BIBLIOGRAPHIC REFERENCE Bignall, G.; Milicich S. D. 2012. Kawerau Geothermal Field: Geological Framework, GNS Science Report 2012/33. 35 p. G. Bignall GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3350, New Zealand. [email protected] S.D. Milicich GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3350, New Zealand. [email protected] © Institute of Geological and Nuclear Sciences Limited, 2012 ISSN 1177-2425 ISBN 978-1-972192-20-7 CONTENTS ABSTRACT ...........................................................................................................................II KEYWORDS ......................................................................................................................... V 1.0 INTRODUCTION ........................................................................................................ 1 2.0 REGIONAL GEOLOGICAL SETTING ........................................................................ 1 3.0 KAWERAU SURFACE GEOLOGY ............................................................................ 3 4.0 KAWERAU SUBSURFACE GEOLOGY ..................................................................... 3 5.0 STRUCTURE OF THE KAWERAU GEOTHERMAL FIELD ..................................... 10 6.0 STRUCTURAL - STRATIGRAPHIC CONTROLS ON PERMEABILITY ................... 12 6.1 Stratigraphic permeability ..............................................................................13 6.2 Structural permeability ...................................................................................13 7.0 HYDROTHERMAL ALTERATION ............................................................................ 14 8.0 GEOLOGICAL HAZARDS ....................................................................................... 15 9.0 GEOLOGICAL MODEL OF THE KAWERAU GEOTHERMAL FIELD ..................... 16 10.0 REFERENCES ......................................................................................................... 18 FIGURES Figure 1. Location of Kawerau Geothermal Field, showing mapped faults, inferred caldera boundaries (Gravley et al., 2007) and outcropping Mesozoic greywacke. ............................................................ 2 Figure 2. Kawerau Geothermal Field, showing geothermal well locations and well tracks ................................ 5 Figure 3. Schematic geological cross-section of the Kawerau Geothermal Field, showing the relationship of the main stratigraphic units ....................................................................................... 10 Figure 4. Aerial photograph of the Kawerau area, showing inferred fault locations......................................... 11 Figure 5. Conceptual model of the Kawerau Geothermal Field ....................................................................... 17 TABLES Table 1. Generalised stratigraphy of the Kawerau Geothermal Field. .............................................................. 7 APPENDICES Appendix 1 Geothermal (Geoscience) Glossary ................................................................................................. 24 GNS Science Report 2012/33 i ABSTRACT The Kawerau Geothermal Field is one of the longest-lived active geothermal systems in the Taupo Volcanic Zone (TVZ) and is comparable in size to a number of other New Zealand geothermal fields developed for electrical power generation. Geological information on the Kawerau Geothermal Field has markedly increased over the last 30 years, and there is now a clearer understanding of the field compared with the state of knowledge when exploration drilling began in the 1950s. The conceptual model of the Kawerau geothermal system points to deep reservoir fluid moving towards the surface (and laterally) via NE-trending normal faults (and NW-trending cross-faults) and fractures of high local permeability, which provide drilling targets within otherwise relatively impermeable ignimbrite, andesite and basement greywacke. The faults and fractures pass into overlying pyroclastic units and lacustrine sediments that infill the downfaulted Whakatane Graben, where hot water spreads laterally (northwards) along subhorizontal permeable volcanic and sedimentary formations. Intra-formation sediments, welded ignimbrite and hydrothermal eruption breccias are important aquicludes that separate near-surface aquifers from deeper reservoir fluids. Geological Setting: • Geothermal activity at Kawerau occurs at the southern end of the Whakatane Graben (an area of active extension, faulting and regional subsidence), where the NE-striking active rift of the TVZ intersects N-trending strike-slip faults of the North Island Shear Belt. • During the last ~1 million years, Mesozoic greywacke basement within the Whakatane Graben has downfaulted to 1-2 km below sea level, with the resultant NE-trending fault- bounded structural depression infilled by a sequence of locally-erupted Quaternary rhyolite, dacite and andesite lavas, ash-flow (ignimbrite) and lacustrine sediments. • Hydrothermal eruption breccias have been inferred in the Kawerau stratigraphy, overlain by Matahina Formation ignimbrite, which point to geothermal activity in the Kawerau area for possibly >320,000 years. • The Whakatane Graben is an area of active extension and faulting. Geodetic surveys reveal an average spreading rate of ~7-8 mm/year across the structural depression. Kawerau Surface Geology: • The Kawerau area is covered by debris from recent Mt. Tarawera eruptions; deposits from Tarawera River flooding, and hydrothermal eruption breccias (associated with ~15,000 to 9,000 year-old hydrothermal events). • Putauaki (Mt. Edgecumbe) is a multiple vent dacite-andesite volcano, near the southern boundary of the Field, with a youngest dated eruption of 2,400 years B.P. The main cone is <5,000 years old, which is much younger than the age of the geothermal system. Kawerau Stratigraphy: • The stratigraphy of the Kawerau Geothermal Field has been established from the study of core and cuttings from about 60 deep exploration, production and injection geothermal wells (KA1-50 and PK1-8) and several shallow monitor holes (KAM1-11). GNS Science Report 2012/33 ii • Rock units in the near surface, or outcropping at the surface, include: Recent Alluvium, Putauaki Volcanics, Hydrothermal Eruption Breccias, Unconsolidated Pyroclastics (commonly referred to as “Rotoiti Breccia”) and Onepu Formation (rhyodacite domes). • To ~500 m depth, the field stratigraphy comprises welded ash-flow and pumice lapilli tuff of the 320,000 year old Matahina Formation ignimbrite; unwelded pumice-rhyolite lapilli and intercalated water-laid(?) tuff of the Tahuna Formation (including units previously logged as Huka Group sediments, Tahuna Breccia and Onepu Ash), and buried brecciated to massive rhyolite domes of the Caxton Formation. • With increasing depth, the stratigraphy at Kawerau comprises Karaponga Formation (tuffaceous sandstone, siltstone and partly welded crystal-vitric tuff), Onerahi Formation (muddy breccias, tuffaceous sandstone and siltstone), Kawerau Andesite (lava and breccias), Raepahu Formation (welded crystal-lithic tuff/breccia), Tasman Formation (breccia, sandstone and siltstone), Te Teko Formation (lenticulitic ignimbrite, crystal-lithic tuff and breccia) and Tamurenui Subgroup (including terrestrial sedimentary units of the Rotoroa Formation, and greywacke conglomerate/gravels of the Waikora Formation). • Interbedded sandstone and argillite, hardened by low grade regional metamorphism (Torlesse greywacke basement), has been drilled in 29 wells from Kawerau, from about -670 mRL on the upfaulted south-eastern margin of the Whakatane Graben (e.g. from - 681 mRL in KA26, -667 mRL in KA29) and at greater depth towards the north-western margin (e.g. -1078 mRL in KA21, -1110 mRL in KA28 and -1285 mRL in KA48). Structure of the Kawerau Geothermal Field: • The greywacke basement at Kawerau is step-faulted on NE-trending normal faults, downthrown to the north-west. Combined with cross-cutting NW-trending faults this has produced a series of NE-plunging fault blocks. Some faults have been intersected by drillholes, which show basement faulting mainly occurred before eruption of the Kawerau Andesite, since inferred displacements decrease upward along the fault planes. • The surface locations of many faults are not known, as young sediments mask their traces. The Rotoitipaku Fault (~2 km NW of Kawerau) reveals at least 5 major, normal displacement events during the last 8,500 years, with slip rates of 1-11 mm/year, and evidence movement may have been be associated with hydrothermal eruption events. • The Edgecumbe Fault ruptured during the 1987 Edgecumbe Earthquake (with 0.4 m downthrow to its NW-side). This highlights the potential for movement on existing faults within the geothermal system, which could enhance permeability within the field. Structural and Stratigraphic Controls on Permeability: • The production area at Kawerau is characterised by several permeable, formation- hosted reservoirs, and low permeability units (e.g. lacustrine sediments of the Tahuna Formation) that separate near-surface waters from deep reservoir fluids. Lithological permeability is mainly confined to fractured rock units (e.g., Kawerau Andesite), and some sedimentary units within the Tamurenui Subgroup (including Waikora Formation). • Welded ignimbrite can form barriers restricting vertical fluid flow, although unwelded layers also exist (e.g. in Matahina Formation), which allow lateral fluid flow. Fracture GNS