The Analysis Of Dolomite Genesis Of Zhuozishan Formation, Oridovician, The North Of Tianhuan, Ordos

At present, the practice form Chinese marine carbonate oil and gas explorationshows that reef,grain bank, dolomite,and paleokarst are the most major types of oiland gas carbonate reservoir. Apart from paleokarst, the form of the remainings’carbonate reservoir has intimate relationship with dolomitisation. Therefore, theresearch on dolomite has become an important part of the carbonate reservoir studies,while dolomite diagenesis become the core of the research. The essence of dolomitediagenesis is the genesis of dolomite.So, to clarify the origin of dolomite isparticularly important for the understanding of dolomite and carbonate reservoirswhich associated with oil and gas exploration. Based on predecessors, usingobserbation of slice, SEM and X-ray, trace elements, REE; Carbon,Oxygen,Strontiumstable isotopes test techniques, by studying dolomite petrological characteristics andgeochemistry of Zhuozishan Formation,the may conclusions and new recognitions asfollows：(1). In combination with previous researches in Ordovician Ordos Basin, usingthe latest strata (core, logging, section measurements, etc.), combing the western ofOrdos Basin Ordovician and in the eastern part of the Ordovician stratum comparativeand divide, Establish the multiple stratigraphic classification scheme.(2). Systematic analysis of the northwestern margin of the basin ZhuozishanFormation sedimentary facies and distribution characteristics, and the mapping thesedimentary facies, proposed in the study area in the Middle-Late of Early Ordoviciansedimentary model diagram.(3). The conclusions from trace element analysis shows that the dissolveddolomite with high Fe, Mn and low Sr content that shows it has been leached byfreshwater for long-term; crystalline dolomite with high K, Na content and mediumFe,Mn which indicates that this rock types’ diagenetic fluid is salt water or dilute saltwater. The values of Elements Ca/Mg, Sr/Ba speculate that the residual dolarenite dolomite formed by the seepage reflux effect, crystalline dolomite formation in burialdolomitization.(4). The Rare earth element analysis showed that various types of dolomite andmicrite have the same similar distribution patterns, and most of them have differentdegrees of reduction in the total REE; the light rare earth elements are stronger thanthe heavy rare earth elements in the migration and dilution; various types of dolomiteshowed negative Ce anomalies, indicates dolomite formed in the weak reductionenvironment, various types of dolomite shows Eu anomaly realized not unified, theEu positive and negative anomaly is very evident. It proved that dolomite has formedin the environment with a great change in temperature.(5). The character of Carbon,Oxygen stable isotopes shows the sources of allkinds of dolimite dolomitisation fluid is similar.which maybe come from or relatedwith sea water. Crystalline dolomite, dissolved dolomite, residual dolarenite dolomitehave a special diagenesis which can represent a special Diagenetic environment.(6). The character of Strontium stable isotopes shows that cystalline dolomite hasa high87Sr/86Sr isotopic composition, which is a good evidence that the process ofdolimite dolomitisation has been affectd by fresh water, some samles may be affectedby aluminosilicate. Some dolomite dolomitisation fluid may be similar with sea water.The residual dolarenite and dissolved dolomite had experienced some degreesaffected by freash water.(7). Integrated characteristics of petrology and geochemistry of dolomite analysisshows that the study area dolomite early diagenesis impact by fresh water. Burialdolomitization has imparted on the western side of the study area while mixed waterdolomitization is the major diagenesis of the eastern.