The Stevns peninsula, situated in the eastern Danish Basin at the fringe of the Baltic Sea, is an ideal target for studying the early stages of diagenesis that affected the Upper Cretaceous chalk during its burial to 500-1400m depth. For the present study onshore and offshore high-resolution seismic reflection profiles, high-resolution wireline logs and sedimentological data from a fully cored borehole, and field information from quarry and coastal cliff exposures were taken to inform about early diagenetic features. This integrated approach for the first time enabled placing different processes operating in the chalk sediments at widely different scales into a single diagenetic model:
At Stevns the chalk is affected by an extensive polygonal fault system which is expressed in onshore and offshore seismic profiles. Smaller scale contractional features like deformation bands (hairline fractures), stylolites and fluid escape structures can be studied using outcrop and core data. The spatial relationship between stylolites and fractures suggests pressure solution as trigger for shear failure that in turn led to the initiation of the polygonal fault system.
Early diagenetic structures strongly affect reservoir properties of the chalk both by establishing compartments and vertical connections. A better understanding of these reservoir modifications will be critical for improving the predictive capability of models describing the behaviour of drinking water and hydrocarbons hosted in chalk sedimentary rocks.