Thermal cracking for upgrading medium-low maturity shale oil: evolution of organic matter occurrence

The evaluation of the basic properties and hydrocarbon generation potential of medium-low maturity shale oil serves as a critical link between geological resources and engineering development. This study focuses on JY oil shale, utilizing analytical techniques including vitrinite reflectance (R₀), total organic carbon (TOC), thermogravimetric analysis (TGA), and Rock-Eval pyrolysis to systematically characterize its geochemical properties. Based on sample characteristics, an in-situ upgrading simulation system was developed and optimized. By comparing nuclear magnetic resonance (NMR) T₁-T₂ spectrum and pyrolysis-gas chromatography/mass spectrometry (PY-GC-MS) results before and after thermal cracking, the research further elucidates the mechanism by which thermal cracking influences the occurrence state of organic matter. Results indicate that the JY shale samples have a TOC content ranging from 1.94% to 2.57%, a total hydrocarbon generation potential (S₁ + S₂) of approximately 21 mg/g, R₀ between 0.66% and 1.18%, and a sapropelinite content in kerogen as high as 90.33%. These parameters classify the kerogen as Type0 I, belonging to medium-low maturity source rocks with high organic matter abundance, favorable pyrolysis characteristics, and significant hydrocarbon generation potential, thus qualifying for in-situ upgrading development. Simulation experiments conducted within the temperature range of 400 °C to 600 °C show that under isothermal conditions at 450 °C for 12 h, the organic carbon degradation rate can reach about 40%, and fracturing measures can enhance the in-situ upgrading effect. After thermal cracking, the proportion of kerogen decreases by approximately 10%, the proportion of adsorbed hydrocarbons increases by about 5%, and the proportion of free hydrocarbons increases by roughly 3%. PY-GC-MS analysis further reveals that the proportion of light hydrocarbons increases significantly by about 20%, while the proportion of heavy hydrocarbons decreases by over 20%, validating the conversion sequence of "heavy hydrocarbons → medium hydrocarbons → light hydrocarbons" during thermal cracking. This study methodologically integrates multi-scale analysis with a self-developed simulation system and mechanistically clarifies the evolution pathway of the three-phase state of organic matter and the trend toward lighter hydrocarbon compositions. It provides theoretical and experimental support for assessing the feasibility of in-situ upgrading development and optimizing techniques for medium-low maturity shale oil, offering valuable insights for mitigating development risks.