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In this work, Huadian oil shale was extracted by subcritical water at 365 °C with a time series (2–100 h) to better investigate the carbon isotope fractionation characteristics and how to use its fractionation characteristics to constrain the oil recovery stage during oil shale in situ exploitation. The results revealed that the maximum generation of oil is 70–100 h, and the secondary cracking is limited. The carbon isotopes of the hydrocarbon gases show a normal sequence, with no “rollover” and “reversals” phenomena, and the existence of alkene gases and the CH4-CO2-CO diagram implied that neither chemical nor carbon isotopes achieve equilibrium in the C-H-O system. The carbon isotope (C1–C3) fractionation before oil generation is mainly related to kinetics of organic matter decomposition, and the thermodynamic equilibrium process is limited; when entering the oil generation area, the effect of the carbon isotope thermodynamic equilibrium process (CH4 + 2H2O ⇄ CO2 + 4H2) becomes more important than kinetics, and when it exceeds the maximum oil generation stage, the carbon isotope kinetics process becomes more important again. The δ13CCO2−CH4 is the result of the competition between kinetics and thermodynamic fractionation during the oil shale pyrolysis process. After oil begins to generate, δ13CCO2−CH4 goes from increasing to decreasing (first “turning”); in contrast, when exceeding the maximum oil generation area, it goes from decreasing to increasing (second “turning”). Thus, the second “turning” point can be used to indicate the maximum oil generation area, and it also can be used to help determine when to stop the heating process during oil shale exploitation and lower the production costs.

Constrain on Oil Recovery Stage during Oil Shale Subcritical Water Extraction Process Based on Carbon Isotope Fractionation Character