Fluid Mineral Equilibria in Geothermal Reservoirs from the Great Basin: Implications for Chemical Geothermometry

Abstract

Partial to nearly full fluid-mineral equilibrium exists between common aqueous species in reservoir fluids and the mineral assemblage quartz (chalcedony), Na-feldspar, K-feldspar, K-mica, Mg-chlorite (clinochlore), calcite, and heulandite in Great Basin geothermal systems. For chemical geothermometry, the silica geothermometer based on quartz/chalcedony solubility appears to be the most reliable indicator of reservoir temperature. Although Na/H vs K/H ratios plot on the Na-feldspar-K-feldspar equilibrium line (~250°C), application of the Na-K geothermometer overestimates reservoir temperature, possibly because equilibration occurs at conditions that are hotter and deeper than existing feed points in geothermal wells. A linear trend in Na/K versus temperature permits formulation of a new empirical geothermometer that has yet to be tested and proven. Modest to low confidence in the application of the K-Mg geothermometer is expected because the reservoir K/Mg ratios are scattered with respect to Mg-chlorite-K-mica-K-feldspar-quartz equilibria.