Study on thermal cracking upgrading experiment and changes in organic matter occurrence state of medium-low maturity shale oil

Abstract The assessment of medium-low maturity shale oil's basic properties and hydrocarbon generation potential forms a critical link between geological resources and engineering development. This study focuses on JY shale oil, employing analytical techniques including Rₒ, TOC, TGA, and Rock-Eval pyrolysis to systematically characterize its geochemical properties. An in-situ upgrading simulation system was developed and optimized based on sample properties. Using NMR T1-T2 and PY-GC/MS analyses before and after thermal cracking, the research further investigates the mechanism by which thermal cracking affects the occurrence state of organic matter.The research results show that the JY shale oil samples have a TOC content ranging from 1.94–2.57%, a total hydrocarbon generation potential (S1 + S2) of approximately 21 mg/g, a vitrinite reflectance (Rₒ) of 0.66–1.18%, and a sapropelinite content in kerogen as high as 90.33%. These data indicate that the kerogen type of JY shale oil is Type Ⅰ, belonging to medium-low maturity source rocks, with high organic matter abundance, good pyrolysis characteristics, and high hydrocarbon generation potential, thus having the potential for in-situ upgrading development. Simulation experiments within the temperature range of 400℃–600℃ show that under the condition of isothermal treatment at 450℃ for 12 hours, the degradation rate of organic carbon 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 reveals that the proportion of light hydrocarbons increases significantly by about 20%, while the proportion of heavy hydrocarbons decreases by more than 20%, verifying the conversion sequence of "heavy hydrocarbons → medium hydrocarbons → light hydrocarbons" during thermal cracking.​This study offers theoretical and experimental support for the efficient development and technical optimization of medium–low maturity shale oil, providing valuable references for assessing the feasibility of in-situ upgrading and mitigating development risks.