Characterization and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben

In the Valdemar Field of the Danish Central Graben, production is from the Upper Hauterivian-Aptian succession (Tuxen and Sola formations) which comprises interlayered pelagic/hemipelagic chalks, marly chalks and marlstones. Based on core data, the reservoir chalks (6-10% insoluble residue (IR)) and marly chalks (10-30% IR) possess porosities in the 20-48% range and matrix permeabilities of 0.1-4 mD. The porosity (φ) of these chalks correlates negatively with the IR (particularly clay) content. A permeability cut-off of 0.1 mD was defined for reservoir studies, corresponding to a φ of 20-30% and an IR of c. 35%. A detailed reservoir zonation, based on integration of core and petrophysical data constrained within a sequence stratigraphic framework, illustrates the stratigraphical compartmentalization of the field. The distinctive nature of this heterogeneous argillaceous chalk reservoir is illustrated by comparison with Maastrichtian-Danian chalks of the Central Graben. For a given porosity, the matrix permeability of the Valdemar Field chalks is a factor of ten lower than that of an equivalent Maastrichtian chalk, the irreducible water saturation is a factor of ten higher and the capillary entry pressure of the Lower Cretaceous reservoir is 2-3 times that of the younger chalks.