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Chimneys and pipe structures have been observed in the caprock above the Utsira Aquifer in the North Sea. The caprock is of Pleistocene age and the chimneys appear to have been formed by natural hydraulic fracturing towards the end of the last glaciation. We study six different models for the pressure build-up in the Utsira Aquifer with respect to chimney formation. The first two models produce overpressure by a rapid deposition of glacial sediments. Using these two models, we show that the caprock permeability must be as low as 100 nD for sufficiently strong overpressure to develop. This value seems to be one order of magnitude lower than the measured permeabilities of the caprock. The four remaining models produce overpressure by a glacial loading of the caprock and the aquifer. This study shows that a 1-D model of a caprock with soil properties cannot produce conditions for chimney formation unless the least horizontal compressive stress is much less than the overburden. Furthermore, a 1-D poroelastic model of glacial loading of an aquifer and a caprock cannot produce conditions for chimney formation based on available geomechanical data. However, we demonstrate that a 2-D poroelastic model can produce conditions for chimney formation with glacial loads that partially cover the surface.

Models of overpressure build-up in shallow sediments by glacial deposition and glacial loading with respect to chimney formation