Characteristics and Controlling Factors of Deep Buried Reservoir in the Southern Margin of Junggar Basin

Summary The Junggar basin is a large petroliferous, superimposed basin in Northwest China. The burial depth of Cretaceous Qingshuihe Formation sandstone reservoirs in the southern margin is 5500-7000m.Although the burial depth is large, the sandstone reservoirs are of good physical properties. The Qingshuihe reservoirs have mineralogical immaturity and moderate textural maturity. The pore system is mainly primary intergranular pore and a few secondary dissolution pore. Microfractures are developed in some grains. More initial porosity lost by compaction than cementation. Eodiagenesis in Qingshuihe sandstones is dominated by mechanical compaction, cementation by calcite and analcime. Mesodiagenesis mainly consist of further mechanical compaction, dissolution of feldspars and cements, formation of illite and mixed-layered I/S, Fe-calcite cementation. The formation of deeply buried high-quality reservoirs in the Qingshuihe Formation is controlled by four key factors. Firstly, grain size and rock composition played a significant role on initial porosity. Secondly, early long⁃term shallow burial and late rapid deep burial is conducive to the preservation of primary pores in the reservoir. Another factor is intragranular micro-fractures, which can increase the permeability of the reservoir and the overpressure also is an important factor for the formation of high-quality reservoirs.