Abstract: The oil source of JZ25 reservoir,one of the significant oil and gas discoveries in Bohai Sea,comes from the north depression in Liaoxi depression.JZ25 reservoir is segmented into several complex fault block reservoirs after the tectonic movements of multi-phases,and the block reservoirs are heterogeneous in hydrocarbon fullness.Analysis on homoge- nization temperatures of fluid inclusions,burial history and geothermal temperature history indicates that JZ25 reservoir was filled into reservoir in the first stage at the end of Dongying formation.Because the matching of the hydrocar- bon filling period and the active stage of fractures,the cas- cade migration path system formed between the faults of mi- gration path and the sandstone strata in the downthrown, and the hydrocarbon migrated preferentially towards the blocks in the upcast side till the reservoir strata were full and then filled the high position of the downthrown,which result in the complicated fault block reservoir of high fullness in the downthrown and low fullness in the upcast side.