Thermal evolution of pore structure and its implications for shale oil mobility in the Lucaogou formation of the Jimsar Sag

Thermal evolution modifies both shale pore systems and hydrocarbon properties, with implications for shale oil occurrence and mobility. This study integrates organic geochemical, mineralogical and petrophysical data, scanning electron microscopy, organic extraction, thermal simulation, low-temperature N 2 adsorption, nuclear magnetic resonance (NMR) and liquid-product viscosity measurements for the Lucaogou Formation in the Jimsar Sag. Unextracted and preheated, extracted aliquots were compared as experimental analogues of hydrocarbon retention and expulsion. Intergranular/intercrystalline and intragranular dissolution pores dominate the observed reservoir space. The largest extraction-related changes in N 2 -accessible pore volume occur at pore diameters of 2–100 nm, indicating substantial hydrocarbon occupancy in this range. Pore volume and specific surface area exhibit an increase–decrease–increase pattern in the unextracted series and a decrease–increase–slight decrease–increase pattern in the extracted series. In n-dodecane-saturated plugs, movable-fluid saturation increases overall from 24.49% to 57.60% as the assigned vitrinite reflectance (Ro) increases from 0.73% to 2.71%, while the transverse-relaxation-time cutoff decreases from 89.02 to 3.43 m. Separately, the viscosity of the thermal-simulation liquid products decreases. These complementary observations support a conceptual framework linking pore accessibility and connectivity with fluid-property evolution. Three stages are distinguished: low mobility at Ro < 0.73%, moderate mobility at 0.73% ≤ Ro < 1.32%, and relatively high mobility at Ro ≥ 1.32%. The boundaries describe the investigated experimental sequence; the NMR measurements quantify a model-fluid response and do not directly determine native-oil recoverability.