Reservoir Properties of Upper Jurassic to Lower Cretaceous Formations in the Northern Dutch Offshore
MetadataShow full item record
In the northern Dutch offshore, Upper Jurassic to Lower Cretaceous formations form an important target for hydrocarbon exploration. During the Late Jurassic to Early Cretaceous period, rifting in the Southern North Sea accommodated the deposition of three transgressive mega-sequences, which reflect a general change in energy conditions from proximal towards distal facies. The reservoir sands vary widely in presence, thickness, and reservoir quality, a factor which is still poorly understood. A study of reservoir properties was therefore performed with the aim of constraining the main processes controlling reservoir quality. Core data was used from 45 exploration, appraisal, and production wells containing a total of 1426 m of core length including 4066 plug sample measurements on porosity, permeability, grain density, and petrographic data such thin sections. All cores were cut in the Lower Graben Subgroup and the Scruff Group, and located in the Dutch Central Graben, Terschelling Basin, and Schill Grund Platform. In addition, facies data was correlated with 2066 plug samples in order to map the depositional environments of the formations. Correlating the facies data with the plug sample data not only showed clear differences in reservoir quality due to different depositional environments (eogenesis), but also the progression of these environments through time and the combined effects of burial diagenesis (mesogenesis) on different lithologies. The most important factor determining reservoir quality is the environment of deposition, which controls energy conditions that influence clay content. The Friese Front Formation shows the best reservoir quality in clean, well-sorted channel sands that indicate high energy conditions, which lead to lower clay content. Alluvial fan deposits in the Friese Front Formation also show good reservoir quality. Other reservoir sands with good quality are from marginal marine settings such as sands deposited in beach-barrier complexes as encountered in the Terschelling Sandstone Member, and tidal channel sands in the Lower Graben Formation in the F03- FB field. Marginal marine sands of the Middle and Upper Graben formations also show good reservoir quality. The shallow marine Noordvaarder Member has good reservoir quality with potential as it can be correlated with similar sands in the UK, Danish, and probably also the German section of the Central Graben. Depositional settings with low energy conditions lead to high clay content and a poor reservoir quality, as seen in the brackish lagoon/bay deposits of the Lower Graben Formation. Lower shoreface sands of the Scruff Greensand Formation have poor permeability but good porosity due to the dissolution of sponge spicules. Lithologies consisting of primarily clay such as the Kimmeridge Clay Formation and the Lutine Formation show a poor reservoir quality. Processes related to burial diagenesis significantly influence reservoir quality after initial deposition. Porosity decreases linearly with depth due to mechanical compaction and burial cementation. Permeability shows no clear trend with depth. However, core data from the F03-FB field show rapid losses of porosity and permeability at depths greater than 3100 m, which suggests chemical diagenesis as the dominant control on reservoir quality. Permeability losses at depths greater than 3100 m are in 1 to 2 orders of magnitude, which is attributed to chemical diagenesis due to the precipitation of clays such as illite at increased pressures and temperatures.