Cracks in our foundations: The nature and origin of fissures in the Kortrijk Formation

The stiff Ypresian clays of the Kortrijk Formation occur extensively throughout the subsurface of the Princess Elizabeth Zone (PEZ), North Sea. The formation is pervasively deformed with large-scale polygonal fault networks, so-called Clay Tectonic Features (CTF; e.g. Verschuren, 2019, Marine Geology). Moreover, recently acquired samples from the Kortrijk Formation in the PEZ suggest a heavily fissured internal structure of these clays at the centimeter scale. The presence of faults and fissures in this formation have strong implications for its geotechnical properties, such as strength and stiffness, which may pose challenges for the foundations of the planned offshore wind energy farms.

With this in mind, we study the physical, mineralogical and chemical properties of the Kortrijk Formation in high-resolution using a multi-methodological approach including X-ray CT scanning, organic and inorganic geochemical analyses (LOI, organic material, calcimetry, pH, stable carbon isotopes, pXRF, XRD and ICP-OES) and sedimentological investigations (grain size, thin sections). The first samples were collected from a 20m deep borehole with alternating rotary coring and hydraulic push sampling in Rumbeke (from a section that is considered stratigraphically equivalent to the PEZ) and multiple drilling campaigns at other locations are planned.

Initial X-Ray CT scans of these samples reveal a heterogenous internal architecture containing four main feature types: bioturbation, concretions, fissures, and faults. Bioturbation occurs throughout the cores, often appearing as millimeter-thick, centimeters-long, high-density features, likely reflecting the presence of precipitated minerals such as pyrite, following microbially-mediated sulfate reduction. In contrast, concretions (siderite-fluorapatite) are rare in the core sections, consistent with their observed scattered presence in land-based observations. Fissures are recognized as low CT-density features which do not occur throughout all the core sections but are concentrated in localized zones, leaving intervening volumes of clay intact. The observed cm-scale normal faulting structures point to a local extensional regime. The geometry, pattern, and textures of the observed fissures and fractures are tested against established criteria (e.g. radial and axisymmetry, bending near the core rim, etc.) to conclusively differentiate natural features from coring-induced artifacts (Adriaens et al., 2024, Geoenergy). To quantitatively analyze all features, the X-ray CT data are processed using a comprehensive workflow involving filtering, segmentation, and grouping of features based on multi-ROI analysis using 3D connectivity. Following isolation, we perform a detailed analysis of the morphological characteristics (e.g., volume, surface area) and the three-dimensional orientation of the segmented features. The high-resolution 3D model of the features in the clay derived from CT scanning will be used to inform numerical models which will test the stiffness and long-term mechanical stability of the Kortrijk formation clays under different geotechnical loading scenarios.

By combining detailed sedimentological, mineralogical and geochemical characterization with the high-resolution CT-based structural analysis, we aim to establish the origin of the fissures and faults in the Kortrijk formation, thereby providing the geological context for their impact on geotechnical stability.