The history of hydrocarbon filling of Danish chalk fields

Abstract

It is well established that dynamic conditions expressed as tilted fluid contacts characterize most hydrocarbon accumulations in North Sea Chalk reservoirs. Chalk is a low-permeability, high-porosity rock and properties grade smoothly from reservoir over baffle to seal. The natural dynamic conditions prevail because pressure dissipation takes place through the rockmatrix, as fracture-supported flow often is minimal. The dynamic conditions are imposed by processes occurring on a geological time-scale and result mainly in lateral pressure differences in the water zone and even in lateral pressure differences in the oil zone. Re-equilibration of fluid contacts also occurs on a geological time-scale. These processes are of paramount importance for trap definition and impose severe restrictions on migration distances. Reservoir simulation techniques are applied, in combination with back-stripping, to the simulation of geological time-scale secondary migration and trapping. Flow simulation of the filling dynamics of a chalk reservoir shows a complex filling geometry due to the high capillary entry pressures in the low-permeability chalks. Such internal barriers will re-direct hydrocarbons and residual oil can be left on the migration route. The process of hydrocarbon charging is slow and equilibration of hydrocarbons with respect to pressure gradients, therefore, also occurs very slowly. The Kraka Field and the Dan and Halfdan fields are subjected to studies of primary oil charging and re-migration in this paper and the dynamic oil on Dan Field west flank is successfully mimicked. Results show that a time span in the order of 2 Ma is required for the hydrocarbons to reach the summit in an approximately equilibrium state from a flanking position. However, even dynamic equilibrium may not be fully obtained due to re-perturbation by tectonic movements and changing water zone pressure gradients. Results show that saturation profiles in drilled wells may appear in drainage equilibrium while under partial re-imbibition, which impairs saturation modelling.