Geophysical, geochemical, and structural investigations of natural non-volcanic degassing in Oliveto Citra area (southern Apennines): Understandings of structural controls on CO2 leakage

We present a multidisciplinary study on natural non-volcanic CO₂ degassing vents in the southern Apennines, aiming to investigate gas leakage mechanisms related to tectonic structures. The studied degassing areas are located in the Sele River Valley, north and east of Oliveto Citra town. The Sele River Valley features multiple cold and hot springs and frequently aligned gas vents emitting CO₂ and noble gases. We performed structural-geological mapping and geochemical investigations (soil pH and CO₂ mapping) in three key areas of the Sele River Valley. These were implemented by geophysical surveys, including 2D Electrical Resistivity Tomography (ERT), Induced Polarization (IP) Tomography, 2D Seismic Refraction Tomography (SRT), Magnetometry (MAG), and Self Potential (SP) mapping in a single sector (area 1) included in the Mofeta del Vecchio Mulino vents north of the Oliveto Citra town. The results of this multidisciplinary study indicate that most of the gas emissions are along the intersection between the major faults that crosscut a tectonic pile formed by limestones tectonically covered by an oceanic succession made of clays and marls. In area 1, ERT, IP, and SRT profiles mark a vertical conduit where the fluids migrate upward, corresponding to a major fault zone that lowered the tectonic pile to the north. The MAG and SP maps also show anomalies highlighting uprising fluids along the intersection between major faults. CO₂ flux maps of three areas embedding the major vents show that the geogenic emissions are widespread, with the highest values reaching 2256 gm²d⁻¹. Generally, the degassing vents form about circular areas of 10 m in diameter. Also, the pH map indicates acid soil anomalies close to the major emission vents. The novelty of this work is the multidisciplinary approach, which uses different methodologies to reconstruct the buried tectonostratigraphic architecture and the articulated pathways for fluid migration, highlighting that once fluids move toward the surface, they follow the main fault zones. Furthermore, their migration and leakage are controlled mainly by the surficial segmentation of faults and local permeability paths. The procedures applied in this study can be helpful for investigations in other natural degassing areas or the CO₂ storage industry to investigate the seeping and leakage processes and mitigate the gas migration.