Differential Diagenesis And Its Influence On Pore Structure Of Chang 7 Sandstones In The Ordos Basin

The Ordos Basin is a typical tight oil producing area in China.The exploration and quantitative characterization of the microscopic pore-throat structure of the reservoir will help the production of tight oil.Based on the logging data and the integration of core observation,combined with rock thin section,casting thin section,scanning electron microscope,electron probe analysis and X-ray diffraction,physical property test and high-pressure mercury injection experiment,this study analyzes the sedimentary reservoir characteristics of Chang 7 sandstone reservoirs;studys the microscopic pore structure of the reservoirs and its effects on reservoir petrophysical properties;calculates the effects of different diagenesis on porosity quantitatively.The results show that the reservoirs are dominated by lithic quartz sandstone and feldspar lithic quartz sandstone.The reservoirs mainly experienced 4diagenetic phases,namely eogenesis A,eogenesis B,mesogenetic A₁ and mesogenetic A₂.Four types of diagenetic facies were identified according to their specific diagenetic evolution pathways,including primary intergranular pore-chlorite film cementation facies,secondary dissolution pore-feldspar quartz dissolution facies,micropore-weak dissolution facies and micropore-tight cemented facies.The pore types include residual intergranular pores,intragranular dissolved pores,intergranular dissolved pores and fractures.The reservoirs can be divided into four types according to the pore structure and petrophysical properties,and the petrophysical properties of reservoirs are mainly controlled by pore throat connectivity and pore throat radius.Differential diagenesis has different effects on pore throat size,reservoir property and permeability.The results show that compaction and cementation decrease porosity greatly,while dissolution increases porosity.The primary intergranular pore-chlorite film cementation facies and secondary dissolution pore-feldspar quartz dissolution facies are the high-quality reservoirs in the study area,while the micropore-weak dissolution facies and micropore-tight cemented facies are poor in quality.