FLUID STORAGE AND MIGRATION PROPERTIES OF FRACTURED AND KARSTIFIED CARBONATE AQUIFERS

This dissertation investigates the Lower Jurassic to Cretaceous fractured shallow-water carbonate aquifer of Viggiano Mt., located on the eastern edge of Italy's High Agri Valley Basin. Covering approximately 13 km² with a thickness of 500-600 meters, this unconfined aquifer represents a significant hydro-structure bordered by semi-permeable to impermeable siliciclastic rocks and basinal carbonates. The entire succession resides within the Southern Apennines fold-and-thrust belt. This aquifer’s hydrodynamics is defined by an intricate network of fractures, faults, and karst features, associated with the East Agri Fault System (EAFS) and characterized by a polygonal-like structural configuration, in which the N90-120E, N20-30E, and N130-150E high-angle sets are the most common. Recharge is predominantly driven by meteoric fluids entering the aquifer through these faults, while major springs serve as primary discharge points. The boundary of the hydro-structure is defined by the contact between limestone and flyschoid sediments, as well as by the overthrust plane of the Apennine platform on Miocene basinal units. To advance the innovative exploration and sustainable utilization of groundwater within these carbonate aquifers, this study integrates field and digital outcrop structural survey techniques for a multiscale spatial, geometrical, and dimensional analysis of fractures and faults, to assess the overall contribution provided by mesoscale fracture network to aquifer storage and transport properties. Field data were collected through linear and circular scanlines, complemented by drone surveys, with structural data extracted via specialized software (Openplot®). By assessing the influence of carbonate lithofacies, structural position, and observation scales, this research quantifies the 1D and 2D fracture density and intensity and characterizes the dimensional properties of individual fault and fracture sets.Additionally, field and digital structural data were used as input parameters for Discrete Fracture Network (DFN) models of geocellular volumes reflecting the observed bed packages (5m volumes), outcrops (50m volumes), and reservoir-scale carbonate cliffs (500m volumes). DFN models, calibrated with minimum mechanical aperture values from field data for porosity and theoretical hydraulic aperture values for permeability, address challenges posed by weathering and dissolution on field-measured apertures. High-resolution DFN models simulate the impact of varying aperture values under depth-equivalent stresses, analysing their effects on effective permeability and solute transport. Our first assessment enabled us to compute a 2D fracture intensity (P21) of ca. 20 for over 3 orders of magnitude, while P20 (2D fracture density) values exhibited inconsistency. Without considering the control exerted by the single carbonate lithofacies, at a reservoir scale, we document local variation in the computed P21 values due to small-scale faults which impacted computed results. Also, we recorded notable variability in fracture density and intensity across carbonate lithofacies. Secondly, our findings reveal that a scale-dependent geometry characterizes the Cretaceous carbonates over three orders of magnitude. These results support previously published data, and contrast with those obtained for the Lower Jurassic carbonates, which suffer of some bias due to the quality of the exposures. DFN modelling indicates that fracture porosity is predominantly due to SB and NSB fractures, with increased equivalent permeability within faulted rock volumes. In terms of horizontal permeability, which is the most reliable result after DFN modelling, we document near isotropic horizontal permeability ellipses at all scales of observations. At individual outcrops, we note that the permeability ellipses are elongated parallel to the dominant fault sets highlighting the influence of strain on fluid flow directionality. The results are summarized in form of a conceptual model that illustrates themodalities of meteoric fluid infiltration through the vadose zone, and then subsequent horizontal flow in the phreatic zone present within the fracture carbonates at shallow depths. Stressed DFN models demonstrate enhanced anisotropy and early solute particle breakthroughs due to channelized flow paths within stressed fracture networks, underscoring the dynamic fluid transport properties of fractured carbonates under realistic stress conditions. This work contributes a framework for groundwater exploration within similar geologic settings by demonstrating the spatial variability, petrophysical and hydromechanical behaviour of fractured carbonates across multiple scales. The contribution provided by this research provides high resolution data to better optimize results from hydrogeological studies, and their application to the assessments of groundwater flow dynamics within the Viggiano mt. aquifer, especially pertaining to groundwater contamination and remediation. The outcome finds application in similar hydrogeological settings, where the Mesozoic carbonate aquifer is characterized by a network of multiple fracture sets and is not heavily karstified.