Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand

Poorly permeable basement rocks commonly occur in geothermal regions around the world, and the Quaternary Taupo Volcanic Zone (TVZ) of New Zealand is no exception. Production from basement terrane requires detailed knowledge of its geological and geophysical parameters, as shown by the history of Kawerau and Ohaaki, the only geothermal fields in the TVZ where Mesozoic Torlesse terrane greywacke (litharenite) basement is commonly penetrated at drilled depths of 1–2.5 km. In both fields the basement is step-faulted down into the TVZ. Although hot and hydrothermally altered, the greywackes have little permeability. Some production wells feed from elusive basement faults at Kawerau, but rarely at Ohaaki. Greywackes at Ohaaki are of “granite-rhyolite” provenance, and have more interbedded argillite than the “andesite-dacite” derived Kawerau greywackes. In consequence, the Kawerau basement may sustain brittle fracture at higher temperatures and depths than the more ductile Ohaaki basement, allowing convective circulation of higher enthalpy fluids into permeable Quaternary aquifers.