3d visualisation model of the taupo volcanic zone …...calderas were modelled as closed and...

1 New Zealand Geothermal Workshop 2011 Proceedings

21 - 23 November 2011

Auckland, New Zealand

3D VISUALISATION MODEL OF THE TAUPO VOLCANIC ZONE BASEMENT

S.A. Alcaraz1, 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,

Taupo Volcanic Zone, Torlesse Supergroup, Leapfrog

Geothermal

ABSTRACT

The Taupo Volcanic Zone (TVZ; ~350 km long, ~60 km

wide) constitutes the southern portion of the active 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 is

infilled by Quaternary volcanic rock and sediments, with

the top of underlying basement greywacke displaced up to

1-2 km below sea level.

A geological basement model of top surface of the Torlesse

greywacke in the TVZ is presented. The 3D model,

generated using Leapfrog Geothermal software, is based on

revised interpretation of acquired TVZ gravity data, and

constrained by geological information from the recently

updated regional 1:250,000 (QMAP), geological maps of

the area and geothermal drillhole logging data collected

over the last 60 years from several geothermal fields

(including Kawerau, Ngatamariki, Ohaaki and Wairakei-

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

volcaniclastic sandstones of the Torlesse and Waipapa

terranes (Adams et al., 2009). These greywacke rocks and

inferred intrusive igneous rocks supply heated fluids to the

TVZ geothermal systems (Figure 1, Rowland and Sibson,

2001). The TVZ has been drilled for geothermal and

mineral exploration, with recent drilling including

exploration, production and injection of deep geothermal

boreholes. These boreholes are providing new information

on the geology and structure of several TVZ geothermal

systems, including Wairakei-Tauhara (Rosenberg et al.,

2009; Bignall et al., 2010; Alcaraz et al., 2010), Ohaaki

(Milicich et al., 2008; Milicich et al., 2010b), Kawerau

(Milicich et al., 2010a; Alcaraz, 2010) and Ngatamariki

(Bignall, 2009).

In recent years, the New Zealand geothermal community

has come to consider the potential of untapped, deeper and

hotter geothermal resources in the TVZ – i.e. beyond the 1

to 3 km depth interval that defines most of the >240°C

reservoirs currently developed for electricity generation. A

barrier to development of deeper geothermal resources,

however, is the ability to identify and target deep-seated

structural and stratigraphic permeability. There are

technical challenges due to the temperature increase with

depth, and extraction becomes more difficult as rocks are

less porous at high pressures and temperatures, thus

reducing fluid flow.

Figure 1: Map showing the location of TVZ geothermal

systems, and extent of the 3D basement model

described in this paper.

New Zealand Geothermal Workshop 2011 Proceedings

21 - 23 November 2011


Our work contributes to understanding the evolution and

structure of the TVZ, but also provides the New Zealand

geothermal industry with a higher level of confidence to

develop deep geothermal reservoirs – especially systems

hosted by fractured Torlesse greywacke (e.g. Kawerau,

Ohaaki, Rotokawa), and advance deeper exploration in

areas where greywacke occurs at unknown depth. Our 3D

geological model of the Torlesse greywacke basement uses

actual well data allowing the evaluation of geophysical

models in some areas (e.g. Bertrand et al., 2011;

Soengkono, 2011).

In the future, it is expected that deep-seated, fractured

greywacke-hosted geothermal systems will be explored for

their resource potential. Information on structural-controls

on permeability and fluid flow, rheology and depth to

basement require 3D model visualisation for designing

exploration drilling strategies.

2. MODEL CREATION

2.1 Leapfrog Geothermal

Leapfrog Geothermal is a 3D modelling and visualisation

software package developed by ARANZ Geo in

cooperation with GNS Science to meet the need of the

geothermal industry for an integrated interface (Alcaraz et

al., 2011). Leapfrog Geothermal is being adopted within the

industry, both in New Zealand and internationally, as an

innovative resource management tool.

The new release of Leapfrog Geothermal 2.2 enhances its

structural geological modelling capabilities, in particular

the modelling of faults and fault blocks. These are now

processed using a chronological table which has enabled the

modelling of complex geometries such as the TVZ’s faulted

Torlesse greywacke basement.

2.2 Input data

This preliminary construction of a TVZ basement model

used various sources of information (Figure 2):

• Topographic data (GNS’s Digital Terrain Model &

topographic data from Land Information New Zealand)

• Geothermal and mineral borehole data

• QMAP Rotorua 1:250,000 geological map (Leonard et

al., 2010), includes surface geology, faults, calderas,

structural measurements and cross sections

• GNS Science’s Active Faults Database

• Basement surface interpreted from gravity data

(Soengkono, 2011).

Borehole data from TVZ geothermal fields, including

Kawerau, Waiotapu, Orakei Korako, Te Kopia,

Ngatamariki, Mokai, Ohaaki, Rotokawa and Wairakei-

Tauhara, were used to constrain the depth of the Torlesse

greywacke. Currently, the model incorporates data from

441 wells, but is updated as new data becomes available

from ongoing TVZ geothermal drilling.

Boreholes location, survey and geology datasets have been

collected from public sources, or provided by courtesy of

Mighty River Power Ltd. and Contact Energy Ltd. for

research purposes. Each well geometry has been associated

to an interval table classified in two categories: basement or

cover. Leapfrog Geothermal honours stratigraphic contact

points, which means all known contacts between cover and

basement provides a factual depth as input to model

surfaces. The Leapfrog Geothermal modelling technique

also uses well data that do not intersect the greywacke, to

force the surface underneath the borehole maximum depth.

Recently published geological map data for the Taupo

Volcanic Zone (Leonard et al., 2010) have been used to

model the basement. The new Rotorua geological map is

part of the GNS Science’s QMAP series and has been

compiled from new field data supplementing legacy

information from more than 100 published and unpublished

geological maps. The QMAP Geographic Information

System (GIS) dataset consists of numerous layers such as

geological map units, faults and structural measurements,

and caldera outlines. Features within these are

comprehensively described by attributes such as rock type,

feature name, stratigraphic affiliation, age and orientation.

Relevant GIS data have been incorporated into the 3D

geological model (notably the geological contact between

the greywacke basement and overlying volcanic sequences,

major faults and calderas).

The published geological map portrays the subsurface

geology in four cross sections that were incorporated into

the 3D model as georeferenced images. The contacts and

major faults shown in the cross sections have been used to

constrain the modelled surfaces at depth.

The GNS Science New Zealand Active Faults Database

(GNS Science, 2011) was used to complement QMAP data.

The Active Faults Database contains detailed information

compiled from field measurements of offset features,

trenching and dating. The database also contains

interpretation in the form of recurrence interval, slip rate

and date of last movement. This database played a major

role in the selection process of the principal faults to be

integrated in this model.

Also used as an input in the model is a 3D basement model

interpreted from gravity data (Figure 2c & e, from

Soengkono, 2011). It was created using a "density layer"

defined by 250-metre-wide elevation grids which covers the

whole of the TVZ. The 250 metre digital topographic model

(DTM) of the TVZ from Land Information NZ is used for

the upper elevation grid of the model. The bottom

elevation grid, representing the greywacke basement

surface, was adjusted until the theoretical gravity effects of

the density layer, computed using a density contrast of -470

kg/m3 (representing the average density difference between

TVZ Quaternary volcanic infill and greywacke basement)

matched the residual gravity anomalies obtained from

actual gravity measurements over the entire TVZ.

2.2 Torlesse greywacke depth

Torlesse greywacke occurs at surface in the ranges at the

margin of the TVZ (Grindley, 1960; Healy et al., 1964;

Leonard et al., 2010). Within the TVZ itself, the depth to

the basement is locally constrained by 69 deep drillholes,

intersecting greywacke as shallow as -666 mRL in

Kawerau, and as deep as -3015 mRL in Ngatamariki.

Of the other 372 deep boreholes that do not intersect the

greywacke basement, some are deep (> 2.5 km) and provide

minimum depths of the basement. The deepest boreholes

imported in the model reach -3015 mRL (Ngatamariki) and

~-2920 mRL (Mangakino). While the first one does

intersect the basement, the Mangakino well (located in the

Mangakino caldera) did not reach it but provides valuable

information to constrain the depth of the caldera.


21 - 23 November 2011


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).

e. d.

c.

b.

a.


21 - 23 November 2011


2.3 Structural Framework

The model presented in this paper includes information for

40 major regional faults and 7 calderas (Figure 3).

Fault trends are dominantly NE-SW within the TVZ. Most

of the faults in the basement model presented here were

created by digitising the surface traces from the QMAP

Rotorua dataset and/or the GNS Active Faults Database,

using dip direction and dip angle attributes where available.

Principal faults were selected based on their geographical

extent, data confidence, and their activity. A few major

faults were modeled on the edges of the TVZ, including:

Awakeri, Te Whaiti, Wheao and Whakatane Faults.

Within the TVZ, major faults modeled include: Kaingaroa,

Aratiata, Whakaipo, Orakei Korako, Thorpe-Poplar,

Ngapouri, Paeroa Fault Zone, Tumunui, Whirinaki,

Edgcumbe, Rotoitipaku, Omeheu, Braemar, Matamata

Fault Zone and Ngakuru faults. Local faults were added in

Ohaaki and Rotokawa areas to accommodate major offsets

in the greywacke basement between nearby drillholes (e.g.

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.

2.4 3D basement model from gravity data

As discussed in Soengkono (2011), there is a strong

correlation between inferred caldera locations and gravity

basement depressions (Figure 4), especially the Mangakino,

Rotorua and Okataina centres. South-east of the Rotorua

caldera is a major gravity anomaly that is consistent with

the inferred location of the Kapenga caldera. The caldera is

likely to have displaced the greywacke basement surface

but has not yet been added to the model.

The basement surface currently modeled is consistent with

the depth of the greywacke from borehole data at Kawerau

(Figure 4b), but will need some adjustment in the southern

part of the TVZ. The modeled surface there happens to be

shallower than the maximum depth of wells that in fact do

not intersect the basement (Soengkono, 2011; figure 4c).

The modeled surface interpreted from gravity data is used

as an indicator of the basement depth, at regional (TVZ)

scale, and provides useful guidelines where there is no

surface or available downhole data.

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.

a.

b.

New Zealand Geothermal Workshop 2011 Proceedings

21 - 23 November 2011


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

The TVZ basement model has been built following a

standard workflow within Leapfrog Geothermal. The first

step in building such a model is to define the area of

interest, import the topography and various input data to be

included in the interface (i.e., GIS datasets, as well as the

QMAP geological map, geo-referenced QMAP cross-

sections and borehole data).

The model encompasses the fault-bounded margins of the

volcano-tectonic depression. The resolution of the model

was set to 1,000 metres to limit processing time. However,

Leapfrog Geothermal implicit modelling techniques allow

scale refinement at any stage if required. For ease of

construction, the TVZ model was split into 3 distinct areas,

but this does not affect the coherency of the final combined

model.

The second step consisted of building the intricate fault

network presented in Figure 3. Hierarchy and relationships

between faults and calderas were carefully defined within

Leapfrog Geothermal in order to create a geologically

consistent and realistic structural framework that honours

the QMAP and Active Faults datasets.

The next stage in building the model was to generate the

Cover-Basement contact surfaces, starting in areas where

contact points are relatively well constrained by factual data

(geothermal wells; outcrop).

Finally, basement surfaces in remaining fault blocks were

edited using guidelines such as contacts between

Quaternary volcano-sedimentary infill and Torlesse

greywacke from QMAP interpretive cross-sections. In areas

having neither borehole data nor QMAP information, the

basement surface generated from modelled gravity data was

used as an indication of the basement depth. Gravity model

results were especially useful in the area bounded by Lake

Rotorua, Lake Tarawera and the Ohakuri and Reporoa

calderas where there is little constraint on greywacke depth.

Figure 5 shows the preliminary basement model of the

Taupo Volcanic Zone, as built in Leapfrog Geothermal 2.2.

a.

b. c.


21 - 23 November 2011


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.

a.

b.

c.

d.


16 – 18 November 2009

Rotorua, New Zealand

2. CONCLUSION

The geological model presented in this paper is a

preliminary 3D interpretation of the upper surface of the

Torlesse greywacke basement in the TVZ, built using

Leapfrog Geothermal version 2.2 modelling software. The

ability of the software to handle the TVZ’s complex

structural framework has enabled a realistic 3D model to be

constructed. Whilst the model presented here is still “work

in-progress’, it already provides a geologically plausible 3D

view of what the TVZ basement might actually look like.

The model is expected to foster discussion and assist with

identifying new research opportunities. Leapfrog

Geothermal is a highly effective 3D interface to integrate

multi-disciplinary datasets and will be used in the future to

correlate new scientific findings and test possible scenarios

including structural and stratigraphic controls on heat and

fluid flow in the TVZ.

Our current research programme focuses on the delineation

of New Zealand’s deep geothermal resources, with the aim

of providing developers with a higher level of confidence

and to reduce exploration risk in deep drilling. Improved

geoscientific knowledge, including insights on the basement

structure and evolution of the Taupo Volcanic Zone are also

expected. These insights will help quantify the location and

permeability structure of the deep resources, and help reduce

barriers to development.

The current research programme is also including a

magnetotelluric-seismic model of TVZ mid-crustal structure,

to identify deep permeability and fluid flow. The goal is to

produce a map of inferred temperature and depth to the

brittle-ductile transition. The 3D greywacke basement model

will also be part of collaborative work to develop a time-

series reconstruction of TVZ depression evolution, and

model for the modern strain distribution across the TVZ.

By the conclusion of the current research programme, we

will have greatly extended our knowledge of the deep

hydrology and structure of geothermal systems in the TVZ,

such that an industry-research consortium will be well

placed to target a deep exploration well to test the

permeability structure and deep geothermal resource

potential of the TVZ. The 3D greywacke basement model,

presented here, will very likely be an important tool in

deciding the location of any future industry- / ICDP-

supported, international deep geoscience TVZ-Deep

Geothermal Drilling Project (i.e., a proposed deep hole

(prospectively to 4-5 km depth), The findings of TVZ-

DGDP, and drilling / engineering experience gained, will be

invaluable to the international EGS research and

development community.

ACKNOWLEDGEMENTS

Mighty River Power Ltd. and Contact Energy Ltd. are

acknowledged for their continued support of the GNS

Science geothermal research programme. Graham Leonard

assisted with interpreting caldera geometries. This work was

originally supported by the Foundation for Research Science

and Technology PROJ-20199-GEO-GNS “Harnessing New

Zealand’s Geothermal Resources: Hotter and Deeper”,

which has (from 1 July, 2011) been incorporated in the GNS

Science CSA (Core Science Area) Geothermal Research

Programme. This work has also benefited from strong

collaboration with ARANZ Geo, for continued development

of Leapfrog Geothermal software. We thank the reviewers

for their helpful comments.

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3d visualisation model of the taupo volcanic zone …...calderas were modelled as closed and...

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