Structural analysis of the Smeaheia fault block, a potential CO2 storage site, northern Horda Platform.

 

Mulrooney, M.J.1, Osmond, J.L.1, Skurtveit, E.1,2, Faleide, J.I.1 & Braathen, A.1

 

1 Department of Geosciences, University of Oslo (UiO), PO Box 1047, Blindern, 0316, Oslo, Norway

2 Norwegian Geotechnical Institute (NGI), PO Box 3930, Ullevål Stadion, 0806, Oslo, Norway

 

Smeaheia, a prominent fault block located on the Horda Platform, northern North Sea is identified as a potential subsurface CO2 storage site. We utilise the GN1101 3D and regional 2D seismic surveys to generate a high-resolution subsurface geomodel to inform the structural style and evolution of the fault block, to investigate geological controls on proposed CO2 storage and provide a geometric framework as a basis for future analyses. Two basement-involved (first-order) north-south trending fault systems, the Vette Fault Zone (VFZ) and the Øygarden Fault Complex (ØFC), bound the 15 km-wide fault block. Apart from activity during the Permo-Triassic (Rift Phase 1) and the Late Jurassic–Early Cretaceous (Rift Phase 2), we present evidence that rifting in this part of the North Sea continued into the Late Cretaceous with minor reactivation in the Palaeocene–Eocene. Two segments of the VFZ interacted and linked at a relay ramp during Rift Phase 2. Second-order (thin-skinned) faults show basement affinity and developed during Rift Phase 2 in two distinct pulses. A population of polygonal faults intersects the overburden and developed during the Eocene to middle Miocene. We have revised the areal extent of two structural closures that define the Smeaheia fault block, Alpha (VFZ footwall) and Beta (ØFC hanging wall) which consist of Upper Jurassic Viking Group target formations. Cross-fault juxtaposition analysis of the VFZ and second-order intra-block faults are presented and inform pressure communication pathways  between the Smeaheia and Tusse fault block, as well as reservoir integrity and compartmentalisation. The geomodel further identifies important geological controls on CO2 storage in the fault block including a thinning caprock above the Alpha structure, and identification of hard-linkage between deep tectonic faults and shallow polygonal faults. Fault reactivation analysis was conducted on depth-converted faults to determine the risk of up-fault CO2 migration. Hydrostatic and depleted scenarios were modelled. Faults are modelled as classic cohesionless structures but also utilising parameters (cohesion and friction angle) derived from host rock mechanical analysis.