Fluid geochemistry of the Acqui Terme-Visone geothermal area (Piemonte, Italy)

The main geothermal reservoir of Acqui Terme-Visone hosts Na-Cl waters, whi ch are in chemical equilibrium at 120-130 degrees C with typical hydrotherm al minerals including quartz, albite, K-feldspar, illite, chlorite (or smec tite), anhydrite, calcite and an unspecified Ca-Al-silicate. In the Acqui T erme-Visone area, these geothermal waters ascend along zones of high vertic al permeability and discharge at the surface almost undiluted or mixed with cold, shallow waters. To the SW of Acqui Terme, other ascending geothermal waters, either undiluted or mixed with low-salinity waters, enter relative ly shallow secondary reservoirs, where they reequilibrate at 65-70 degrees C. Both chemical and isotopic data indicate that bacterial SO4 reduction affec ts all these waters, especially those discharged by the secondary reservoir s. Therefore, geothermal waters must get in contact with oil, acquiring the relatively oxidized organic substances needed by SO4-reducing bacteria. Th is oil-water interaction process deserves further investigations, for poten tial economic implications. (C) 2000 Elsevier Science Ltd. All rights reser ved.