The Diagenetic Environment Of Upper Paleozoic Sandstone And Its Influence On Reservoir Quality In Xiasiwan District,Northern Shaanxi

The sandstone reservoirs in the Xiasiwan area of the Ordos Basin are low-porosity and low-permeability reservoirs.This paper combines the basic knowledge of sedimentary petrology,petroleum geology,and previous research results,through the analysis of cores,thin sections,clay minerals and scanning electron microscopy.The reservoir characteristics,diagenesis and diagenetic environment in the study area are studied,and the influence of diagenetic environment on reservoir quality is analyzed.The Upper Paleozoic rock types in the study area are mainly quartz sandstone,lithic quartz sandstone and lithic sandstone.The Shanxi Formation is dominated by lithic quartz sandstone with high content of fine sandstone,and the Shihezi Formation is dominated by lithic quartz sandstone and lithic sandstone.The sandstone content is high.The sorting performance is generally moderate,the debris structure is mainly supported by particles,and the contact methods are point contact,line contact and point-line contact.The average porosity of Shanxi Formation is 2.78%,and the average permeability is 0.2658×10-3μm2.The average porosity of the Shihezi Formation is 5.41%,and the average permeability is0.3049×10-3μm2.The porosity and permeability in the sandstone reservoirs of the Shanxi Formation and Shihezi Formation are relatively low and they are positively correlated,and the correlation is good.The pore types are dominated by secondary pores,including intergranular pores and intragranular dissolved pores,with developed fractures.The types of diagenesis in the study area are complex,including compaction,pressure solution,cementation,metasomatism and dissolution.The effect of compaction and dissolution is mainly manifested in the bending of plastic particles,the directional or semi-directional arrangement of debris particles,and the line contact or concave-convex contact between quartz particles.Dissolution cementation is characterized by the dissolution of easily accessible components such as cuttings,feldspar,and tuffaceous matter,forming secondary pores,and siliceous,clay minerals and carbonate cements fill the pores.The metasomatism exists in the form of iron dolomite and iron calcite metasomatism detrital particles.The sandstone is in the middle diagenetic stage B.By analyzing the formation mechanism of the diagenetic environment and combining the identification characteristics of the acid-alkali diagenetic environment,the diagenetic environment of the sandstone in the study area is mainly acidic,and the alkaline environment is weak.By analyzing the influencing factors of the reservoir quality of the Upper Paleozoic Shanxi Formation and Shihezi Formation in Xiasiwan District,it is believed that the mineral composition and diagenesis of sandstone have different effects on the quality of the reservoir.The content of quartz particles is negatively correlated with the porosity and permeability of the reservoir.The higher the content,the worse the physical properties of the reservoir.The dissolution of cuttings,feldspar and tuff is beneficial to the formation of secondary pores.The average porosity reduction rate caused by sandstone reservoir compaction in Shanxi Formation is 75.1%,the average porosity reduction rate caused by sandstone reservoir compaction in Shihezi Formation is 74.9%,the cementation rate of sandstone reservoir in Shanxi Formation is 24.3%,and the sandstone reservoir in Shihezi Formation is 24.3%.The cementation rate of the reservoir is 27.3%.By evaluating the relative importance of sandstone reservoir compaction and cementation in the study area,it can be seen that both compaction and cementation reduce the porosity of sandstone reservoirs,but the effect of compaction is more obvious.Dissolution The Shanxi Formation is higher than the Shihezi Formation.The sandstone reservoir of the Shanxi Formation has a porosity increase rate of 15.7% due to dissolution,and the sandstone reservoir of the Shihezi Formation has a porosity increase rate of 12.9%.The mineral components of the fluid are dissolved,reflecting a strong acidic environment.