Seismic cycle-controlled hydrocarbon vertical migration through carbonates in the Ragusa Oil Field (SE Sicily, Italy)

Hydrocarbon seepage at the Earth’s surface provides crucial insights into subsurface petroleum systems. This study investigates the role of seismic cycle dynamics in controlling vertical hydrocarbon migration by studying the Ragusa Oil Field, a long-exploited petroleum district in the Hyblean foreland domain of south-eastern Sicily (Italy). Inspired by oil spilling in the area that followed a seismic sequence in February 2016, a multiscale structural analysis was undertaken to explore the relationship between fault activity and oil mobilization. This study integrates mesoscale structural measurements, microstructural analysis of bitumen-bearing fault breccias, and 3D Dilation Tendency modelling under paleo- and present-day stress conditions to build a dynamic model of upward hydrocarbon migration and seepage at the Earth’s surface during the seismic cycle in carbonate-hosted normal faults in a foreland setting. Evidence from abandoned asphalt mines and active seep sites reveals both stratigraphic layer-impregnation and localized fault/fracture-controlled oil pathways. Field observation and Dilation Tendency analysis indicate that vertical hydrocarbon migration may predominantly occur by fractures instability during seismic rupture allowing overpressured fluids to migrate vertically, mainly at fault in- tersections. These findings highlight the role of seismic deformation in controlling fractures instability and transient permeability changes which, in turn, facilitate hydrocarbon mobilization and leakage. Calcite clast aggregates within hydrocarbon-filled voids observed during microstructural investigations confirm episodic, pressure-driven fluidization consistent with co-seismic mobilization. Stratigraphic evidence of repeated seepage events in Quaternary alluvial deposits supports a model of cyclic hydrocarbon migration linked to stress vari- ations during the seismic cycle. The novelty of this paper is that we document an hydrocarbon seepage process associated with modern seismicity, filling the gap of previous observations of hydrocarbon seepage speculatively associated with fossil earthquakes without a direct cause-effect link.