Identifying known unknowns in estimating regional methane fluxes from natural seeps and anthropogenic well leakage in the marine environment

Seabeds are a geological source for methane via natural release at seeps, dispersive methanogenic degradation of sedimentary organic matter as well as anthropogenic well leakage. Global estimates of methane seepage fluxes vary around severals tens of Tg/y. The associated release to the atmosphere is lowered by attenuation in the water column. Large uncertainties are associated with these figures as emphasized by the researchers that performed these investigations. Our goal is to investigate which quantitative data is available on both natural seepage and anthropogenic well leakage and which unknowns emerge from this for upscaling to regional basin scale and beyond. We focused on the Gulf of Mexico (GoM) and the North Sea basin (NSB) as these are relatively well studied marine areas. For the two areas together, the number of quantitative studies on natural seepage is about 30 for seabed fluxes and 6 for sea/air emissions. One unknown that emerges is what flux data should be used for further upscaling: that from the studied region or from a global data-set as done before? The population statistics are different for the two. Another unknown for GoM is the fate of methane released at deep seeps (> 500 m below sea surface) that dissolves in seawater and does not reach the sea surface (as bubbles): does it get oxidised, does it interfer with biological methane, does it stay dissolved at large depth? Several quantitative studies focus on the bubble fluxes and neglect mass transport of dissolved methane. How justified is this when we recall that methane from rising bubbles dissolves into surrounding seawater and incidental storms mix seawater at shallow depth enabling transfer to air. Globally, most (quantitative) studies on natural seepage happened at the Northern hemisphere. Natural seeps are commonly found in oil and gas producing basins and oil and gas exploration in several basins at the Southern hemisphere has been performed only recently. This means that natural seepage at such basins is probable as well but has been neglected so far. This puts another unknown forward. The emission related to exploitation and transport of fossil fuels is a major, global anthropogenic source. For onshore wells, the contribution of subsurface leakage has obtained large attention in the past years with considerable numbers of local to national investigations. However, the contribution is far from clear for offshore wells whereas offshore wells may show more frequent well integrity issues as well as barrier integrity issues. Two blowouts have been studied in detail: the Deep Water Horizon blowout in GoM that was capped after 84 days and the UK22/4b blowout in NSB that is continuously leaking since 1990. Additionally, methane fluxes at three leaking, abandoned Norwegian wells were quantified. The data gap for GoM and NSB as well as globally will be illustrated taking into account data on well integrity issues and blowouts. In conclusion, several major unknowns exist on methane fluxes associated with natural seeps and anthopogenic well leakage. These should be addressed to further constrain their contribution to the global methane budget