Geochemistry of gas and water at the CA1 borehole (Alban Hills volcano, central Italy): new insights on deep fluids circulation and origin

Abstract

In the framework of a multidisciplinary project funded by the Italian “Dipartimento della Protezione Civile”, focused on the Alban Hills volcanic district (Central Italy), a 350 m deep borehole (named CA1) was drilled for geophysical and petrographic explorations (Mariucci et al., 2008). The borehole is located near Santa Maria delle Mole village (onwards SMM), adjacent to the western rim of the Tuscolano-Artemisio caldera, where several phenomena of unrest recently occurred (i.e. seismicity, episodes of gas exhalation from soil and/or gas burst, Figure 1). Gas exhalation can occurs due to i) human activity (removal of the shallow impermeable fine-grained sediments that discontinuously cover the area) and drillings (mainly for water supply) intercepting pressurised gas pockets that form at depth (the last episode occurred few kilometres from the CA1 well in June 2008) and ii) natural causes (i.e. opening of new cracks and variation of the fracture networks in the pervious shallow sediments, mainly clays) due to changes in the local/regional stress field and/or local seismic activity. All these phenomena caused in the past illness and casualties among local inhabitants and animals, marking the considered area as exposed to a high Natural Gas Hazard. During the phase of hydraulic fracturing tests at the CA1 borehole, in a sandy unit at a depth between 345 and 350m, a blow-out occurred (Figure 2), causing the collapse of its deepest part. In order to emphasise the origin of the fluids emitted from the CA1 borehole (gas mainly, with minor water) and to highlight the main gas-water-rock interaction processes that account for the observed chemistry, gas and water were collected and analysed for their chemical and isotopic characterisation. Moreover, for a general discussion on the fluid geochemistry of the Alban Hills and on the ongoing degassing processes, water and gas compositions are discussed and compared together with other new data (samples collected in 2006-2007). New geochemical data provided additional information about both the deep volcanic circulation of fluids and their possible connection to a deep-seated magma chamber. Furthermore, CA1 borehole provided very important information for identifying the depth, the thickness and the spatial location of both the fine-grained sediments and the impervious layers in order to mitigate and, possibly, prevent further harmful gas escapes. All these geological and stratigraphical data were still unknown for the SMM sector; the CA1 borehole provided very useful information and filled this gap of knowledge.