Effect of Hydrothermal Alteration on Rock Properties in the Tauhara Geothermal Field, New Zealand

Authors: Philipp MIELKE, Angela PRIETO, Greg BIGNALL, Ingo SASS
Keywords: Wairakei-Tauhara, permeability, porosity, thermal conductivity, specific heat capacity, tuff, hydrothermal alteration
Conference: World Geothermal Congress Session: Geology
Year: 2015 Language: English
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 ( less than 200°C) depths to propylitic-type alteration in core from deeper and hotter (200 to more than 300°C) parts of the reservoir.
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