Because ethane acts apart from methane as a significant component in shale gas, knowledge of the competitive adsorption behavior of methane and ethane in shales, primarily under supercritical conditions, is crucial to understanding the fundamental mechanisms governing fluid storage, transport, and hydrocarbon production. Using grand canonical Monte Carlo simulations, we studied the adsorption behavior of single and binary mixtures of methane and ethane in montmorillonite slits having apertures ranging from 1.1 to 3.0 nm, for pressures up to 40 MPa over a temperature range of 333-393 K. The dependences of adsorption isotherms and selectivity on pore size, pressure, temperature, fluid composition, and water content were examined. With increasing pressure, the selectivity of ethane relative to methane decreases and tends to approach a constant smaller than unity, indicating that the adsorption propensity of ethane fails at higher pressures. The weaker interactions among adsorbate molecules, posterior adsorption saturation, and smaller molecular size of methane lead to its higher adsorption affinity at elevated pressures. Enlarging the pore or increasing the water content suppresses the selectivity of ethane over methane; selectivity decreases more steeply with pressure at lower temperatures. While there is only negligible selectivity variation with fluid composition at low pressures, the preferential adsorption of ethane at high pressures is facilitated by decreasing its mole fraction in the bulk fluid. We also discussed the implications of our work on shale gas exploitation. This study provides better insight into the storage mechanisms of shale gas and sheds light on the reliable estimation of gas-inplace and, more generally, competitive adsorption of mixtures in nanoporous materials.