To achieve net-zero carbon emissions by 2050, gigatonnes of CO2 must be captured and stored in the subsurface. Screening and exploration of prospective storage sites have thus gained momentum in recent years. The Miocene-age Lille John Member in the Danish Central Graben represents a promising, yet underexplored, CO2 storage candidate due to its lack of commercial hydrocarbon potential. This study integrates high-resolution 3D seismic data, core analyses, and wireline logs within a sequence stratigraphic framework to characterise the depositional environment within the targeted Miocene interval. Seismic attributes such as RMS amplitude and spectral decomposition are used to define the three-dimensional architecture of the geobodies and evaluate their potential for CO2 storage by comprehending reservoir distribution, heterogeneity, and connectivity. The reservoir consists of two unconsolidated sand units, informally termed the lower and upper sand units, separated by a mudstone interval. The lower sand unit represents a basin floor fan emplaced by gravity flows during the falling stage systems tract, while the upper unit comprises unconfined gravity flow deposits associated with the lowstand systems tract. The reservoir sands of the Lille John Member are predominantly localised in the southeastern portion of the Central Graben at depths suitable for storing supercritical CO2. Theoretical P50 storage capacity is estimated at approximately 1108 million tonnes for the lower sand unit and 51 million tonnes for the upper unit. Heterogeneities such as silt beds, mudstones, and carbonate concretions may act as flow baffles, enhancing storage efficiency through plume dispersion, residual trapping, CO2 dissolution, and geochemical interactions. This study situates the Lille John Member within a broader regional framework by integrating a larger 3D seismic dataset with advanced seismic interpretation workflows, extending beyond the scope of previous investigations. The results provide new insights with implications for unlocking CO2 storage potential in analogous depositional settings.

In June 2021, a novel Danish national carbon capture and storage strategy was ratified by the Danish Parliament, and this was followed by the initiation of the project ‘CCS2022–2024’, led by the Geological Survey of Denmark and Greenland. In collaboration with other institutions, we acquired and interpreted new 2D seismic data between 2022 to 2024 to investigate and mature eight sites for potential subsurface storage of CO2 in Danish onshore and offshore areas. This Bulletin contains a series of papers that present important results of the work. In this introduction paper, we provide an overview of seismic acquisitions and the interpretation of seismic data together with existing deep wells. The study sites selected are large subsurface structures located in onshore Jylland, Sjælland and Lolland and offshore Denmark in the eastern North Sea. The onshore targets are the Gassum, Havnsø, Rødby, Stenlille and Thorning structures, while the offshore sites comprise the Inez, Jammerbugt and Lisa structures. The project work comprises a series of reports regarding extensive seismic acquisition, processing and interpretation of the new and pre-existing seismic data as well as other publications emanating from the project. This Bulletin and the technical reports present an improved understanding of the formation, composition and geometry of the investigated structures. The studies include the mapping of the reservoir and seal formations, identification of principal faults, interpretation of the stratigraphic and structural development, reservoir and seal characterisation and estimates of the static storage capacity. Hence, this research provides a significant step forward concerning characterisation of the geology and maturation of the potential storage sites. In addition, it has inspired new ideas, including an updated regional stratigraphic interpretation of the Triassic succession of the Danish Basin and correlation with adjacent basins.

This study documents a variety of deposits created by submarine landslides within the Upper Cretaceous to lowermost Paleocene Chalk Group in the Danish Central Graben and investigates the impact of remobilization on porosity. Improved visualization of the landslides in 3D seismic data compared with previous studies was facilitated by better seismic data quality for the Chalk Group, the availability of a large stack of stratigraphy-consistent horizons and the use of spectral decomposition data. The illustrated examples are chosen to reflect the spectrum of deformation styles seen in the chalk and all have a well penetrating the affected succession. They include a large collapse (375 km(2)) of an inversion ridge within the Kraka and Gorm formations, a field of large slide blocks (100-1000 m, 10-26 m) of likely lowermost Danian age embedded in the uppermost Ekofisk Formation, a debris flow system within the uppermost Tor Formation probably originating from the Ringkobing-Fyn High and fine-grained bottom current sediment waves within the lowermost Danian Ekofisk Formation. In general, porosities are higher (10-25 porosity units) in the remobilized chalk compared with time-equivalent pelagic chalk in nearby reference wells. In earlier studies this has been linked to lack of bioturbation (resulting in limited grain repacking) in the remobilized chalks owing to high sedimentation rates, resulting in a relatively open fabric during initial burial. In contrast, surrounding and covering pelagic deposits could be much more effectively bioturbated, leading to tighter grain packing during burial. The insights of this study help in the seismic characterization of mud-grade carbonate oozes and have important applications in the reservoir modelling of mud-grade carbonate reservoirs (also in light of carbon capture and storage), and in palaeo-reconstructions of pelagic seafloors since submarine landslides provide kinematic indicators.

Summary Denmark has a large potential to utilize geothermal energy as a baseline heating and cooling solution, due to the occurrence of widespread sandstone reservoirs and a large portion of the households are connected to district heating networks. However, data coverage onshore Denmark is generally poor so that using geostatistical methods alone to estimate reservoir properties æleads to large uncertainties. To overcome this challenge, we demonstrate the use of forward stratigraphic modelling on the most promising geothermal reservoir of Denmark, the Upper Triassic-Lower Jurassic Gassum Formation, which has been studied extensively for geothermal purposes and more recently for CO2 storage. Model parameterization was therefore following previous geological and sequence stratigraphic knowledge of the Gassum Formation, and around 40 boreholes were available for calibration to arrive at a best-fit model. The results are a 4D (3D space and geotime) grid (5x5km) that contains quantified information on sediment proportions, sedimentation rates, water depths, river water flow etc. Sensitivity and risk analysis, which sample the parameter uncertainty ranges using several 100s of models, help to quantify the importance of different parameters on sand thickness, and probabilities of having a sand of certain thickness. The results are useful to update the geothermal webportal.

In Denmark, Geological Carbon Storage (GCS) has been prioritized as an immediate solution for climate action. The Havns & Oslash; domal structure has been identified as one of the most promising locations for GCS because its size and properties are believed to be suitable for GCS. However, the preliminary assessments, based mainly on old, sparse, and low-quality seismic data, are uncertain regarding the prospective storage resource and the integrity of the structure. To enable informed decisions and planning of the storage operations and as part of a large-scale acquisition campaign targeting several similar onshore structures throughout Denmark, a seismic data acquisition work was conducted in 2022 in the area. The purpose of the survey was to delineate the structural closure and map possible geologic features, such as faults, that could jeopardize GCS operations. In total, 132 km of highfold and high-resolution 2D profiles were acquired using an innovative dual-element recording system for both deep and shallow subsurface imaging purposes. The recording comprises two vibrating sources and a combination of nodal recorders spaced at 10 m, and 2-m-spaced microelectromechanical systems (MEMS)-based recorders attached to a moving landstreamer. The seismic data contain information on all horizons of interest for GCS. The structure is estimated as a well-defined four-way closure, where the reservoir is continuous. A thick, mostly uniform sealing rock is interpreted and no large-scale faults are found in the near surface. The results, supported from existing background information, provide crucial information to assist further decisions and actions related to future storage operations in Havns & Oslash;.

Summary This study is a first example of application of forward stratigraphic modelling in order to characterize a reservoir and a seal for CO2 storage purposes. The Gassum and the Fjerritslev formations onshore Denmark are an excellent candidate for reservoir and seal respectively for CCS. However, their distribution, quality, and thickness are not very well characterized due to the lack of 3D seismic, low quality 2D seismic, and a small number of wells. Therefor we applied forward stratigraphic modelling in order to generate reservoir and seal net thickness and net to gross maps that help identify potential areas for further data acquiring and eventually CO2 storage.

Depleted chalk oilfields and chalk structures in the Danish Central Graben, North Sea, are potential CO 2 storage sites. In most of these fields, the main reservoir is the Upper Cretaceous – Danian Chalk Group and the Eocene – Miocene mudstones of the Horda and Lark Formations constitute the primary seal. In a few fields, the reservoir is composed of the Lower Cretaceous Tuxen and Sola Formations. Here the main seal is assumed to be the Chalk Group which however has poor gas sealing characteristics; the Horda and Lark Formations constitute an efficient secondary seal although they are quite high in the section. This study documents a workflow that may help to evaluate the seal integrity of the structures from an integration of mud gas data from wells with seismic data. Mud gas data provide detailed information about the distribution and types of gas (biogenic or thermogenic) throughout the seal section and overburden. The presence of higher carbon number gases (C 3 –C 5 , propane to pentane) in the seal indicates migration of thermogenic gas into the thermally immature sealing mudstones; whereas the dominance of C 1 (methane) and partly C 2 (ethane) likely reflects the presence of in situ generated biogenic gas in the mudstones, thus indicating that there are no seal integrity issues. The vertical thermogenic gas migration front has been determined, and a “traffic light” indicator system has been used for seal integrity evaluation. Where no or minor migration of thermogenic gas into the primary seal has occurred and a primary seal >30 m thick is present, the seal is considered to have good matrix seal integrity (green). If some significant thermogenic gas migration has occurred into the primary seal but more than 30 m of primary seal is present above the thermogenic gas migration front, the seal integrity is reduced (yellow). In structures where thermogenic gas migration is recorded through the primary seal and into the overburden, seal integrity is considered to be poor (red). In areas where significant leakage of thermogenic gas has occurred into the seal, high density, low porosity carbonate beds frequently occur encapsulated within the sealing mudstones and are interpreted to be composed of methane‐derived authigenic carbonates (MDACs). Seismic data show that there is a convincing correlation between leakage as indicated from mud gas data and the presence of vertical wipe‐out zones (gas chimneys), bright zones (gas‐charged sediments or MDACs), and depressions (pockmarks). In general, potential CO 2 storage sites in the study area in tectonically inverted structures show good seal integrity, but this may locally be reduced and require additional analyses. Storage sites associated with salt diapirs generally show poor seal integrity and are likely to be poor candidates for CO 2 storage. In combination, mud gas and seismic data are therefore powerful tools to investigate (palaeo‐) leakage phenomena and provide support for seal integrity evaluation at local to regional scales.

The alternating marlstone and chalk of the Lower Cretaceous succession in the Danish Central Graben (DCG) are important for the understanding of the evolution of the larger North Sea Basin. This study focusses on the clay mineral assemblages of the upper Hauterivian – lower Aptian in the DCG and Danish Basin (DB) and their implications. Clay mineral assemblages are predominantly used to assess palaeoclimate. In this study, however, they were additionally used in a source-to-sink context. Kaolinite was found to form a dominant component of the clay mineral assemblage in the sampled wells of the DCG and in the DB, suggesting that a feldspar- or kaolinite-rich source was present and actively eroded in the region during the Early Cretaceous. Moreover, a decreasing gradient west to east of average kaolinite content is observed in the three studied wells for the early Hauterivian to late Barremian (BC9-BC17), with the highest content observed in the North Jens-1 well (av. 74%), followed by the Boje-2C well (av. 49%) and lastly in the Vinding-1 well (av. 39%). Due to the relatively rapid settling of kaolinite in marine environments compared to other clay minerals, this gradient suggests that the main clay mineral source was located in the south-western part of the DCG. Isochore maps, a new palaeogeographic map of the DCG and the western part of the German sector of the North Sea illustrates where Lower Cretaceous rocks are absent in this region, due to either erosion or non-deposition. Potential subaerially exposed highs included the distant Baltic Shield to the north, the Ringkøbing–Fyn High to the east and the Heno Plateau within the DCG, with the latter being located closest to the North Jens-1 well and containing feldspar-rich sandstones of the Heno Formation (upper Kimmeridgian – lowermost Volgian/Tithonian). During the Early Cretaceous, part of the Heno Formation was potentially subaerially exposed or subject to wave reworking/erosion in parts of the Danish and German sectors. The sandstones could weather into kaolinite and this structural high is therefore suggested to have been the main source area for this part of the DCG, with minor sediment influxes from the Ringkøbing–Fyn High and Baltic Shield. In addition, the overall decrease in kaolinite in the DCG from the late Hauterivian to the late Barremian indicates a climatic change towards drier conditions, with some minor, slightly more humid periods.

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 lowporosity 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.

Summary We use colour processing and ant-tracking techniques to resolve subtle features on 3D seismic data of the strata overlying a salt pillow in the Danish Central Graben. This reveals a series of subtle pipes and planar zones of high intensity signal, which are resolved as inclined lineations on vertical sections and form a polygonal pattern when projected onto stratigraphic horizons. Although these show some similarities in location and orientation with resistive fractures seen on borehole images and large cemented fractures seen on core, there is not a 1:1 correspondence, and the lineations are broader and more diffuse than the fractures. They are more likely to represent zones of low porosity surrounding fractures and small faults, resulting from compaction and/or cementation caused by escape of overpressured fluids through the fractures.

A quantitative seismic interpretation of the Gassum Formation at the onshore aquifer gas storage near the Danish town of Stenlille is presented with its implications for exploiting the gas storage facility as a potential CO 2 demonstration site. Our objective was to better outline the reservoir heterogeneity of the Gassum Formation based on a 3D seismic volume and 20 wells available from the Stenlille gas storage facility. We derived new absolute and relative P-impedance inversion products, which are useful for delineating lithological distributions of thin sandstone reservoirs and shaly beds within the Gassum Formation. Our results broadly agree with previously published seismic interpretations, and build upon these by the identification of deviations in parts of the Gassum interval that should be considered in any subsequent development of a static reservoir model. Hence, this work is an important contribution to the planning of drilling new CO 2 injection wells as part of the development and management of the Stenlille CO 2 storage demonstration site. Nevertheless, we also recommend a modern seismic reprocessing of the 3D seismic data combined with newly acquired 2D data from the Stenlille structure as input into further quantitative seismic interpretation studies to refine reservoir characterization and reduce associated uncertainties. Supplementary material: video is available at https://doi.org/10.6084/m9.figshare.c.6662413

The mapping of faults provides essential information on many aspects of seismic exploration, characterisation of reservoirs for compartmentalisation and cap-rock integrity. However, manual interpretation of faults from seismic data is time-consuming and challenging due to limited resolution and seismic noise. In this study, we apply a convolutional neural network trained on synthetic seismic data with planar fault shapes to improve fault mapping in the Lower and Upper Cretaceous sections of the Valdemar Field in the Danish North Sea. Our objective is to evaluate the performance of the neural network model on post-stack seismic data from the Valdemar Field. Comparison with variance and ant-tracking attributes and a manual fault interpretation shows that the neural network predicts faults with more details that may improve the overall geological and tectonic understanding of the study area and add information on potential compartmentalisation that was previously overlooked. However, the neural network is sensitive to seismic noise, which can distort the fault predictions. Therefore, the proposed model should be treated as an additional fault interpretation tool. Nonetheless, the method represents a state-of-the-art fault mapping tool that can be useful for hydrocarbon exploration and CO2 storage site evaluations.

The Python script that calculates the bearing between two points (start and end point of the long axis) and visualizes the orientations using the Python library mplstereonet (https://github.com/joferkington/mplstereonet)

A quantitative seismic interpretation of the Gassum Formation at the onshore aquifer gas storage near the Danish town Stenlille is presented with its implications for exploiting the gas storage facility as a potential CO2 demonstration site. An available dataset including a 3D seismic volume and 20 wells was used to interrogate the following questions: (1) Is the seismic response of the natural gas stored in the Stenlille aquifer similar to a modelled CO2 fluid at equivalent burial depths?; (2) Can we map the injected natural gas distributions stored within the various reservoir zones and compartments in the Stenlille aquifer?; and (3) Can we better resolve the spatial reservoir heterogeneities and zone boundaries in the Gassum Formation? Our approach involves employing a set of quantitative interpretation tools based on the available 3D seismic post-stack volume and well logs. The quantitative interpretation results yield additional reservoir insights useful for building a robust static model for subsequent CO2 storage and de-risking. The present study is a part of recent projects (ConsenCUS and CCS2022−2024) carried out at GEUS.

Summary The Tuxen formation in the Danish central graben is an intercalation of chalk and marl that include some important yet challenging reservoirs. In this study we used forward stratigraphic modelling to model the deposition of this formation and understand its vertical and lateral heterogeneities. Additionally, we used an innovative response surface modelling approach to perform a thorough sensitivity and risk analysis to quantify the effect of uncertain input parameters on output properties such as reservoir facies net thickness and net to gross for each studies sequence and on the full study area.

A novel direct probabilistic inversion using seismic pre-stack data as input to characterize a wedged chalk reservoir prospect was demonstrated from the Upper Cretaceous unit, Danish North Sea. The objective was to better resolve the lateral extent and pinch-out of the chalk prospect in a frontier exploration setting and compare the results with a more traditional deterministic inversion and geostatistical reservoir modeling. The direct probabilistic inversion results provided additional reservoir insights that were challenging to obtain from the more traditional workflows and are also more flexible for associated uncertainty assessments. Hence, this study demonstrates the usefulness of such direct probabilistic inversions even with suboptimal data availability.

Fault mapping provides important information for defining compartments in reservoirs and for investigating caprock integrity. However, due to complex fault geometries, manual interpretations based on seismic data and seismic attributes can be timeconsuming and ambiguous. In this study, a convolutional neural network (CNN) trained on synthetic data is applied to 3D post-stack seismic data from a Danish onshore aquifer gas storage facility in the town of Stenlille, which is currently being considered as a demonstration site for geological storage of CO2. Comparison with a manual fault interpretation based on traditional seismic attributes shows that the neural network predicts faults with more details and faults that were overlooked in the manual interpretation. The neural network predictions are, however, in some cases patchy and lack coherence, which may lead to erroneous fault predictions. Therefore, the CNN model should be treated as an additional fault interpretation tool for the interpreter to quality check in a critical manner. Nonetheless, the method represents a novel fault mapping tool that can be useful for de-risking future geothermal and carbon capture storage and utilization prospects.

This study proposes a unique workflow to unravel complex burial diagenetic histories of overpressured basins based on the integration of seismic and well log data, biostratigraphy, petrography, clumped isotope analyses and basin modelling. This approach is demonstrated with an example from the Chalk Group in the Danish Central Graben, where a seismic‐scale palaeo‐lithification front has been observed and studied in detail to elucidate the timing of the establishment of overpressured conditions and its relation to changing diagenetic activity. The palaeo‐lithification front separates high‐porosity chalks above, that dominantly underwent mechanical compaction and contact cementation, from low‐porosity chalk below, that dominantly underwent severe pressure dissolution and pore‐filling cementation. Analysis of chalk buried under hydrostatic conditions shows a strikingly similar lithification front between 1000 and 1200 m burial, much shallower than the lithification front in the Danish Central Graben at a current depth between 2100 and 2400 m below seafloor. The discrepancy of 1200 m is due to the establishment of overpressured conditions that limited the increase in effective stress as burial continued, finally halting burial compaction when formation fluids started to carry the lithostatic weight. Basin modelling data indicate that this occurred at the end of the Oligocene for large parts of the Danish Central Graben, which is much earlier than the Middle Miocene timing that is currently assumed. The results imply a regional occurrence of a relict lithification front in the North Sea Basin, its position guided by stratigraphy, but mainly dependent on the maximum effective stress experienced during its burial history. The study shows that the porosity bipartition is a remnant of the past and not from ongoing compaction as has previously been suggested. Since it was established before the thermal maturity of the main source rocks, chalk below the lithification front must have formed a sealing unit during hydrocarbon migration. The recognition of the lithification front is also of importance to velocity modelling and depth‐conversion since a non‐linear increase between velocity and depth is expected across this boundary. The methodology may be applied in other overpressured basins where the diagenetic state of reservoir rocks at the end of hydrostatic conditions must be constrained.