Upscaling Carbon Capture and Storage requires identification of suitable storage sites, with robust reservoir seals. The Utsira Formation in the northern North Sea has been flagged as a target for further storage. However, there are no regional studies of seal variability addressing heterogeneities that could facilitate seal bypass. This study aims to: (a) identify, assess and map the elements that promote or restrict fluid migration, (b) develop a matrix to regionally map containment confidence (CC) and (c) rank the different areas for CO2 containment across the Utsira Formation. The seal and overburden were mapped using a high-resolution, pre-stack depth-migrated 3D broadband seismic reflection dataset and 141 exploration wells. Seal geometry, sandstone presence and sandstone connectivity in the seal and overburden were assigned relative CC scores, which were summed to map overall CC of the Utsira Fm. Indicators for shallow gas and migration were mapped and correlated with the other elements. Areas with the lowest CC are in the west of the Utsira Fm. Here, sandstones within the Seal Interval are connected through the overburden via sandy submarine fans. In the southeast, dipping stratigraphy downlaps onto the Utsira Fm., increasing the potential for connection with glacially-derived channel-lobe systems in the overburden. The areas with the highest CC are the central and northeast parts of the Utsira Fm., where the Seal Interval is mudstone-dominated and parallel to the reservoir, and channel-lobe systems identified in the Overburden Interval are disconnected from the reservoir. This area coincides with a thick depocentre of the northern Utsira Fm. These results can be used to inform CO2 storage site selection and constrain future CO2 plume simulation analyses for the Utsira Fm. The CC matrix outlined here can also be adapted and applied to regionally assess the containment of other potential CO2 storage reservoirs in any setting.

Interactions between footwall‐, hangingwall‐ and axial‐derived depositional systems make syn‐rift stratigraphic architecture difficult to predict, and preservation of net‐erosional source landscapes is limited. Distinguishing between deposits derived from fault‐scarp degradation (consequent systems) and those derived from long‐lived catchments beyond the fault block crest (antecedent systems) is also challenging, but important for hydrocarbon reservoir prospecting. We undertake geometric and volumetric analysis of a fault‐scarp degradation complex and adjacent hangingwall‐fill associated with the Thebe‐2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, using high resolution 3D seismic data. Vertical and headward erosion of the complex and fault throw are measured. Seismic‐stratigraphic and seismic facies mapping allow us to constrain the spatial and architectural variability of depositional systems in the hangingwall. Footwall‐derived systems interacted with hangingwall‐ and axial‐derived systems, through diversion around topography, interfingering or successive onlap. We calculate the volume of footwall‐sourced hangingwall fans (V HW ) for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up‐dip fault‐scarp (V FW ). This analysis highlights areas of sediment bypass (V FW > V HW ) and areas fed by sediment sources beyond the degraded fault scarp (V HW > V FW ). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault block crest that fed the hangingwall basin. One source persisted throughout the main syn‐rift episode, and its location coincided with: (a) an intra‐basin topographic high; (b) a local fault throw minimum; (c) increased vertical and headward erosion within the fault‐scarp degradation complex; and (d) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through‐going sediment input points could be applied to other rift basin‐fills. We highlight implications for hydrocarbon exploration and emphasize the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono‐stratigraphic models for rift basins.

Subsurface CO2 storage is considered a key element of reducing anthropogenic emissions in virtually all scenarios compatible with limiting global warming to 1.5°C. The Utsira-Skade Aquifer (Utsira, Eir and Skade Formations), northern North Sea, has been identified as a suitable reservoir. Although the overall storage capacity of the full aquifer has been estimated based on regional data, it is lacking an integrated assessment of containment and internal heterogeneity, to identify optimal areas for injection and for calculation of site-specific storage capacities. A high-resolution, broadband 3D seismic reflection dataset, full waveform inverted velocity data and 102 exploration wells are utilised to provide a catalogue of CO2 storage prospects in the northern Utsira-Skade Aquifer. This is achieved through: 1) definition of the aquifer’s spatial limits; 2) calculation of porosity distribution; 3) assessment of the extent, geomorphology, thickness variability, and containment confidence (CC) of mudstones; and 4) mapping of closures through fill-to-spill simulations. CO2 storage capacity was calculated for the prospects using two approaches; using the full reservoir thickness (FRT) beneath the closures and using only the thickness from the closure top to the spill point (TSP), i.e., within structural traps. Porosity ranges from 29 to 39% across the aquifer and is higher in the Utsira and Eir Fms. relative to the underlying Skade Fm. The mudstone separating the Skade and Eir/Utsira Fm. has a thickness > 50 m, and is a potential barrier for CO2. Other intra-aquifer mudstones were mainly interpreted to act as baffles to flow. Structural traps at the top Utsira and Skade Fms. yield fifteen prospects, with criteria of > 700 m depth and FRT storage capacity of > 5 Mt CO2. They have a combined storage capacity of 330 Mt CO2 (FRT) or 196 Mt CO2 (TSP). Five prospects have a positive CC score (total capacity: 54 Mt CO2 FRT or 39 Mt CO2 TSP). Additional storage capacity could be achieved through more detailed analysis of the seal to upgrade the CC scores, or through use of a network of the mapped closures with a fill-to-spill approach, utilising more of the aquifer.

Estimations of CO2 storage capacities for saline aquifers, particularly the Utsira Formation (northern North Sea) have previously been calculated using a variety of numerical approaches. These are mainly based off reservoir depth maps and averaged petrophysical properties. In these first-pass estimations, a thick shale succession in the overburden is assumed to form the top seal. This is unlikely to be representative of the true, regional lithological heterogeneity and 3D variability of stratigraphic architecture, which may promote CO2 migration out of the reservoir during injection. This study utilises a recently acquired regional high-resolution 3D broadband seismic dataset (37,500 km2) and >200 wells in the North Viking Graben, with the aim to fully characterise the overburden of the potential CO2 reservoir (Northern Utsira Formation). The objectives are to analyse: i) the presence and spatial extent of sandstone bodies in the overburden and their connectivity with the reservoir; ii) the presence of sand-filled slope channels on the clinoform foresets that may act as migration pathways; iii) evidence of previous fluid migration through the overburden. Manual seismic interpretation and well correlation is augmented by automated horizon propagation (Palaeoscan) to map individual clinoforms across the region. This is integrated with seismic attribute analysis, frequency decomposition and automated well lithology extraction to understand regional sand distribution and feature analysis (e.g. identification of channels and their fill, and possible shallow gas). Large fan-shaped sandstone bodies (10s km-scale) are identified in the lower foresets and bottomsets of the clinothems. In the west, these are in connection with the Utsira Fm., or separated from it by a thin (<10 m) shale layer. These sands can be both beneficial to the storage capacity by producing additional gross reservoir volume (if sealed and below the critical depth for CO2), or detrimental to it if they provide a path to bypass the Utsira Fm. top seal. In the south east, sand-filled slope channels and lobes (km-scale) are recorded in the prograding clinothems but are not observed to be in connection with the Utsira Fm. (located >100 m above top Utsira Fm.). No sand-filled channels were identified in the north east from seismic attribute analysis, however the well lithology extraction for this region contained ~3% sand, thus there is a possibility of sub-seismic resolution features. In the south, foresets directly downlap the Utsira Fm. This geometry juxtaposes several individual clinothems against the reservoir, increasing the likelihood of migration if there is sand presence. This contrasts with the scenario in the north, where the bottomset of a single clinothem disconnects the reservoir from younger clinothems and restricts potential migration. The outcome of this study is an integration of each of the regional feature maps to generate: i) a seal thickness map between the Utsira Fm. and the first overlying sand body; ii) the first leakage risk map of the Utsira Fm. that captures geological geometry and lithology distribution. These can be incorporated into any future storage estimations and identification of potential injection sites.