Seismically induced fault leakage from the Val d'Agri hydrocarbon reservoir (Southern Italy)

Hydrocarbon leakage from reservoirs poses significant environmental and societal problems, including the potential contamination of drinking water and agricultural soils. Distinguishing between natural and anthropogenic leakage is challenging but crucial for assessing human impacts on reservoir seal integrity and developing effective mitigation strategies. To address this issue, this study presents an innovative approach that combines traditional groundwater analyses with fluid inclusion investigation to evaluate hydrocarbon seepage in both present and past shallow fluids, providing insights into long-term sealing history. The Val d'Agri oil field in Southern Italy, an environmentally sensitive area where hydrocarbon leakage is suspected, serves as an ideal case study. Raman spectroscopy of fluid inclusions in Pleistocene-Holocene fault-related calcite veins reveals the entrapment of saturated hydrocarbons with minor aromatic components, similar to those in the current reservoir. This indicates that temporary seal breaches occurred naturally in the geological past (earlier than oil extraction), likely due to fault-valve action during strong earthquakes. Hydrogeochemical analyses of groundwater indicate that the San Giovanni spring, located near productive oil wells, exhibits negative δ³⁴S(SO₄) (-14.20 ‰) and δ¹³C_deep (-11.8 ‰) values, along with a high CO₂ concentration (5.4 mmol/L). These results indicate methane oxidation in the San Giovanni spring, related to long-term mixing between hydrocarbons, likely released during strong seismic events, and meteoric fluids. These findings highlight the importance of reconstructing pre-production processes to assess environmental hazards more effectively, particularly in seismically active, hydrocarbon-rich regions such as the Val d'Agri Basin.