Impact of order–disorder in secondary minerals on the multicomponent geothermometry of formation waters and geothermal waters

Multicomponent geothermometry is a critical tool for estimating subsurface temperatures, but its accuracy can be significantly affected by the crystalline order–disorder state of minerals, which is often inadequately represented in standard thermodynamic databases. This study addresses this limitation by systematically incorporating variably ordered forms of key secondary minerals. We demonstrate that the availability of these forms, characterized by the order–disorder parameter Q, and the ability to select the mineral form with the right degree of order–disorder, leads to substantial improvements in the application and reliability of multicomponent geothermometry. This was achieved by first calculating the thermodynamic properties of disorder (Gibbs free energy, enthalpy, entropy, volume) for a suite of common secondary minerals including feldspars (adularia, albite, anorthite), carbonates (calcite, dolomite, ankerite), micas (muscovite, paragonite, phlogopite), iron oxides (hematite, magnetite), iron sulfides (pyrrhotite), sphene, and quartz using Landau theory, for Q values ranging from 0 (completely disordered) to 1 (completely ordered). These data were then used to significantly expand the SUPCRT92 thermodynamic database and subsequently the PHREEQC-LLNL database. The enhanced geothermometric approach, utilizing PHREEQC with the modified database, was then applied to two formation waters from the Temblor Formation in the Kettleman North Dome oil and gas field, California, and fifteen geothermal waters from the geothermal fields of Reykjanes and Svartsengi (Iceland), Tauhara (New Zealand), Miravalles (Costa Rica), and Kizildere (Turkey). In these case studies, selecting the mineral form with the appropriate Q value resulted in improved convergence of mineral saturation indices (log Q/K) at plausible equilibrium temperatures. This often led to better agreement with independently estimated reservoir conditions or known authigenic/hydrothermal mineral assemblages compared to calculations using only fully ordered or unspecified mineral forms. This achievement is crucial, as crystalline order–disorder significantly impacts the thermodynamic stability and reactivity of many solid phases. These minerals, present as diagenetic (authigenic) phases in oil-and-gas field aquifers and as hydrothermal alteration products in geothermal reservoirs, play a key role in dictating fluid-rock equilibrium. Neglecting their specific order–disorder state can therefore lead to erroneous temperature estimations and misinterpretation of reservoir conditions. The results underscore the importance of considering order–disorder phenomena for robust geochemical modeling and highlight the utility of the expanded thermodynamic dataset for future geothermometry applications.