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

Abstract

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.