Diagenetic Facies And Pore Evolution Of Chang 7 Tight Reservoir In Heshui Area,Ordos Basin

Since entering the new century,the focus of oil and gas exploration in China has shifted from conventional reservoirs to unconventional reservoirs,which greatly increases the difficulty of exploration.Exploration research extends from sedimentary facies to diagenetic facies.Fundamentally,the degree of oil and gas enrichment in reservoirs is mainly controlled by"facies",and diagenetic facies is the core factor determining reservoir performance and oil and gas enrichment.Based on a large number of microscopic identifications,analyses and tests,this paper deeply studied the petrological characteristics,diagenesis types and identification characteristics of the tight reservoir of the Chang 7 oil layer group in the Heshui Area,Ordos Basin,divided different diagenetic facies based on diagenesis,and calculated the pore evolution of different diagenetic facies.The stratum thickness of the Chang 7 oil layer group in the Heshui Area is 90-120m,The average thickness of each sublayer is 34-36m;Lithic feldspathic sandstone and feldspathic lithic sandstone are the main lithology,the particle size is mainly fine sand（0.125～0.25mm）,the sorting is medium poor,sub angular,and the reservoir is dense with low porosity and permeability.According to the porosity and permeability level,it belongs to ultra-low porosity and ultra-low permeability reservoir.The reservoir space type is mainly feldspar solution pore and other secondary pores;The types of diagenesis are complex,mainly being compaction,cementation and dissolution.The first two are the main destructive diagenesis,and the latter is the main constructive diagenesis;According to the division table of diagenetic stages of clastic rocks,the diagenetic stage in this area belongs to middle diagenetic A1stage;The compaction porosity reduction rate of the Chang 7₁ is 45%,the cementation porosity reduction rate is 48%,and the dissolution porosity increase rate is 23%;The compaction porosity reduction rate of the Chang 7₂ is 52%,the cementation porosity reduction rate is 43%,and the dissolution porosity increase rate is 24%;Combined with the analysis of diagenesis and its strength,diagenetic minerals and sedimentary conditions,five diagenetic facies were divided:unstable component dissolution facies,clay mineral cementation dissolution facies,clay mineral cementation facies,carbonate rock cementation facies and compacted dense facies;the pore loss of different diagenetic facies in different diagenesis was calculated.Six logging curves such as AC,CNL,den,GR,RT and SP were selected to statistically summarize the logging identification characteristics of different diagenetic facies,established the identification chart and spider web diagram,and used their characteristics to draw the single well diagenetic facies histogram and the diagenetic facies plan of small layers of the Chang7₁ and the Chang 7₂.The unstable component dissolution facies are where the dominant reservoir are located.For example,the West and south of the Xiaozui town in the Chang7₁,the east of the Jincun town,the north of the Panke town and the south of the Heshui county,and the north of the dianzi town.