Pressure variations in the northern part of the Danish Central Graben, North Sea

Abstract

The overpressure variation in the Cenozoic to Jurassic succession in the northern part of the Danish Central Graben may broadly be divided into three major compartments: An upper hydrostatically pressured unit comprises the post-mid Miocene to recent succession down to ∼ 1200 m depth in the North and ∼ 700m in the South part of the Danish Central Graben. The second compartment comprises the mid-Miocene smectite rich clays down to and including the upper Cretaceous chalk. There, the Paleogene to Lower Miocene succession provides the seal. The third compartment constitutes the Jurassic succession with pressure above hydrostatic that may exceed twice that seen at the upper chalk level. Pressure levels can be estimated using Eaton's approach for the second compartment in agreement with pressure data. Modeling of the transient pressure development in the Cretaceous — mid-Miocene succession broadly complies with Eaton estimates and show main overpressure build-up to occur within the last 10 Ma. The overpressure in this succession may be mapped using methods that exploit correlations between fluid pressure and degree of consolidation, - while the Jurassic cannot. However, the lateral variation of the upper Jurassic overpressure correlates broadly with the maturity of the Upper Jurassic source rock allowing the pressure variation to be mapped.Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressureSupplementary material:https://doi.org/10.6084/m9.figshare.c.5797068

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

“…porosity reduction) in the underlying material. Until this question is clarified, it seems reasonable for Quaternary sea floor sediments in the Central Graben to be given bulk density and porosity values which are similar to those of Quaternary sediments at the ODP site 646 -in the order of 1.50-1.60 g/m 3 and 0.70-0.8, respectively, and not 1.92 g/m 3 and 0.47 as suggested by Vejbaek (2022).…”

“…Vejbaek (2022) derived a bulk density of about 1.92 gm -3 and 0.47 porosity at zero effective stress (i.e. at sea floor: see figure 8 in Vejbaek, 2022), assuming the presence of a 500 m thick ice cover during the Pleistocene. We obtained similar values -porosity of 0.45-0.50 and density of about 1.9 g/m 3 -by extrapolating the data from the Central Graben wells in our dataset (see the purple trend lines in Fig.…”

“…A number of wells in the Danish Central Graben have reported overpressures in the clay-rich Cenozoic sedimentary succession (e.g. Japsen, 1999;Jensen, 2004, Regel et al, 2017Vejbaek, 2019Vejbaek, , 2022. Krygier et al (2020) found that wellbore instability related to drilling through overpressured formations in the North Sea has resulted in significant economic loss in the order of several million USD.…”

“…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.…”

“…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.…”