Diagenesis Of The Fifth Member Of Ordovician Majiagou Formation In The South Ordos Basin

The region of interest is located in the south of Yishan slope, day ring sag, west marginthrust belt,Nearly all of the weibei uplift and part of LiuPanShan basin and Fennutrient-laden graben. The fifth members of majiagou formation consist of mainly dolomite,intercalating anhydrock, anhydrite dolomite and limestone.The fifth members diagenesisincludes compaction, pressure solution, dolomitization, cementation, replacement, dissolution,karstification and dedolomitization. The dolomitization, dissolution, karstification areconstructive for reservoirs. The average of δ18O of Dolomite composition is-8.2‰which is lower than Middle ordovician Marine dolomite, it illustrates that the dolomite in the studyarea is affected by the fresh water leaching and buried. The average of13C is-0.69‰explains that That carbonate was not affected by the presence of organic sources of CO2. Theaverage of the degree of order for dolomite is0.69and the content of MgCO3is48.52mol%.Most of the dolomite has low degree of order and content of MgCO3due to therapid precipitation. Because of leaching of fresh water and burial diagenesis. Theconcentration of Fe and Mn is high, and concentration of Sr and Na is obviously low.Theaverage of Sr isotope ratios is0.710853, becanse of in burial diagenetic process the overlyingstrata of aluminum silicate minerals dissolved to provide Marine carbonate mineral strontiumradioactive source. In the study area dolomite formed in the penecontemporaneous stages orunder the condition of the oxidation of the early diagenetic stage, thus has darkcathodoluminescence.But the calite in the dolomite karst holes precipitated in weak reductionin early stage of diagenesis thus has the bright cathodoluminescence. Dolomitization occurredin the limited higher salinity of the epicontinental sea environment, formed by microbialdolomitization. Main reservoir space is moldic pores of gypsum and non-fabric selectivedissolution pores, and secondary reservoir space is dissolution intercrystalline pores. Theporosity ranges from0.08%to18.6%, averaging2.66%, and the permeability ranges from0.001×10-3m2～1343.4×10-3m2, averaging2.54×10-3m2. The reservoir rocks arecharacterized with low porosity and permeability, and high anisotropism.