Dempsey D.E.,University of Auckland |
Simmons S.F.,Hot Solutions Ltd. |
Archer R.A.,University of Auckland |
Rowland J.V.,University of Auckland
Journal of Geophysical Research: Solid Earth | Year: 2012
Hydrothermal convection in the Taupo Volcanic Zone (TVZ), New Zealand, is driven by heat extracted at the brittle-ductile transition, which is in turn supplied by magmatic intrusion of the lower-crust. We present a numerical model that approximates this circulation in a statistical sense, being constrained by TVZ dimensions and mean thermal properties, but incorporating a permeability distribution of arbitrary heterogeneity. A particle tracking methodology that accounts for advective and dispersive transport is introduced, and ensembles of several tens of thousands of flow paths are constructed for each of the modeled geothermal systems. These flow paths reveal the nature of mass recharge in multicell convective systems, and suggest the ages of waters in TVZ geothermal systems vary between 5 and 50 kyr. Flow path ensembles are used to delineate catchments for each of the modeled systems and a method for calculating their areas is introduced. Catchment area is shown to be proportional to system heat output and area, which is consistent with a 1-D analytical model of heat and mass transfer. As an approximation to catchments delineated by particle tracking, we outline a method of Voronoi tessellation based on the positions and heat outputs of geothermal systems. This method is used to delineate catchments for geothermal systems in the TVZ and Iceland. © 2012. American Geophysical Union. All Rights Reserved.
Sepulveda F.,Contact Energy Ltd |
Rosenberg M.D.,Institute of Geological & Nuclear Sciences |
Rowland J.V.,University of Auckland |
Simmons S.F.,Hot Solutions Ltd
Geothermics | Year: 2012
Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field were used for geostatistical predictions using Kriging. In order to adequately constrain Kriging models, anisotropy and trends associated with temperature and stratigraphy were studied using standard variogram analysis, in combination with new regional and local structural data, revised gravity, and available geoscientific and reservoir data. This combined analysis lead to the incorporation of horizontal anisotropy (horizontal to vertical correlation ranging from 8:1 for regional stratigraphic units to 4:1 for local rhyolite bodies) in the case of stratigraphic models and variable anisotropy in the case of temperature models. In the latter, the variable anisotropy was represented by two end members: an isotropic model (horizontal to vertical correlation of 1:1) representative of depths >2000. mGL, and an anisotropic model (horizontal to vertical correlation of 3:1) representative of depths <1000. mGL. Kriging models of temperature also incorporated a vertical trend which is a combination of two end members at Wairakei: Boiling-Depth-Point Curve (convective) and linear (conductive). The Kriging models succeeded in identifying the primary geological controls on temperature distribution: major upflows largely controlled by structures at depth (>1000. m depth) and shallow (<1000. m depth) outflows stratigraphically channelled through formation contacts and rhyolite edges. A combination of stratigraphy and faults explain local cold downflows in shallow (750-1000. m depth) parts of the field. © 2012 Elsevier Ltd.
Rowland J.V.,University of Auckland |
Simmons S.F.,Hot Solutions Ltd
Economic Geology | Year: 2012
Geologic controls on development of high-flux hydrothermal conduits that promote epithermal ore formation are evaluated at large and small scales for geothermal systems of the Taupo Volcanic Zone, New Zealand. Most geothermal systems occur within a rifted volcanic arc (~150 km long) dominated by silicic volcanism, and they occur in association with major faults near caldera structures or within accommodation zones that transfer extension between rift segments. The geothermal systems are hosted in a thick sequence (1->3 km) of young volcanic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and graywacke. Flow regimes and permeability controls in one extinct (Ohakuri) and six active (Broadlands-Ohaaki, Waiotapu, Rotokawa, Waimangu, Te Kopia, and Orakeikorako) geothermal systems show that in general, hydrothermal fluid flow is controlled by (1) heat from magmatic intrusions which drives convective circulation; (2) intergranular host-rock porosity and permeability; (3) fault-fracture network permeability produced by tectonism, volcanism, and/or diking; (4) pipelike vertical conduits produced by volcanic and hydrothermal eruptions; and (5) hydrothermal alteration and mineral deposition that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: (1) a feed zone (>2,000 m depth), (2) an epithermal mineralization zone (<200-2,000 m depth), and (3) a discharge zone (0-200 m depth). Within the deepest part of the feed zone, hydrothermal fluid flow is influenced by magmatic intrusions guided by faults, which localize convection cells, and the brittle-ductile transition at the base of the seismogenic zone, which limits downflow of meteoric water. Hydraulic connectivity through low-permeability Mesozoic rocks is favored along NNE- to ENE- and WNW- to NNW-striking structures given the NW-SE direction of maximum extension (~10 mm/yr). In the epithermal mineralization zone, high-flux structures extend upward from the feed zone and transmit fluids to shallow depths, analogous to a geothermal production well. The host stratigraphic interval is dominated by porous pyroclastic deposits and distributed flow can be widespread until the intergranular permeability is reduced by hydrothermal alteration or where dense, low-porosity, high-tensile strength rocks exist. Distributed fluid-flow accounts for large volumes of hydrothermal alteration extending 10 to >100 km 3 that encloses geothermal reservoirs and high-flux fluid conduits. Fracture-dominated flow becomes important with decreasing porosity induced by hydrothermal alteration. In the discharge zone, the reduction in confining pressure, combined with mineral deposition and alteration, hydrothermal eruptions, and interplay of hot and cold waters create complex, but strongly localized flow paths that feed hot springs. The permeability structure conducive to epithermal vein formation is analogous to a geothermal well: short in horizontal dimension (10s-100s m) but long in vertical dimension (>1,500 m) and possibly pipelike in shape. Episodic high-flux occurs over time scales of tens to thousands of years to accumulate sufficient amounts of gold and silver to form orebodies. During these episodes when faults and fractures are dilated, development of an upward-expanding column of boiling fluid promotes rapid ascent and high mass flow but also promotes silica and calcite precipitation, which can quickly reduce hydrothermal flow. Seismic activity and/or dike intrusion create and reactivate these high-flux pathways through extension and extensional shearing, caused by low differential stresses. The Taupo Volcanic Zone is highly prospective for epithermal-style mineralization, but the predominance of weak porous host rocks at shallow depths is prone to disseminated-style mineralization (e.g., Ohakuri). Structurally controlled mineralization forms in volcanic rocks where they have been embrittled by silicification through seismicity and fault displacement, caldera-forming eruptions, and dike intrusion. © 2012 Society of Economic Geologists, Inc.