Assessment of subsurface pressure regimes onshore Denmark derived from formation-water salinity and artesian flow

This study presents a first-order assessment of reservoir pressure regimes across onshore Denmark, focusing on hydrostatic conditions and occurrences of natural overpressure. Using a newly compiled database of brine salinity measurements from 28 wells, hydrostatic pressures were estimated based on salinity-derived density profiles. Validation against in situ pressure measurements confirms the reliability of this approach. Additional analysis of historical artesian deep wells documents local overpressured reservoirs within specific units such as the Zechstein Group, Bunter Sandstone and Ørslev Formations. These overpressured compartments suggest that reservoirs may be hydraulically isolated, potentially impacting storage capacity in CO₂ sequestration projects. Overpressure is also observed in the overburden within the Chalk Group. We suggest that regional Neogene uplift has further shaped hydraulic heads in the Chalk Group, introducing semi-regional pressure compartments. The main objective of this study was to establish a robust regional baseline for hydrostatic pressure conditions while highlighting historically underappreciated overpressure phenomena relevant to risk assessment in subsurface CO₂ storage, geothermal utilisation and underground energy storage. We calculate the pressure required to lift brine to the surface, potentially allowing interaction with groundwater and thereby providing practical constraints for risk assessments and subsurface pressure management in prospective CO₂ storage sites. In order to distinguish between open versus closed aquifer behaviour and identify potential brine discharge pathways, the sealing capacity of the Quaternary cover above the sparse natural outcrops of the Gassum and Skagerrak Formations, together with the conductivity of fault systems, must be assessed. These results establish a pressure baseline and provide critical input for future pressure communication models, supporting site selection, operational safety and coordinated subsurface resource management during Denmark’s green energy transition.