The history of hydrocarbon filling of Danish chalk fields

The modelling reported here demonstrates that oil accumulations in chalk may require several million years to equilibrate following perturbations resulting from primary migration or reservoir tilting, if matrix permeability governs fluid flow. Since naturally occurring disequilibrium oil accumulations dominate the Danish chalk fields, it must be concluded that matrix flow dominates fluid dynamics. The modelled filling scenarios are intended to illustrate the general aspects of geological timescale oil-water dynamics in chalk reservoirs. The scenarios are not considered to represent actual filling histories, as they are constrained by relatively simple model assumptions, but they are geologically plausible. Due to the long equilibration times, it can be dangerous to interpret tilted contacts as reflecting only dynamic equilibrium, as they may be fully dynamic and still actively flowing. This is revealed locally by non-equilibrium between Danian and Maastrichtian oil where they are seen to have different FWLs. It is important to try to understand fluid dynamics during exploration work, since this strongly affects trap definition and volumes. The project shows that with simple and geologically based assumptions, a reasonable filling history can be modelled quantitatively. A reasonable end-result can be produced that has many similarities with present-day hydrocarbon configurations. With the methods developed in the project, even a fully dynamic system (with both oil and water moving), as for example in the Dan-Halfdan field system, may be explained.