Nearshore shallow gas around the U.K. coast

Abstract

The occurrence of shallow gas in seafloor sediments within 50 km of the coast of the British Isles is generally attributed to the degradation of organic matter within post glacial deposits. However there is evidence that buried rockhead topography may also be a factor governing the existence of shallow gas in some localities. In such cases gas may be escaping from the rock through fissures. Where channels have been cut into the rockhead and are now filled with unconsolidated sediments, gas often accumulates in the central portion of the channels. This may be due to the greater thickness of sediment in the centre of the channels generating sufficient gas to be detectable in seismic profiling data. The process of compaction causing gas to be squeezed out of the pore spaces may also be a contributory factor. In other localities no such relationship appears to exist and the gas is present at very shallow levels (within 5 m of the seabed) in the superficial sediment column. Detailed analyses of high resolution continuous seismic reflection profiles from Cardigan Bay, the northeast Irish Sea and the Firth of Forth, would appear to permit the categorization of nearshore shallow gas zones around these parts of the U.K. continental shelf and probably elsewhere, in order to assist with the explanation of their occurrence and the likely hazard that the zones present to seabed and sub-seabed structures.