The partitioning and conversion between adsorbed and free oil are critical controls on shale oil mobility and recoverability. In this study， a combined thermogravimetric analysis-nuclear magnetic resonance （TGA-NMR） approach was employed to investigate lacustrine shales from the Chang 7 （Ch-7） submember of the Yanchang Formation， Ordos Basin， China.
n-dodecane was used as a probe fluid to characterize the contents， proportions， pore-scale distributions， and temperature-induced conversion behaviors of adsorbed and free oil at 60， 80， and 100 ℃. TGA results indicate that increasing temperature continuously promotes the conversion of adsorbed oil into free oil， accompanied by a progressive expansion of the pore size range hosting free oil， whereas adsorbed oil remains the dominant fraction， accounting for more than 50% of the total oil even at elevated temperatures. Integrated analyses of NMR T
2 spectra and T
1-T
2 maps further indicate that adsorbed oil is primarily retained in micropores， whereas free oil is distributed across pores spanning the full pore-size range. By coupling TGA-and NMR-derived parameters， the density and thickness of the adsorbed oil layer were quantitatively constrained， and the temperature threshold for complete conversion of adsorbed oil to free oil under in-situ reservoir conditions was determined. The results demonstrate that shale oil mobility enhancement is mainly governed by the progressive redistribution of oil occurrence states rather than by complete conversion of adsorbed oil. A robust linear relationship is observed between the adsorbed oil ratio and temperature. An adsorbed oil ratio of 0.5 is identified as an experimentally constrained threshold for pronounced mobility enhancement within the
n-dodecane-based system. These findings provide experimentally constrained criteria for predicting shale oil mobility and optimizing thermal parameters under in-situ upgrading conditions.
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