Disequilibrium compaction as the cause for Cretaceous–Paleogene overpressures in the Danish North Sea

Abstract

Cretaceous–Paleogene overpressure distribution in the Danish Central Graben shows a remarkable coincidence with the thickness of the rapidly deposited middle Miocene to Holocene succession. Slow deposition of smectite-dominated clays in a deep-marine environment occurred from the late Paleocene until the middle Miocene, and the resultant mudstone succession constitutes the main barrier that delays pressure dissipation. Between the late Miocene and the Holocene, the Upper Cretaceous–Paleogene succession became overpressured, probably because of accelerated depositional rates. Quantification of this disequilibrium compaction mechanism relies mainly on a determination of permeability and effective compressibility of the Paleogene shales. This article shows that realistic permeabilities can be assumed, provided that compressibilities describing the plastic process of compaction are used in the pressure equation instead of the elastic compressibilites that, for instance, can be derived from log data. One-dimensional (1-D) modeling is applied in two cases: a well from the Dan chalk field, where accelerated deposition since the Tortonian (11.2 Ma) produced a present-day overpressure of 7.97 MPa (1156 psi); and a well from the South Arne chalk field, where accelerated deposition since the early Serravallian (14.6 Ma) produced a present-day overpressure of 13.9 MPa (2016 psi). This is based on an identical set of parameters and compares with the observed 7.7 and 14.8 MPa (1117 and 2147 psi ) overpressure at the two locations. The modeled development of the pressure profiles shows that an effective stress minimum occurred in the upper part of the Paleogene succession. This is consistent with the observed ubiquitous intraformational faulting at that level. About 80% of the added Neogene load is estimated to have been converted to overpressure.