SEAL INTEGRITY OF THE LOWER NORTH SEA GROUP IN THE BROAD FOURTEENS BASIN (DUTCH OFFSHORE): Seismic Characterisation, polygonal fault systems, and quantitative analysis

Abstract

This study evaluated the sealing capacity of the Lower North Sea Group (LNS) in the Broad Fourteens Basin, North Sea, Netherlands, for long-term CO2 storage. The work integrated multi-attribute seismic interpretation (variance, coherence, and ant-tracking), well-log analysis, gamma-ray characterisation, and quantitative fault-seal evaluation to address three research questions: (1) Do key shale layers within the LNS exhibit a mappable seismic signature across the basin? (2) Can laterally discontinuous polygonal fault systems and fluid migration pathways be systematically characterised? (3) Do these clay-rich intervals constitute effective caprocks, or are significant risks present for CO2 containment? Seismic and well-log analyses reveal that the principal caprocks—the Ieper Clay Member and Rupel Clay Member—display high shale volumes (Vsh 59–100%) and robust regional thickness (85–240 m, mean 155 m), establishing regional seal continuity suitable for carbon capture and storage (CCS) applications. Multi-attribute interpretation delineated 12,458 polygonal fault segments organised into two stratigraphic tiers. Shale Gouge Ratio (SGR) analysis demonstrates that Tier I faults within the Ieper Clay retain high membrane sealing capacity (SGR > 100% m), whilst Tier II faults within the heterogeneous Landen Formation display moderate risk (SGR 22–88%). Critically, no upward fault propagation is observed, supporting a syn-depositional (compaction- driven) origin rather than tectonic control. High-amplitude anomalies consistent with past fluid migration are spatially restricted to < 5% of the survey area and concentrate predictably within 500 m of mapped Zechstein salt diapirs and major fault corridors. This spatial clustering enables risk mitigation through careful site selection. Basin-centre locations with thick Ieper Clay (> 150 m), minimal polygonal faulting, and distance > 1 km from salt structures offer strong geological conditions for secure, long-term CO2 containment. The quantitative site-selection framework and sealing metrics developed here directly support Dutch CCS deployment timelines (Porthos 2026, Aramis 2030) and contribute to national climate transition objectives. These findings confirm technical viability for multi-megatonne annual CO2 storage in the Broad Fourteens Basin and provide a reproducible screening template for regional CCS site assessment. However, site-specific core testing, geomechanical characterisation, and dynamic reservoir simulation remain essential before operational deployment.