The high pCO_2 Caprese Reservoir (Northern Apennines, Italy): Relationships between present- and paleo-fluid geochemistry and structural setting

摘要

The chemical and isotopic compositions of (i) CO_2-rich fluids exploited from the ~5,000m deep Pieve Santo Stefano 1 (PSS1) borehole located in the Upper Tiber Basin (Northern Apennines, Italy) and (ii) natural gas seeps located in its surroundings, mainly in the Mt. Fungaia ophiolitic-bearing complex (Ligurian Unit), are presented and discussed. Deep seismic profiles have allowed to identify a thrust-related antiform (Caprese Antiform), a regional geological structure trapping the pressurized deep fluids. Fluids from the Caprese Reservoir (CR) consist of a CO_2-, N_2-rich gas phase sourced by both mantle degassing and thermal degradation of carbonate rocks and organic-rich sediments of the Umbro-Tuscan sedimentary series and a Na-Cl saline (up to 82g/L of TDS) brine. Gases naturally discharging in the study area are related to those present in the CR. These gases, during their uprising toward the surface, mix with shallow aquifers. Addition of (i) H_2 and H_2S, derived by the interaction with ophiolite-bearing Ligurian Units, and (ii) thermogenic hydrocarbons, originated from the degradation of organic matter contained in the turbiditic (sandstone-rich) formations (Cervarola-Falterona Unit) was also evidenced. Fluid inclusions, trapped in calcite and quartz crystals from the PSS1 drill-core at the depth of 3864 to 3867m, contain CO_2-N_2 and H_2O-NaCl phases. The different paleo CO_2-N_2 contents and densities found with respect to the fluids currently exploited from the CR have been related to the evolution of the reservoir throughout the time. Compositional data of fluids exploited from PSS1 borehole provide a unique opportunity to better characterize chemical-physical processes and source regions of past and present deep fluids circulating in Northern Apennines.