Oil and gas-bearing shale formations have received a great deal of interest in recent years, especially in North America. Oil and gas from shale could make a significant contribution to global hydrocarbon production. However, their development has been hindered by the complexity of drilling and completion strategies, which must be adapted in response to shale's mineralogy and physical properties. Matrix acidizing is commonly used as a pre-flush to the hydraulic fracturing stimulation of shale formations. The process dissolves sediments and mud solids that inhibit the permeability of the rock, enlarging the natural pores of the reservoir and stimulating flow of hydrocarbons. In some plays it is used as the main stimulation technique (e.g. Monterey shale, California). In this paper, the mineralogical and physical responses to matrix acidizing of several important North American shale formations were evaluated, and the effects on their physical properties are described. Samples of Eagle Ford, Mancos, Barnett and Marcellus shale formations were exposed to different concentrations of HCl (1-3 wt%). Based on experimental observations, mass loss, and X-ray diffraction studies, the samples lost as much as 16 wt. % calcite when treated with 3 wt% of HCl for three hours. Post-reaction measurements show increases in porosity in all experiments, which was attributed to calcite dissolution in Eagle Ford samples and to crack development in Mancos, Barnett, and Marcellus samples. Eagle Ford samples showed the greatest reduction in Young's Modulus ranging from 25-82% when exposed to 1-3 wt% HCl solutions and 75% when exposed to 5 wt% NaCl saline solution at 200°F.