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 Ka werau and Ohaaki, the only geothermal fields in the TVZ where Mesozoic Torl esse terrane greywacke (litharenite) basement is commonly penetrated at dri lled depths of 1-2.5 km. In both fields the basement is step-faulted down i nto the TVZ, Although hot and hydrothermally altered. the greywackes have l ittle permeability. Some production wells feed from elusive basement faults at Kawerau, but rarely at Ohaaki. Greywackes at Ohaaki are of "granite-rhy olite" provenance, and have more interbedded argillite than the "andesite-d acite" derived Kawerau greywackes. In consequence, the Kawerau basement may sustain brittle fracture at higher temperatures and depths than the more d uctile Ohaaki basement, allowing convective circulation of higher enthalpy fluids into permeable Quaternary aquifers. (C) 2001 CNR. Published by Elsev ier Science Ltd. All rights reserved.