This study presents a broad set of prior experimental data for shale membrane efficiency when interacting with water-based and oil-based drilling fluids. The assessment of the experimentally measured membrane efficiencies of shale reveals that shales are leaky semi-permeable membranes. Results suggest that shale permeability and cation exchange capacity play a determining role in its membrane efficiency, as higher cation exchange capacity and lower permeability are strongly consistent with higher membrane efficiency of shale. Furthermore, it has been found that the ratio of hydrated ionic size relative to shale pore throats’ size regulates the movement of ions into the shale, thus directly controlling its membrane efficiency. Ions with large hydrated diameters exhibit higher membrane efficiencies than those with small hydrated diameters. Results also show that the measured membrane efficiencies of oil-based muds were higher than those reported for shales. While shale gets its membrane quality from its tiny pores and inherent surface charge, oil-based mud gets its membrane quality from the stability of its emulsion. We believe that when using oil-based muds to drill shale formations, the emulsifier assumes responsibility for screening out ions, however, if the emulsion stability is breached, shale will become the regulator for ions flow.