Mixed siliciclastic-carbonate sediments occur broadly in modern and ancient systems. Studies on mixing processes began in shallow shelf environments; however, the genetic model of marine mixed sediments is difficult to apply to continental rift basins due to the complex palaeogeographic environment. We identified three mixing types in the first and second members of the Palaeogene Shahejie Formation (E2s1–2) in the Qinhuangdao area of the Bohai Sea: (1) mixed fan delta, (2) retrogradation mixed sheet, and (3) mixed sheet without siliciclastic influx. Tectonic stability, arid climate, and saline lakes are prerequisite conditions for the development of mixed sediments, whereas the palaeogeomorphologic unit should be the critical factor. We also concluded that the primary sedimentary material contains near-source coarse terrestrial debris, and the advantageous lithologic facies producing biological components are the foundation for high-quality mixed reservoirs, which are characterized by thick layers and favorable porosities and permeabilities. The micritic coatings and early dolomitization against the background of a saline lake environment favored the preservation of primary pores, whereas the leaching by atmospheric water and organic acid erosion during thermal evolution of the source rock created many secondary pores. In addition, hydrocarbon charging protected the reservoir space from carbonate cementation.