Effect of hydrothermal alteration on rock properties in the Tauhara Geothermal Field, New Zealand, Proceedings, World Geothermal Congress,

ABSTRACT Hydrothermal alteration records physicochemical changes to a rock, including dissolution and/or precipitation of new mineral phases, by its interaction with (typically) hot mineralised fluid. Transient geochemical processes can alter petro-and thermophysical rock properties. A database of hydraulic (e.g. permeability, porosity) and thermophysical parameters (e.g. thermal conductivity, heat capacity) has been established for ~450 drill cores from seven geothermal/monitor wells (TH18, THM12, THM13, THM14, THM17, THM18 and THM19) in the Tauhara Geothermal Field (Taupo Volcanic Zone (TVZ), New Zealand), including for (a) clay-altered tuff and intercalated siltstones (cap rock for the Tauhara geothermal system); (b) tuffaceous sandstones (reservoir-hosting units); and (c) rhyolitic and andesitic lavas, and their associated breccias. Hydrothermal alteration determined from XRD and shortwave infrared analysis (SWIR), optical microscopy and clay mineral characterisation, ranges from an argillic-type assemblage at shallow (< 200°C) depths to propylitic-type alteration in core from deeper and hotter (200 to >300°C) parts of the reservoir