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Mighty River Power Limited is a New Zealand electricity generation and electricity retailing company. It was formed from the breakup of the Electricity Corporation of New Zealand in 1999 as a result one of the reforms of the New Zealand Electricity Market and corporatised to become a state-owned enterprise with its own board of directors and Ministerial shareholders. A state-owned enterprise from its founding, it was partially privatised by the Fifth National Government in May 2013, with the government retaining a 51.78 percent shareholding and the remaining 48.22 percent listed on the stock market.The company owns and operates the hydroelectric generating stations on the Waikato River as well as geothermal plants in the Taupo area, the gas fired Southdown plant in south Auckland and the largely unused plant at Marsden Point near Whangarei.In 2013, Mighty River Power generated 15% of the country's electricity and had a market share of 19%. According to its own website, the company supplies 22% of New Zealand peak energy demand, with about 80% of this coming from hydro-power. Wikipedia.

Milicich S.D.,Institute of Geological & Nuclear Sciences | Clark J.P.,Mighty River Power | Wong C.,Mighty River Power | Askari M.,Mighty River Power
Geothermics | Year: 2015

Drawing on earlier work of scientists and engineers, along with results from recent drilling, this paper describes the current knowledge of the geology, geophysics and fluid chemistry of the Kawerau Geothermal Field. The historical and recent data have been evaluated and integrated to present a consistent hydrogeological model of the field.The field extends over an area of 22km2, with a natural surface to near-surface heat flow of 100-150MWt (thermal) (Bromley, 2002). Drilling began at Kawerau in 1951 to supply steam to the Tasman pulp and paper processing plant and for power generation, with continued drilling since this time resulting in more than 70 wells being drilled. There is currently ~121MWe (electrical) generation commissioned at Kawerau.Deep hot water (>300. °C), sourced in the south, moves across and flows north into the field, through faults and fractures in otherwise impermeable basement greywacke into the overlying volcano-sedimentary sequence. The hot fluids spread laterally into permeable zones along sub-horizontal volcanic and sedimentary layers, with lacustrine sediments, and welded intervals within some ignimbrite units, acting as localised aquitards. © 2015 Elsevier Ltd.

Cole J.W.,University of Canterbury | Spinks K.D.,Mighty River Power | Deering C.D.,University of Canterbury | Nairn I.A.,45 Summit Road | Leonard G.S.,Institute of Geological & Nuclear Sciences
Journal of Volcanology and Geothermal Research | Year: 2010

The Okataina Volcanic Centre (OVC) contains the northeasternmost caldera complex in onshore Taupo Volcanic Zone (TVZ), New Zealand, sited largely within the Taupo Rift. Pyroclastic fall deposits and ignimbrites deposited in the Bay of Plenty coast area between 420 and 625 ka were probably erupted from OVC, and these provide a maximum age for the centre. The earliest ignimbrite for which there is good evidence for eruption from OVC is the c.550 ka Quartz-biotite Ignimbrite, exposed to the west of Lake Okataina and to SE of OVC. This ignimbrite probably correlates with one of the early ignimbrites found in the Kawerau geothermal wells, and is large enough to have been accompanied by caldera collapse. It was followed by extensive rhyolitic explosive eruptions of the Murupara Subgroup, culminating with eruption of the Matahina Ignimbrite at c.325 ka. This c.160 km3 (magma volume) eruption was accompanied by caldera collapse to form the southern part of the present day Okataina caldera complex. A long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of OVC, before eruption of the > 100 km3 Rotoiti Pyroclastics at c.61 ka was accompanied by caldera collapse on the northern side of the centre. The Rotoiti episode was followed by an intensive period of intra-caldera volcanic activity which is still going on today. The Mangaone Subgroup pyroclastics were erupted between 40 and 31 ka, and include the c.33 ka Kawerau Ignimbrite (∼ 20 km3), large enough to have caused further minor caldera collapse. In the last 26 ka, nine rhyolite eruption episodes have built the Haroharo and Tarawera lava and pyroclastic massifs (> 85 km3 magma volume) within the caldera complex. The structural boundaries of the OVC calderas are buried by the products of later eruptions, but are probably controlled by regional tectonic features. Both the Matahina and Rotoiti calderas appear to have embayments which represent downsags where magma has migrated along regional structures associated with the Taupo Rift. OVC is sited at a major offset within the young Taupo Rift and represents a structurally complex transfer zone. Some early rhyolite domes are aligned north-northwest suggesting control by structures in the subvolcanic basement, while more recent domes are aligned northeastwards, reflecting the orientation of the Taupo Rift. Southwestward propagation of the axial rift of the Whakatane segment and northeastward propagation of the Kapenga segment have created two linear vent zones through OVC (Haroharo and Tarawera). At Tarawera, fissures and near surface dikes formed during the 10 June 1886 basalt eruption are oblique to the vent lineation suggesting some near surface strike-slip component consistent with OVC being in a zone of transtension. © 2009 Elsevier B.V. All rights reserved.

Milicich S.D.,Victoria University of Wellington | Milicich S.D.,Institute of Geological & Nuclear Sciences | Wilson C.J.N.,Victoria University of Wellington | Bignall G.,Institute of Geological & Nuclear Sciences | And 4 more authors.
Journal of Volcanology and Geothermal Research | Year: 2013

Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons from altered volcanic units penetrated by drillholes at Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ), New Zealand. Drillholes penetrate 700-1300. m of volcanic rocks and sediments before reaching the basement Mesozoic greywacke. Twenty-seven samples of altered volcanic lithologies and two surficial, fresh rock units have been studied in order to constrain ages of the major stratigraphic units. Within the volcanic/sedimentary pile the oldest in-situ ignimbrites that can be widely correlated have ages of ~. 1.45. Ma. Between them and the basement greywacke is a variable thickness of sediments, mostly greywacke gravels and minor volcaniclastic units, reflecting localised basinal deposition associated with strike-slip faulting. Two ignimbrites within this sequence yield age estimates of c. 2.4 and 2.1. Ma, consistent with these being distal southern Coromandel Volcanic Zone deposits, pre-dating TVZ activity. Below the regional marker plane of the 0.32. Ma Matahina ignimbrite, three main ignimbrite groups occur, with ages around 1.45. Ma, 1.0. Ma and 0.6-0.5. Ma, which are separated by sediment-dominated intervals and andesite volcanics. All of these ignimbrites represent marker horizons from other volcanic centres and do not reflect the presence of local magmatic heat sources.Numerous bodies of coherent rhyolite, previous labelled as Caxton and Onepu rhyolites, have been intersected at all pre-Matahina ignimbrite levels (including within the basement greywacke) and reflect earlier local magmatic heat sources. Our geochronological data resolve these rock bodies into three packages. The youngest is represented by the surficial rhyodacite Onepu domes, 40Ar/39Ar dated at 0.138±0.007Ma. U-Pb ages on zircons from dome material yield a spectrum that can be matched (consistent with petrography) with two dikes intersected at 880m and 2.67km depth, and with an estimated age of 0.15±0.01Ma (Onepu Formation). The older two packages consist of older crystal rich (~15%) and younger crystal-poor (~5%) rhyolite, here grouped as Caxton Formation and with eruption/intrusion age of 0.36±0.03Ma. The shallowest Caxton rhyolite bodies are interpreted to be domes, whilst deeper intersections are inferred to be sills based on the lateral extents relative to thicknesses.Net subsidence rates inferred from depths to key units do not reflect the present-day situation. Modern rates of subsidence (2 ± 1. mm/yr) associated with TVZ rifting processes can have been active for no more than ~. 50,000. years, based on elevation differences of the Matahina ignimbrite top surface. An inferred change in intrusion geometry from sill (Caxton) to dike (Onepu) indicates a change in principal stress orientations reflecting onset of the modern Whakatane Graben. This change is dated at ~. 0.37. Ma in coastal sedimentary sequences 23. km to the north of Kawerau, consistent with our age data. Although previously interpreted to be a long-lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence. © 2012 Elsevier B.V.

Mcnamara D.D.,Institute of Geological & Nuclear Sciences | Massiot C.,Victoria University of Wellington | Lewis B.,Institute of Geological & Nuclear Sciences | Wallis I.C.,Mighty River Power
Journal of Geophysical Research B: Solid Earth | Year: 2015

Geometric characterization of a geothermal reservoir's structures, and their relation to stress field orientation, is vital for resource development. Subsurface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand, have been studied, for the first time, using observations obtained from analysis of three acoustic borehole televiewer logs. While an overall NE-SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well due to proximity to variously oriented, graben-bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures clusters within certain depth intervals have antithetic dip directions to the well's dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general SHmax orientation of NE-SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling-induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE-SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates. ©2014. American Geophysical Union.

Chambefort I.,Institute of Geological & Nuclear Sciences | Lewis B.,Institute of Geological & Nuclear Sciences | Wilson C.J.N.,Victoria University of Wellington | Rae A.J.,Institute of Geological & Nuclear Sciences | And 3 more authors.
Journal of Volcanology and Geothermal Research | Year: 2014

Recent drilling at the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand has provided new constraints on the stratigraphy and volcanic evolution of the region. Over 2800m thickness of volcanic products are present at Ngatamariki, mainly comprised of rhyolitic ignimbrites linked to large caldera-forming events at sources outside the field area, but locally sourced andesite and rhyolite lavas and domes are also encountered. Most of the rocks are allocated to the pre-0.35Ma Tahorakuri Formation. Crystallisation age spectra (and consequent best estimates of eruption age) have been obtained by U-Pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is an ignimbrite, which yields an eruption age estimate of 1.85±0.06Ma. This ignimbrite, plus comparable-aged units dated at the adjacent Rotokawa and Ohaaki geothermal fields, are interpreted to represent the oldest silicic volcanic deposits in the area, and onlap the basal andesite lava pile that is best developed at Rotokawa to the south. Other pyroclastic units and associated volcaniclastic sediments (with another intercalated andesite lava unit) return age estimates between 1.85±0.06 and 0.701±0.039Ma. Between ~0.7 and 0.35Ma, contemporaneous surface lithologies in the Ngatamariki area are dominated by sediments, with subordinate lava domes. Between 0.716±0.017 and 0.655±0.016Ma at least three shallow (to<2km depth) intrusions were emplaced under the northern part of the field: a diorite, microdiorite and a large tonalite body totalling >5km3. The intrusions generated a large alteration halo (~25km3 minimum) and intense silicification of the wall rocks. At 0.35 and 0.34Ma the area was buried by two ignimbrite packages of the Whakamaru Group, erupted from sources just west and well north, respectively, of the field. Ignimbrites of the Waiora Formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0.282±0.011 and 0.257±0.011Ma, coeval with more extensive volcanic activity in the Maroa dome complex west of the field. Sediments of the Huka Falls Formation and deposits of the 25.4±0.2ka Oruanui eruption then cap and seal the system. The new U-Pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the Ngatamariki Geothermal Field. The field, despite >2.8km of subsidence, does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of Quaternary age. Two chemically and temporally distinct hydrothermal events are located at the Ngatamariki field, with no evidence of continuity between the two. © 2014 Elsevier B.V.

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