geoscience Australia implies that these potential shale gas plays are likely to extend from the northern Lawn Hill Platform, where they have been primarily identified to date, underneath the South Nicholson Basin and into the Carrara Sub-basin, significantly increasing their lateral extent. <b>Citation:</b> A. H. E. Bailey, A. J. M. Jarrett, L. Wang, B. L. Reno, E. Tenthorey, C. Carson & P. Henson (2022) Shale brittleness within the Paleoproterozoic Isa Superbasin succession in the South Nicholson region, Northern Australia, <i>Australian Journal of Earth Sciences, </i>DOI: 10.1080/08120099.2022.2095029

Exploring for the Future (EFTF) is an Australian Government initiative focused on gathering new data and information about potential mineral, energy and groundwater resources across northern Australia. This area is generally under-explored and offers enormous potential for industry development, as it is advantageously located close to major global markets, infrastructure and hosts many prospective regions. In June 2020, the Hon Keith Pitt MP, Minister for Resources, Water and Northern Australia, announced a four year extension to this program with an expansion in scope to cover the whole of Australia. The energy component of EFTF aims to improve our understanding of the petroleum potential of frontier Australian basins. Building an understanding of geomechanical rock properties is key to understanding both conventional and unconventional petroleum systems as well as carbon storage and sedimentary geothermal systems. Under EFTF, Geoscience Australia has undertaken geomechanical work including stress modelling, shale brittleness studies, and the acquisition of new rock property data through extensive testing on samples from the Paleo- to Mesoproterozoic South Nicholson region of Queensland and the Northern Territory and the Paleozoic Kidson Sub-basin of Western Australia. These analyses are summarised herein. Providing baseline geomechanical data in frontier basins is essential as legacy data coverage can often be inadequate for making investment decisions, particularly where unconventional plays are a primary exploration target. As EFTF increases in scope, Geoscience Australia anticipates expanding these studies to encompass further underexplored regions throughout Australia, lowering the barrier to entry and encouraging greenfield exploration. <b>Citation:</b> Bailey Adam H. E., Jarrett Amber J. M., Wang Liuqi, Dewhurst David N., Esteban Lionel, Kager Shane, Monmusson Ludwig, Carr Lidena K., Henson Paul A. (2021) Exploring for the Future geomechanics: breaking down barriers to exploration. <i>The APPEA Journal </i><b>61</b>, 579-587. https://doi.org/10.1071/AJ20039

<p>The Isa Superbasin is a Paleoproterozoic to Mesoproterozoic succession (approximately 1670-1575 Ma), primarily described in north-west Queensland. Despite the basin’s frontier status, recent exploration in the northern Lawn Hill Platform has demonstrated shale gas potential in the Lawn and River supersequences. Here, we characterise the unconventional reservoir properties of these supersequences, providing new insights into regional shale gas prospectivity. <p>The depths, thicknesses and mappable extents of the Lawn and River supersequences are based on the 3D geological model of Bradshaw et al. (2018). Source rock net thickness, total organic carbon (TOC), kerogen type and maturity are characterised based on new and existing Rock-Eval and organic petrology data, integrated with petroleum systems modelling. Petrophysical properties, including porosity, permeability and gas saturation, are evaluated based on well logs. Mineralogy is used to calculate brittleness (see also Jarrett et al., 2019, this volume). Regional stress and pressure regimes are also reviewed. <p>Abundant source rocks are present in the Isa Superbasin succession. Overall, shale rock characteristics were found to be favourable for both sequences assessed; both the Lawn and River supersequences host thick, extensive, and organically rich source rocks with up to 7.1 wt% total organic carbon (TOC) in the Lawn Supersequence and up to 11.3 wt% TOC in the River Supersequence. Net shale thicknesses demonstrate an abundance of potential shale gas reservoir units across the Lawn Hill Platform. <p>With average brittleness indices of greater than 0.5, both the Lawn and River supersequences are interpreted as likely to be favourable for fracture stimulation. As-received total gas content from air-dried samples is favourable, with average values of 0.909 scc/g for the Lawn Supersequence and 1.143 scc/g for the River Supersequence <p>The stress regime in the Isa Superbasin and the surrounding region is poorly defined; however, it is likely dominated by strike-slip faulting. Modelling demonstrates limited stress variations based on both lithology and the thickness of the overlying Phanerozoic basins, resulting in likely inter- and intra-formational controls over fracture propagation. No evidence of overpressure has been observed to date, however, it is possible that overpressures may exist deeper in the basin where less permeable sediments exist. <p>This review of the shale reservoir properties of the Lawn and River supersequences of the Isa Superbasin significantly improves our understanding of the distribution of potentially prospective shale gas plays across the Lawn Hill Platform and more broadly across this region of northern Australia.

Shale gas plays require technology such as fracture stimulation to increase rock permeability and achieve commercial rates of flow. The brittleness of shales are a major control on the ease of fracture stimulation. The Brittleness Index (BI) is a proxy for rock strength, based on geomechanical parameters, and/or rock mineralogy, and provides an indication of hydraulic stimulation effectiveness. Legacy drill core does not always have the geophysical logs needed for assessment of shale brittleness, therefore mineralogical and geochemical derived proxies for shale brittlenesss are often used with varying success. Shales from the Paleoproterozoic Lawn Hill Platform of north-west Queensland and the Northern Territory are known to contain organic-rich sedimentary units with the potential to host shale-gas plays. The Egilabria 2 DW1 well demonstrated a technical success in flowing gas from the Lawn Supersequence and recent geomechanical logging in the Egilabria prospect have demonstrated the presence of brittle rocks favourable for fracture stimulation with similarities between logged geophysics and X-Ray Diffraction (XRD) derived brittleness (Bailey et al., 2019). In this study we assess the brittleness of shales from twelve wells across the Isa Superbasin that do not have geophysical log data using XRD, major elemental analyses from X-Ray Fluorescence (XRF) and mineral carbon (MinC) techniques. The results of this study demonstrate highly variable mineral components between all analysed supersequences. The brittleness index (BI) of the shales derived from XRD ranges from ductile to brittle with zones of brittle shales present in all supersequences. Shale brittleness is controlled by increasing quartz and decreasing clay content with little influence from carbonates. Major elements based on XRF showed moderate to poor correlations to mineral type and the brittleness index was significantly higher than the results calculated by XRD. XRF analyses are not recommended as a proxy for shale brittleness in the Isa Superbasin region. The MinC parameter was highly effective as a proxy for carbonate based on comparisons with XRD derived carbonate concentrations. An important finding in this study is the variations in BI and TOC between supersequences, and within supersequences. This is evident down-hole and spatially across the area sampled. Future work is needed to investigate sweet spots for shale gas production.

Seismic reflection mapping, geochemical analyses and petroleum systems modelling have increased our understanding of the highly prospective Mesoproterozoic and Paleoproterozoic source rocks across northern Australia, expanding the repertoire of exploration targets currently being exploited in Proterozoic petroleum systems. Data collected during the Exploring for the Future program have enabled us to redefine and increase the extent of regional petroleum systems, which will encourage additional interest and exploration activity in frontier regions. Here, we present a review of the Paleoproterozoic McArthur and Mesoproterozoic Urapungan petroleum supersystems, and the most up-to-date interpretation of burial and thermal history modelling in the greater McArthur Basin (including the Beetaloo Sub-basin), South Nicholson Basin and Isa Superbasin. We also present potential direct hydrocarbon indicators imaged in the 2017 South Nicholson Deep Crustal Seismic Survey that increase the attractiveness of this frontier region for hydrocarbon exploration activities. <b>Citation:</b> MacFarlane, S.K., Jarrett, A.J.M., Hall, L.S., Edwards, D., Palu, T.J., Close, D., Troup, A. and Henson, P., 2020. A regional perspective of the Paleo- and Mesoproterozoic petroleum systems of northern Australia. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

The Energy component of Geoscience Australia’s Exploring for the Future (EFTF) Programme is aimed at improving our understanding of the petroleum resource potential of northern Australia, in partnership with the state and territory geological surveys. The sediments of the Mesoproterozoic South Nicholson Basin and the underlying Paleoproterozoic Isa Superbasin in the Northern Territory and Queensland are amongst the primary targets of the EFTF Energy program as they are known to contain organic rich sedimentary units with the potential to host unconventional gas plays, although their subsurface extent under the cover of the Georgina Basin is presently unknown. In order to economically produce from unconventional reservoirs, the petrophysical rock properties and in-situ stresses must be conducive to the creation of secondary permeability networks that connect a wellbore to as large a reservoir volume as possible. This study utilises data from the recently drilled Armour Energy wells Egilabria 2, Egilabria 2-DW1, and Egilabria 4 to constrain rock properties and in-situ stresses for the Isa Superbasin sequence where intersected on the Lawn Hill Platform of northwest Queensland. These results have implications for petroleum prospectivity in an area with proven gas potential, which are discussed here in the context of the rock properties and in-situ stresses desired for a viable shale gas play. In addition, this has relevance to potential future exploration across the broader Isa Superbasin sequence.

The energy component of Geoscience Australia’s Exploring for the Future (EFTF) program aimed to improve our understanding of the petroleum resource potential of northern Australia. The sediments of the Mesoproterozoic South Nicholson Basin and the Paleoproterozoic Isa Superbasin on the northern Lawn Hill Platfrom (nLHP) are primary targets of the EFTF program, as they are known to contain highly prospective organic-rich units with the potential to host unconventional gas plays. A defining feature of shale gas plays is that they require technological intervention to increase bulk rock permeability and achieve commercial flow rates. The Egilabria prospect, intersecting nLHP sediments in northwest Queensland, flowed gas to surface from a fracture-stimulated lateral well, demonstrating a technical success. Elsewhere in the region, shale gas prospectivity is limited by a lack of well data. Shale rock brittleness in the nLHP part of the Isa Superbasin was analysed in two studies under the EFTF program. These studies showed that shale brittleness ranges from ductile to brittle; zones of brittle shales were present in all supersequences. Shale brittleness is controlled by increasing quartz and decreasing clay content, with carbonate content proving insignificant. Organic-rich target zones in the Lawn and River supersequences are demonstrated to be brittle and favourable for fracture stimulation. <b>Citation:</b> Bailey, A.H.E., Jarrett, A.J.M., Wang, L., Champion, D.C., Hall, L.S. and Henson, P., 2020. Shale brittleness in the Isa Superbasin on the northern Lawn Hill Platform. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.