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

Sardinia is typically seismically quiescent, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. This evidence may be in agreement with the presence of a ductile layer in the northern sector of the island, as suggested by the He isotopic signature in fluids rising to the surface through quiescent fault systems. The fault systems have been found to be ‘‘segmented’’ and therefore isolated in fluid circulation. The study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as Rn-indoor, or to define geological structures like potential CO2 storage sites. Both of these have been recently requested by the exploitation in Italy of the Euratom Directive and the evolution of the KyotoProtocol policy. Four water-dominated hydrothermal areas of Sardinia, located along regional fault systems, were considered: Campidano Graben, Tirso Valley, Logudoro and Casteldoria. A fluid geochemical survey was carried out taking into account physical–chemical and environmental parameters, major elements within gaseous and liquid phases, a few minor and trace elements, selected isotope ratios (2H, 18O, 13C, 3He/4He), 222Rn concentration, and some dissolved gases. 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.