Abstract: Petroleum accumulation mechanisms for two oil/gas fields, the Puguang gas field in the Sichuan basin and the MXZ oil field in the Junggar basin, were analyzed and compared, and factors controlling petroleum accumulation and occurrence in superimposed basins were discussed. In view of petroleum accumulation, the superimposed basins are characterized by the multiple petroleum sources (multiple source kitchens, multiple source rock intervals and multiple petroleum precursors such as kerogen, accumulated oils and various bitumens), the spatial and temporal diversities in fluid conduit systems and pressure, temperature and stress fields, and the complicated post-accumulation physicochemical processes such as chemical alteration of the petroleum reservoirs and fluid re-migration and re-accumulation. The manners and styles of the superimposition of accumulation processes during multiple-stage tectonic movements control the enrichment or dilution of petroleum. Successive or enhanced focusing of petroleum originated from different kitchens or source rock intervals is the important mechanism for petroleum enrichment in superimposed basins, whereas dispersal migration and adjustment of accumulated petroleum may cause petroleum dilution.