Diagenetic facies and pore evolution of tight sandstone reservoirs of the Jiamuhe formation in the Shawan sag, Junggar basin.

The Jiamuhe Formation in the northwest margin of the Shawan Sag comprises tight sandstone reservoirs, and its distribution has obvious regional differences, but the reasons for this difference are not clear. This study aims to clarify the diagenetic facies and pore evolution patterns by integrating petrographic thin sections, fluorescence thin sections, X-ray diffraction, porosity-permeability data. The objective is to identify the causes of these differences, establish a theoretical evolutionary model, and quantitatively characterize pore evolution. The study area is dominated by lithic sandstone, feldspar lithic sandstone, and lithic feldspar sandstone. These rocks are characterized by relatively low textural and compositional maturity. The primary storage space is predominantly composed of secondary pores formed through dissolution, with minor microfractures present. The findings indicate that dissolution is the principal factor contributing to the physical properties of reservoirs observed between the delta plain and delta front. The texture of the cement and secondary pores govern the variations in physical properties within the same facies zone. The diagenetic sequence includes compaction→early chlorite coating→early meteoric water dissolution→early calcite/kaolinite cementation→middle-stage analcime/philipsite/zeolite cementation →middle-stage organic acid dissolution→microfractures. Quantitative analysis indicates that the original porosity decreased 23.89% by compaction and 7.94% by cementation. Conversely, dissolution and tectonic fracturing enhance porosity by 5.87% and 5.10%, respectively. The theoretical calculations align well with the experimental results. Based on the cementation-sedimentation-pore type, the Jiamuhe Formation in the study area is classified into four distinct diagenetic facies, type I is strong cementation - outer delta front - dissolution pores. Type II is strong cementation - inner delta front - intra granular dissolution pores with moderate dissolution. Type III is poor cementation - delta plain - intergranular pores with strong dissolution. Type IV is strong compaction - delta plain - intergranular dissolution pores with moderate cementation. The evolutionary history of each facies was reconstructed by using porosity evolution curves. Type I and Type III were demonstrated significant reservoir potential, offering a reference for predicting favorable reservoir distribution in future studies.