The Bunter Sandstone CO₂ storage complex of the Southern North Sea: reservoir and seal characterisation

The Lower Triassic Bunter Sandstone Formation and the overlying Haisborough Group represent a key prospective CO₂ storage interval in the Southern North Sea, and will play an integral role in the UK’s Net Zero Strategy. The interval is central to the North Sea Transition Authority’s CO₂ storage licensing strategy. Despite increased interest in the formation’s CO₂ storage potential and numerous wellbore penetrations, there remains notably little published information on reservoir and seal composition and heterogeneity at a regional scale and reservoir provenance.

The Bunter Sandstone Formation and equivalent Sherwood Sandstone in the study region comprise laterally variable 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 evaporites of the overlying Haisborough Group and the equivalent onshore Mercia Mudstone Group.

Here we present preliminary results from a multi-disciplinary study of core and cuttings material from selected UK Southern North Sea wells as well as a previously unstudied UK onshore borehole. All core has been logged at 1:100 scale and facies interpretations made. Sampling of core and cuttings has enabled a suite of analytical techniques to be employed including scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), handheld XRF and poro-perm measurements on reservoir and sealing elements. These data are supplemented by provenance techniques including conventional and single grain heavy mineral analysis including U-Pb zircon and apatite geochronology and apatite geochemistry.

The results will be used to characterise the reservoir and seal units as well as to identify likely sediment source regions, together these data will contribute to a better understanding of sediment transport pathways, depositional and diagenetic controls on reservoir quality. Detailed investigation of the seal complex will allow for a better understanding of seal integrity and capacity. The results of this work contribute to a better understanding of the sediment source regions, 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.