3D VISUALISATION MODEL OF THE TAUPO VOLCANIC ZONE BASEMENT S.A. Alcaraz 1, M.S. Rattenbury 2, M.D. Rosenberg 1, S. Soengkono 1, G. Bignall 1 and H. van Moerkerk 3 1 GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand 2 GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand 3 ARANZ Geo Ltd., PO Box 3894, Christchurch 8140, New Zealand [email protected] Keywords: 3D modelling, 3D visualisation, calderas, 2001). The TVZ has been drilled for geothermal and Taupo Volcanic Zone, Torlesse Supergroup, Leapfrog mineral exploration, with recent drilling including Geothermal exploration, production and injection of deep geothermal boreholes. These boreholes are providing new information ABSTRACT on the geology and structure of several TVZ geothermal The Taupo Volcanic Zone (TVZ; ~350 km long, ~60 km systems, including Wairakei-Tauhara (Rosenberg et al., wide) constitutes the southern portion of the active Lau- 2009; Bignall et al., 2010; Alcaraz et al., 2010), Ohaaki Havre-Taupo extensional back arc basin, and formed by (Milicich et al., 2008; Milicich et al., 2010b), Kawerau extension of crust above the Hikurangi subduction zone in (Milicich et al., 2010a; Alcaraz, 2010) and Ngatamariki the central North Island. The fault-controlled depression is (Bignall, 2009). infilled by Quaternary volcanic rock and sediments, with the top of underlying basement greywacke displaced up to In recent years, the New Zealand geothermal community 1-2 km below sea level. has come to consider the potential of untapped, deeper and hotter geothermal resources in the TVZ – i.e. beyond the 1 A geological basement model of top surface of the Torlesse to 3 km depth interval that defines most of the >240°C greywacke in the TVZ is presented. The 3D model, reservoirs currently developed for electricity generation. A generated using Leapfrog Geothermal software, is based on barrier to development of deeper geothermal resources, revised interpretation of acquired TVZ gravity data, and however, is the ability to identify and target deep-seated constrained by geological information from the recently structural and stratigraphic permeability. There are updated regional 1:250,000 (QMAP), geological maps of technical challenges due to the temperature increase with the area and geothermal drillhole logging data collected depth, and extraction becomes more difficult as rocks are over the last 60 years from several geothermal fields less porous at high pressures and temperatures, thus (including Kawerau, Ngatamariki, Ohaaki and Wairakei- reducing fluid flow. Tauhara). The 3D model of the TVZ Torlesse greywacke basement upper surface provides a preliminary visualisation of the geological and structural framework of the TVZ, and enhances our understanding of the deep rheology and controls on deep-seated permeability. The model also represents an important output from an integrated, multi- disciplinary study that aims to support future development of New Zealand’s high enthalpy (>250°C) geothermal resources, which are hosted in some fields (e.g. Kawerau) by fractured Torlesse greywacke. 1. INTRODUCTION The Taupo Volcanic Zone (TVZ; ~350 km long, ~60 km wide) constitutes the active southern portion of the Lau- Havre-Taupo extensional back arc basin, and formed by extension of crust above the Hikurangi subduction zone in the central North Island. The fault-controlled depression began forming <2 Ma ago in response to regional crustal extension. Through a process of active faulting and caldera formation and collapse the depression has deepened but simultaneously infilled by volcanic rock deposits and sediments up to 3 km thick, with the upper surface of the underlying basement greywacke faulted to 1-2 km below sea level. Basement rocks of the TVZ comprise Mesozoic Figure 1: Map showing the location of TVZ geothermal volcaniclastic sandstones of the Torlesse and Waipapa systems, and extent of the 3D basement model terranes (Adams et al., 2009). These greywacke rocks and described in this paper. inferred intrusive igneous rocks supply heated fluids to the TVZ geothermal systems (Figure 1, Rowland and Sibson, 1 New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand Our work contributes to understanding the evolution and also uses well data that do not intersect the greywacke, to structure of the TVZ, but also provides the New Zealand force the surface underneath the borehole maximum depth. geothermal industry with a higher level of confidence to develop deep geothermal reservoirs – especially systems Recently published geological map data for the Taupo hosted by fractured Torlesse greywacke (e.g. Kawerau, Volcanic Zone (Leonard et al., 2010) have been used to Ohaaki, Rotokawa), and advance deeper exploration in model the basement. The new Rotorua geological map is areas where greywacke occurs at unknown depth. Our 3D part of the GNS Science’s QMAP series and has been geological model of the Torlesse greywacke basement uses compiled from new field data supplementing legacy actual well data allowing the evaluation of geophysical information from more than 100 published and unpublished models in some areas (e.g. Bertrand et al., 2011; geological maps. The QMAP Geographic Information Soengkono, 2011). System (GIS) dataset consists of numerous layers such as geological map units, faults and structural measurements, In the future, it is expected that deep-seated, fractured and caldera outlines. Features within these are greywacke-hosted geothermal systems will be explored for comprehensively described by attributes such as rock type, their resource potential. Information on structural-controls feature name, stratigraphic affiliation, age and orientation. on permeability and fluid flow, rheology and depth to Relevant GIS data have been incorporated into the 3D basement require 3D model visualisation for designing geological model (notably the geological contact between exploration drilling strategies. the greywacke basement and overlying volcanic sequences, major faults and calderas). 2. MODEL CREATION 2.1 Leapfrog Geothermal The published geological map portrays the subsurface geology in four cross sections that were incorporated into Leapfrog Geothermal is a 3D modelling and visualisation the 3D model as georeferenced images. The contacts and software package developed by ARANZ Geo in major faults shown in the cross sections have been used to cooperation with GNS Science to meet the need of the constrain the modelled surfaces at depth. geothermal industry for an integrated interface (Alcaraz et al., 2011). Leapfrog Geothermal is being adopted within the The GNS Science New Zealand Active Faults Database industry, both in New Zealand and internationally, as an (GNS Science, 2011) was used to complement QMAP data. innovative resource management tool. The Active Faults Database contains detailed information compiled from field measurements of offset features, The new release of Leapfrog Geothermal 2.2 enhances its trenching and dating. The database also contains structural geological modelling capabilities, in particular interpretation in the form of recurrence interval, slip rate the modelling of faults and fault blocks. These are now and date of last movement. This database played a major processed using a chronological table which has enabled the role in the selection process of the principal faults to be modelling of complex geometries such as the TVZ’s faulted integrated in this model. Torlesse greywacke basement. Also used as an input in the model is a 3D basement model 2.2 Input data interpreted from gravity data (Figure 2c & e, from This preliminary construction of a TVZ basement model Soengkono, 2011). It was created using a "density layer" used various sources of information (Figure 2): defined by 250-metre-wide elevation grids which covers the • Topographic data (GNS’s Digital Terrain Model & whole of the TVZ. The 250 metre digital topographic model topographic data from Land Information New Zealand) (DTM) of the TVZ from Land Information NZ is used for • Geothermal and mineral borehole data the upper elevation grid of the model. The bottom • QMAP Rotorua 1:250,000 geological map (Leonard et elevation grid, representing the greywacke basement al., 2010), includes surface geology, faults, calderas, surface, was adjusted until the theoretical gravity effects of structural measurements and cross sections the density layer, computed using a density contrast of -470 • GNS Science’s Active Faults Database kg/m 3 (representing the average density difference between • Basement surface interpreted from gravity data TVZ Quaternary volcanic infill and greywacke basement) (Soengkono, 2011). matched the residual gravity anomalies obtained from actual gravity measurements over the entire TVZ. Borehole data from TVZ geothermal fields, including Kawerau, Waiotapu, Orakei Korako, Te Kopia, 2.2 Torlesse greywacke depth Ngatamariki, Mokai, Ohaaki, Rotokawa and Wairakei- Torlesse greywacke occurs at surface in the ranges at the Tauhara, were used to constrain the depth of the Torlesse margin of the TVZ (Grindley, 1960; Healy et al., 1964; greywacke. Currently, the model incorporates data from Leonard et al., 2010). Within the TVZ itself, the depth to 441 wells, but is updated as new data becomes available the basement is locally constrained by 69 deep drillholes, from ongoing TVZ geothermal drilling. intersecting greywacke as shallow as -666 mRL in Kawerau, and as deep as -3015 mRL in Ngatamariki. Boreholes location, survey