The potential for supercritical geothermal resources in the Taupō Volcanic Zone

Supercritical geothermal resources beneath conventional high-enthalpy geothermal fields have recently received strong attention to boost geothermal power generation. Although several high-enthalpy geothermal fields have been operating in the Taupō Volcanic Zone (TVZ) for decades, the potential for supercritical resources in New Zealand remains unclear, and is possibly hampered by the theoretically low brittle-to-ductile transition temperature (TBDT) of silicic rocks and by the low primary permeability of the basement. We use numerical models to explore the supercritical resource potential of two endmember magmatic scenarios in the TVZ: (1) a deep (>7 km), extensive, long-lived (crystallizing over >100 kyr) magma mush and (2) smaller, shallow (∼3-5 km depth) and short-lived (crystallizing over 5-15 kyr) intrusions emplaced in the upper part of the TVZ basement.Modelled shallow intrusions drive hotter geothermal systems than the deep mush can. The latter requires an optimistically high host rock permeability (10-16 m2) and elevated TBDT (400°C) of the TVZ greywacke basement to initiate high-enthalpy geothermal systems in the shallow volcanic infill at all. The hottest geothermal systems above shallow intrusions consist of a central up-flow plume with temperatures following the boiling curve down to more than about 1 km, with a supercritical resource present within the basement near the interface between the basement and the overlying permeable volcanics. The formation of supercritical resources in silicic host rocks requires that the onset of permeability decrease due the brittle-to-ductile transition occurs at temperatures above the critical point of water (T > 374°C), i.e., just ca. 20 °C above the commonly assumed TBDT for quartz-rich rocks.