Quantitative analysis of natural fracture networks in Maastrichtian chalk: Insights from a 7,000 m2 digital outcrop model of the Rørdal Quarry, Denmark

This study presents a comprehensive analysis of over 27,400 individual fractures from a digital outcrop model (DOM) of Maastrichtian-age chalk in the Rørdal Quarry, northern Jutland, Denmark. The extensive dataset, covering approximately 7,000 square meters, provides a detailed mapping of fracture networks representing a more extensive and continuous area than traditional scanlines and smaller aerial sampling methods. Analysis reveals significant spatial variations in fracture orientations and lengths, influenced by local fault systems and regional tectonics within the Sorgenfrei-Tornquist Zone (STZ). The resulting fracture network supports anisotropic fluid flow, with NE-oriented extensional fractures serving as primary pathways and NW-NNW and NNE-oriented shear fractures enhancing connectivity. Comparisons with other Danish chalk quarries, such as Sigerslev, Hillerslev, and Nye Kløv, highlight both similarities in regional trends and local variations due to factors like salt tectonics. The fractures in Rørdal exhibit a log-normal spacing distribution across the quarry, indicating a regularly spaced pattern. These findings underscore the potential of using outcrop analogues to inform subsurface models, particularly for predicting anisotropic fluid flow. However, careful consideration is required when applying outcrop data, ensuring that variations in fracture connectivity, stress regimes, and scale-dependent characteristics are accurately integrated to optimize carbon storage strategies and other subsurface applications.