Evolution of Occurrence States of Retained Hydrocarbons in Organic-Rich Shales During Thermal Maturation: Evidence from Semi-Open System Pyrolysis

Understanding the dynamic evolution of occurrence states of retained hydrocarbons at different thermal maturation stages is crucial for assessing shale oil mobility and enhancing oil recovery. Semi-open pyrolysis experiments were conducted on organic-rich shales from the Chang 73 sub-member of the Ordos Basin. Based on semi-open system hydrocarbon generation and expulsion thermal simulation experiments, combined with sequential multi-solvent extraction, gas chromatography–mass spectrometry (GC–MS), Fourier transform infrared spectroscopy (FTIR), and low-temperature N2 adsorption, this study systematically characterized the composition, occurrence pore size, occurrence state, and quantitative evolution of retained hydrocarbons in organic-rich shales across a maturity range of vitrinite reflectance (Ro) = 0.53–1.59%. The results indicate that maturity significantly influences hydrocarbon generation and pore structure, which in turn affects the occurrence states and contents of retained hydrocarbons. As maturity increases, the contents of various occurrence states reach their peak at around Ro = 0.88%, with free oil predominating at a proportion as high as 86.57%. Beyond Ro = 0.88%, the proportion of free oil decreases rapidly, while the proportions of adsorbed oil, adsorbed–miscible oil, and miscible–adsorbed oil all increase. The content of free oil is not only related to pores in the 20–100 nm range but also to the molecular structure of kerogen. It is proposed that the Ro = 0.74–1.03% stage represents the golden window for free oil yield, with favorable exploration and development potential. These findings provide a theoretical basis for the exploration and development of organic-rich shale oil and the assessment of movable resource potential.