Characteristics of two natural gas seepages in the North Sea

Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block 19, has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of ∼ 100 m. It is estimated to produce ∼ 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material. The second occurrence is located in U.K. block 1525 (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of ∼ 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.

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