Oil Migration and Dynamic Traps in Chalk, Danish North Sea
Apollo Kok and Michael Arnhild
Maersk Olie og Gas, Copenhagen, Denmark
The pace of lateral oil migration in chalk is less than that of continental drift and - like plate tectonics - oil migration is still happening, making the present day situation only a snapshot in time. However, over geological time, the oil can move significant distances and the fact that the majority of discoveries in the Danish Sector of the North Sea has been made in four-way closures, indicates that a substantial part of the oil has managed to migrate to these structural highs. The discovery of the Halfdan Field / Dan West Flank in 1998 proved that at least some oil is in non-structurally or dynamically trapped accumulations. The question is whether there may be more of these accumulations and to go even a step further, accumulations where there is no or only little oil in the structure up-dip, i.e. a fully “on-the-move” oil system. In combination with recent charge, the chances of encountering such a dynamically trapped oil accumulation may increase significantly.
While several papers have addressed the various aspects of oil migration and tilting oil-water contacts in chalk, the current analysis focuses on quantifying “on-the-move” oil accumulations and in particular those that may have originated from recent local charge. Recent charge from a local origin is supported by under-filled structures in the Southern part of the Danish Sector and geochemistry indicating that the oil in these fields is less biodegraded than the oil from more Northern fields. The Jurassic source rock in the area has only reached maturity in the last couple of million years and may generate up to 1000 rb/year based on basin modelling. Such high charge rates are considered feasible if compared to the filling history of the Northern fields in the Danish sector. If these fields were to be filled within a reasonable time frame, the charge rates must have been in the range of 100 - 1000 rb/y. With little known about the vertical migration from the source rock to the chalk layers, the oil is assumed to enter the base chalk at certain leak points at a specified rate. The leak points can either be faults or fractures, but also areas where the Lower Cretaceous is thin or absent. Depending on the actual location of the leak point and timing of the charge, some of the charge may still be located in synclines or just started to migrate laterally towards structural highs. The lateral migration occurs in the Tor formation as the overlying Ekofisk has much lower permeabilities and the height of the oil column in the Tor is usually not sufficient to overcome the capillary entry pressure of the Ekofisk formation. In circumstances where a sufficient column in the Tor can accumulate, the Ekofisk starts to fill from below in the crest, which in itself can take several million years to complete.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012