Abstract: Based on lots of logging and core data, the Nantun formation in Beixi area of Beier sag (western Beier sag) is divided into four three-order sequences (SQ1-SQ4), among which five types of sedimentary facies are identified, i.e. fan delta, braided channel delta, neritic subsea apron, slump turbidite fan and lake ones. In this studied area, the sedimentary characteristics in different periods are mainly controlled by the tectonic evolution. In SQ1 period, Beixi area is divided into south and north parts, dominated by fan delta, braided channel delta and lake facies; in SQ2 period, continuous raise of the lake base level forms a unified lake basin with sedimentary characteristics like SQ1, except for forming new fan delta in the southwestern margin of this sag for existence of provenance; in SQ3 period, the lake base level reaches the highest with obvious shrinkage of sizes of the fan (braided channel) delta, dominated by slump turbidite fan; in SQ4 period, the tectonic activities are active, causing shrinkage of the lake area and not big change of deposit sizes in northwester and northern margin of the sag except its southeaster part for insufficient provenance, which leads to the disappearance of the central and southeastern braided channel delta.

Key words: Beier sag, Beixi area, Nantun formation, sedimentary facies, evolution