The Formation Mechanism Of Carbonate Cement In The Sandstone Of Yanchang6₃Formation In Huaqing Area And Its Effects On Reservoir Quality

It is mainly talk about the formation mechanism of carbonate cements insouthwest of Yanchang6₃formation of ordos basin and their effects on reservoirquality in this paper.By use of thin-section analysis, cathodeluminecenme analysis, and isotopeanalysis, chemical analysis and so on, we research mineralogy, petrology andgeochemistry of carbonate cements of the sandstone reservoir in Yanchang6₃formation, the southwest of ordos basin, and discuss their formation mechanism andeffects on reservoir quality.The primarily petrology types of the sandstone in Yangchang6₃formation ofHuaqing area include feldspar sandstone, lithic feldspar sandstone. Fine grain size,riching feldspar, high plastic debris and matrix content are the characteristics ofYanchang6₃formation reservoir sandstone. The composition maturity of sandstonehas obvious zonztion. The north of area is low（the average value is less than0.8）,while the south of area is high（the average value is between0.8-1）. Reservoirsandstone has poor structural maturity and the average content of matrix is7.4%，alsohas much authigenic mineral, and its average content achieves7.91%. Includingauthigenic carbonate mineral, authigenic quartzite and clay mineral.Carbonates are the most abundant cements in sandstone of Yanchang6₃formation, which mainly consist of calcite, dolomite and siderite. The north of areamainly calcite cements, which is cemented in micritic-micro-grainular and poikilitic.Dolomite is the main cements in the south of area, they are hosted in intergranulardissolution porosity.The value of δ¹³C of calcite cement is-0.39‰（-2.19‰and1.08‰）, near0‰，So, its display of inorganic carbon source carbon isotopic characteristics; The value ofδ¹⁸O of calcite cement is also higher than of dolomite, compared with dolomitecement, the forming tempture of calcite cement is lower and the forming time isearlier. Dolomite cement is shaped in granularity, replaces clastic, fills second pores,and widly occupies the solutional pore of feldspar and later than the quartzovergrowth. On the basis of geologic setting, province and buried history, we study in thesolutional medium in deposition process, the material resource and the formationmechanism, which is of carbonate cement. The calcite cement in study area is formingin the depositional stage and the early diagenetic A stage. The forming temperature ofcalcite is lower. The negative cement porosity of calcite cement is higher, In additionto the matrix of syndepoitional lake facies, the early calcite cements are in connectionwith the hydration of aluminum silicate minerals, metamorphic rocks and magmaticrocks. The formation of ferrocalcite cements are closely related to the aluminumsilicate minerals and the transformation of clay mineral and occurred during the earlydiagenetic B stage. The （iron）dolomite cements distributed in the southern of area isforming in the middle diagenetic A stage. The southern area has higher carbonatedetritus（in dolomite detritus mainly）, the material sources of the dolomite cementshould be from the non-coetaneous dolomite dissolution.The value of porosity and permeability in reservoir sandstone of Yanchang6₃formation are very small(average value9.29%and0.19×10^-3μm²). The study areabelongs to compact sandstone reservoir. Compaction processes and cementation, asthe two most important diagenesis effect factors on reduction the porosity, are judgedhere, and showed that compaction processes is more important for the reservoirsandstone on reduction porosity than cementation. The reservoir porosity andpermeability reduction because of the carbonate cements filling pore, further reducingthe storage space. The most cementation of the carbonate are disadvantageous for thereservoir properties, its a destractive diagenesis.