Diagenesis Of Carbonate Rocks Of Yingshan Formation, Ordovician In North Slope Of Middle Tarim

Taking the carbonate rocks of Yingshan Formation, Ordovician in the north slope of Middle Tarim in Tarim Basin as the research object, based on core observation and thin section analysis, and combined with scanning electron microscope, energy spectrum analysis, cathode luminescence, electron probe, inclusion thermometry, fluorescence analysis, carbon & oxygen isotope test, trace elements tests and other means, studies are carried out as described in this paper on reservoir petrological characteristics, types of diagenesis and main diagenesis mechanisms.The main rock types of carbonate reservoirs in Yingshan Formation, Ordovician in the study area are limestone, dolomite and silicified rocks. The limestones include oolitic limestone, microcrystalline limestone, dolomitic(dolomite containing) limestone, and siliceous(silica containing) limestone; the dolomites include micritic-silty dolomite, fine-grained dolomite, medium-grained dolomite,(limy) lime-dolomite, siliceous dolomite; the silicified rocks include fine-grained silicified rocks and(limy) lime-silicified rocks.The main types of reservoir space in the study area include intergranular pores, intercrystalline dissolution pores, interparticle dissolution pores, caves and fractures. The range of porosity in the lower member of Yingshan Formation is 0.1%~2.40%, and the average porosity is 1.13%. The permeability is around 0.001~30.20×10-3?m2, and the average permeability is 1.24×10-3?m2 the porosity of the upper member of Yingshan Formation is 0.20%~20.50%, and the average porosity is 7.36%. The permeability is around 0.001×10-3?m2~73.40×10-3?m2, and the average permeability is 17.18×10-3?m2. Reservoirs are generally poorly developed, and the statistical results indicate that the thickness and amount of reservoirs in Yingshan Formation in the study area are mainly of type III reservoirs.Syngenetic karst, hypergene karst(interstratal karst) and burial dissolution( hydrothermal fluid silicatization) are developed in the carbonate reservoirs in Yingshan Formation in the study area. Various types of dissolution contributed actively to the formation of reservoirs, and hypergene dissolution has the greatest impact on the development of reservoirs in the area.Geochemical characteristic analysis results for different types of dolomites indicate that the silty and fine-grained dolomites in plaque silty and fine-grained dolomite and dolomite limestone are mainly products of evaporation pump dolomitization of the pene-sedimentary stage. The fine-grained and medium-grained dolomites distributed along the suture line are products of dolomitization under a shallow burial environment in the early diagenetic stage. The medium- coarse grained dolomite and saddle shaped dolomite were formed under the burial environment of the diagenetic stage.It indicates from main & microelement analysis and noble gas isotope analysis for silicified rocks of Yingshan Formation that the siliceous minerals in the hydrothermal fluid was not entirely derived from mantle magma, but the hydrothermal fluid silicatization which occurred in the process of upward migration of a hydrothermal fluid getting out from magmatic differentiation in Permian, when silicite layers in the bottom of Cambrian were leached, and a silicon rich hydrothermal fluid was brought into the strata of Yingshan Formation.Diagenesis of carbonate rocks of Yingshan Formation, Ordovician in the study area was complex and diversified, mainly including consolidated filling, compaction pressure solution, fracturing, dissolution, dolomitization, and hydrothermal silicification. The main destructive diagenesis was dissolution, and the main destructive diagenesis was cementation.