Tectonostratigraphy and structural evolution of the Stenlille Structure in Zealand, Denmark – a site for natural gas and CO₂ storage

The Stenlille structure in central Zealand may be the first demonstration site onshore Denmark for geological storage of CO₂ and is a structure, where natural gas has been safely stored and used for consumer supply for more than 30 years. It is the onshore structure in Denmark covered by the most comprehensive subsurface dataset including twenty wells and 2D/3D seismic data. Thus, it is a very good site to study the geological evolution and to define a geological model, which can be used as an analogue for similar sites with potential for CO₂ storage. The Stenlille structure is situated in the eastern part of the Danish Basin and is underlain by Paleozoic sedimentary rocks resting on a crustal basement. Structures include Paleozoic normal faulted rift blocks and later mobilized salt pillows, which shaped the Mesozoic overburden. Sandstones with reservoir quality are known in Denmark from Lower Triassic Bunter Sandstone Formation and the Upper Triassic−Lower Jurassic Gassum Formation, which form regionally well-known reservoirs used for production of geothermal energy and possess a large potential for CO₂ storage. The Gassum Formation sandstones and interbedded mudstones of Late Triassic age in the Stenlille area, formed during several depositional events influenced by relative sea-level fluctuations and sourced from structural highs in the Fennoscandian shield to the north and east, and structures in the south. Detailed studies of the Stenlille dataset show channel systems partly formed around crestal parts of the structure and indicating early paleo-topography highs. Later, during Early Jurassic time the Gassum Formation deposits were transgressed and covered by thick marine clays of the Fjerritslev Formation at present forming the main seal succession, though with thin silty and sandy layers. Subsidence and sediment loading through Jurassic caused deeper burial and conditions for mobilization of Zechstein salt. The regional tectonism of the mid-Cimmerian (mid-Jurassic) phase may also have been a factor that triggered faulting and mobilization of salt from the end of Early Jurassic time. Tectonism is reflected by a number of normal faults, most of which can be traced to near the top of the Fjerritslev Formation. Faults also occur in the Upper Cretaceous Chalk Group, some with deeper connections into the Fjerritslev Formation. The uppermost part of the preserved Fjerritslev Formation at the crest of the Stenlille structure is likely of Toarcian age. The formation is truncated by a marked unconformity, which is onlapped by the Lower Cretaceous Vedsted Formation. From regional correlation the upper part of the Fjerritslev Formation (the major part of member F-IV) and Middle−Upper Jurassic successions are missing at this unconformity, indicating a major hiatus. The Top Fjerritslev surface is approximately concordant with the topography of the Top Zechstein (top of the Stenlille salt pillow) and the Stenlille structure was mainly formed by salt pillow growth and uplift causing top erosion from late Early Jurassic time. The recent studies also show that some of the faults may compart the Gassum Formation reservoir and seem to restrict modelled gas in some parts of the structure. The results from the Stenlille structure may be applied to similar structures such as the Havnsø structure, located about 25 km northwest of the Stenlille structure and is also underlain by a salt pillow and affected by Jurassic and/or later reactivated faults.