Indications for Intra-Chalk Seals in the F-blocks of the Dutch Offshore (2014)

This MSc thesis has been written by former intern Maarten Huijgen.

A study on the Dutch Cretaceous Chalk Group is performed during a six month internship at EBN. The study area is the F-Blocks in the northern Dutch offshore. The focus of this study is to evaluate if there are indications of hydrocarbon seals present in the Chalk.

These seals act as intra-Chalk permeability barriers and can be formed by hardgrounds and tight zones.

Permeability barriers may be associated with: (1) the presence of unconformities; (2) a sudden rise in pressure within the Chalk; (3) the presence of hydrocarbon shows within the Chalk. In the study area a few indications for intra-Chalk permeability barriers have been observed.

(1) Several regional unconformities are present in the Dutch Chalk. A Late Campanian regional unconformity was seismically mapped throughout the study area. This unconformity is tied with various well logs and biostratigraphic well data.

(2) Hydrocarbon shows within the Chalk are also evaluated in order to see if the shows are located intra-Chalk as this would suggest an intra-Chalk permeability barrier. Almost all shows in the Chalk are sealed by the overlying Tertiary. Well F06-02 has a hydrocarbon column located under Top Chalk but examination suggests that the overlying Chalk is a waste zone and the column is sealed by the Tertiary.

(3) There is a sudden pressure increase in well F06-02 with hydrocarbon shows located underneath it. This sudden pressure increase coincides with the mapped Late Campanian unconformity and a well log character that suggest denser, less permeable, chalk.

This outcome of this study shows that there is weak evidence that the Late Campanian Unconformity is able to act as a permeability barrier for hydrocarbons. Intra-Chalk structural traps have not been identified. It may be possible that an intra-Chalk permeability trap in the form of a constriction trap exists but no well tests are available.