Structural characterization and across-fault seal assessment of the Aurora CO2 storage site

The proposed Aurora CO2 storage site is located in the Horda Platform, northern North Sea. Within the storage site, two storage aquifers and an overlying seal, which form part of the Lower Jurassic Dunlin Group, represent a CO2 storage complex. The storage site is bordered by the thick-skinned (first-order) Tusse and Svartalv fault zones. In addition, numerous thin-skinned (second-order) faults displace the storage complex within the projected up-dip CO2 migration path. Faults play an essential role within a CO2 storage site as they can serve as barriers or conduits to fluid flow, thus affecting the migration of injected CO2. To determine their possible role and improve the geological understanding of the storage site, assessment of fault geometries (strike, dip, and throw) is performed applying a detailed structural geomodel created using the GN10M1 3D seismic survey, regional 2D seismic lines, and well data. Thereafter, the geomodel is used to assess across-fault juxtaposition and membrane seals, using the shale gouge ratio (SGR) method, assess the spatial and temporal evolution of faults, and discuss plausible CO2 migration paths and gross rock volumes (GRV) of small-scale structural closures. Results herein show that first-order faults are predominately N–S striking, W-dipping, and formed during Permian to Triassic rifting (Rift Phase 1). In contrast, second-order faults show predominately N–S to NW–SE strikes, varying dip directions, and formed during the Early to Middle Jurassic or the Middle Jurassic to Early Cretaceous (Rift Phase 2). The storage complex is laterally extensive and dips (2°) to the south, and, therefore, injected CO2 is expected to migrate northwards and encounter the Svartalv Fault Zone from the footwall side. The fault zone offsets the seal unit (throw >80 m) thus juxtaposing the storage units against shallower successions and potentially allow across-fault migration. However, SGR values exceeding 30% are present across the fault zone potentially preventing this. Second-order faults exhibit throws less than the thickness of the primary seal (<80 m). Results from across-fault seal assessment of the second-order faults show that E and NE dipping faults are more likely to baffle fluid flow, compared to W and SW dipping faults. Three triangular two-way closures, are present north of the injection well and possess a combined GRV of 68 × 106 m3 in the primary and 93.6 × 106 m3 in the secondary storage unit, potentially increasing the storage capacity of the Aurora storage site. Results herein show that faults within the Aurora storage site may have an overall positive impact on CO2 storage by creating small baffles and likely preventing migration out of the storage units. However, higher uncertainties are related to the sealing potential of the Svartalv Fault Zone, which offsets the primary seal, due to challenges in assessing membrane seals using present-day methods.