Petrophysical Cutoffs and Reservoir Grading Criteria for Qingshankou Formation Shale in Qijia–Gulong Sag, Songliao Basin

Petrophysical cutoffs and reservoir-grading criteria were investigated for shale oil in the first member of the Qingshankou Formation in the Qijia–Gulong Sag, Songliao Basin. A total of 186 shale core samples were analyzed using scanning electron microscopy, contact-angle measurements, and high-pressure mercury intrusion. The connected pore-throat system was characterized from mercury-intrusion data, and the pore-throat limits associated with hydrocarbon storage and effective flow were determined using the water-film-thickness method and permeability-contribution analysis, respectively. Mercury-intrusion-derived permeability was further calibrated against measured permeability for reservoir evaluation. The shale samples are predominantly water-wet, with contact angles mainly concentrated at 20–25°. Considering the actual formation-pressure range, the storage-related pore-throat cutoff is approximately 2.95–3.60 nm, whereas the minimum effective-flow pore-throat radius is approximately 7 nm, indicating that hydrocarbon storage and effective flow correspond to different pore-throat conditions. MICP-curve characteristics and fractal analysis further distinguish four reservoir types, showing progressively reduced pore-throat size, connectivity, and permeability from Type I to Type IV. Based on corrected permeability and average pore-throat radius, Grade I reservoirs are defined by >0.30 mD and >300 nm, Grade II by 0.09–0.30 mD and 30–300 nm, Grade III by 0.04–0.09 mD and 7–30 nm, and Grade IV by <0.04 mD and <7 nm. The results distinguish the lower pore-throat condition for hydrocarbon storage from that required for effective flow and establish a quantitative relationship between microscopic pore-throat characteristics and reservoir quality, providing practical criteria for shale-reservoir evaluation in the Qijia–Gulong Sag.