Assessing the sealing capacity of the Triassic Haisborough Group, Southern North Sea: insights from a continuous core succession in North Yorkshire

Successful geological carbon storage (GCS) depends on the co-occurrence of a suitable reservoir and an effective seal. In the Southern North Sea region, the Lower Triassic Bunter Sandstone Formation and the overlying Middle to Upper Triassic Haisborough Group form such a couplet, and are a major GCS target in the UK and beyond. Nevertheless, due to a dearth of core material, the mudstones and evaporites of the Haisborough Group are grossly understudied; their lithostratigraphy is largely based on wireline logs, and their actual sealing capacity is poorly known. The SM14 core from the Anglo American Woodsmith Mine site in the Cleveland Basin north of Scarborough, North Yorkshire, UK, offers a unique opportunity to address these issues. The core preserves a complete, near 300-m-thick succession of the Haisborough Group, which serves as an immediate analogue for potential GCS storage sites offshore.

CASP has documented the entire succession via sedimentary logging at 1:100 scale and high-resolution facies analysis, and has taken a total of 455 hand-held X-ray fluorescence (XRF) measurements directly from the core. For further analysis, 64 more or less evenly spaced large core samples, which are representative of the facies heterogeneity of the Haisborough Group, were collected. Quantitative X-ray diffraction (QXRD) analysis, optical petrography and scanning electron microscope energy dispersive spectroscopy (SEM-EDS) are combined with the facies scheme and XRF data to accurately characterise the rocks. These techniques provide insights at different scales and on different components, and thus are complementary rather than interchangeable. Porosity-permeability and mercury injection capillary pressure (MICP) analyses are coupled with geomechanical analytical results from fresh core to assess the suitability of the Haisborough Group strata as a seal. To facilitate robust correlation to the offshore subsurface, all results will be integrated with palynostratigraphy from more than 130 samples and with a comprehensive suite of wireline log data. In a next step, direct comparison of the SM14 dataset to select cores and samples from offshore wells in the Southern North Sea will be made. In parallel, sister projects of CASP investigate the reservoir properties of the underlying Bunter Sandstone Formation, but also extend the seal studies into the overlying Lower Jurassic succession of the Cleveland Basin.

Logging and facies analysis identified a conformal boundary between the Bunter Sandstone Formation and the overlying Haisborough Group. A preliminary lithostratigraphy for the latter has been developed, focusing on sediment textures and the abundances of halite, anhydrite and dolomite cement for subdivision. Throughout the succession, optical petrographic, SEM-EDS and QXRD analyses identify anhydrite, clay minerals, dolomite, quartz and feldspar as common mineral components at varying proportions. Halite is largely confined to the Röt Halite Member of the Dowsing Formation. Elsewhere, compositional variation is mostly driven by the abundance of anhydrite, which occurs either finely dispersed, or concentrated in streaks, veins or nodules, often also healing fractures. QXRD analysis further records widely varying clay contents (10 to 65%), mainly illite + smectite, but also significant proportions of muscovite, chlorite and corrensite. Porosity-permeability and MICP analyses, together with a variety of rock strength tests, yield encouraging results with regard to validating the suitability of the Mercia Mudstone Group as a seal.