The Lower to Middle Triassic Bunter Sandstone CO₂ storage complex of the Southern North Sea: multi-disciplinary reservoir investigation

The Lower Triassic Bunter Sandstone represents a key CO₂ storage reservoir in the Southern North Sea, and will play an integral role in the UK’s Net Zero Strategy. The unit forms the main interval of interest for the Endurance structure (UK Blocks 42/25 and 43/21), one of two CO₂ storage sites selected by the UK Government for initial development, and is central to the North Sea Transition Authority’s first carbon storage licensing round.

The Bunter Sandstone Formation and its equivalents comprise laterally variable continental red bed sequences deposited under arid to semi-arid conditions in a low-energy, ephemeral fluvial to aeolian-sabkha environments. The formation is sealed by mudstones and evaporites of the overlying Middle Triassic Dowsing Formation (Haisborough Group).

Despite recent interest in the interval’s CO₂ storage potential and numerous wellbore penetrations, there is notably little published information on its chronostratigraphy, composition and variability. A thorough understanding of these factors, as well as how the reservoir and seal will behave when subjected to the injection of CO₂ remains essential.

Here we present preliminary results from a multi-disciplinary studies of core and cuttings material from selected UK onshore and Southern North Sea wells (UK and Dutch sectors). New sedimentological, petrographic and provenance data are placed within the context of a palynologically constrained cyclostratigraphy, which provides a framework for assessing the variability in spatio-temporal sediment routing, lateral compositional and net-to-gross ratio, reservoir quality and seal capacity. The results of this study will better inform the assessment of Lower to Middle Triassic strata in the Southern North Sea for future CO₂ storage.