Study On Sedimentary Facies And Geochemistry Of Trace Elements Of Carboniferous-Permian Shale In The Eastern Ordos Basin

The paper does researches on coal-bearing shale of the Late Paleozoic Carboniferous Taiyuan Formation and the Permian Shanxi Formation,which are selected from the northern,central,southern part of Eastern Ordos Basin,namely,Ha'ersu area,Linxian area,Xixian area.The rock mineral composition of samples is obtained by X-Ray Diffraction(XRD)and scanning electron microscopy(SEM).The paper identifies sedimentary facies and establishes sequence framework based on the former production and the theory of sedimentology,geophysics,petrology,geochemistry and coal geology,outcrops,cores,logging and other geological data.At the same time,quantitative analysis of sedimentary facies is made by trace element geochemical analysis.By doing those,it is possible to clarify the rules of sedimentary facies distribution and variation in level and vertical and change of geochemical index within sequence framework.According to previous studies and knowledge form core,outcrop and logging in research areas,Taiyuan Formation develops marine facies and transitional facies,mainly including carbonate platform,barrier island,lagoon,tidal flat and delta.However,Shanxi Formation matches delta deposit,which can be subdivided into microfacies,such as flood basin,diversion channel,diversion bay,peat swamp,etc.Carbonate deposition mainly occurs in northern and southern Eastern Ordos Basin,instead of the northern like Ha'ersu area.The northern develops delta well,which is proven to be long-duration and wider.Sequence framework is established vertically:2 long-term base-level cycles(LSC1,LSC2),divided by interface between Taiyuan Formation and Shanxi Formation,and further identified 4 middle-term base-level cycles(MSC1 ~ MSC4).XRD and SEM are used to analyses the mineral composition.The XRD results show that coal-bearing shale is dominated by quartz and clay minerals,which is also consistent with the results from SEM.Kaolinite and I/S mixed-layer minerals play a vital role in clay minerals,indicating a warm and humid deposit environment.X-ray fluorescence(XRF)provides a good way to measure major and trace elements.The depositional environment of coal-bearding shale is rebuilt by trace elements,such as redox conditions,paleoclimate,paleosalinity and paleoproductivity.The results show that sedimentary environment is weak oxidation–reduction,warm and humid,weak seawater and higher paleoproductivity.The rare earth element(REE)of samples is of higher contents.The source,the deposition rate,palaeoclimate and paleosalinity may make a difference.The variation of paleosalinity and redox conditions is similar to that for middle-term base-level cycles within the sequence,while paleoproductivity and?REE are opposite.