Shale brittleness within the Paleoproterozoic Isa Superbasin succession in the South Nicholson region, Northern Australia

Abstract

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: X-ray diffraction (XRD) mineralogy, X-ray fluorescence (XRF) major-element geochemistry and derived elastic 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 derived elastic properties agrees closely with XRD mineralogy derived brittleness. As brittleness derived from XRF major-element geochemistry provides brittleness estimates that are excessively high and with minimal overlap with the other two brittleness datasets, the use of these data to calculate brittleness in the South Nicholson region is not recommended. 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 highlight intervals of exploration interest within these supersequences, being those depths where high-organic-content, brittle rocks are identified. The rocks that meet this criterion are primarily constrained to the already-known potential shale gas plays of the River and Lawn supersequences. Recent data from Geoscience Australia imply a significantly increased lateral extent of these potential shale gas plays, likely extending from the northern Lawn Hill Platform, underneath the South Nicholson Basin and into the Carrara Sub-basin. KEY POINTS This study utilises three different methods for calculating proxy values for shale brittleness: XRD mineralogy, XRF major-element geochemistry and derived elastic properties. Calculated brittleness indices should be considered for their utility as screening tools rather than as accurate representations of brittleness. Results indicate highly variable mineralogy within the analysed samples, demonstrating heterogeneity in shale brittleness throughout the studied supersequences. 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.