Research paperThe significance of the 22/4b blow-out site methane emissions in the context of the North Sea

- Atmospheric methane concentrations were measured on two North Sea surveys.
- Concentrations were highest near the site of a 1990 blowout.
- Distant & local sources, natural & anthropogenic, affect methane concentrations.
- The blow-out site is significant, but not the only strong North Sea methane source.

Atmospheric methane concentrations were measured close above the sea surface during surveys in 2011 and 2012 using a Picarro cavity ring-down spectrometer. These surveys covered parts of the UK, Dutch, German, Danish and Norwegian sectors of the North Sea. The highest concentrations were recorded in block UK22/4b in a survey area around the site of blowout that occurred in 1990, and which still is known to be actively seeping gas. Variations in the methane concentration recorded during transits between ports and this and other survey areas (Sleipner and Juist) were examined in order to assess whether or not the 22/4b emissions are significant within the general context of the North Sea.The provenance of the air mass passing over the ship evidently influences the background methane concentration with additional methane introduced by local (natural and/or anthropogenic) sources; this is made apparent by the large discrepancy between pre- and post-survey measurements made in port. Background variations, estimated by smoothing the raw methane concentrations (taking the hourly average), are significant, and likely reflect both air mass provenance and local contributions. Residual values (the difference between raw and background) reflect deviations from the background, attributable to local sources. 15 residual anomalies were identified; some associated with natural sources, others with petroleum industry sources. Although these comprise lower methane concentrations than those associated with the 22/4b blowout site, they were recorded at transit speed rather than the slower survey speed, or whilst the ship was engaged in seabed operations. The speed and the chance encountering of these anomalies suggest that direct comparisons with the 22/4b methane concentrations are not valid. It is concluded that, whilst the 22/4b emissions are significant, they are not unique. Other methane sources, natural and anthropogenic, are present in the North Sea; the strength of the anomalies suggests that some may be potent.