Chemical Geothermometers and Their Application to Formation Waters from Sedimentary Basins

Chemical geothermometers, based on the concentration of silica and proportions of sodium, potassium, lithium, calcium, and magnesium in water from hot springs and geothermal wells, have been used successfully to estimate the subsurface temperatures of the reservoir rocks. Modified versions of these geothermometers and a new chemical geothermometer, based on the concentrations of magnesium and lithium, are developed to estimate the subsurface temperatures (30°C to 200°C) in sedimentary basins where water salinities and hydraulic pressures are generally much higher than those in geothermal systems. The new Mg-Li geothermometer, which can be used to estimate subsurface temperatures as high as 350°C for waters from sedimentary basins and geothermal systems, is given by: $$t = \frac{{2200}} {{\log \left( {\frac{{\sqrt {Mg} }}{{Li}}} \right) + 5.47}} - 273,$$ where t is temperature (°C) and Mg and Li concentrations are in mg/L.Quartz, Mg-Li, Mg-corrected Na-K-Ca, and Na-Li geothermometers give concordant subsurface temperatures that are within 10°C of the measured values for reservoir temperatures higher than about 70°C. Mg-Li, Na-Li, and chalcedony geothermometers give the best results for reservoir temperatures from 30°C to 70°C. Subsurface temperatures calculated by chemical geothermometers are at least as reliable as those obtained by conventional methods. Chemical and conventional methods should be used together where reliable temperature data are required.