Preservation mechanism of pores in middle and deep sandstone reservoirs of Cretaceous Bashijiqike Formation in Yingmaili area, Kuqa Depression, Tarim Basin

The sandstone reservoirs of the Cretaceous Bashijiqike Formation in the Yingmaili area on the southern slope of the Kuqa Depression within the Tarim Basin demonstrate favorable physical properties and considerable potential for oil and gas exploration. However, they are characterized by strong heterogeneity and unclear patterns of oil and gas distribution. In this study, the lithology and physical properties of the middle and deep reservoirs of the Bashijiqike Formation in the Yingmaili area were analyzed using core observation, a series of thin section analyses(standard, casting, cathodoluminescence, and inclusion thin sections), scanning electron microscopy(SEM), physical property testing, X-ray diffraction, and diagenesis reconstruction and physical property recovery techniques. It aims to explore the pore characteristics and preservation mechanisms, classify reservoir types, and clarify the distribution patterns and controlling factors of favorable reservoirs.The results show that the sandstone is mainly composed of feldspathic lithic sandstone and lithic feldspathic sandstone with low matrix content and medium maturity in both composition and structure. The primary pore type of the reservoir is residual primary pores, followed by secondary pores, including intergranular and intragranular dissolution pores, classifying the reservoir as a medium-to-high porosity and permeability type. The preservation of the primary pores in the middle and deep sandstones of the Bashijiqike Formation was mainly attributed to the depositional environment and subsequent diagenetic and reservoir evolution. The sandstone was initially formed in the microfacies of distributary channels at the front edge of a braided river delta with high hydrodynamics. The constantly overlapping channels formed thick and stable composite sand bodies. The strong hydrodynamics in the area led to high concentration and good sorting of sandstone clastic particles, providing the material basis for the formation of primary pores. The burial evolution process involved early long-term shallow burial and late-stage rapid deep burial, resulting in weak compaction transformation of the sandstone. Meanwhile, late-stage deep overpressure greatly enhanced the sand body's resistance to compaction, allowing for the preservation of residual primary pores. The continuously decreasing paleogeothermal gradient in the depression further contributed to the effective preservation of residual primary pores.