A comprehensive seabed baseline for platform abandonment

Before abandoning hydrocarbon production sites, solid indicators of present and post-seepage are important. Here we present a multi-disciplinary baseline designed to characterize pre-production seepage conditions and assess potential future changes associated with possible reservoir seal failure. The study involved two oil/gas production sites in the Danish North Sea, and one off-platform site and differentiates anthropogenic impact from natural gas seepage. Our study integrates shallow-marine geophysical data with ¹⁴C based sediment core chronologies, incorporating multiple proxies including geochemistry, microbiology, and marine fauna. None of the study sites showed evidence of ebullition of thermogenic gas. No methane seepage has been encountered at platform 1 or at the off-platform site. In contrast, data from platform 2 suggests that methane seepage has been active for at least 6000 years indicating presence of naturally occurring, non-anthropogenic methane sources. At both platform sites the impact of hydrocarbon production was identified just below the sea floor, but no gas seepage related to human activities. Environmental conditions are the key controlling factors for the differences in fauna composition between the localities. Thus, the study highlights the importance of site-specific baseline studies. As an approach for future monitoring of seepage from abandoned sites we recommend multidisciplinary data collection, including sediment coring, to produce a site-specific baseline. This initial campaign may be followed by semi-annual collection of surface samples for analyzing changes in living foraminifera, microbial fingerprints and gas composition.