Rock properties of the Fjerritslev Formation seal, Danish North Sea, from cuttings elemental data

Rock properties, including geomechanical and petrophysical characteristics, are crucial for assessing CO2 storage safety and viability. In Denmark, the Lower Jurassic Fjerritslev Formation serves as the main seal for key target reservoirs, yet its properties are poorly understood, particularly in the North Sea and western onshore Denmark. Most exploration wells in this area were drilled between the 1950s and 1980s, with limited cores and inadequate log suites. We present an assessment of the Fjerritslev Formation’s rock properties in three wells based on a recent screening of cuttings using HH-XRF. Using advanced multilinear regression models, we provide a harmonized dataset that includes volumes of clay, quartz, and carbonate, grain densities, and rock mineral brittleness. This approach offers calibrated measures of Vclay from wireline logs, overcoming traditional pitfalls in defining values and harmonizing between different log types. The results indicate that the Fjerritslev Formation is a thick mudstone with a clay content of 30–50% and total porosities of 10–25%. The rock has a mineralogical brittleness index of 0.4–0.5. Porous beds occur within the shales, highlighting its regional complexity and relevance for CO2 storage.