Cp. Wood et al., Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand, GEOTHERMICS, 30(4), 2001, pp. 461-481
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.