Seismic geomorphology of the Upper Triassic – Lower Jurassic Gassum Formation – Improved reservoir characterization in the Stenlille (Denmark) CCS demonstration site

The Danish subsurface has the potential to store significant amounts of CO₂ by having porous siliciclastic reservoirs at depths below 800 m (where pressure and temperature allow CO₂ to be in supercritical dense form) that are covered by thick sealing mudstones and occur within numerous structural closures formed by salt tectonics. The Upper Triassic – Lower Jurassic Gassum Formation that is widespread throughout Denmark is used for geothermal energy extraction and natural gas storage and is now also considered for Carbon Capture Storage (CCS). The formation thickness is relative homogenous between 50 – 200 m. It comprises 5 – 30 m thick successions of alternating fluvial and shoreface sandstones with excellent reservoir properties (porosity between 15 to 35% porosity, permeability up to several Darcies) and m-scale intercalated marine and lagoonal muds. The Stenlille natural gas storage facility is located 60 km west of Copenhagen, where gas has been seasonally stored for over 30 years within the Gassum Formation above a gentle salt structure and is now also considered as a CCS demonstration site. Due to the current role as a gas storage, the area of Stenlille is covered by a rich subsurface dataset: 1) a 60 km2 high-quality 3D seismic survey; 2) 20 wells with wireline logs and a large amount of core material largely covering the Gassum Formation. In order to accurately simulate CO₂ injection and storage over a prolonged period, it is crucial to construct static reservoir models that reflect the heterogeneity of the Gassum Formation in terms of reservoir and baffle architecture. We therefore utilized a seismic geomorphological approach, whereby a dense set of sequence stratigraphic seismic horizons were constructed from a 3D Relative Geological Time model (RGT) and tied to the wells, followed by displaying seismic attributes to highlight acoustic heterogeneities that were interpreted as depositional elements. We then tied those depositional elements to depositional facies that had been interpreted from core and wireline logs to verify the interpretation and spatial distribution of the facies. Our integrated seismic geomorphological analysis identified four major depositional environments (from proximal to distal): 1) fluvial landscapes with meandering rivers and pointbars; 2) lagoons and tidal creeks; 3) shoreface sandplates ; 4) deep-water offshore environments. From the base to the top of the Gassum Formation, four major flooding events can be observed that forced the shoreline landward and caused the deposition of offshore marine mud in the entire study area, at present forming baffles in the formation. The fluvial landscapes were succeeded by lagoonal environments, shoreface, and finally deep-water offshore environments. Using our improved understanding of the Gassum Formation within the Stenlille area, we are now able to better construct the reservoir model by extracting reservoir bodies directly from the 3D seismic or use stochastic modelling tools and choosing parameters such that they reflect the heterogeneities for each of the different landscape elements. The importance of this work goes well beyond the Stenlille CCS demonstration site, since the 3D seismic survey allows to construct a rich catalogue of landscape elements and their 2D expression that can occur within the Gassum Formation, which then can be used for other green energy projects (geothermal, heat storage, Power-to-X). Since onshore Denmark is mainly covered by 2D seismic data and a new data acquisition is under way, such a catalogue will help to interpret depositional environments and construct better reservoir models in areas where such exploitations of the subsurface are planned.