Seal Integrity of the Agardhfjellet Formation, Svalbard: Influence of Mineralogical and Depositional Environment on Geomechanical Properties

Summary The Agardhfjellet Formation (Fm) in Svalbard, a middle Jurassic to lowermost Cretaceous mudstone-dominated unit, is investigated for its geomechanical properties critical for CO2 storage seal integrity. This study focuses on how mineralogy and depositional environment influence the formation’s geomechanical behavior, using borehole data from fully cored drill cores in central Spitsbergen. The Agardhfjellet Fm, partially equivalent to the Fuglen and Hekkingen formations in the Barents Sea, is analyzed for its lithology, grain size, and redox conditions through XRF-derived trace element ratios (Zr/Rb, U/Th, V/(V+Ni)). These indicate an oxic to anoxic depositional environment, with the coarser-grained Oppdalssåta Member contrasting the finer-grained Slottsmøya and Lardyfjellet Members. Geomechanical properties, assessed via log-derived Young’s modulus (E) and Poisson’s ratio (ν), reveal that the Oppdalssåta Member is more brittle, while Slottsmøya and Lardyfjellet Members are more ductile, indicating a correlating between Zr/Rb ratio and elastic response. Further, higher gamma ray values suggest more ductile behavior, however no clear trends between organic content (resistivity) and elastic properties are observed. Initial interpretation of these findings suggest that grain size and mineralogy significantly influence failure mode and fracture potential, with Zr/Rb ratios potentially serving as a useful proxy for assessing seal integrity in a CO2 storage context.