Early diagenetic evolution of Chalk in eastern Denmark

The genesis of polygonal faults is an intriguing diagenetic phenomenon. This study discusses their origin in carbonate mudstones together with other associated diagenetic features. In the eastern Danish Basin, at the fringe of the Baltic Sea, the Stevns peninsula offers a unique opportunity to study the early diagenesis of Upper Cretaceous Chalk deposits, buried between 500 m and 1400 m. This paper combines data from onshore and offshore high-resolution seismic reflection profiles, a fully cored borehole with high-resolution wireline logs and quarry and coastal cliff outcrops to study early diagenetic features at different scales. Chalk is affected by an extensive polygonal fault system that is detected in onshore and offshore seismic data. Outcrop and core data provide a better understanding of the distribution of contraction-related features like deformation bands (hairline fractures), stylolites and fluid escape structures. An original model of genetic relationships between these different diagenetic processes is documented for Chalk. The spatial relationships between stylolites and fractures suggest that pressure-solution processes triggered shear failure that initiated the polygonal fault systems. The early diagenetic processes affect the reservoir properties of Chalk by creating compartments and vertical connections. Taking these features into account will allow for a more detailed understanding of early diagenesis and better models for exploiting drinking water or hydrocarbons hosted in Chalk.