Structural deformation and mineralogy of the Agardhfjellet and Rurikfjellet formations in central Spitsbergen, Svalbard

The Upper Jurassic-Lower Cretaceous Agardhfjellet and Rurikfjellet formations comprise a 450 m thick shale-dominated succession. This succession is present across central parts of Spitsbergen, Svalbard, and is the targeted caprock for the Longyearbyen CO2 Lab. The focus of this study has been on the Slottsmøya Member of the Agardhfjellet Formation and the Wimanfjellet Member of the Rurikfjellet Formation. Structural measurements collected from two drill cores and two outcrops, located 15 km apart, are the basis of this study. Additionally, X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses have been conducted on two samples from one of the drill cores. The drill cores exhibit high fracture frequencies, dominated by low-angle shear fractures seen as polished surfaces with slickensides. The outcrops show highly fractured beds, with mainly high-angle open and shear fractures. Mode I and Mode II fractures are interpreted in both drill core and outcrop data. Contractional, meso-scale structures are observed in the field area. The recorded deformation is interpreted to be the result of tectonic events that has affected Spitsbergen after the deposition of the Agardhfjellet and Rurikfjellet formations. These events include the Cretaceous magmatic event which resulted in uplift, Paleogene transpression and Cenozoic deglaciation and erosion. The mineralogy is consistent with a typical shale composition, where clay minerals and quartz constituents show the highest fractions. XRD analyses determine that clay miner- als comprise 35% and 51% of the Agardhfjellet and Rurikfjellet formations, respectively. The study shows that the Agardhfjellet and Rurikfjellet formations are highly fractured, with a de ́collement zone present along the boundary. This is interpreted as the primary reason for the highly deformed rock mass. However, the interval appears to have a good sealing property seen as a vertical pressure difference exists between the targeted reser- voir and caprock succession. The reason for the functional seal is not fully understood, but appears to be related to several factors such as: (i) thickness resulting in satisfying confining pressure, (ii) large amounts of clay minerals, (iii) preferential orientation of existing fractures in relation to current stress regime, (iv) and a décollement zone forming a barrier for vertical fluid flow