Kerogen can release both liquid and gaseous hydrocarbons during thermal maturation. In this study, the maturation and hydrocarbon generation of a low-maturity type II kerogen were simulated via temperature-programmed gold tube closed system (GTCS) pyrolysis at two different heating rates. In addition to the yields of C-1-C-5 gaseous hydrocarbons, H/C atomic ratios and delta C-13 values were also obtained on the kerogen residues from GTCS pyrolyses. The remaining C-1-C-5 gaseous hydrocarbon and C-6(+) liquid hydrocarbon generation potentials in both the original kerogen and the kerogen residues of various thermal maturities were determined by quantitative flash pyrolysis-gas chromatography (Py-GC) and were used to estimate the loss of aliphatic carbons in artificially matured kerogens compared with that in the original kerogen. The yields of C-1-C-5 gaseous hydrocarbons from GTCS pyrolysis were compared with the lost amount of C-1-C-5 gaseous hydrocarbon potentials within the kerogen structure at the same maturities. The results indicate that the H/C atomic ratios of kerogen residues decreased and the delta C-13 values of kerogen residues became progressively heavier (i.e., being enriched in C-13) with increasing maturity in the oil generation window (OGW) but with delta C-13 becoming slightly lighter above the calculated R-o (Calcd R-o, namely, Easy R-o or equivalent vitrinite reflectance) of 2.5%. It is worth noting that, in the OGW, the yields of C-1-C-5 gaseous hydrocarbons from GTCS pyrolyses were significantly lower than the amounts of lost C-1-C-5 potentials indicated by Py-GC. We speculate that hydrocarbon gases generated in OGW may either be partially adsorbed within kerogen and dissolved in oil or participate in the thermal degradation of kerogen during GTCS pyrolyses, resulting in lower yields of C-1-C-5 gaseous hydrocarbons than the decrease in kerogen C-1-C-5 potentials. The Py-GC results of kerogen residues also show that the rates of loss for C-1, C-2, and C-3 hydrocarbons are about 92%, 98%, and 99%, respectively, at 1.3% Calcd Ro. This indicates that most of the C29 alkyl side chains can be cleaved from the kerogen structure within OGW. In comparison, at a Calcd R-o of 1.3%, the formation rate of methane (C-1) from GTCS pyrolysis of kerogen at a heating rate of 2 degrees C/h only amounted to about 32%. The results from this study appear to suggest that, in GTCS pyrolysis, most C-1 generated from type II kerogen at the overmature stage cannot be attributed to primary kerogen cracking but to gradual secondary cracking of hydrocarbons with high molecular weights.