Geochemistry of natural gas manifestions from the Upper Tiber Valley (central Italy)

Six natural gas manifestions from the upper Tiber Valley at Caprese Michelangela and Pieve S. Stefano (Arezzo) and at Umbertide (Pertugia) localities have been analysed for major, minor, trace gas compositions, as well as for ??13C in CO2 and CH4, ??15N in N2 and 3He/4He isotopic ratios. All gas emissions are CO2-rich (???94%), with N2 contents of 4-5%. Those from Caprese and Pieve S. Stefano have very peculiar compsitions when compared to other gases from northern-central Apennines. Apart from CO2, these gases show relatively high contents of He (with crustal isotopic ratios) and medium to high temperature-related gases such as CO, H2 and C6H6. Although located quite far from the geothermal areas in Tuscany, the application of several gas geothermetric techniques suggest for these gases deep equilibrium tempratures of about 300??C. Moreover, the ??13C in CO2 and CH4 (~.4.0% and -38.0%, respectively) and the ??13N values in N2 (+0.064 to +0.84%) would seem to imply a multiple deep source for these gases.