Circulation of fluids along the Val d’Agri extensional fault systems

Faults and fault zones can modulate fluid flow as passive barriers/conduits or in a dynamic manner, causing the redistribution of fluids from different sources. The potential involvement of deep fluids in the seismic cycle represents a frontier in earthquake studies, as it could open the possibility of using hydrogeochemical modifications of groundwater as seismic precursors. A multidisciplinary approach that combines structural and geochemical analyses applied to syn-kinematic calcite mineralizations enables the reconstruction of paleo-fluid circulation along faults, providing relevant insights into the intricate understanding of fluid-fault relationships. This is especially important in the vicinity of productive oil fields where induced seismicity and fault leakage of hydrocarbon reservoirs pose potential threats. Thus, the Val d’Agri Basin (southern Italy), which hosts the largest onshore oil field in western Europe and whose oil extraction activities induce low-magnitude seismicity, represents a perfect case study. This basin is bounded on the northeastern and southwestern sides by, respectively, the East Agri (EAFS) and Monti della Maddalena (MMFS) extensional-transtensional fault systems. Both strong historical (e.g., M=7 in 1857) and instrumental low magnitude (M<2.5) earthquakes demonstrate the seismic activity of these faults. Despite its economic importance, the nature, location, and attitude of the main seismogenic fault system, as well as the tectonic architecture and evolution of the basin, remain open questions. Moreover, present-day fluid circulation is not well constrained, and the assessments of threats related to induced seismicity and potential environmental contamination are still under debate. In this thesis, through a multidisciplinary approach, I provide new significant insights into the comprehension of fluid-faulting relationships in the Val d’Agri Basin. The two Val d’Agri fault systems (i.e., EAFS and MMFS) exhibit different architectures due to the heterogeneous thickness of the clay-rich overpressured reservoir seal (i.e., Irpinia mélange; as discussed in Chapter 2). Moreover, the litho-stratigraphical and mechanical characteristics of this tectonic mélange play a key role in syn-kinematic fluid flow. The ascent of deep crustal fluids during post orogenic tectonics manifests differently within the basin. It is frequently identified along the southwestern side of the basin, whereas it occurs rarely along the northeastern side, where the productive oil field is currently located (as discussed in Chapter 3). The deep crustal fluid temperatures suggested that their source was the hydrocarbon reservoir rocks (i.e., fractured carbonates of the Apulian Platform). Therefore, my aim has been to identify geochemical signals related to hydrocarbons in both present- and paleo-fluids. Present-day fluids were investigated through regional and detailed near-surface soil gas surveys (as discussed in Chapter 4) and groundwater monitoring (as discussed in Chapter 5). Instead, paleo-fluids were analyzed through a multi technique approach applied to fluid inclusions within syn-kinematic calcite (as discussed in Chapter 5). Combining these results, I revealed the capability of these faults to establish temporary or permanent connections between the near-surface and the hydrocarbon reservoir, both in the present and in the past.