New perspectives on the analysis of active fluid seepages in offshore Sicily

The analysis of active fluid seepages in the Sicily offshore, allows to differentiate processes that generate seabed structures (mounds and pockmarks) from buried structures (active fluid seepage, pressurized reservoirs, and gas chimneys). This distinction could be useful to assess the slope instability and the explosive potential in relation to the different physiographic environment and morphobathymetry and then the preparatory and predisposing factor of the areas to ground instability. Seismic data acquire in different OGS, CNR and University of Palermo surveys in the Sicily Channel and Gulf of Palermo show numerous marker reflectors related to gas accumulation: bright spots (positive amplitude anomalies caused by gas and water accumulation), flat spots (horizontal seismic reflections attributed to an interface between two fluids), and Bottom Simulating Reflectors both along fluid migration paths and within shallow confined reservoirs. The evaluation of the role of each parameter will be performed through Sensitivity Analysis. The data will be processed with geotechnical models to evaluate the stability threshold of continental slopes (Factor Of Safety) in each case study and the limit pressure levels of reservoirs and gas chimneys, in relation to geomechanical variables (e.g., intergranular cohesion, internal friction angle, undrained shear strength, etc.) on the basis of which a fragile response of the geological volume can be expected. For example, a gas chimney with height of 1 m and Pcrit ≥ 15 kPa within a sedimentary body with internal friction angle φ of 25° at 45 mbsf may generate a seafloor shear failure. All the results regarding this morphological and stratigraphic study of the structures linked to fluid escapes and the experimental approach of the geo-mechanical study of the process will be summarized in a table representing all the fluid escapes structures.