Keywords: 
• Beier Depression /
• Nantun Formation /
• base level cycle /
• sedimentary system /
• source and reservoir distribution

Abstract: On the basis of core logging,well logging and other data sources,this study examined the mode of high-resolution sequence development and how the base level cycles evolved at all levels in the Beixi area of the Beier De-pression in the Hailar Basin. The results showed that five types of sequence boundary developed in the Nantun Forma-tion within the study area. These are divided into three long-term base-level cycles and 12 medium-term base-level cy-cles. Stratigraphic thickness is controlled by changes in paleogeomorphology and datum surface. The ancient land-scape has had a great influence on thickness variation,forming the ‘south-thick gradual transition to north-thick’ stratum model. As an example,the effect of datum elevation on phase sequence,phase domain and source reservoir distribution was analyzed in LSC1. It is considered that during LSC1 deposition,the change of base level comprised an early slow-rising stage,a fast-rising stage,a further slow-rising stage,a slow decline stage,and finally a rapid de-cline stage. A fan delta,a lake bed fan and a lake depositional system were developed in the study area. The sandbod-ies in MSC1,MSC2 and MSC5 are well developed and have beneficial physical properties. The geometrical shape is predominantly multi-layered. Thick-layer distributary channel deposits constitute the main reservoir sandbodies. MSC3 represents the longest time period of the largest lake,forming high-quality hydrocarbon source rocks. The source reservoir is close to the bottom, and a superior reservoir formation has resulted.