Joint Mercury-Injection Porosimetry–NMR Evaluation and Sweet-Spot Identification of High-Clay Siltstone Interlayers: A Case Study of Well PⅡ in the Qing-1 Member, Gulong Depression

The high-clay siltstone interlayers of the Qing-1 Member in the Gulong Depression (Songliao Basin) act as key carriers for micro-scale oil migration and local enrichment in shale-oil reservoirs.Under strong clay constraints, the coupling between clay minerals and pore–throat structure often leads to oil-bearing yet immobile behavior.Focusing on within-well applications, we construct a joint evaluation framework combining high-pressure mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) to clarify the structure-mobility relationship and to establish empirical thresholds for sweet-spot identification.To ensure sample comparability, group-aligned sampling within the same well and lithofacies was adopted and specimens with a Mineral Difference Index (MDI)≤15% were selected.Key parameters include the median pore–throat radius r₅₀, sorting coefficient σ, mercury withdrawal efficiency, the T₂cutoff, the saturated movable fluid saturation (SMF), and the saturated geometric mean of T₂.A Composite Movable Oil Index (CMOI) was defined and applied as CMOI=So×SMF/100 to unify the assessment of oil content and mobility.CT and SEM observations reveal clay fabrics (films/bridges) that narrow throats and restrict connectivity, providing mechanistic support for the clay-induced reductions in SMF and CMOI.Results show that, under high-clay constraints, the MIP–NMR combination provides good discrimination between movable and immobile fluids; increasing clay content corresponds to smaller r₅₀, poorer sorting, higher displacement pressure, and decreases in SMF and CMOI, while the geometric mean T₂ correlates positively with SMF and CMOI and thus captures the combined effects of pore-throat size and relaxation environment.Cross-verification within the well interval yields empirical discriminant thresholds of r₅₀>0.4μm, SMF>40%, and CMOI>20%, which show good indicative performance for sweet-spot intervals in Well PⅡ and support the subdivision of sweet spots into A/B/C types according to pore–throat quality and mobility.These thresholds and conclusions apply only to this well interval and lithofacies and are not extrapolated regionally;statistical relationships constrained by limited sample size are used for trend indication only.This work establishes a within-well workflow of “MIP–NMR joint evaluation with CT/SEM-supported, CMOI-based sweet-spot identification”, providing practical guidance for method selection and sweet-spot ranking in high-clay siltstone interlayers.