Hot or cold: a review of geothermometry at the Kawerau and Nga Tamariki fields, Taupo Volcanic Zone, New Zealand

Abstract

Geothermometers are widely employed to determine reservoir enthalpy in both exploration and production scenarios. This paper evaluates the conditions under which silica and cation geothermometers provide valid temperatures, with case examples from the Kawerau and Nga Tamariki reservoirs. Geothermometry temperatures are presented in conjunction with tracer flow testing (TFT) and downhole survey results (i.e. PTS). Notably, at both fields, enthalpy deficit conditions are identified, where enthalpy
determined from quartz solubility (i.e. Fournier and Potter, 1982a,b) exceeds TFT and PTS values. In this paper, an iterative spreadsheet-based approach is demonstrated for correcting enthalpy deficit. The spreadsheet method presented here provides comparable outputs to the WATCH geothermal chemical speciation software package, with corrected TQtz using either approach within <1°C. At both Kawerau and Nga Tamariki, corrected quartz thermometry provides valid characterization of near-well reservoir conditions, as established by comparison to feedzone PTS temperatures. At Kawerau, cation geothermometer (NaK, NaKCa) results are lower than TQtz and PTS temperatures. Lower cation geothermometer temperatures likely indicateincomplete equilibration of cation ratios as fluids transit from
lower temperature injection zones to production areas, while quartz temperatures demonstrate that enthalpy remains stable in production areas. These examples highlight that correcting for enthalpy deficit improves the accuracy of quartz geothermometry. In cases where equilibrium is not attained, geothermometers may be used to diagnose the impact of ongoing changes in reservoir state due to production, reinjection, or marginal fluid incursion. Incorporating geothermometry into routine monitoring can enable better conceptual understanding of changes in reservoirs over time.