Structural control of gas migration pathways in the hydrocarbon-rich Val d'Agri basin (Southern Apennines, Italy).

Val d'Agri is a seismically active intermontane basin in southern Italy known for hosting the largest on-shore hydrocarbon reservoir in western Europe. Despite extensive study of the basin, important questions remain regarding fluid circulation and the possible link between deep hydrocarbon reservoirs, faults, natural and induced seismicity, and gas migration towards the surface. To address some of these issues we performed near-surface gas geochemistry and structural geology surveys throughout the basin at both the regional and local scale. While carbon dioxide data are due to shallow, biological processes in the soil, anomalous results for other gas species are interpreted as being linked to structural discontinuities. Coincident methane and ethane anomalies, which imply a deep thermogenic origin, occur primarily in the northern part of the Val d'Agri basin. The most dominant alignment of these gases starts from a surface hydrocarbon seep and extends above a buried, NE-SW-striking fault that transects the valley. In contrast, radon anomalies are localized in the southern part of the basin along the western border, in correspondence with the Monti della Maddalena Fault System (MMFS) and a cluster of background natural seismicity. The origin and implications of the observed anomalies are discussed. • Gas geochemistry / structural surveys were performed at Val d'Agri, host of Europe's largest onshore hydrocarbon reservoir. • Coincident methane and ethane anomalies along known / inferred faults imply upward migration of reservoir hydrocarbons. • hydrocarbon gas alignment above buried fault shows possible link between exploited reservoirs on either side of the valley. • The correlation between radon anomalies and recent low-level seismicity implies fracture enhanced emanation. [ABSTRACT FROM AUTHOR]