Formation Mechanism and Effect on Petroleum Accumulation of the Weathering Crust, Top of Jurassic, in the Hinterland of Junggar Basin
Clay-layer of the weathering crust, top of Jurassic, in the hinterland of Junggar Basin, is characterized by enriched Al_ 2O_ 3, Fe_ 2O_ 3 and TiO_ 2. The weathering crust can be classified into two types according to maturity index (SiO_ 2/Al_ 2O_ 3). Type Ⅰ shows high maturity, with SiO_ 2/Al_ 2O_ 3 between 2.7~4.0, and type Ⅱ shows low maturity, with SiO_ 2/Al_ 2O_ 3 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 J_ 3q in Late Jurassic. Truncate-reservoirs can be formed in the ridge of the paleohigh as weathering crust developed upon sandbody of J_ 1s directly. In other area, weathering crust is an important seal for petroleum migration and accumulation.