Shale formation has a broad pore size distribution with both micropores (<2 nm) and mesopores (2–50 nm) in the kerogen matrix in addition to the macropores and fractures. Methane and ethane are two primary hydrocarbon species of shale gas, and the competitive sorption behavior in kerogen micropores and mesopores is not well understood. In this work, grand canonical Monte Carlo (GCMC) was used to study the sorption behavior of methane, ethane, and their binary mixtures (from 1:9 to 9:1) in kerogen nanopore systems with micropores and mesopores at 327 K and pressures ranging from 1 to 50 MPa. Three zones (Zone I: free gas, Zone II: adsorbed gas, and Zone III: absorbed gas) were defined to describe the sorption in micropores and mesopores. The results show different selectivity characteristics of ethane over methane for three different zones. The mass densities of gas molecules in different zones were obtained, which can be used for gas-in-place estimation. The calculated sorption isotherms of shale samples show reasonably good agreement with previous experiments. We show that the Langmuir model can be used to describe the absolute sorption isotherms of pure methane and ethane. For binary mixtures, the absolute sorption of individual gases predicted by the multicomponent Langmuir (ML) model shows a deviation from GCMC simulation results. However, with two correction coefficients, the modified ML models match the absolute sorption amount from the GCMC very well. Finally, a micropore-mesopore (MM) model was derived for the excess-absolute sorption relationship, which significantly improves the prediction accuracy of the absolute sorption in shale gas systems. This research highlights the importance of using kerogen nanopore systems with both micropores and mesopores.