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

Abstract

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.

How do fault systems and seafloor bathymetry influence the structure and distribution characteristics of gas chimneys?

“…Overburden in sedimentary layers produces lithostatic pressure that modulates overpressure accumulation within deep fluid‐source layers through a pressure transfer mechanism (Tingay et al, 2007; Zhao et al, 2018), which affects the occurrence probability and hydraulic fracturing degree of gas chimneys (Cartwright & Santamarina, 2015; Waage et al, 2019; Zhang et al, 2017). Model results show that glacial loads with an average thickness of 1000 m on the seafloor could produce an increase in overpressure in the aquifer, which could impact gas chimney formation (Wangen, 2022). In terms of density, sedimentary rocks are roughly 2–3 times heavier than glaciers (according to Normandeau et al, 2021, ρ ice = 920 kg m −3 ; ρ sandstone = 2200–2700 kg m −3 ; ρ mudstone = 2000–2500 kg m −3 ; ρ shale = 2100–2800 kg m −3 ).…”

“…beneath IN and/or FD regions). 5.2.3 | Distribution pattern of gas chimneys Increasing height differences between canyon ridges and valleys should contribute to more serious overburden pressure differences, then affect the compressibility of the fluid-bearing layer and the local distribution of overpressure accumulation (Hustoft et al, 2009;Wangen, 2022). Gas chimneys may have been concentrated in the IT zones because of the ridged seafloor topography (Figures 3, 7, 9, 10, and 14): thicker sediment layers correspond to a higher overburden pressure at the shallow, a higher accumulation of overpressure in the deep, and a higher intensity of fluid upward migration, which may have contributed to gas chimneys appearing more frequently and having the better breakthrough capability.…”