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The Lower Cretaceous Captain Sandstone Member of the Inner Moray Firth has significant potential for the injection and storage of anthropogenic CO2 in saline aquifer parts of the formation. Pre-existing faults constitute a potential risk to storage security owing to the elevated pore pressures likely to result from large-scale fluid injection. Determination of the regional in situ stresses permits mapping of the stress tensor affecting these faults. Either normal or strike-slip faulting conditions are suggested to be prevalent, with the maximum horizontal stress orientated 33°–213°. Slip-tendency analysis indicates that some fault segments are close to being critically stressed under strike-slip stress conditions, with small pore-pressure perturbations of approximately 1.5 MPa potentially causing reactivation of those faults. Greater pore-pressure increases of approximately 5 MPa would be required to reactivate optimally orientated faults under normal faulting or transitional normal/strike-slip faulting conditions at average reservoir depths. The results provide a useful indication of the fault geometries most susceptible to reactivation under current stress conditions. To account for uncertainty in principal stress magnitudes, high differential stresses have been assumed, providing conservative fault-stability estimates. Detailed geological models and data pertaining to pore pressure, rock mechanics and stress will be required to more accurately investigate fault stability.\ud\udLarge-scale deployment of CO2 storage as a strategy for reducing greenhouse gas emissions will rely on the integrity of sealing strata overlying the storage reservoirs to ensure that the captured CO2 is permanently isolated from the atmosphere (IPCC 2005; Chadwick et al. 2009a; Holloway 2009). The existence of pre-existing fault systems of varying dimensions is a common feature throughout the subsurface, and the efficacy of seals may potentially be compromised by any enhanced transmissibility associated with fault zones. Within the Moray Firth, the Lower Cretaceous Captain Sandstone Member of the Wick Sandstone Formation has been proposed as a suitable storage reservoir candidate (SCCS 2011; Shell 2011a; Akhurst et al. 2015). Storage potential exists within depleting hydrocarbon fields (Marshall et al. 2016), while significant additional capacity is available in the surrounding saline aquifer volume. Regional top seals include the Cretaceous Rodby, Carrack and Valhall formations. Simulation studies of CO2 injection identified the storage capacity of the Captain Sandstone to be between 358 and 2495 Mt (Jin et al. 2012). As the injection of CO2 is reliant on the displacement of existing pore fluids, large-scale injection results in increased pore-fluid pressure, the effects of which will be felt across large areas in well-connected aquifer systems (Chadwick et al. 2009b; Jin et al. 2012; Noy et al. 2012).\ud\udIt is well documented that some faults are transmissible to fluid flow, while others act as effective capillary seals (Caine et al. 1996; Aydin 2000; Faulkner et al. 2010). Whether cross-fault flow occurs depends on the juxtaposition of lithologies in the footwall and hanging-wall blocks, as well as the composition of the fault zone and any differential pressure across the fault. In addition, reactivation of previously stable faults caused by increasing pressure, and therefore a reduction in the effective stress, could allow faults to become transmissive to buoyant fluids, such as supercritical CO2, due to the opening of flow pathways during failure (Streit & Hillis 2004). It is this aspect of fault stability that forms the focus of this study, with respect to the Captain Sandstone of the Inner Moray Firth, and utilizing an adaptation of the geological model presented by Jin et al. (2012).\ud\udAnalysis of the geomechanical stability of faults offsetting the Captain Sandstone requires the contemporary stress field affecting the basin to be characterized, in order to resolve the shear and normal stresses acting on mapped faults and to determine which faults, or segments of faults, are most susceptible to becoming reactivated if pore-fluid pressures in the basin are increased as a result of CO2 injection. In order to do so, detailed knowledge of the pore-pressure conditions at depth, the magnitude and orientations of the principal stresses, and the properties of the faults is required

Carbon dioxide storage in the Captain Sandstone aquifer: determination of in situ stresses and fault-stability analysis