Keywords: 
• Junggar Basin /
• weathering crust /
• formation mechanism /
• petroleum accumulation

Abstract: Clay-layer of the weathering crust, top of Jurassic, in the hinterland of Junggar Basin, is characterized by enriched Al₂O₃, Fe₂O₃ and TiO₂. The weathering crust can be classified into two types according to maturity index (SiO₂/Al₂O₃ ). Type Ⅰ shows high maturity, with SiO₂/Al₂O₃ between 2.7~4.0, and type Ⅱ shows low maturity, with SiO₂/Al₂O₃ between 4.0~5.0. Difference of the maturity of weathering crust (MWC) is mainly caused by tectonic environment and geological time. In the ridge area of Che-Mo Paleohigh, MWC is low, as weathering crust developed progressively down into underlying strata during uplift of the Paleohigh in Late Jurassic. In the flank area of the paleohigh and the area unaffected by the paleohigh, MWC is high, as weathered crust developed in relatively stable environment and experienced longer time. MWC is low in the area of Dong 1 well, as weathered time is much shorter caused by the deposit of J₃q in Late Jurassic. Truncate-reservoirs can be formed in the ridge of the paleohigh as weathering crust developed upon sandbody of J₁s directly. In other area, weathering crust is an important seal for petroleum migration and accumulation.