Tectonic stress and entrapped fluids in the Röt Halite Formation, Southern North Sea

The Schooner gas field was discovered in 1987, first gas was produced in October 1999 and decommissioning began in December 2021. As part of the plug and abandonment operations, high-resolution ultrasonic logs were acquired across the caprock of the Bunter Sandstone to assure containment for planned CO 2 geological storage projects. The main caprock unit is the Röt Halite formation, a relatively thin (∼100 m) evaporite layer of Middle Triassic age. Halite is an excellent caprock lithology because viscoplastic creep allows it to clamp casing and cement and provide robust, long-term well integrity. To understand its operating limits, we need to know its state of stress. A second goal of this study is to better explain the nature and dynamics of fracture-like linear features that were encountered near the base of the Röt Halite in two of the wells; these were associated to casing ovality high enough to affect access. Wells in salt are at a higher risk of casing collapse; therefore, this analysis can help shed light about a possible failure mechanism that could also affect well operation and containment in future CO 2 storage projects. Six of the logs cover the Röt Halite and bordering formations and two logs extend to the previous casing set at around 500 m. An ultrasonic casing caliper is part of the data set, it allows us to calculate casing ovality, which increases as the pipe is deformed by formation creep. We developed a method to extract from it the direction of the three principal stresses and suggest constraints on their ratios. Five of the six wells agree that the maximum horizontal stress S Hmax is oriented in an NNE direction. Stress in the 6 th well is rotated 90°, but this can be explained by the presence of carnallite, an evaporite with much lower shear stress than halite that is more sensitive to local topography. The minimum horizontal stress seems to be equal to the overburden stress S v , whereas S Hmax =1.3-1.4 S hmin . The stress direction doesn't match values reported in the literature for the pre-Zechstein clastic rocks but is orthogonal to the fold axes in the post-Zechstein cover. This is explained by buckling following the tilting and shortening process that started during the Miocene and seems to be ongoing based on the results or our study. The principal stress direction is constant through the mudstone layers above the Triassic Bunter Sandstone and the lower part of the Upper Cretaceous Chalk Group: this reveals the absence of mechanical detachment, especially in the Röt Halite, and that these formations are ductile. In the second part of this study, we considered three different scenarios to explain the linear features: hydraulic fractures, stylolites and fluid inclusions. Only drilling mud inclusion elongated by localised salt creep in an anisotropic stress field can explain the observations. The reasons why the linear features are only found in two of the six wells studied (a third well may also be affected by much shorter inclusions) are unclear. Possible causes are local mineralogical differences in the Röt Halite and partial cement coverage in deviated wells.