Abstract

Using three-dimensional seismic and well data from the northern North Sea, we describe a large (10 km3) body of sand and interpret it as extrusive. To our knowledge, this is the world's largest such sandbody. It would bury Manhattan, New York (60 km2), under 160 m of sand, or the whole of London, UK (1579 km2), under 6 m of sand. This sand vented to the seafloor, when it was more than 500 m deep, during the Pleistocene glacial period. The sandbody (1) covers an area of more than 260 km2, (2) is up to 125 m thick, (3) fills low areas around mounds, which formed when underlying sand injectites lifted the overburden, (4) wedges out, away from a central thick zone, (5) is locally absent along irregular ditches, 20 km long and up to 50 m deep, which overlie feeders on the flanks of the mounds, and (6) consists of fine-grained to medium-grained, sub-rounded to rounded grains. We compare the distribution of the sand with the results of scaled physical experiments. In our interpretation, high fluid pressure fractured the regional Hordaland Group seal in the study area, so that fluidized sand moved rapidly to the seafloor through fissures on the flanks of underlying mounds, mixed with seawater, and formed lateral gravity currents. These transported the sand as much as 8 km away from the blow-out fissures and formed extruded sand sheets. Large extrusive sands represent a new type of economically interesting reservoir.

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