Micro-Nano Space In-Situ Phase Behavior and Reservoir Type Evaluation of Shale Oil

Addressing challenges such as sampling difficulty, large surface ratio errors, and significant nano-pore effects in evaluating the in-situ phase behavior and reservoir types of shale oil, this study proposes an evaluation method based on pressure-retaining core pyrolysis gas chromatography analysis. This method aims to achieve efficient and accurate evaluation of shale oil phase behavior and reservoir types under micro-nano spatial conditions. Original core samples were obtained using liquid nitrogen freeze pressure-retaining coring technology, combined with pyrolysis gas chromatography quantitative analysis to obtain molecular composition and thermodynamic parameters. The PVTsim software was used to generate P-T phase diagrams for shale oil. By integrating data on formation temperature, pressure, and pore size distribution, the in-situ phase behavior and reservoir types were determined. Experiments showed that the pressure-retaining core pyrolysis gas chromatography spectrum can accurately characterize hydrocarbon components, and its P-T phase diagram revealed that nano-pores (10–150 nm) significantly reduce critical temperature and pressure, with shale oil phase behavior transitioning from black oil to condensate gas as pore size decreases. For the Gulong shale oil reservoir (Ro > 1.4%) with a main pore size range of 2–30 nm, the in-situ phase behavior is dominated by condensate gas, corresponding to condensate gas reservoirs. This method addresses issues of light hydrocarbon volatilization and pore effects in traditional evaluations through pyrolysis gas chromatography quantitative analysis and nano-spatial thermodynamic models, providing efficient and low-cost technical support for shale oil exploration and development phase behavior evaluations.