Abstract: The Fuyang oil layer in the Sanzhao Sag, northern Songliao basin, represents a classic below-source tight oil reservoir with significant resource potential. However, its complex migration and accumulation mechanisms hinder further refined exploration, necessitating dynamic tracing via fluid geochemistry. By integrating formation water geochemistry, crude oil biomarker fingerprints, composite conduit frameworks, and migration pressure fields, this study systematically elucidates the migration pathways, charging processes, and differential accumulation mechanisms of hydrocarbons in the Fuyang oil layer. The key findings are as follows: ① Formation water types and salinity exhibit clear spatial zonation. From the sag center to its margins, the fluid system transitions from high-salinity jump/hydrothermal-altered water to low-salinity NaHCO₃-type primary water, defining the Yushulin and Songfangtun primary preservation zones, the Xujiaweizi hydrothermal alteration zone, and the Shengping transitional zone. ② Spatial gradients of crude oil physical properties and biomarkers (MDR, 1,4-/2,4-DMBT, C₁₃/C₁₉TT, etc.) reveal a dynamic migration scheme: late-stage, highly mature crude oils undergo overpressure-driven vertical downward charging in the sag center, whereas early-stage, medium-to-low-maturity oils experience long-distance lateral migration toward slope zones and structural highs. ③ A "dual-source supply, fault-sandstone composite, and differential accumulation" model is established. Hydrocarbons undergo vertical/lateral direct charging in near-source zones, cross-strata migration via conduit faults in source-separated zones, and stepwise fault-sandstone composite transport in distal zones. Notably, early-charged deep natural gas exerts a fluid repulsion effect against subsequent crude oil charging. Coupling formation water fields with crude oil geochemical fields provides a robust approach for target prediction in tight sandstone reservoirs.