A redeposited chalk reservoir (upper Maastrichtian-Danian) in the Oseberg Field, Northern North Sea

Setting and Stratigraphy
The northern limit of the Upper Cretaceous chalks in the North Sea occurs at 60–61°N where carbonates interdigitate with mud-rich siliciclastics. This facies transition, coinciding with marked submarine palaeotopography descending from the Horda Platform to the Viking Graben, is recorded primarily by well-log data which demonstrate the temporal shifts in the position of the chalk:mudstone facies boundary. The Oseberg Field, 130 km west of Bergen (Norway), straddles this regional facies front; declining production from the primary Jurassic reservoirs in this field has focussed attention on hydrocarbon-bearing carbonates in the uppermost Shetland Group, mainly in the south of the field. Of particular interest is a 10–30 m thick chalk-rich section that caps the upper Campanian–Maastrichtian Hardråde Formation; a thin Danian carbonate unit (equivalent to the Ekofisk Formation farther south) is also recognisable locally, resting on a hiatal surface spanning the latest Maastrichtian (nannofossil subzone UC20d) and early Danian (subzones NNTp1–4E).