Hydraulic fracturing of unconventional shale reservoirs increases the fracture network surface area to access hydrocarbons from the low permeability rock matrix. Porosity and permeability of the matrix, through which hydrocarbons migrate to fractures, are important for determining production efficiency and can be altered by chemical interactions between shale and hydraulic fracturing fluids (HFFs). Here, we present results from an experimental study that characterizes the thickness of the alteration zone in the shale matrix after shale-HFF interactions. Experiments were conducted with whole cores submerged in HFF both with and without added barium and sulfate to promote barite scale formation. After 3 weeks of reaction at 77 bar and 80 degrees C, the cores were characterized using X-ray microtomography, synchrotron X-ray fluorescence microprobe imaging, and synchrotron X-ray absorption spectroscopy. Our results show that the thickness of the altered zone depends on shale mineralogical composition and varies for different chemical reactions. For reactions between the low-carbonate Marcellus shale and HFF, pyrite (FeS2) oxidation manifests as both a thick zone of sulfur oxidation (>0.5 cm) and a thinner zone of iron oxidation (100-150 mu m). Carbonate dissolution extended 100-200 mu m into the matrix, with the resulting observable secondary porosity localized at the shale-fluid interface where mineral grains were removed by either dissolution or mechanical erosion. In solutions oversaturated with respect to barite, barite precipitates were observed in the reaction fluid and at the shal HFF interface. In contrast, the carbonate dissolution zone in the high-carbonate Eagle Ford was only 30-40 mu m thick, within which a uniform texture of increased porosity was observed. Pyrite oxidation in the Eagle Ford was evident from an iron oxidation zone (150-200 mu m thick), while sulfur oxidation was minor and hard to observe. Barite precipitation extended 1-2 mm into the matrix when the initial HFF was oversaturated with respect to barite, filling shale microcracks down to the submicrometer length scale. Our findings provide a scientific basis to predict the extent of chemical alteration in shale reservoirs during hydraulic fracturing and its impacts on hydrocarbon production.