Theoretical Chemical Thermometry on Geothermal Waters: Problems and Methods

Abstract

Using a synthetic geothermal water, we examine the effect of errors in Al analyses on theoretical chemical geothermometry based on multicomponent chemical equilibrium calculations of mineral equilibria. A new approach named FixAl that entails the construction of a modified Q/K graph eliminates problems with water analyses lacking Al or with erroneous analyses of Al. This is made possible by forcing the water to be at equilibrium with a selected Al-bearing mineral, such as microcline. In a FixAl graph, a modified Q/K value is plotted against temperature for Al-bearing minerals. Saturation indices of nonaluminous minerals are plotted in the same way as in an ordinary Q/K graph. In addition to Al concentration errors, degassing of CO 2 and dilution of reservoir water interfere with computed equilibrium geothermometry. These effects can be distinguished in a Q/K graph by comparing curves for nonaluminous minerals to those of aluminous minerals then correcting for CO 2 loss and dilution by a trial and error method. Example geothermal waters from China, Iceland, and the USA that are used to demonstrate the methods show that errors in Al concentrations are common, and some are severe. The FixAl approach has proved useful for chemical geothermometry for geothermal waters lacking Al analysis and for waters with an incorrect Al analysis. The equilibrium temperatures estimated by the FixAl approach agree well with quartz, chalcedony, and isotopic geothermometers. The best choice of mineral for forced equilibrium depends on pH. For most neutral pH waters, microcline and albite work well; for more acidic waters, kaolinite or illite are good choices. Measured pH plays a critical role in computed equilibria, and we find that the best pH to use is the one to which the reported carbonate also applies. Commonly this is the laboratory pH instead of field pH, but the field pH is still necessary to constrain CO 2 degassing. Calculations on numerous waters in the 80–180°C reservoir temperature range indicate that mineral-aqueous equilibrium is probably nearly always achieved, but is obscured by short time-scale processes of dilution or degassing of CO 2 in the near-surface environment.