Quantifying Adsorbed Oil Content and Controlling Factors in Lacustrine Shales of the Paleogene Shahejie Formation: Insights from T 1 –T 2 NMR Experiments, Multitemperature Pyrolysis and Solvent Extraction Experiments

Accurate evaluation of adsorbed oil heterogeneity in laminated shales is critical for optimizing shale oil recovery and challenging assumptions of inherently low recovery potential in such reservoirs. This study integrates geochemical, petrophysical, and microscopic analyses to systematically investigate the compositional heterogeneity, storage mechanisms, and controlling factors of adsorbed oil within the Paleogene Shahejie Formation (Bohai Bay Basin), implementing T1–T2 NMR at tailored relaxation time thresholds for adsorbed oil characterization. Results reveal pronounced heterogeneity, with adsorbed oil concentrations significantly higher in organic- and clay-rich bright laminae than in massive or bioturbated facies, which act as preferential storage sites due to enhanced nanoporosity (10–50 nm mesopores), surface area (R2 = 0.77), and synergistic mineral-organic interactions (TOC: R2 = 0.75; clay content: R2 = 0.88). Adsorbed oil is enriched in C19+ n-alkanes, resins, and asphaltenes, retained via π-π stacking, hydrogen bonding, and van der Waals forces. Methodologically, multitemperature pyrolysis (S′2–1+S′2–2) yields movable oil of 0.06–8.82 mg/g (average 2.14 mg/g) but underestimates adsorbed oil by 40–50% (vs NMR), while solvent extraction also shows substantial deviations; T1–T2 NMR (0.013–0.022 mg/L, average 0.0165 mg/L) demonstrates superior sensitivity, primarily due to hydrocarbon volatilization (50–60% loss) during pyrolysis sample preparation. These findings refine sweet spot identification in laminated shale reservoirs, underscoring the need for method-specific calibration and correction factors to reconcile analytical discrepancies for improved movable oil estimation.