Natural leakage of helium from Italian sedimentary basins of the Adriatic structural margin - Perspectives for geological sequestration of'carbon dioxide

Soil gas surveys were performed along the Adriatic foredeep (Vasto, Ferrandina, Pisticci basins), in order to evaluate the relationship between neotectonics and gas leakage from hydrocarbon reservoirs. More than 4000 soil gas samples were collected in the area and analyzed for helium, which is a good fracture tracer due to its chemical inertness and high mobility. Furthermore, helium is enriched in more than 90% of known reservoirs, and displays many characteristics of the ideal geochemical tracer for buried faults and gas and oil reservoirs (abiogenic, non-reactive, and mobile). Statistical analysis shows an average helium concentration of 5.7 ppm in the foredeep basin areas (atmospheric value is 5.2 pprn). This value is higher than that calculated using over 30,000 soil gas samples collected throughout Italy in different geological scenarios (about 5.4 ppm). Results obtained using a geostatistical approach are consistent with the presence of high helium concentrations as linear or spot anomalies due to irregular, channelled flow along faults above hydrocarbon accumulations. Considering the Plio-Miocene age of the Adriatic foredeep reservoirs, the magnitude of these diffuse gas microseepages highlights that losses from hydrocarbon reservoirs should be low. Results indicate that despite the fact that the area is heavily faulted and that gas seepage has Occurred from the reservoir over geological periods of time, no environmental effects are observed at surface. This fact lends support to the idea that geological sequestration Of CO2 within a less structurally-active area would result in the safe, long-term isolation of this green-house gas.