Structural architecture and maturity of the seismically active and hydrocarbons productive Val d'Agri basin (Southern Italy)

Structural analyses of exposed faults provide information as fault lengths, downdip rupture length, bottomset depth, and structural maturity, which is relevant for seismic hazard evaluations (e.g., Wells & Coppersmith, 1994). Motivated by these topics, we study the Val d’Agri area, which consists of an intermontane basin within the southern Apennine fold-and-thrust belt (Italy) hosting a giant oilfield. The basin is bounded by two extensional fault systems respectively labelled as the East Agri Fault System (EAFS), along the eastern side of the valley, where productive hydrocarbon wells are located, and the Monti della Maddalena Fault System (MMFS) along the western side of the valley. The seismic activity of these fault systems is marked by paleoseismological evidence, strong historical earthquakes (Mw ≤ 7.1), and both natural and induced instrumental seismicity. At present, geometry, kinematics, and location of main seismogenic structures within the Val d’Agri basin are still debated. In this work, we characterize the shallow-to-deep fault architecture and structural maturity of the fault systems bordering the Val d’Agri Basin through a multi-scale structural analysis. Results show that the fault attitude and kinematics of the MMFS are consistent with the current NE-SW extensional stress regime (Mariucci and Montone, 2020). Differently, those of the EAFS are non-systematic. Contrary to previously geomorphological studies (Zembo et al., 2009), the fault length analysis evidenced a higher structural maturity of the MMFS than the EAFS. Such differences are interpreted as due to the 3D geometry of the seal rocks topping the hydrocarbon reservoir at depth. The seal rocks include low permeability overpressured deepwater mudstone-siltstones, forming a ductile package. This package is thick below the EAFS, where it acts as an efficient decoupling horizon, while it thins below the MMFS (Catalano et al., 2004), where it is unable to kinematically decouple the sub- and supra-seal layers. Furthermore, U-Pb dating on syn-tectonic calcite slickenfibers show the slip longevity of the MMFS, characterized by a pre- to post- orogenic extensional activity. In conclusion, our results suggest that the MMFS could be interpreted as the main seismogenic source for the Val d’Agri Basin, in accordance with previous seismological studies (e.g., DISS, 2021). Moreover, we shed new light on the structural configuration of the depth carbonates hosting the hydrocarbon and of the allochthonous terranes topping the seal rocks. Since the fault architecture influences the integrity of deep reservoir and fluid migration, these outcomes of the work can have an important implication for environmental studies and hydrocarbon exploration.