Fracture permeability in basement greywacke for supercritical drilling planning

Abstract

Supercritical geothermal resources have potential to decarbonise industry and power sustainable economic growth opportunities. It is inferred that fractured, low permeability metasedimentary basement rocks that underly much of the Taup? Volcanic Zone (TVZ), New Zealand, would host supercritical geothermal resources. These rocks have complex fracturing patterns resulting from a long history of deposition, burial, tectonism and hydrothermal activity. Therefore, the work described here is a first step to understand fracture distributions and fracture permeability in TVZ basement rocks. Fractures were characterised from a resistivity borehole image acquired from a >2 km-deep well in the Kawerau Geothermal Field; the deepest direct fracture dataset collected in the TVZ basement rocks. Basement greywacke comprises metasedimentary sandstones (greywacke) and mudstones (argillite) of the Waipapa Composite Terrane. Rock type through the imaged interval was split into dominantly sandstone, dominantly argillite, and an interbedded mixture of both lithologies.
Potentially permeable fractures (i.e., conductive, conductive fractures with resistive halo and micro-faults) have orientation consistent with current tectonics. Resistive fractures have moderate dip magnitude and no preferred strike orientation, consistent with them being mostly of metamorphic origin. Conductive fractures are interpreted as a combination of metamorphic and tectonic origins, with some likely to have been of metamorphic origin reactivated under current tectonics. Overall, there are more fractures in sandstone than argillite, but this is most pronounced for (mineralised) resistive fractures and (potentially permeable) resistive fractures with conductive halo than conductive fractures. Differences between lithologies are less pronounced when considering volumetric densities.
The lithological controls on fracturing differ amongst the three permeable zones. Two permeable zones occurred in a mixture of the three rock types while the third, major permeable zone was in dominantly sandstone and an interbedded mixture. Potentially permeable fractures of elevated thickness (>20 mm) are more common within the two minor feed zones than outside feed zones. Fracture density is higher in the major feed zone than outside feed zones. These results can be used as a baseline for fractures and fluid movement in basement rocks. This can be used to inform future reservoir models to explore supercritical resources and aid with planning future drilling.