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Spontaneous imbibition (SI) has been used widely to characterize shale wettability and pore structures, which can significantly affect the migration and accumulation processes of shale oil. However, the application of SI in the enrichment mechanism of shale oil is quite limited. Therefore, focusing on the Permian Fengcheng Formation shale deposited in an alkaline lake environment in the Mahu Sag of Junggar Basin, this study conducts systematic SI experiments with complementary experiments such as contact-angle measurement, optical microscopy, field emission scanning electron microscopy, micro-CT, high-pressure mercury intrusion porosimetry, and N 2 adsorption to clarify the controlling factors of SI behavior of different fluids in shale and attempts to evaluate the migration and accumulation potential of shale reservoirs using SI. The results show that the more hydrophobic the sample is, the stronger the absorption of n-decane is, and the SI of n-decane reaches equilibrium quickly. The development of an alkaline mineral layer that is several millimeters thick in Fengcheng Formation shale could promote hydrocarbon migration due to the large particles of alkaline minerals (200−400 μm) and more development of intergranular microfractures, which can be indicated by the higher SI slope and imbibed oil volume. Using SI parameters, the shale hydrocarbon migration−accumulation index H m was proposed in this study; the greater the H m of the shale reservoir is, the more conducive to the migration and accumulation of shale oil the shale reservoir is. Four migration−accumulation patterns were established for different lithofacies in the study area, and the migration and accumulation potential of different lithofacies of shale from strong to weak is in the order of siltstone, shale with an alkaline mineral layer, laminated shale, and then massive shale, which is generally in line with the order of shale oil content. The validity of the proposed shale hydrocarbon migration−accumulation index is also confirmed using data from the literature.