Ekofisk Formation

White, chalky limestones containing rare white and grey nodular and bedded chert layers and thin, grey to green clay laminae. Some glauconite can occur in the basal interval. The unit displays a characteristic gradual increase in acoustic- and resistivity-log readings.

Relatively stable, low-energy conditions in carbonate-shelf and upper bathyal environments. The rocks consist essentially of pelagic, biogenic remains, which settled from suspension. A variety of subfacies has been recognised, reflecting variation in sedimentation rate and water depth. In many localities Campanian-Maastrichtian fossil associations indicate considerably shallower water conditions than Turonian-Santonian associations (e.g, lower plankton/benthos ratio). Particularly in the northern A and B blocks, where the formation may have been deposited in deeper water, redeposition by gravitational mass flow occurred. This is reflected in thin sections by a typical brecciated microstructure of the rocks. The Ekofisk Formation represents post-inversion deposits which onlap onto and overstep inverted basins.

Characterised by a change to a slightly higher response and somewhat more serrate pattern of the gamma-ray curve, and a lower porosity in the basal Ekofisk Formation, as can be observed on the appropriate wire-line logs when compared to the underlying Ommelanden Formation. In general, most of the formation shows a somewhat higher porosity than the uppermost Ommelanden Formation. Very rarely a high peak is seen on the gamma-ray log, representing a Cretaceous/Paleogene boundary clay layer. Locally, a densely cemented hardground level, overlain by a glauconite-rich bed, may occur around this boundary.

The boundary with the overlying sands and clays of the Lower North Sea Group is expressed clearly on wire-line logs by the characteristic pattern: a low gamma-ray response and a high acoustic velocity.