Diagenesis and pore evolution of tight sandstone reservoir in Ordos Basin:taking the seventh Member of Yanchang Formation of Qilicun Oilfield as an example
Ordos Basin is one of the basins in China with the most abundant tight oil resources,having large resources which are widely distributed,and broad prospects for exploration and development.However,research on tight sandstone reservoir of Qilicun Oilfield is relatively weak.By comprehensively analyzing the logging curves,thin sections,scanning electron microscopy,cathodo-luminescence and other microscopic observation data in the study area,combined with measured experimental data such as whole rock X-ray diffraction,this paper finely studied the petrologic characteristics,physical properties,diagenetic types and pore evolution laws of the Chang 7 Member reservoir in Qilicun Oilfield.The results show that the Chang 7 Member reservoir in the study area is mainly composed of feldspar sandstone,with characteristics of extra-low porosity and ultra-low permeability and ultra-low porosity and ultra-low permeability.The main pore types are dissolution pores and intergranular pores,with a small amount of intercrystalline pores and microfractures.Diagenesis has a significant influence on pore evolution,and during the diagenetic process,compaction and cementation reduce the physical properties of the reservoir,while dissolution plays a constructive role to the reservoir.Quantitative calculation of pore evolution in the Chang 7 Member reservoir in the study area shows that the average original porosity is 36.54％,and the porosity reduction rate caused by compaction is 51.60％,while the porosity reduction rate caused by cementation is 43.75％.Dissolution has a construc-tive effect on the physical properties of the Chang 7 Member reservoir,with the porosity increase rate of 11.97％.
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