CO₂ and Heat Release from Magmatic Hydrothermal Systems: Insights from CO₂ Flux and Other Modern Methods

Abstract

Magmatic hydrothermal systems are of interest as a renewable energy source. In areas of high permeability, these systems are commonly manifested at the Earth’s surface as areas of steam heated ground. Ground based heat flux and CO2 flux surveys typically result in 100’s or 1000’s of individual flux measurements. These datasets are usually integrated to provide measures of total flow for the surface area under investigation (i.e. total heat flow, total CO2 flow). Yet very few studies have investigated the relationship between these datasets and the deep geothermal reservoir, which provides the research gap and focus of this research. The goal of this thesis is to determine whether statistical analysis of CO2 flux and heat flux datasets can provide information on the deep reservoir; to extend the interpretation of survey results beyond consideration of integrated, bulk flow. Chapter 3 shows CO2 flux datasets may be related to the specific geological and hydrological setting of each system, and recharge in particular. In Chapter 4, CO2 flux is combined with 13C isotope measurements at Te Mihi, Wairakei. Isotope analysis allows the relative proportions of biological and geothermal CO2 in a flux measurement to be determined. Results show the well-developed hydrothermal capping formation at Wairakei may prevent widespread diffuse degassing, at least at flux-levels that would allow the signal to be distinguished from the biological background. Chapter 5, demonstrates a new methodology for collection of aerial thermal infrared (TIR) imagery; a thermal camera mounted on a drone provides a 2.2 km2 high-resolution TIR orthophoto of the Waikite thermal area. Temperature calibration of the orthophoto allows a probabilistic estimate of heat loss (36 MW) from thermal waters in the survey area. In Chapter 6, TIR imagery is combined with CO2 flux and heat flux surveys at Wairakei, Tauhara and Reporoa thermal areas. Surface temperature, CO2 flux and TIR imagery are spatially correlated, which shows steam and CO2 follow a similar pathway from reservoir to surface. CO2/H2O vapour ratios, derived from CO2 flux and heat flux, were transformed to units of temperature using a pre-existing CO2 geothermometer. Results show good correlation between geothermometer temperatures and measured aquifer temperatures at Wairakei-Tauhara. Geothermometer results at Reporoa suggest high reservoir temperatures. Chapter 7 applies the geothermometer (TCO2 flux) to a variety of other systems with known deep reservoir temperatures. Results confirm CO2 flux and shallow temperature datasets provide a novel geothermometer with application to geothermal surface exploration. In summary, the thesis shows CO2 flux datasets may be related to the specific geological and hydrological setting, and reservoir temperature of each system.