Diagenesis and pore evolution of tight reservoirs in glutenite section of Bachu Formation, Tahe Oilfield

The tight reservoirs in the glutenite section of the Carboniferous Bachu Formation in the Tahe Oilfield represent an important future exploration target. However, research on these reservoirs remains quite limited, particularly regarding diagenesis and pore evolution, which severely restricts the exploration and development of tight oil and gas in the glutenite section of the Bachu Formation. Based on the analysis of lithology and sedimentary facies of the Bachu Formation, this study systematically compared diagenesis, pore evolution, and their differ-ences between glutenite and sandstone in the western and eastern areas of the Tahe Oilfield. The analysis integrated core observations, X-ray diffraction, cast thin sections, scanning electron microscopy, and cathodoluminescence data. The rock types in the glutenite section of the Bachu Formation in Tahe Oilfield were diverse, mainly composed of sandstone and glutenite. The compositional maturity, structural maturity, and physical properties of the reservoirs in the western area were higher than those in the eastern area. Reservoir diagenesis in the glutenite section was mainly characterized by compaction, cementation, and dissolution. In the western area, the main diagenetic processes were strong compaction, intense dissolution, and moderate to strong cementation. In the eastern area, compaction was also strong, cementation was even stronger, but dissolution was relatively weak. Based on vitrinite reflectance (R_o), the proportion of montmorillonite in the mixed layers of illite and montmorillonite, and the occurrence of iron-bearing calcite, it was concluded that the glutenite in both the western and eastern areas of the Tahe Oilfield entered the middle diagenesis stage B. Pore evolution was closely related to diagenesis. In the western area, the porosity of sandstone decreased from 38.8% to 8.8% after diagenesis, while the porosity of glutenite decreased from 30.5% to 4.5%. In the eastern area, the porosity of reservoirs in the glutenite section decreased from 27.0% to 4.0% through diagenetic evolution. This study provides a reference for the evaluation, exploration and development, and sweet spot selection of tight oil reservoirs in the glutenite section of the Bachu Formation in the Tahe Oilfield.