Reservoir Quality Preserving Processes in Lower Jurassic Cook Formation of Veslefrikk Area

Two wells from the Veslefrikk Field area (30/3–4 and 30/3–5S) and one well from the Oseberg field area (30/6–17A) located 30 kilometers south of the Veslefrikk field in the northern North Sea was studied. Reservoir depth ranges from 2800 to 3200 meters for the Veslefrikk wells while the Oseberg well is buried to about 2400m. The aim of this study was to improve the understanding and prediction of reservoir quality in deeper buried Jurassic sandstones within the Dunlin group (Cook Formation) in nearby areas.

28 thin sections were produced from core samples of the three wells and studied for optical point counting and SEM imaging. In addition stubs were studied in the SEM and XRD analysis was performed. Furthermore, sedimentological logging of the cores was performed. Petrographically 300 points were counted on each thin section to estimate grain size, matrix, sorting, porosity and intergranular volume (IGV). SEM analysis showed partial to complete chlorite coatings in deeper samples from the Veslefrikk area. Based on the results of this study the Cook Formation is a moderate to good reservoir rock but with a good potential for preservation of reservoir quality when deeply buried due to pervasive chlorite coatings when present.