Abstract: The southern margin of Junggar Basin is the area that developed seismic velocity anomaly in general. By analysing the geologic structures and lithologic characteristics of Fukang fault zone, it is ascertained that layer veloci­ties of enormously thick thrust block (Upper Paleozoic-Mesozoic)and alluvial fan body distributed along the foot of the mountain are very high(>3 900m/s),they were two important factors which formed seismic velocity pitfalls. Con­sidering the characteristics of steepness in lateral seismic velocity variations, the seismic velocity trends in southern margin of Junggar Basin were ascertained using interactive processing for layering to draw velocity sections, thus im­proving processing quality of seismic data and reflecting subsurface structural patterns more correctly. Seismic velocity pitfalls of Well Tai 3 area. Xiaoquangou anticline and Dubei anticline were removed and structural patterns of Dunan anticline and Xihu anticline further determined. It is advantageous to guide oil-gas exploration work in this area.

Key words: Southern margin of Junggar Basin, Seismic velocity anomaly, Thrust block, Alluviat fan