Sedimentary gas ebullition in shallow waters of the Central North Sea (German Dogger Bank)

In the North Sea a number of surveys have turned their attention to the extent of methane emissions from abandoned or decommissioned wells that may provide gas migration pathways for shallow biogenic gas accumulations. As part of the diverse and partly contradicting investigations, the research cruise MSM98 was to study sedimentary gas ebullition and the consequent distribution and fate of methane (CH₄) within the water column in the German EEZ, particularly at the eastern Dogger Bank in the German EEZ, where a number of abandoned wells and natural gas ebullition sites are present. During MSM98 in January 2021, sedimentary gases were sampled from two different ebullition sites using an ROV. Their main component was biogenic methane, isotopically light ethane with concentrations of 50 - 125 ppm and traces of propane. The area of the Berta salt dome on the eastern Dogger Bank was the only study site with significant elevations of methane in the water column. Four additional abandoned well sites in the German EEZ had methane concentrations close to the background values, with methane profiles at two sites showing a slightly irregular pattern. Methane maxima at known ebullition sites were significantly lower compared to an earlier study in summer 2019, when the water column was stratified with a clear thermocline. During the first three days of sampling at the eastern Dogger Bank, trace amounts of ethane and propane were detected in water samples throughout the water column showing decreasing concentrations with time. For these thermogenic gases, the analysis of currents and hydrographic properties indicated a source outside the working area. In conjunction with ventilated offshore waters coming into the working area these allochthonous light hydrocarbons disappeared except for background concentrations of methane. Short-term changes, likely horizontal input of light hydrocarbons, and rapid mixing of the complete water column did not allow for any quantitative statement with regard to sedimentary input of methane into the water column or export of methane into the atmosphere.