Abstract: Through analysis of source rocks,sedimentary system and reservoir seal assemblage of eastern Maichen sag in Beibuwan Basin, it is revealed that there are two kinds of source rocks in Maichen sag, including lacustraline facies source rock in Liushagang formation and coal-bearing source rocks in Weizhou formation. There are two sets of different sedimentary systems including large rivers-delta depositional system in north slope zone and multi-phase superimposed underwater fans in southern fault zone. There are two sets of reservoir-cap combination, Liushagang Fm reservoir-cap combination and Weizhou Fm reservoir-cap combination. Because of the diversity in hydrocarbon sources,sedimentary systems,reservoir-cap combinations and trap type, reservoir types are quite different in different place such as faulted nose-fault block reservoirs controlled by faults in north slope zone and subtle reservoirs controlled by fans in southern fault zone.