Numerical forward modelling of the overpressure build-up in the Cenozoic section of the Central Graben in the North Sea

Abstract

One of the most widespread hypotheses for the origin of the present-day overpressure in the shale Post-Chalk section in the North Sea is the very rapid sedimentation from Neogene to present day. We tested this hypothesis by the means of numerical forward finite elements modelling and successfully simulated the overpressure build-up during the Cenozoic filling of the North Sea with relatively simple model set-up. Our model shows that overpressure of approximately 28% above hydrostatic developed in the Neogene, while during the Quaternary, it reached up to 36% above hydrostatic. At present day, the predicted onset of overpressure starts at about 800–1000 m below seafloor, while the maximum (magnitude about 1.36 sg, i.e. 36% above the normal hydrostatic pressure) is at approximately 2100 m. This overpressure profile fits reasonably well with data from wells drilled in the Central Graben. The exact magnitude of the overpressure depends on the used assumptions, the model set-up and the values of the input parameters. Especially the dynamic interaction between high sedimentation rates, clay permeability and low horizontal pressure gradient seems to be a key factor in the efficiency of dewatering of saturated clays during burial. The results indicate that, the assumption of horizontal stress isotropy results in nearly no horizontal fluid flow, despite the same magnitude for the vertical and the horizontal permeability. In these conditions, the vertical permeability plays much bigger role than the horizontal one in the effective de-watering of the sediments during burial. Further investigation is needed to explore the role of horizontal pressure gradient in fluid migration in passive sedimentary basins.

Influence of Post‐danian Sea‐level Changes and Variations in Sedimentation Rate on Overpressure Build Up in the Clay‐rich Overburden in the Danish Sector of the North Sea Central Graben

“…Changes in mechanical rock properties, strainstress fields and in pore fluid flow due to sediment consolidation and compaction during burial are described in the . Further details can be found in papers by Peric and Crook (2004), Crook et al (2006a,b), Thornton and Crook (2014) and Orozova-Bekkevold and Petersen (2021). Material properties and parameters used in the modelling and in the Appendix equations are given in Table 1 and Table 2. Only mechanical compaction (i.e.…”

“…3b shows that there is little decrease in porosity between approximately 700 and 2400 m, indicating that a significant amount of fluid is trapped in the pore space, thus preventing compaction. The high volume of pore fluid present in the sediments leads to the development of abnormal pore pressures (Dickinson, 1953;Gibson, 1958;Audet and Fowler, 1992;Wangen, 1993;Bowers, 1995;Osborne and Swarbrick 1997;Burrus, 1998;Fowler and Yang, 1998;Japsen, 1999;Swarbrick et al 2002;Yang and Aplin 2007;Vejbaek 2008Vejbaek , 2019Vejbaek , 2022.…”

“…It is a widely accepted that rapid deposition of material upon fine-grained, fluid-saturated sediments with low permeability may result in inefficient pore fluid expulsion from these formations which can therefore become overpressured (e.g. Gibson, 1958;Audet and Fowler, 1992;Wangen, 1993;Osborne and Swarbrick, 1997;Fowler and Yang, 1998). Previous studies have investigated the relationship between the overpressures observed in the clay-rich succession overlying the Chalk Group in the North Sea and the high post-Danian sedimentation rates (Osborne and Swarbrick, 1997;Holm, 1998, Swarbrick et al, 2002Nordgård Bolås et al, 2004;Goffey et al, 2018;Vejbaek, 2008Vejbaek, , 2019Vejbaek, , 2022Orozova-Bekkevold and Petersen, 2021).…”

“…Gibson, 1958;Audet and Fowler, 1992;Wangen, 1993;Osborne and Swarbrick, 1997;Fowler and Yang, 1998). Previous studies have investigated the relationship between the overpressures observed in the clay-rich succession overlying the Chalk Group in the North Sea and the high post-Danian sedimentation rates (Osborne and Swarbrick, 1997;Holm, 1998, Swarbrick et al, 2002Nordgård Bolås et al, 2004;Goffey et al, 2018;Vejbaek, 2008Vejbaek, , 2019Vejbaek, , 2022Orozova-Bekkevold and Petersen, 2021). Vejbaek (2019Vejbaek ( , 2022 and Goffey et al (2018) concluded that overpressures in the Cenozoic succession in the Danish Central Graben developed mostly during the last 10 million years, starting in the Late Miocene.…”

“…Orozova-Bekkevold and Petersen (2021) did not take account of variations in water depth in their modelling; in addition, they did not differentiate between the Eocene and the Oligocene, or between the Miocene and the Pliocene, but considered these intervals together as "Paleogene" and "Neogene" respectively. In the present study, we have refined the depositional model proposed by Orozova-Bekkevold and Petersen (2021) by introducing sea level changes together with a more detailed representation of sedimentation processes in the Danish Central Graben from the Selandian (Mid Paleocene) to the present, with a particular focus on the Miocene. We consider this model to be more realistic than those previously proposed by Goffey et al (2018), Vejbaek (2019Vejbaek ( , 2022 and Orozova-Bekkevold and Petersen (2021) because it takes into consideration both sea level changes and variations in sedimentation rates in the studied period.…”