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The Australian Structural Permeability Map


A decline in conventional hydrocarbon reserves coupled with technological advances and growing energy demand has driven a shift in exploration of energy rich Australian Basins, with a progressive focus on unconventional energy sources (e.g. Coal Seam Gas, Shale Gas and Enhanced Geothermal Systems). Understanding natural fractures is critical to assessing the prospectivity of unconventional plays, as structural permeability in the form of interconnected natural fracture networks commonly exert a prime control over fluid flow in reservoir units due to low primary permeabilities. Structural permeability in the Northern Perth, South Australian Otway, and Northern Carnarvon basins is characterised using an integrated geophysical and geological approach combining wellbore image logs, core, 3D seismic attribute analysis and detailed structural geology. Integration of these methods allows for the identification of faults and fractures over a range of scales (mm-km), providing crucial permeability information. New stress orientation data is also interpreted, allowing for stress-based predictions of fracture reactivation. The resulting fracture orientations from each basin are compiled into a map of structural permeability of the Australian continent, demonstrating orientation variations which cannot be explained through fracture formation and reactivation prediction based on known stress orientations. The importance of validating remotely sensed fractures is demonstrated in the Otway Basin; analysis of core shows open fractures are rarer than image logs indicate, due to the presence of fracture-filling siderite, an electrically conductive cement which may cause fractures to appear hydraulically conductive in image logs. Although the majority of fractures detected are favourably oriented for reactivation under in-situ stresses; fracture fills primarily control which fractures are open, demonstrating that lithological data is often essential for understanding potential structural permeability networks and the orientations at which open fractures may form. The Carnarvon Basin is shown to host distinct variations in fracture orientation; a result of the in-situ stress regime, regional tectonic development, and local structure. A detailed understanding of the structural development, from regional-scale (100s km) down to local-scale (km), is demonstrated to be important when attempting to understand natural fracture orientations, and hence, structural permeability.