Lower Buntsandstein Subgroup sealing potential in the southern Dutch offshore

This study evaluates the sealing potential of the Main Claystone Formation (RBSM) of the Lower Buntsandstein Subgroup in the southern Dutch offshore, a key consideration for CO₂ storage in depleted Permian reservoirs lacking the conventional top seal, the Zechstein evaporites.

Detailed stratigraphic, structural and compositional analyses were performed on over 160 NL and UK, both offshore and onshore, wells using Petrel and QGIS, integrating thickness and depth mapping, lithological interpretation and overpressure analysis. Regional sealing capacity evaluation followed, incorporating legacy structural data of depleted fields to infer the maximum column heights of CO2 that can be maintained as well as direct core sealing capacity measurements in the laboratory using Mercury Injection Tests (MIT).

The RBSM comprises deeply buried, illite- and anhydrite-rich claystones with minimal porosity and permeability, whose sealing capacity is primarily governed by lithology, anhydrite content, compaction and stratification. Structural controls, notably early Cretaceous uplift and erosion associated with the Winterton High’s presence, locally compromise seal integrity, while central and eastern areas dominated by massive-blocky or laminated claystone-shale facies support CO₂ columns of 360–465 m, exceeding 465 m toward the north and northeast. Sealing effectiveness diminishes to ≤145 m in regions with sandier facies or severe erosion (Winterton High). Overpressure indicators identified within the formation also reinforce the robust sealing behavior regionally.

The findings redefine the Main Claystone as a reliable regional top seal and support its inclusion in risk assessments for CO₂ storage, with implications for expanding storage capacity in the southern Dutch offshore.