Cost-effective and relocatable monitoring of natural hydrocarbon seepages in the Italian Offshore

Hydrocarbon seepage is overlooked in the marine environment, mostly due to lack of marine exploration data, especially over the continental shelf and upper slope, where data may be available but not at adequate resolution. Shallow marine environments, such as the north and the central Adriatic Sea, are ideal for the formation of hydrocarbons, because organic matter sinks to the sea bottom where undergoes rapid burial and anaerobic degradation. On the other hand, deeply-trapped hydrocarbons tend to migrate to shallower sedimentary horizons, they may pierce the seabed, giving rise to peculiar seafloor morphology, and, under particular conditions, fluids escape the seabed to form gas plumes in the water column. The latter can be accurately detected by modern multibeam sonar systems as 3D density anomalies, which sometimes can reach high at the sea surface. This contribution is about the geophysical and geochemical investigation of two seepage sites on the shelf of the Adriatic Sea. The study areas are represented by: an oil spill off Civitanova Marche, at water depth of 10 m; scattered biogenic seeps offshore Mt. Conero, at water depth of 84 m. Dissolved benthic fluxes of nutrients, metals and DIC have been measured by in situ deployment of a benthic chamber equipped with an automatic water sampler and a multi-parameter probe. Geochemical analysis was performed to identify the presence of and characterize hydrocarbons in the water samples and provide insights into the origins of the organic matter. Concentration of Polycyclic Aromatic Hydrocarbons and major and trace elements was analyzed to provide an estimate of hydrocarbon contamination in the surrounding sediment. We conducted these research activities under the umbrella of a technical agreement between ISMARCNR and the Italian Ministry of Economic Development, Directorate General for Safety - National Mining Office for Hydrocarbons and Georesources, within the Clypea Innovation Network, which seeks to increase the safety, also in terms of environmental protection, of offshore oil & gas exploration and exploitation, using innovative approaches. This study aims at: i) establishing a cost-effective and relocatable geochemical and geophysical monitoring system for natural hydrocarbon seepage to mitigate the adverse effects of hydrocarbon spill and discharge, especially if located near the coast or nearby human activities; ii) delivering a better understanding of the characteristics and origins of the petroleum system, adding a valuable layer of information to more conventional offshore exploration data; iii) to unravel the relationship of fluid seepage with geohazards, notably softsediment deformation, and sea level changes as triggering mechanism.