3D Visualisation Model of the Taupo Volcanic Zone Basement S.A

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 and geology datasets have been collected from public sources, or provided by courtesy of Of the other 372 deep boreholes that do not intersect the Mighty River Power Ltd. and Contact Energy Ltd. for greywacke basement, some are deep (> 2.5 km) and provide research purposes. Each well geometry has been associated minimum depths of the basement. The deepest boreholes to an interval table classified in two categories: basement or imported in the model reach -3015 mRL (Ngatamariki) and cover. Leapfrog Geothermal honours stratigraphic contact ~-2920 mRL (Mangakino). While the first one does points, which means all known contacts between cover and intersect the basement, the Mangakino well (located in the basement provides a factual depth as input to model Mangakino caldera) did not reach it but provides valuable surfaces. The Leapfrog Geothermal modelling technique information to constrain the depth of the caldera.

New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand

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d. e. Figure 2: Input data entered in Leapfrog Geothermal to build the 3D model of the Taupo Volcanic Zone (TVZ) greywacke basement. a: DTM, TVZ outline, geothermal fields and borehole data. b: QMAP Rotorua data wrapped on topography, with highlighted faults and cross sections (after Leonard et al., 2010). c: Basement surface interpreted from gravity data (after Soengkono, 2011). d&e: Cross sections looking west showing borehole data (d), and basement surface interpreted from gravity data (e; x5 vertical exaggeration). 3 New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand 2.3 Structural Framework 2.4 3D basement model from gravity data The model presented in this paper includes information for As discussed in Soengkono (2011), there is a strong 40 major regional faults and 7 calderas (Figure 3). correlation between inferred caldera locations and gravity basement depressions (Figure 4), especially the Mangakino, Fault trends are dominantly NE-SW within the TVZ. Most Rotorua and Okataina centres. South-east of the Rotorua of the faults in the basement model presented here were caldera is a major gravity anomaly that is consistent with created by digitising the surface traces from the QMAP the inferred location of the Kapenga caldera. The caldera is Rotorua dataset and/or the GNS Active Faults Database, likely to have displaced the greywacke basement surface using dip direction and dip angle attributes where available. but has not yet been added to the model. Principal faults were selected based on their geographical extent, data confidence, and their activity. A few major The basement surface currently modeled is consistent with faults were modeled on the edges of the TVZ, including: the depth of the greywacke from borehole data at Kawerau Awakeri, Te Whaiti, Wheao and Whakatane Faults. (Figure 4b), but will need some adjustment in the southern part of the TVZ. The modeled surface there happens to be Within the TVZ, major faults modeled include: Kaingaroa, shallower than the maximum depth of wells that in fact do Aratiata, Whakaipo, Orakei Korako, Thorpe-Poplar, not intersect the basement (Soengkono, 2011; figure 4c). Ngapouri, Paeroa Fault Zone, Tumunui, Whirinaki, Edgcumbe, Rotoitipaku, Omeheu, Braemar, Matamata The modeled surface interpreted from gravity data is used Fault Zone and Ngakuru faults. Local faults were added in as an indicator of the basement depth, at regional (TVZ) Ohaaki and Rotokawa areas to accommodate major offsets scale, and provides useful guidelines where there is no in the greywacke basement between nearby drillholes (e.g. surface or available downhole data. Milicich et al., 2010b; Alcaraz et al., 2011).

Calderas were modelled as closed and cylinder shape faults. Seven were included in the current TVZ basement model: Mangakino, Ohakuri, Okataina, Oruanui, Reporoa, Rotorua and Whakamaru calderas.

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Figure 3: Structural framework of the 3D TVZ model, illustrating fault and caldera geometry and relationships. a: The model includes 40 major regional faults (in green) and 7 calderas (in blue). b: Resulting faulted blocks based on the regional fault network.

4 New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand a.

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Figure 4: 3D basement surface computed from gravity data (after Soengkono, 2011). a: in relation to the calderas included in the compiled model of the greywacke basement surface presented in this paper. b&c: Local observation of basement surface from gravity data in relations to boreholes observations (contact points are highlighted). b:Looking south towards Kawerau. c: looking North towards Rotokawa & Ohaaki.

2.5 Preliminary model Leapfrog Geothermal in order to create a geologically The TVZ basement model has been built following a consistent and realistic structural framework that honours standard workflow within Leapfrog Geothermal. The first the QMAP and Active Faults datasets. step in building such a model is to define the area of interest, import the topography and various input data to be The next stage in building the model was to generate the included in the interface (i.e., GIS datasets, as well as the Cover-Basement contact surfaces, starting in areas where QMAP geological map, geo-referenced QMAP cross- contact points are relatively well constrained by factual data sections and borehole data). (geothermal wells; outcrop).

The model encompasses the fault-bounded margins of the Finally, basement surfaces in remaining fault blocks were volcano-tectonic depression. The resolution of the model edited using guidelines such as contacts between was set to 1,000 metres to limit processing time. However, Quaternary volcano-sedimentary infill and Torlesse Leapfrog Geothermal implicit modelling techniques allow greywacke from QMAP interpretive cross-sections. In areas scale refinement at any stage if required. For ease of having neither borehole data nor QMAP information, the construction, the TVZ model was split into 3 distinct areas, basement surface generated from modelled gravity data was but this does not affect the coherency of the final combined used as an indication of the basement depth. Gravity model model. results were especially useful in the area bounded by Lake Rotorua, Lake Tarawera and the Ohakuri and Reporoa The second step consisted of building the intricate fault calderas where there is little constraint on greywacke depth. network presented in Figure 3. Hierarchy and relationships between faults and calderas were carefully defined within Figure 5 shows the preliminary basement model of the Taupo Volcanic Zone, as built in Leapfrog Geothermal 2.2.

New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand a.

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Figure 5: Model of the TVZ basement (green: Torlesse Supergroup, yellow: volcanic infill). a: Greywacke basement surface, looking towards NNE. b & c & d: Model sliced NW-SE and looking to the NNE. b: Sliced through Kawerau. c: Sliced through Ohaaki. d: Sliced through Taupo.

6 New Zealand Geothermal Workshop 2011 Proceedings 21 - 23 November 2011 Auckland, New Zealand 2. CONCLUSION REFERENCES The geological model presented in this paper is a Adams, C.J., Mortimer, N., Campbell, H.J. and Griffin, W. preliminary 3D interpretation of the upper surface of the L.: Age and isotopic characterisation of Torlesse greywacke basement in the TVZ, built using metasedimentary rocks from the Torlesse Supergroup Leapfrog Geothermal version 2.2 modelling software. The and Waipapa Group in the central North Island, New ability of the software to handle the TVZ’s complex Zealand. New Zealand Journal of Geology and structural framework has enabled a realistic 3D model to be Geophysics , 52:2, pp. 149-170. (2009). constructed. Whilst the model presented here is still “work in-progress’, it already provides a geologically plausible 3D Alcaraz, S.,A., Lane, R., Spragg, K., Milicich, S., view of what the TVZ basement might actually look like. Sepulveda, F. and Bignall, G.: 3D Geological The model is expected to foster discussion and assist with Modelling using new Leapfrog Geothermal Software. identifying new research opportunities. Leapfrog Proc. 36 th Workshop on Geothermal Reservoir Geothermal is a highly effective 3D interface to integrate Engineering , Stanford University, California. (2011). multi-disciplinary datasets and will be used in the future to correlate new scientific findings and test possible scenarios Alcaraz, S.A., Sepulveda, F., Lane, R., Rosenberg, M.D., including structural and stratigraphic controls on heat and Rae, A.J. and Bignall, G.: A 3-D representation of the fluid flow in the TVZ. Wairakei Geothermal System (New Zealand) using "Earth Research" geothermal visualisation and Our current research programme focuses on the delineation modelling software. Transactions. Geothermal of New Zealand’s deep geothermal resources, with the aim Resources Council, 34, pp. 1119-1123. (2010). of providing developers with a higher level of confidence and to reduce exploration risk in deep drilling. Improved Alcaraz, S.A.: 3D geological visualisation of the Kawerau geoscientific knowledge, including insights on the basement Geothermal Field. GNS Science consultancy report structure and evolution of the Taupo Volcanic Zone are also 2010/29LR. Confidential Report to Mighty River Power expected. These insights will help quantify the location and Limited. (2010). permeability structure of the deep resources, and help reduce barriers to development. Bertrand, E. A., Caldwell, T. G., Hill, G. J., Wallin, E. L., Bennie, S. L., Cozens, N., Onacha, S. A., Ryan, G. A., The current research programme is also including a Walter, C., Zaino, A., and Wameyo, P.: magnetotelluric-seismic model of TVZ mid-crustal structure, Magnetotelluric imaging of upper-crustal convection to identify deep permeability and fluid flow. The goal is to plumes beneath the Taupo volcanic zone, New produce a map of inferred temperature and depth to the Zealand. Geophysical Research Letters . In progress. brittle-ductile transition. The 3D greywacke basement model (2011). will also be part of collaborative work to develop a time- series reconstruction of TVZ depression evolution, and Bignall, G.: Ngatamariki Geothermal Field geoscience model for the modern strain distribution across the TVZ. overview. GNS Science consultancy report 2009/94. 35 p. Confidential Report. (2009) By the conclusion of the current research programme, we will have greatly extended our knowledge of the deep Bignall, G., Milicich, S.D., Ramirez, L.E., Rosenberg, M.D., hydrology and structure of geothermal systems in the TVZ, Kilgour, G.N. and Rae, A.J.: Geology of the Wairakei- such that an industry-research consortium will be well Tauhara Geothermal System, New Zealand. placed to target a deep exploration well to test the Proceedings Worlds Geothermal Congress , 25-30 permeability structure and deep geothermal resource April, 2010, Bali, Indonesia. Paper 1229. (2010). potential of the TVZ. The 3D greywacke basement model, GNS Science: New Zealand Active Faults Database. presented here, will very likely be an important tool in http://data.gns.cri.nz/af/ . (2011). deciding the location of any future industry- / ICDP- supported, international deep geoscience TVZ-Deep Grindley, G.W.: Taupo, Geological map of new Zealand, Geothermal Drilling Project (i.e., a proposed deep hole sheet 8, scale 1;250,00 , Department of Scientific and (prospectively to 4-5 km depth), The findings of TVZ- Industrial Research, Wellington, New Zealand. (1960). DGDP, and drilling / engineering experience gained, will be invaluable to the international EGS research and Healy, J., Schofield, J. C., and Thompson, B.N.: Rotorua, development community. Geological map of new Zealand, sheet 5, scale 1;250,00, Department of Scientific and Industrial ACKNOWLEDGEMENTS Research, Wellington, New Zealand. (1964). Mighty River Power Ltd. and Contact Energy Ltd. are acknowledged for their continued support of the GNS Leonard, G.S., Begg, J.G. and Wilson, C.J.N. (compilers). Science geothermal research programme. Graham Leonard Geology of the Rotorua area: scale 1:250,000. Lower assisted with interpreting caldera geometries. This work was Hutt: Institute of Geological & Nuclear Sciences originally supported by the Foundation for Research Science Limited . Institute of Geological & Nuclear Sciences and Technology PROJ-20199-GEO-GNS “Harnessing New 1:250,000 geological map 5 . 99 p. + 1 fold. Map Zealand’s Geothermal Resources: Hotter and Deeper”, (2010). which has (from 1 July, 2011) been incorporated in the GNS Science CSA (Core Science Area) Geothermal Research Milicich, S.D., Rae, A.J., Rosenberg, M.D. and Bignall, G.: Programme. This work has also benefited from strong Lithological and structural controls on fluid flow and collaboration with ARANZ Geo, for continued development hydrothermal alteration in the western Ohaaki of Leapfrog Geothermal software. We thank the reviewers Geothermal Field (New Zealand) – insights from for their helpful comments. recent deep drilling. Transactions . Geothermal Resources Council, 32, pp. 303-307. (2008).

7 New Zealand Geothermal Workshop 2009 Proceedings 16 – 18 November 2009 Rotorua, New Zealand Milicich, S.D., Fruetsch, F., Ramirez, L.E., Rae, A.J., Rosenberg, M.D., Bignall, G. and Rae, A.J.: The geological Alcaraz, S.A., Kallenberg, B., McCoy-West, A.J. and framework of the Wairakei-Tauhara Geothermal Bignall, G.: Stratigraphic correlation study of the System, New Zealand. Geothermics , 38, pp. 72-84. Kawerau Geothermal Field. GNS Science Consultancy (2009). Report 2010/23 , 61 p. Confidential Report to Mighty River Power Limited. (2010a). Rowland, J.V. and Sibson, R.H.: Extensional fault kinematics within the Taupo Volcanic Zone, New Milicich, S.D., van Dam, M., Rosenberg, M.D., Rae, A.J. Zealand: soft-linked segmentation of a continental rift and Bignall, G.: “Earth Research” 3-Dimensional system. New Zealand Journal of Geology and Modelling of Geological Information from Geothermal Geophysics , 44, pp. 271–284. (2001). Systems of the Taupo Volcanic Zone, New Zealand – a New Visualisation Tool. Proceedings World Soengkono, S.: Deep interpretation of gravity and magnetic Geothermal Congress , 25-30 April, 2010, Bali, data of the central Taupo Volcanic Zone . New Zealand Indonesia. Paper 3201. (2010b). Geothermal Workshop, Auckland, In progress. (2011).

8 New Zealand Geothermal Workshop 2009 Proceedings 16 – 18 November 2009 Rotorua, New Zealand