In the Danish North Sea, the chalk of the Danian Ekofisk Formation is largely pelagic and includes horizons of clay-sized quartz and/or clay minerals. The upper part of the Ekofisk Formation (Ekofisk Porous) has reservoir potential, whereas the lower part of the formation (Ekofisk Tight) is of low permeability. The Ekofisk Formation was studied in wells Rigs-1, Rigs-2 and SA-1 in the South Arne field in the Danish North Sea. Clay-poor and clay-rich horizons were sampled in order to evaluate the effect of the mineralogical composition on the reservoir properties of the Ekofisk Formation. The Ekofisk chalk samples were investigated by scanning electron and atomic force microscopy, by thermal analysis and for isotopic composition. Helium porosity, gas permeability and nitrogen adsorption (for specific surface area and fine porosity) were measured to characterize the chalk structure. Calcite was removed by titration and the residues investigated by X-ray diffraction and atomic force microscopy. The clay layers consisting of nano-quartz, clay minerals and carbonates have He-porosity similar to that of the tight chalk matrix and have a large amount of fine pores (<200 angstrom diameter) which favour diffusion. This diffusion has dissolved calcite in the layers and has resulted in the formation of low-porosity chalk due to recrystallization of calcite adjacent to the layers. The chalk matrix of the Ekofisk Formation contains a considerable amount of non-calcite residue, of which the major part (similar to 2/3) is nano-quartz. The nano-quartz is assumed to have formed by sedimentation. For the samples investigated, there is no clear correlation between mineralogy (% residue, % nano-quartz, % clay minerals) and porosity/permeability. However, samples with <20% residue have high He-porosity and relatively high permeability; whereas samples with >25% residue have permeability <0.4 mD. The effect on He-porosity is due to the content of both nano-quartz and clay minerals, whereas the effect on permeability is mainly due to the content of nano-quartz, because the nano-size quartz particles pack between the coccoliths and thereby reduce the effective pore throat size. Consequently, for reservoir evaluation purposes, it is important to distinguish between the clay minerals and the nano-quartz in the clay fraction. However, the high proportion of nano-quartz in the Ekofisk Formation in general, and the relative high content in the Ekofisk Tight in particular, suggest that nano-quartz may have a major impact on the reservoir properties.