After water vapour, CO₂ is the most abundant gas associated with magmatic hydrothermal systems. The
detection of anomalous soil temperature gradients, and/or a significant flux of magmatic volatiles, is
commonly the only surface signature of an underlying high temperature reservoir. For both heat (as water
vapour) and gas to ascend to the surface, structural permeability must exist, as the unmodified bulk
permeability of reservoir rock is too low to generate the focussed fluid flow typical of magmatic
hydrothermal systems. This thesis reports the investigation into the surface heat and mass flow of the
Ohaaki hydrothermal field using detailed surface measurements of CO₂ flux and heat flow. Detailed
surface measurements form the basis of geostatistical models that quantify and depict the spatial
variability of surface heat and mass flow, across the surface of both major thermal areas, as high
resolution pixel plots. These maps, in conjunction with earlier heat and mass flow studies, enable: (i)
estimates of the pre-production and current CO₂ emissions and heat flow for the Ohaaki Field; (ii)
interpretation of the shallow permeability structures governing fluid flow, and; (iii) the spatial
relationships between pressure-induced ground subsidence and permeability.
Heat flow and CO₂ flux surveys indicate that at Ohaaki the soil zone is the dominant (≥ 70% and up to
99%) pathway of heat and mass release to the atmosphere from the underlying hydrothermal reservoir.
Modelling indicates that although the total surface heat and mass flow at Ohaaki is small, it is highly
focused (i.e., high volume per unit area) relative to other fields within the Taupo Volcanic Zone (TVZ).
Normalised CO₂ emissions are comparable to other volcanic and hydrothermal fields both regionally and
globally. Despite 20 years of production, there is little difference between pre-production and current CO₂
emission rates. However, the similarity of CO₂ emission rates masks a 40% increase in CO₂ emissions
from new areas of intense steaming ground that have developed in response to production of the field for
electrical energy production. This increase in thermal ground emissions is offset by emission losses
associated with the drying up of all steam heated pools and alkali-Cl outflows from the Ohaaki West
(OHW) thermal area, in response to production-induced pressure decline. The location of surface thermal
areas is governed by the occurrence of buried or partially emergent lava domes, whereas the magnitude of
CO₂ flux, mass flow, and heat flow occurring within each thermal area is determined by the proximity of
each dome (thermal areas) to major upflow zones.
Buried or partially emergent silicic lava domes act as cross-stratal pathways for fluid flow, connecting the
underlying reservoir to the surface, and bypassing several hundred metres of the poorly permeable Huka
Falls Formation (HFF) caprock. For each dome complex the permeable structures governing fluid flow
are varied. At Ohaaki West, thermal activity is controlled by a deep-rooted concentric fracture zone,
developed during eruption of the Ohaaki Rhyolite dome. Within the steam-heated Ohaaki East (OHE)
thermal area, flow is controlled by a high permeability fault damage zone (Broadlands Fault) developed
within the apex of the Broadlands Dacite dome. Structures controlling alkali-Cl fluid flow at OHW also
iii
appear to control the occurrence and shape of major subsidence bowls (e.g., the Main Ohaaki Subsidence
Bowl), the propagation of pressure decline to surface, and the development and localization of pore fluid
drainage. Across the remainder of the Ohaaki field low amplitude ground subsidence is controlled by the
extent of aquifer and aquitard units that underlie the HFF, and proximity to the margins of the hot water
reservoir. The correlation between the extent of low amplitude ground subsidence and the margins of the
field reflects the coupled relationship between the hot water reservoir and reservoir pressure. Only where
thick vapour-phase zones buffer the vertical propagation of deep-seated pressure decline to the surface
(i.e., OHE thermal area), is ground subsidence not correlated with subvertical structural permeability
developed within the HFF.
This thesis makes contributions to regional and global research on geothermal and hydrothermal systems
by: (i) quantifying the origin, mass, and upward transport of magmatic carbon from geothermal
reservoirs; (ii) assessing the changes to the natural surface heat and mass flow of the Ohaaki Field
following 20 years of production; (iii) establishing the utility of surface CO₂ flux and heat flow surveys to
identify major upflow zones, estimate minimum mass flow, and determine the enthalpy of reservoirs; (iv)
providing insight into the hydrothermal, structural and lithological controls over hydrothermal fluid flow;
(v) demonstrating the influence of extinct silicic lava domes as important structural elements in the
localisation of hydrothermal fluid flow; (vi) identifying the hydrostructural controls governing the spatial variability in the magnitude of pressure-induced ground subsidence, from which predictive models of subsidence risk may be defined, and; (vii) developing new technologies and characterising methods used for detailed assessment of surface heat and mass flow.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/5027 |
| Date | January 2010 |
| Creators | Rissmann, Clinton Francis |
| Publisher | University of Canterbury. Geological Sciences |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Clinton Francis Rissmann, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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