Fluid geochemistry of the Sardinian Rift-Campidano Graben (Sardinia, Italy): fault segmentation, seismic quiescence of geochemically “active” faults, and new constraints for selection of CO2 storage sites

Two different fluids have been recognised as regards both water chemistry and dissolved gases: (i) CO2-rich gases, poor in He and Rn, with a relatively high 3He/4He ratio (up to R/Ra = 2.32), associated with Na-HCO3-(Cl) thermal and cold groundwater; (ii) gases rich in He and N2, poor in CO2 and Rn, with a low 3He/4He ratio, associated with alkaline thermal and cold waters. The distribution of these two groups of fluids characterises the Sardinian tectonic systems. In fact, gas fluxes are not homogeneous, being mainly related to the different fault segments and to the areas where Quaternary basalts crop out. The underground geochemical evolution of the Sardinian fluids, as a function of the geological and tectonic systems, provides some suggestions for solving one of the most important problems: CO2 geological sequestration. In order to reduce the CO2 excess produced by human activity, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, or aquifers characterised by maximum pH buffering capacity of their mineralogical matrix. The knowledge of the role of faults, as permeability barriers or as deep fluid uprising pathways, is prerequisite.