Abstract: Based on the analyses for formation of Renqiu, Dongxin, Gudao, Beidagang and Shuguang-Huanxiling oilfields, it is found that their principal hydrocarbon source rock is Shasanduan unit,the hydrocarbon expelling and secondary migration occurred at about the end of Paleogene,and the forming and finalizing of such oilfields are accomplished in the end of Neogene to Quarternary. The four large oifields along Bohai bay, due to faults connection and communication, the hydrocarbon of Paleogene again migrated into Neogene. The crude oil was also affected by degradation and became more viscous and heavier.

Key words: Bohai bay basin, large scale oilfield, primary reservoir, oil and gas migration, secondary reservoir, reservoir formation, mode