Illite distribution and morphology explaining basinal variations in reservoir properties of Upper Jurassic sandstones, Danish North Sea

Growth of fibrous illite may destroy an otherwise promising sandstone reservoir, even sandstones characterized by microquartz coatings. Therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the Upper Jurassic Heno Formation. The Heno Formation is encountered in large parts of the Danish Central Graben, where its lower part – the Gert Member – was mainly deposited in back-barrier environments and its upper part – the Ravn Member – was deposited in a shoreface environment. In total, 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy. This was supplemented by plug porosity and permeability measurements and X-ray diffraction of bulk and clay fraction. The investigations show, among other things, that microquartz coatings may help to preserve porosity and permeability, whereas the permeability is reduced when microquartz occurs combined with fibrous illite. Illite precipitation is largely governed by the amounts of K-feldspar present during burial. Consequently, the initial abundance of K-feldspar (in the sediment source area) and its alteration during and immediately after deposition, in particular during subaerial exposure (at sequence boundaries), has a major influence on the final reservoir properties of the sandstones. The risk of illite precipitation can largely be predicted from the available sediment source material, depositional environment and burial depth.