Analysis of brittle structures and fluid flow in Mesozoic allochthonous Units outcropping in Agri Valley (Basilicata).

The geological and structural complexity of the High Agri Valley has aroused the interest of authors in the past. Indeed the area allows to investigate different tectonic Units of the Southern Apennines and the presence of recently active normal faults. Further interest is provided by the abundance of natural resources, due to the presence of the largest hydrocarbon field of on-shore continental Europe, coupled with abundant water resources within fractured carbonatic rocks. The Agri valley is located in Basilicata, on the border with the Campania region, and is a NW–SE-trending structural depression filled by Quaternary continental deposits that cover pre-Quaternary sedimentary rocks, represented by the Sicilide and Liguride Complexes, the Apennine Platform and the Lagonegro Units. The superposition of tectonic Units consisting of Meso-Cenozoic sedimentary cover, with eastward vergence, is highlighted by regional thrusts developed since the late Burdigalian. Strike-slip and normal faults are instead the result of a later extensional phase that involved the chain since upper Pliocene. This phase produced the two major fault systems that border the High Agri Valley and specifically, the Monti della Maddalena Fault System (MMFS) to the West and the East Agri Fault System (EAFS) to the East. The PhD project has been carried out by combining field observations, interpretation of subsurface data and 3D computer modelling of geological structures in order to study the fault network occurring in the eastern side of the High Agri Valley. The methods adopted for the research include geological mapping, field structural analyses of the brittle structures and the measurements of dimensional properties of faults. A GIS software has been used for the drafting of a Geological-structural map and for integrating the various data obtained during field analysis. The detailed map pays a greater attention to the differentiation of outcropping lithologies and to the stratigraphic relationships between Formations of the same tectonic Unit. The main ductile and brittle structures have been identified, differentiating the orientations of fold axes, the different sets of faults and distinguishing high- and low-angle faults. The integration of well data with new information provided by the field surveys allowed the construction of 2D geological cross-sections. These last combined with DEM of the High Agri Valley (with the resolution of 5 m) have been used for the realization of a 3D structural model (by means of the Move™ software). The 3D model of the allochthonous Units of the East High Agri Valley allowed to obtain information on the geometries of the rock bodies at depth and the arrangement and geometries of the fault planes. Results of the 3D model allowed the realization of elevation and isopach maps of the investigated area, which has been divided into four smaller areas characterized by a peculiar structural-geological setting. The valley floor of the High Agri Valley has been modelled obtaining information about the threshold zones and the depocentral areas, where the maximum thickness of the Quaternary deposits is observed. A reconstruction of the 3D geometry of the high-angle fault network generated between the upper Pliocene and Pleistocene, crosscutting pre-existing major thrusts in the Agri Valley, has been proposed. In order to realize detailed study of the fault geometry, field and subsurface data allowed to identify faults that compartmentalized different sectors of the allochthonous Units. The analysis of the fault planes selected within the model allowed to visualize the geometries of the displacements with throw profiles maps. A better understanding of the dimensional attributes and the kinematics of the different sets of high-angle faults located in the High Agri valley and their chronological relationships has been acquired by means of the analysis of cut-off lines and throw distribution along selected faults. The results allowed to observe similar features for faults of the same sets. The study of the brittle structures has been carried out, in order to provide information useful for detailed hydrogeological studies in the area, and particularly on the circulation of groundwater. This might be useful when estimating the availability of water resources. Moreover, considering deeper aquifers, such as in the Tramutola area, located on the western side of the High Agri Valley, the relationships between the presence of low enthalpy geothermal fluids and the tectonic structures has been inferred. In particular, the study of the Tramutola well area has allowed to understand the role played by high and low-angle fault systems. The structural geological and geophysical investigations allowed to observe high-angle faults offsetting the pre-existing contractional structures and the low-angle normal faults. The latter produce the local tectonic elision of the Apennine Platform Unit rocks, justifying their lateral thickness variations and their absence in some wells in the investigated area. The upwelling of hypothermal water and hydrocarbons appears to be concentrated at the intersections of the different fault sets. A detailed measurement of structures at the sub-seismic scale on carbonate outcrops of the Calcari con Selce Formation of the Lagonegro Units, representing the main carbonate aquifer of the High Agri Valley. All the attitude and dimensional parameters have been assigned to the outcrop-scale structures that have been grouped in sets. Their persistence has been correlated with large-scale structures present in the studied areas. Discrete Fracture Networks were built using fracture parameters derived by field data. These models allowed to provide preliminary estimates of the secondary porosity as well as the permeability in the Calcari con Selce Formation. The 9 DFN models realized by fracture data allowed to compare permeability and porosity values in the analysed outcrops. The main results indicate that the most pervasive fracture sets are oriented parallel to the large-scale structures in each area. The comparison between the different dataset allowed to observe how the most pervasive fracture sets vary in their orientation, demonstrating the heterogeneity of the tectonic setting in the different areas. Furthermore, a comparison between models that consider only SB fractures, only NSBs and a combination between the two for each area, demonstrates how the latter have the highest porosity and permeability values when compared to the others.