AAPG Bulletin, V. 82 (1998), No. 11 (November 1998), P. 2031-2074.

Regional Velocity-Depth Anomalies, North Sea Chalk: A Record of Overpressure and Neogene Uplift and Erosion¹

Peter Japsen²

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received June 20, 1996; revised manuscript received March 16, 1998; final acceptance April 15, 1998.
²Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark; e-mail: [email protected]

This study was made possible through the generous support of the Carlsberg Foundation and GEUS. Petroleum Information (Erico) is thanked for giving me permission to use Chalk pressure data from its British and Norwegian pressure studies, and for placing the British and most of the Norwegian well velocity data at my disposal; without the backing of Peter Sheil and Stuart Thomas, both Petroleum Information (Erico), this study would not have been possible. Statoil is thanked for giving me access to well data. Christian Hermanrud, Erik Vik, and Lars Wensaas at the Statoil Research Center in Trondheim, Norway, helped me with many basic questions. The Geological Survey of the Netherlands is thanked for giving me access to pressure data. Per Knudsen, National Survey and Cadastre-Denmark, advised me on the kriging technique, and Ida Lind, Danish Technical University, took part in many considerations. I thank colleagues who have supported me in many ways, in particular Torben Bidstrup, Jim Chalmers, Anders Mathiesen, and Jens Jørgen Møller. Jens Clausen, Dopas; Finn Surlyk, University of Copenhagen; and Claus Andersen, Thomas Dons, Jon Ineson, Peter Konradi, and Birger Larsen, all GEUS, provided valuable comments on different parts of the manuscript. Finally, editors and journal referees are thanked for their penetrative and constructive reviews.

Abstract

A normal velocity-depth trend for the Upper Cretaceous-Danian Chalk Group is determined by identifying interval-velocity data that represent maximum burial in areas unaffected by overpressuring; these data are derived from 845 wells throughout the North Sea Basin. Data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths. Positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the North Sea Basin, and reflect regional Neogene uplift and erosion of up to 1 km along the present-day limit of the Chalk. A hiatus at the base of the Quaternary increases in magnitude away from the basin center, where a complete Cenozoic succession is found. This hiatus is consistent in size with the missing section estimated from Chalk velocities when allowance is made for the Quaternary reburial of the Chalk. Negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the Chalk exceed 10 MPa, equivalent to a burial anomaly greater than 1 km relative to the normal trend. The Chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden; therefore, the Chalk is considered to represent a regional aquitard, and the hydrodynamic model of long-distance migration within the Chalk is rejected. The Neogene uplift and erosion of the margins of the North Sea Basin and the rapid, late Cenozoic subsidence of its center fit into a pattern of late Cenozoic vertical movements around the North Atlantic.