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

The Lower Triassic Bunter Sandstone Formation represents a key prospective 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 primary 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 CO₂ storage licensing round. 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.

The Bunter Sandstone Formation comprises continental red bed sequences deposited under fluctuating arid to semi-arid conditions in endorheic ephemeral fluvial/lacustrine-palustrine to aeolian environments. The formation is capped by mudstones and evaporite seals of the overlying Middle Triassic Dowsing Formation (Haisborough Group).

Here we present preliminary results from a multi-disciplinary study of core material from selected UK Southern North Sea wells. Sandstone composition is investigated using a range of techniques including detailed sedimentary logging, portable XRF, poro-perm analysis and scanning electron microscopy–energy dispersive X-ray spectroscopy analysis (SEM-EDS). The XRF and SEM-EDS data, in particular, have revealed a range of different cements, including dolomite, calcite, anhydrite, halite and gypsum; whose relationships can be used to understand the diagenetic evolution of the Bunter Sandstone Formation. These new sedimentological and petrographic data are placed within a cyclostratigraphic framework, against which variations in spatio-temporal sediment routing, composition and net-to-gross ratio can be assessed.

The results of this work will aid the detangling of depositional and diagenetic controls on reservoir quality and provide a sedimentological framework for regional biostratigraphic investigation. In this way we aim to better inform the assessment of Lower to Middle Triassic strata in the Southern North Sea for future CO₂ storage.