NMR Quantification of Movable Pore Throat Thresholds in Shale Oil Reservoirs

Shale reservoirs are characterized by high tightness and strong heterogeneity; determining the lower limit of movable pore throats is a core scientific issue for reservoir evaluation and development optimization. Targeting shale in the Jiyang Depression, Bohai Bay Basin, this study employed mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR) combined with centrifugal experiments, and simulated elastic depressurization production tests to investigate the pore throat structure and fluid mobility of three types of dominant lithofacies. The dominant lithofacies in the study area are laminated cryptocrystalline argillaceous limestone, bedded cryptocrystalline argillaceous limestone, and laminated cryptocrystalline calcareous mudstone, with significant differences in mineral composition among the three lithofacies. Laminated cryptocrystalline calcareous mudstone exhibits the smallest average pore throat radius, yet possesses optimal pore throat connectivity and fluid mobilization capability, with uniform pore throat distribution concentrated in the small pore-size range. The two argillaceous limestone lithofacies develop multi-modal pore systems. Elastic depletion production simulation indicates that the lower limit of movable pore throats for the bedded cryptocrystalline argillaceous limestone lithofacies is 34.6 nm under the tested depletion conditions, demonstrating that stress sensitivity can elevate the actual lower limit of movable pore throats by approximately one order of magnitude; this threshold is lithofacies- and condition-specific and requires validation before generalization. The lower limits of movable pore throats for different lithofacies follow the order: bedded cryptocrystalline argillaceous limestone > laminated cryptocrystalline argillaceous limestone > laminated cryptocrystalline calcareous mudstone.