The permeability and capillary entry pressure of chalk reservoirs are controlled by their porosity and specific surface area. Measured permeabilities are in the range 0.025–5.3 mD and are successfully predicted by use of the Kozeny equation. In this paper we focus on the factors that control specific surface area. Fifty-nine Tor and Ekofisk Formation chalk samples from five North Sea chalk reservoirs were investigated. All contain quartz and clay minerals, most commonly kaolinite and smectite, with trace amounts of illite. The contents of calcite and quartz are inversely correlated and both are independent of the content of clays. We thus infer that the main part of the silica is of biogenic origin.
The specific surface area of the chalk is mainly controlled by clay content. The specific surface area of calcite is determined by the individual calcite crystal size and is not dependent on stratigraphic variations in fossil size. The specific surface area of calcite increases with increasing content of quartz and clays. These constituents may inhibit recrystallization of calcite and thus preserve high specific surface area. Our data accord with the following specific surface areas (m2 g−1): calcite between 0.5 and 3.5, quartz about 5, kaolinite about 15, and smectite about 60.