Research On Characteristics And Sensitivity Of Upper Paleozoic Sandstone Reservoirs In Eastern Ordos Basin

According to the theory of reservoir geology and geochemistry,the characteristics of petrography,diagenesis,diagenetic cement and geochemistry of diagenetic fluid were systematically studied on the Upper Paleozoic reservoirs in eastern Ordos Basin.Seven types of diagenetic facies were classified,and the microstructural characteristics,physical properties,sensitivity and distribution of each diagenetic facies were studied.The reservoirs were classified and evaluated based on the main controlling factors of high quality reservoir.The rock types of the Upper Paleozoic gas reservoirs are simple in eastern Ordos Basin,but the petrological characteristics are very complex.He 8,Shan 2~1 and Shan 2~2 reservoirs are mainly composed of lithic sandstones,almost without quartz sandstones.However,Shan 2~3 reservoirs are mainly composed of quartz sandstone.It indicates that Shan 2~3 reservoirs have different material source or depositional environment compared with the other layers.However,rock debris and argillaceous cement in all the above four layers show obvious alteration characteristics,which indicates that sandstones have experienced metasomatism and alteration after deposition.The diagenesis types of the Upper Paleozoic gas reservoirs in the eastern Ordos Basin are complex and diverse,which have obvious effects on reservoir physical properties.Sandstones have experienced not only normal diagenesis,such as compaction,cementation,recrystallization and dissolution,but also obvious hydrothermal alteration.Content of flexible rock debris and clay matrix are high in most sandstone,and most of the intergranular pores were lost after compaction.Only a certain amount of intergranular pores are retained in the quartz sandstone in Shan 2~3 reservoirs.However,compaction didn't make the reservoir compact.The average residual porosity of the reservoir after compaction is 15.34%.Large number of late calcite cement and a variety of siliceous cement caused by cementing and hydrothermal alteration make reservoirs compact.The remaining porosity after cementation is only 3.3%.However,dissolution of feldspar,clay complex,carbonate cement,volcanic tuff and other soluble substances by organic acid and carbonic acid has improved the reservoir properties.Sandstones show obvious alteration characteristics,ie,feldspar and metamorphic rock debris occurred obvious kaolinization and carbonatation;biotite and volcanic debris were commonly illitized,and so on.The geochemical analysis of authigenic minerals in sandstone showed that illites and kaolinites in the reservoir sandstones are characterized by LREE enrichment,HREE loss,and rare earth element distribution curve is right-leaning.Eu and Ce have mild positive anomaly or mild negative abnormal,indicating that illite and kaolinite have typical characteristics of hot water deposits.The ?13CPDB of iron dolomite cements in sandstone is between-1.18 ‰ and-11.97 ‰,with an average value of-5.57%;?18OPDB is between-2.28 ‰ and-15.10 ‰,with an average value of-12.57%,indicating obviously geochemical characteristics of hot water genesis.Combined with petrological characteristics,diagenesis and pores characteristics,seven types of diagenetic facies were divided in the studied area: ?siliceous cement intergranular pores diagenetic facies(residual primary intergranular pores as main reservoir space),?rock debris dissolution pore diagenetic facies(rock debris dissolution pores as main reservoir space),? matrix dissolution pore diagenetic facies(matrix dissolution pores as main reservoir space),? calcite cement dissolution pore diagenetic facies(calcite cement dissolution pores as main reservoir space),? undissolved calcite cemented diagenetic facies(intergranular micropore as main reservoir space),? matrix filling strong compaction diagenetic facies(intergranular micropore as main reservoir space)and ? fractured diagenetic facies(fracture as main reservoir space).Studies showed that physical properties of sandstones were obvious controlled by diagenetic facies.The speed sensitivity,water sensitivity and acid sensitivity of reservoir sandstones were analyzed and evaluated from the angle of diagenetic facies,and the distribution of reservoir sensitivity were studied.Diagenetic faices ?show medium-weak speed sensitivity,medium to weak water sensitivity,no acid sensitivity.Diagenetic facies ? and ? are with moderate-strong speed sensitive,strong water sensitivity,weak acid sensitivity.Diagenetic facies ? and ?are with medium-weak sensitive,medium-weak water sensitivity,no acid sensitivity.Diagenetic facies ? is with strong speed sensitivity,strong water sensitivity and weak acid sensitivity.The main controlling factors of sensitivity are physical properties,pore structure,content of clay mineral in the reservoir sandstone.There is a significant negative correlation between the sensitivity index of the sandstones and their physical parameters.ie,the worse the physical properties and pore structures,the higher the sensitivity index.Meanwhile,the speed sensitivity is positive related with content of clay minerals,the water sensitivity is positive realted with content of mixed-layer minerals of illite and smectite,the acid sensitivity is positive related with content of chlorite and negative related with content of conbonate cement.According to the results of sensitivity evaluation,measures of reservoir prevention can be considered in the process of reservoir reconstruction and water injection.Based on sedimentary facies,effective sand thickness,diagenetic facies and reservoir properties,the reservoir evaluation standards were set and the main gas production layers were evaluated.The results show that Shan 2~3 reservoirs are the best reservoirs,He 8 taking the second place,shan 2~1 and 2~2 is the worst.In the studied area,the most favorable reservoirs are siliceous intergranular pores diagenetic facies developed in southwest of the Shan 2~3 reservoirs.Meanwhile,the areas of rock debris dissolution pore diagenetic facies,matrix dissolution pore diagenetic facies and calcite cement dissolution pore diagenetic facies are also favorable exploration area.