Studies of thermal waters and thermal wells in the hauraki depression

Abstract

The chemical composition of thermal waters and downhole temperatures of wells has been investigated in the Hauraki Depression. Most of the thermal waters are typically spring or well discharges with low temperatures (< 60°C), low yields (< 3-4 litres/minute), and a sodium-bicarbonate composition. The main compositional differences distinguishing thermal from non thermal waters are their higher concentrations of boron (> 0.4 ppm) and lithium (> 0.15 ppm), low concentrations of sulfate (< 2 ppm), and relatively low ratio values for CI/B (< 40) and Na/Li (< 800). Overlap in these chemical distinctions indicate mixing of thermal and non thermal waters. Downhole temperature profiles show patterns of shallow advective heat transfer in permeable formations at depths less than 350m, indicative of horizontal flow of thermal waters. In contrast to the other thermal waters of the Hauraki Depression, the thermal waters at Miranda have a sodium-chloride composition with very low magnesium concentrations which suggest that they are unmixed. The Te Aroha thermal area is a small, localised, low temperature system which discharges thermal waters of a sodium-bicarbonate composition with high dissolved solids concentrations (≈12 000 mg/kg). Compositional patterns amongst the main thermal discharges reflect a common origin, with varied degrees of mixing with a more dilute shallow water. Geothermometry indicates equilibrium temperatures up to 95°C, which is close to recently measured downhole temperatures. Downhole temperature measurements and surface observations of the geysering wells at Te Aroha have shown the main factors governing their unique geysering process. Hot CO2-rich fluid which enters the well releases bubbles of CO2. As more fluid enters the bore, an increasing volume of degassed fluid is displaced by the rising bubbles of gas until it discharges to the surface. The Te Aroha system is indicated to have a low competence by the decline of spring activity in response to the regular productivity of the geysering bores, and the unfavourable response of the Mokena geyser to minor disturbances.