Preliminary Assessment of the Low Enthalpy Geothermal System at Ohinewai, New Zealand

Abstract

Low enthalpy geothermal systems are widely spread in New Zealand although their exploration and utilization has been relatively limited. There are approximately 105 low-temperature geothermal systems in New Zealand. Approximately thirty of them found within the Waikato region, which is also host to most of the high-temperature systems in New Zealand. This study focuses on Ohinewai low-enthalpy geothermal system, which is located to the North West of the Waikato region in the central north island of New Zealand. A field survey was carried out in Ohinewai geothermal prospect to assess the field potential. Geothermal water samples were collected from self-discharging and pumped warm-water wells. The samples were analyzed for their geochemical signature. The geothermal waters have been classified with respect to their anion and cation contents. The Cl-SO4-HCO3 ternary diagram classifies the geothermal water as dilute alkali chloride waters with relatively lower concentrations of sulphate and bicarbonates. Geothermometers were used for the determination of subsurface or reservoir temperature by assuming equilibrium between specific minerals and the geothermal fluids at depth. Calculated temperature from the geothermometers shows the temperature in the range of 60-100 °C. A geological cross section was developed based on drilling logs/records showed that the Greywacke basement (which is the source of the hot fluid) is located at about 300 to 360 m from ground surface. Isotherm map based on well data indicated that the up-flow zone is likely to be located towards North to North-West of the prospect (with respect to springs in the area). From the geochemical analysis it is inferred that the resource might be small in size. This needs to be confirmed by deep drilling and further detailed geo-scientific studies.