Sedimentology and reservoir properties of the Jurassic Cook Formation, in the Northeastern Norwegian North Sea

Abstract

Porosity and permeability vary significantly within the Lower Jurassic Cook Formation which is a problem when exploring for hydrocarbons or storage of CO2. Reservoir quality varies significantly from well to well, but the reason why this is so, remain unclear. A recent sedimentological study has shown that the best sand units are deposited as tidal ridges in the western margin of the Cook Formation, but several studies suggest that the Cook Formation is sourced from the west coast of Norway. It is clear that there is no common understanding for the sedimentary distribution of the Cook Formation, especially in the eastern margin.

This study will investigate the Cook Formation in cored intervals in the easternmost margin and deduct a sedimentological analysis to understand which sedimentary environments that are present in the proximal region. Additionally, investigate the lateral variations by examining wireline-log from wells without cored intervals, because it provides a greater dataset than just cores. Core-plug data will also be investigated to understand how reservoir properties vary within the study area, and in the different facies’ associations. A total of 192 meters of core have been logged from 6 wells, and wireline-logs from 20 wells selected within the Cook Formation.

The result from this study shows that the eastern margin of the Cook Formation is deposited in a subaqueous tidal regime, with the shelf tidal ridge deposits exhibit the best reservoir properties. The tidal ridge sandstones in well 31/5-7 contains the best reservoir properties, while the deeply buried sandstones in well 35/4-1 exhibit poor reservoir quality. This implies that further exploration of hydrocarbons or storage of CO2 should occur in tidal ridge deposits in the Cook Formation situated at relatively shallow depths.