Introduction.- Thermodynamics, geochemical modeling and related considerations.- The selected reservoir liquids.- The hydrothermal minerals.- The traditional water geothermometers and fCO2-indicators.- The activity-based theoretical Na-K geoindicators.- The activity-based theoretical K-Mg and Na-Mg geoindicators.- The activity-based theoretical K-Ca and Na-Ca geoindicators.- Conclusions and way forwards.- References.