GEOCHEMICAL AND GEOPHYSICAL CHARACTERISATION OF AN ACTIVE CO_2 GAS VENT NEAR THE VILLAGE OF LATERA, CENTRAL ITALY

摘要

The principle goal of CO_2 geological sequestration is that the injected gas will remain trapped at depth, however there is always the potential that the CO_2 may eventually leak to surface due to unforeseen events or pathways. It is thus critical to understand what effects the leaking gas may have in the shallow environment, and the best way to understand these effects is to study natural analogues where geologically produced CO_2 has been leaking to surface over long periods of time. One such site occurs in the extinct volcanic caldera of Latera (central Italy), an inhabited area known for geothermal energy, carbonate rich springs and CO_2-rich gas vents. Some of these vents were studied during the present work in order to better understand their morphology, alteration and gas flow regime. Geochemical studies included soil gas, gas flux and mineralogical measurements, while geophysical work consisted of Ground Penetrating Radar (GPR) and Time Domain Reflectometry (TDR) surveys. The soil gas and gas flux measurements outlined a series of sub-circular vent structures which varied in diameter from 5 to 20 m in diameter. The larger of these vents consisted of a small central core having high flux rates and anomalous soil-gas concentrations of both redox-reactive and non-reactive gas species, surrounded by a halo of low flux values and elevated concentrations of non-reactive species only; the smaller vents often lacked the central core. These results indicate that gas flow along these vents is very spatially restricted, and that the unsaturated vadose zone is an important buffer for the consumption of reactive gas species and the storage of non-reactive species. The GPR and TDR results are in excellent agreement with the geochemical data, defining a central core where there is a total loss of signal (likely due to highly conductive clay minerals formed by alteration) surrounded by a "disrupted" stratigraphy which may be caused by alteration along sub-vertical migration pathways.