Sedimentology and reservoir potential of a ‘marginal facies’ limestone from the Cretaceous of the North Sea

Abstract

The lower Cretaceous section of UK North Sea well 3/16-1, on the eastern flank of the Shetland Platform, drilled by Sovereign Oil & Gas plc, is dominated by marine ‘shelf-type’ limestones with a very low argillaceous content. In this respect it differs both from the typical open-sea coccolith-rich Chalk and from the argillaceous Cretaceous of the Viking Graben (the Cromer Knoll and Shetland Groups). Diagenesis has resulted in crystal overgrowths, corrosion, and development of very fine to coarsely crystalline void filling calcite. Cavities, of stromatactis type and the result of shell dissolution, are filled with radiaxial fibrous calcite. Porosity is mostly intercrystal with possible contributions from fractures and vugs. Hydrocarbon staining is confined to patches of void-filling calcite and partially cemented fractures. No oil staining was observed in the carbonate matrix. It therefore appears that oil migrated through the rock via fractures but was unable to penetrate the main fabric of the rock. The lithological and petrophysical characteristics of the interval are similar to the Chalk Group of Norwegian well 1/3-1. The thickness is considerably less in 3/16-1, probably as a result of deposition on a topographic high such as existed along the margins of the Viking Graben and the East Shetland Platform during the Cretaceous.