Fault leakages from the Val d’Agri hydrocarbon reservoir: a comparison between paleo- and present-day fluids

Assessing leakage mechanisms that compromise reservoir integrity is essential for effective geo-resource management and mitigating environmental risks. Reservoir leakages can occur via both anthropogenic pathways, such as active and inactive wells and pipelines, and natural pathways, including fractures and fault zones. Additionally, fault-valve action can temporarily disrupt sealing layers, allowing trapped fluids to migrate upward. Distinguishing between natural and human-induced causes of reservoir leakage is valuable but often challenging. To address this, we present an innovative approach that compares fluid circulation systems before and after the onset of reservoir exploitation. Present-day fluids are studied using standard groundwater sampling, modelling, and near-surface soil gas surveys. In contrast, paleo-fluids are analyzed using carbonate clumped isotope of fault-related calcite veins, along with fluid inclusion spectroscopy and microthermometry to determine parental fluid temperatures and compositions. We applied this approach to the giant Val d’Agri hydrocarbon reservoir in Southern Italy, a region characterized by: (i) high seismic hazard, with historical earthquakes up to magnitude 7; (ii) recent low-magnitude seismicity induced by oil extraction; and (iii) ongoing debate about industrial activities potentially triggering anthropogenic leakages. From our extensive dataset of fault-related calcite veins, we selected samples from Pleistocene-Holocene extensional-transtensional faults of the northeastern side of the valley, where productive oil wells are located. Carbonate clumped isotope analysis revealed precipitation temperatures of 160-180°C, while micro-Raman spectroscopy of fluid inclusions detected hydrocarbon phases matching those currently extracted from the reservoir. These findings suggest that past faulting, likely associated with strong earthquakes, temporarily breached the thick sealing layer, releasing trapped hydrocarbons. Considering present-day fluids, isotope analyses (carbon, boron, sulfate, and helium) from hydrogeochemical monitoring of nearby springs indicated long-term mixing between these hydrocarbons and shallow fluids. In summary, our multidisciplinary study demonstrates that natural leakage via fault-valve action occurred in the pre-exploitation period. Given the high seismic hazard in this region, we recommend incorporating these natural processes into future assessments to enhance environmental hazard mitigation and support sustainable hydrocarbon production management.