The importance of fault damage zones for fluid flow in low-permeable carbonate rocks – Fault-related compaction fronts in the Danish North Sea

This study documents the timing and driving forces of the formation of fault-related compaction fronts in the Upper Cretaceous to lowermost Paleogene Chalk Group in the southern Danish Central Graben, based on the integration of 3D seismic, petrophysical log and clumped isotope data. The compaction fronts reflect zones in the low permeable Chalk Group that underwent time-transgressive fault reactivation and pore fluid venting driven by movements of deeper salt and inversion movements. The fault-damage zones formed narrow permeability fairways that facilitated better drainage of compaction-driven pore fluids trapped in the matrix, eventually resulting in preferential mechanical compaction of the chalk. Salt doming during the Paleocene – Early Eocene, possibly linked to regional inversion tectonics, led to an initial phase of fault reactivation, offsetting the entire Chalk Group; pockmarks within this interval indicate release of pressurized fluids on the seafloor. Clumped isotope data from calcite-cemented veins associated with these fault-damage zones indicate precipitation from fluids that likely originated from Lower to Middle Jurassic strata at the root of these faults some 1500 m below the Chalk Group. Local thickening of the Paleocene to Lower Miocene Rogaland and Hordaland Groups matches a 20–50 m thinning of the chalk within the compaction fronts. This indicates that preferential drainage and compaction continued as the chalk became buried with clays, which became affected by polygonal-faulting causing episodic leak-offs. The results indicate that fault damage zones in low-permeability rocks may initially act as permeability fairways, but the improved drainage of formation fluids may over time cause preferential mechanical compaction and calcite precipitation. At present, the fault-related compaction fronts form low-porosity chalk bodies that may have acted as seals and/or re-directed fluid migration. The results have important implications for static and dynamic reservoir models, also in the light of Carbon Capture Storage and geothermal energy extraction and storage.