Interplay and feedback between tectonic regime, faulting, sealing horizons, and fluid flow in a hydrocarbon-hosting extensional basin: The Val d'Agri Basin case, Southern Italy

Understanding the factors that govern past fluid circulation in tectonically active and/or hydrocarbon-rich basins is crucial for elucidating present-day fluid-flow scenarios. We investigate the circulation of paleo-fluids in the extensional-transtensional Val d'Agri Basin (southern Italy), home to a giant oil field and significantly affected by both natural and human-induced seismicity. Our aim is to understand how faulting and the variable thickness of the clay-rich tectonic mélange, which constitutes the seal of the hydrocarbon reservoir, influenced past fluid flow under different tectonic regimes. To achieve this, we combined multiscale structural observations with isotope (C, O, clumped, and Sr) and Rare Earth and Yttrium (REY) analyses of fault-related calcite mineralizations. Using analytical methodologies that allow the analysis of sub-milligram samples for carbonate clumped isotopes, we provided a detailed characterization of the variability in precipitation temperatures and composition of parental fluids in both space and time. Our results reveal five main types of parental fluids, ranging from meteoric to intraformational and deep crustal, which were differently involved in the tectonic evolution of the Val d'Agri Basin. During orogenic shortening, vertical fluid circulation was mostly limited and compartmentalized, whereas post-orogenic extensional faulting promoted the ascent of deep fluids. Our findings indicate that the sealing properties of the mélange were likely enhanced locally by increased thickness but were also compromised by fault activity and associated seismic events. Fluid circulation in the study area has been influenced by the prevailing tectonic regime (compressive vs. extensional), stratigraphic-structural architecture, properties of impermeable horizons, and seismic events. The model proposed for the Val d'Agri Basin elucidates past processes that are useful for understanding current fluid circulation in the basin itself and can be applied to other basins where fluid circulation is partly manipulated by human activities.