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Seepage of shallow gas, mainly methane, recently has been documented visually in the central section of the North Sea at water depths of about 70-80 m. The surface expressions caused by these seeps are not dramatic. They mainly consist of small funnel-shaped craters (20 cm across) in the cover sand. However, in deeper parts of the North Sea, where the cover sediments consist of silt and clay, the surface features are more dramatic. Here the craters (pockmarks) caused by shallow gas eruptions or seepages are up to 20 m deep and 100 m wide. In some areas, the pockmarks are paved with a crust of calcium carbonate cement, which is believed to have formed as a result of gas efflux through the seabed. Furthermore, the presence of these calcium carbonate reefs seems to have attr cted a wide variety of marine life. The presence of shallow gas has caused an enrichment of marine life that seems to be significant.
Beside the semicircular and composite pockmarks, gas-induced erosion also has caused elongate depressions. These occur where the top sediment bedding has caused the gas to migrate along certain axes. The mode and speed of formation of the gas-induced erosion features are of major concern to oil exploration and development in the northern North Sea.
Research into these aspects recently has been stimulated by the discovery of one of the world's largest offshore gas reservoirs, which coincidentally lies within a pockmarked area.
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