Exploring Structure and Stress from Depth to Surface in the Wairakei Geothermal Field, New Zealand

GNS Science

Exploring Structure and Stress from Depth to Surface in the Wairakei Geothermal Field, New Zealand

David McNamaraStephen BannisterPilar VillamorFabian Sepúlveda

Twitter: @mcnamadd E-Mail: d.mcnamara@gns.cri.nz

Sarah MilicichSamantha AlcarazCécile Massiot

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Why is Structure Important in Geothermal?

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New Zealand Geothermal – The Taupo Volcanic Zone

Pacific Plate

Australian-Indian Plate

~ 50 km

Taupo Volcanic Zone

~740 MWe from TVZ geothermal

sources

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The Taupo Volcanic ZoneA

location of rift axis with arrows for extension direction

Extension direction from focal mechanismstf – transfer zones

BSE facing faultsNW facing faults

Wilson and Rowland, 2016

Wairakei Geothermal Field

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The Taupo Volcanic Zone

WT

RK

BO

MKRE

KA

TT

RO

OKTK

WW

NG

Seismicity is noted along structures in the Modern Taupo Fault Belt.

Some of this seismicity falls within geothermal fields.

Some geothermal fields are aseismic.

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Wairakei Geothermal FieldStructural components:

1) Located in Modern Taupo Fault Belt (mainly NE-SW, some N-S and E-W striking faults).

2) Located in an inferred accommodation zone – (rooted in deep NW-SE striking basement structure)

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Wairakei Geothermal Field

S – Surficial DepositsHFF – Huka Falls FormationWa – Waiora FormationK – Karapiti RhyolitesWk – Wairakei IgnimbriteP – Poihipi RhyolitesTa – Tahorakuri Formation

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Wairakei Geothermal Field

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Wairakei Geothermal Field

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New Insights – Surface StructuresBEFORE AFTER

Villamor et al., 2015

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New Insights – Subsurface Structures

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New Insights – Subsurface Structures

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Acoustic Borehole Imaging:• Based on how the geology of the

well attenuates sonic waves• Uses sonic travel times to provide

360° caliper of the borehole

N NS WE

New Insights – Subsurface Structures

Massiot et al., 2013

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New Insights – Subsurface Structures

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New Insights – Subsurface Structures

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New Insights – Seismicity

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New Insights – Seismicity

Sepúlveda et al., 2013

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New Insights – Seismicity

Western Fault Central Fault

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Discussion

Definitive 3D interpretation of the structure of Wairakei Geothermal Field is ongoing – but we are getting closer!

Active fault mapping, 3D geological modeling, seismicity, and borehole imagery show dominant NE-SW, and subordinate N-S, and E-W oriented structures in top 3 to 3.5 km of the Wairakei Geothermal Field – the rift trend.

Seismicity shows potential deep (>3 km) structures striking both NE-SW and NW-SE

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DiscussionAll datasets show variation in structure orientation across the Wairakei Geothermal Field and with depth:

• Related to the location of the field in a structurally complex accommodation zone between rift segment?

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DiscussionWhat is the structural data telling us about the nature of the accommodation zone?

Regional NW-SE Fault Model Other Accommodation Fault Models?

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DiscussionAll datasets show variation in structure orientation across the Wairakei Geothermal Field and with depth:

• Linked to stress variation or compartmentalization of the field?

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DiscussionAll datasets show variation in structure orientation across the Wairakei Geothermal Field and with depth:

• Linked to stress variation or compartmentalization of the field?• Linked to variation in the mechanical properties of the fields

lithologies?

Mielke et al., 2015

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DiscussionWhat role is structure playing in permeability?

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Thank You

Acknowledgments:GNS Science - Geothermal Resources of New Zealand’ Direct Core FundingContact Energy Ltd. – Data provision, scientific input