Hydrocarbon gas expulsion from coal source rocks takes place under conditions intermediate between an open and closed system. The compositional and isotopic changes of hydrocarbon gases generated from Paleogene coal with increasing maturity were investigated under semi-open and closed system laboratory pyrolysis to evaluate the maturity level at the time of hydrocarbon gas expulsion. Lower yields of saturated hydrocarbon gases and their higher delta C-13 values were observed in the semi-open system pyrolysis compared to those at the same maturity level in the closed system pyrolysis, since gaseous hydrocarbons depleted in C-13 were removed during semi-open system pyrolysis. The delta C-13 values of hydrocarbon gases generated in the closed system pyrolysis were initially lower than those in the semi-open system pyrolysis, and then became markedly higher at the higher maturity level (vitrinite reflectance [VRr] > 1.2%), indicating significant hydrogenation of isotopically heavier inorganic hydrogen derived from water moisture in the coal matrix. The semi-open and closed system pyrolysis showed that the relationship between the isotopic composition of hydrocarbon gases expelled from coal and the maturity of coal depends on the state of openness or closedness of the hydrocarbon generation environment. Diagrams showing the relationship between the delta C-13 values of hydrocarbon gases expelled from Paleogene coal and the VRr under conditions intermediate between semi-open and semi-closed systems were proposed for evaluating the timing of hydrocarbon gas expulsion in sedimentary basins.