Diagenetic evolution of the Bunter Sandstone Formation and its controls on reservoir quality: implications for CO ₂ injectivity and storage

The Bunter Sandstone Formation (BSF) is a target reservoir for the storage of CO 2 in the UK Southern North Sea (UKSNS). Previous industry studies highlighted diagenetic features that influence fluid flow in the BSF but failed to identify the controls and patterns of regional diagenesis that are now needed to inform more accurate prediction of porosity distribution and connectivity for CO 2 storage. This study presents a regional diagenetic model from the petrographical analysis of 78 samples from 12 wells in the northern UKSNS. Diagenetic cements (carbonates, sulfates and halite) are common. Most are early and episodic, patchy at local and regional scales, with periods of replacement and dissolution. Consequential fine-scale heterogeneous compaction textures are unrelated to current or maximum burial depths. Calcrete and dolocrete layers, associated with the formation of displacive eodiagenetic carbonate nodules, form discontinuous millimetre- to metre-thick vertical flow barriers. Halite and anhydrite are developed preferentially in coarser-grained sandstones, resulting in the ‘reservoir quality inversion’ noted in previous studies. There is abundant evidence for local, late mobilization and dissolution of halite and anhydrite, observed to preferentially affect samples from depths above c .1400 m, restoring some zones to good porosity. Additional high-density sampling and petrography is recommended, however, to provide the predictability required for CO 2 storage.