The water (i.e., base fluid) used in hydraulic fracturing of unconventional reservoirs is a critical component. While base fluids are commonly optimized for stimulation activities, their significance with respect to fluid-rock interactions in the shale reservoir over stimulation and production timescales is not well understood. Variability in chemical compositions of base fluids such as freshwater and treated produced water (e.g., "clean brine") illustrate further need to understand subsurface behavior of base fluids with respect to different shale compositions. Here, we present experimental results for fluid-rock interactions with the goal of improved understanding of strontium (Sr) behavior in unconventional shale reservoirs. In this study, carbonate- and clay-rich cores (ground) from Spraberry and Wolfcamp formations were reacted with freshwater and clean brine base hydraulic-fracturing fluids. Timed addition of chemical additives was used to simulate the hydraulic-fracturing processes that occur in the field. Experiments were conducted at 80° C and two different pressures (1 and 85 bar) for an overall experimental time of three weeks. Samples were analyzed pre- and post-reaction using various laboratory- and synchrotron-based techniques to elucidate Sr behavior with different base fluids. Bulk sample analyses showed Sr leaching from carbonate- and clay-rich samples with freshwater base hydraulic fracturing fluids. In contrast, reactions with clean brine base hydraulic fracturing fluids showed Sr uptake in carbonate-rich shale (14% increase) and significant Sr uptake in clay-rich shale (450% increase). SEM imaging of samples reacted with clean brine base hydraulic fracturing fluids identified precipitation of SrSO4 on the surface, and significantly more precipitation was observed on clay-rich compared to carbonate-rich samples. Strontium speciation for post-reaction clay-rich samples was determined with extended X-ray absorption fine structure spectroscopy and consisted of SrSO4 (50%), SrCl2 (40%), and Sr adsorbed on clay (10%). The results of this study show that the chemical composition of base fluids used in hydraulic fracturing of shale reservoirs has significant effect on Sr cycling in the subsurface. Important fluid-rock interactions are occurring on stimulation and likely production timescales in unconventional shale plays. The type and extent of such interactions appears to be largely influenced by hydraulic fracturing fluid components (e.g., base fluid) and shale mineralogy which has implications for interpretations of flowback and produced fluids.