Ohaaki geothermal system: Refinement of a conceptual reservoir model

Mroczek, Ed; Milicich, Sarah D.; Bixley, Paul F.; Sepúlveda, Fabián; Bertrand, E. A.; Soengkono, S.; Rae, Andrew

doi:10.1016/j.geothermics.2015.09.002

Cited by 30 publications

(16 citation statements)

References 32 publications

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“…Likewise, large displacements of ~350 ka old deposits sealed by ~230 ka old deposits have been interpreted on the eastern side based on seismic reflection and gravity data (Stagpoole, ) (location 6 in Figure b and Text S1). Near the eastern boundary of the modern rift, borehole data at the Ohaaki geothermal field show clear faulting in pre‐350 ka deposits (location 7a in Figure b) and some intra‐350 ka deposits, but no faulting after ~280 ka (Milicich et al, ; Mroczek et al, ). Finally, in the Kawerau geothermal field (location 5), an approximately 2 km inward fault migration from the old to the modern rift boundary can be inferred from interpretation of timing and location of faulting in boreholes (Milicich, Wilson, Bignall, Pezaro, Charlier, et al, ) and aerial photo reviews (this study).…”

Section: Evidence For the Evolution Of The Taupo Riftmentioning

confidence: 99%

Rapid Evolution of Subduction‐Related Continental Intraarc Rifts: The Taupo Rift, New Zealand

et al. 2017

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The evolution of the continental intraarc Taupo Rift in the North Island, New Zealand, is rapid, significantly faster than comparative intracontinental rifts such as the African Rifts. Based on our faulting data and published geological, geophysical, and borehole data, we show that activity in the ~2 Ma Taupo Rift has rapidly and asymmetrically narrowed via inward and eastward migration of faulting (at rates of approximately 30 km Myr−1 and 15 km Myr−1, respectively) and has propagated southward along its axis ~70 km in 350 kyr. The loci of voluminous volcanic eruptions and active faulting are correlated in time and space, suggesting that a controlling factor in the rapid rift narrowing is the presence of large shallow heterogeneities in the crust, such as large rhyolitic magma bodies generated by subduction processes, which weaken the crust and localize deformation. Eastward migration of faulting also follows the eastward migration of the volcanic arc which may be related to rollback of the Pacific crust slab at the Hikurangi subduction zone. Southward propagation of the rift is linked with southward migration of the Hikurangi Plateau/Chatham Rise subduction point and occurs episodically aided by stress changes associated with voluminous local volcanism. The large magma supply during early continental intraarc rift stages explains faster evolution (from tectonic to magmatic) than intracontinental rifts. However, the fast changes in magma supply from the subduction zone can also lead to evolution reversals (more evolved magmatic stages reverting to less evolved tectonic stages), rift cessation, and thus failed continental breakup.

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Section: Evidence For the Evolution Of The Taupo Riftmentioning

confidence: 99%

Rapid Evolution of Subduction‐Related Continental Intraarc Rifts: The Taupo Rift, New Zealand

et al. 2017

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“…Some rhyolite breccias are (or are inferred to be) permeable in TVZ geothermal fields (e.g., Ohaaki [ Mroczek et al , ] and Wairakei [ Milloy and Lim , ]). At Rotokawa, fracture density in cores is higher in lavas than in breccias.…”

Section: Discussionmentioning

confidence: 99%

“…This 6.6 m long core is strongly altered and has a porosity of 25–35% [ Rae et al , ], which is significantly higher than the typical porosity of <10% reported for andesites [ Siratovich et al , ]. In partially welded and strongly altered tuff lithologies, fluid flow is thus likely controlled primarily by matrix porosity rather than by fractures, even when strongly altered, as inferred at the Mokai and Ohaaki Geothermal Fields, TVZ [ Bignall et al , ; Mroczek et al , ].…”

Section: Discussionmentioning

confidence: 99%

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

et al. 2017

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Analysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite‐hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self‐similar behavior in 3‐D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite‐hosted geothermal systems.

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“…Some rhyolite breccias are (or are inferred to be) permeable in TVZ geothermal fields (e.g., Ohaaki [Mroczek et al, 2016] and Wairakei [Milloy and Lim, 2012]). At Rotokawa, fracture density in cores is higher in lavas than in breccias.…”

Section: Effect Of Lithology and Alteration On Fluid Pathwaysmentioning

confidence: 99%

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high-temperature geothermal field

Massiot¹,

Nicol²,

McNamara³

et al. 2021

Preprint

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©2017. American Geophysical Union. All Rights Reserved. Analysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite-hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self-similar behavior in 3-D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite-hosted geothermal systems.

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Ohaaki geothermal system: Refinement of a conceptual reservoir model

Cited by 30 publications

References 32 publications

Rapid Evolution of Subduction‐Related Continental Intraarc Rifts: The Taupo Rift, New Zealand

Rapid Evolution of Subduction‐Related Continental Intraarc Rifts: The Taupo Rift, New Zealand

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high-temperature geothermal field

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