Multicomponent geothermometry applied to a medium-low enthalpy carbonate-evaporite geothermal reservoir

To improve knowledge of the thermal state of medium to low-enthalpy thermal systems hosted in carbonate-evaporite rocks, a mineral-solution equilibrium model was compared to other theoretical geothermometers. We use the GeoT code, which uses as input the chemical composition of water and saturation indices of minerals to calculate water-rock equilibrium over a temperature range of interest. The calculations were applied to the medium and low enthalpy geothermal systems in the Tyrrhenian-Apennine area (central Italy). The lithology consists of a Paleozoic metamorphic basement, overlain by Mesozoic carbonate-evaporite-and Oligocene-Middle Miocene flysch formations, and Quaternary volcanic complexes associated with crustal extension. A regional aquifer is hosted in the carbonate-evaporite formations, and smaller aquifers are hosted in the volcanic rocks. Reservoir temperatures were calculated based on the chemical composition of springs and wells in Central Italy (sampled previously), and in the Cimino-Vicano hydrothermal system (sampled in 2012). Chalcedony and quartz geothermometers provide realistic temperatures. The sensitivity of the model is tested for CO2 degassing and input minerals. The results of optimized GeoT simulations show that all the samples are affected by degassing during their rise to the surface and that for computing a realistic reservoir temperature it is necessary to consider the principal minerals of the geothermal reservoir (particularly gypsum, quartz, dolomite, aragonite and calcite). The equilibrium temperatures range from 48-115C. The statistical approach of "best clustering minerals" solves the problems related to cation or single component geothermometers. Multicomponent geothermometry coupled with optimization provides a reliable approach to reconstruct fluid composition at depth and estimate reservoir temperatures.