Carbon And Oxygen Isotopic Characteristics Of The Permian Dolomitic Rock In The Junggar Basin

The original carbon and oxygen isotopic information of the lacustrine authigenic carbonate rocks can effectively reflect their paleoenvironment and genesis,which is one of the basic methods in the study carbonate rocks.The Permian lacustrine dolomitic reservoirs were developed in the Junggar basin.Previous researches pointed out there exists distinctions in the carbon and oxygen isotopes of the dolomitic rocks between the Fengcheng Formation in the northwestern margin and the Lucaogou Formation in the southern margin,but these studies are scattered mostly and lack of systematic comparisons.The reason for the differences of carbon and oxygen isotopes in the dolomitic rocks between Fengcheng Formation and the Lucaogou formation,which are both lacustrine sediments,has not been discussed yet.This article focus on the Permian dolomitic rocks in the Junggar basin,including the Fengcheng formation in the northwest margin and the Lucaogou formation in the south margin.Based on the analysis of petrography and isotopic geochemistry,the types of dolomitic rocks,sedimentary facies,and characteristics of carbon and oxygen isotopes are finally determined.The formation environment of the dolomitic rocks is revealed,the mechanism of abnormal carbon isotopes in the Lucaogou Formation is discussed,and the effects of different sedimentary environments on the carbon and oxygen isotopes are confirmed.The dolomitic rocks in the Fengcheng formation mainly include dolomite,dolomitic-silicolite or siliceous-dolomitic rocks,dolomitic sandstone and dolomitic siltstone,and a small amount of thin dolomite and mudstone are interbedded.The dolomitic rocks formed in the alkali rich saline lake,where had intermittent hydro--thermal activities.While the dolomites of Lucaogou Formation are mainly composed of grain dolomite,biogenic dolomite and muddy microcrystalline dolomite,which are formed in delta-front facies to deep lacustrine facies.The?¹³C_PDB value of the dolomitic rocks in Fengcheng Formation ranges between-2.81‰and 3.64‰,and the?¹⁸O_PDBvalue is in the range of-13.14‰?0.17‰.All of the?¹³C_PDBvalues overlap with the range of evaporative carbonate,and most of the oxygen isotopes are not affected by diagenesis,and the negative oxygen isotopes may be affected by the hydrothermal fluid.Oxygen and carbon stable isotopic covariation showes that the sedimentary environment was relatively closed,implying that evaporative effects are the primary driver of these covariant trends.Salinity index Z value of dolomitic rocks ranges from 115 to134,indicating that these dolomitic rocks were formed in the alkaline lake at the early stage of salinization.The paleo--temperature determined by the isotope thermometer is approximately 20?up to86?,and there are many hydrothermal vents in the depositional period of dolomitic rocks.The dolomitic rocks of Fengcheng Formation should be mainly deposited in alkaline lake mixing hydrothermal fluid,with frequent hydrothermal activity at the bottom of the lake.The?¹³C_PDB value of the dolomite of Hongyanchi section in Lucaogou Forma--tion ranges between 3.22‰?19.56‰,and the?¹⁸O_PDB value is in the range of-17.42‰?-2.31‰.The?¹³C_PDB value of Shirengou section ranges from 5.10‰to12.87‰,and the?¹⁸O_PDB value is in the range of-13.92‰?-5.15‰.They all show positive carbon isotopes,with a maximum value of 19.56%.The covariation between?¹⁸O and?¹³C values can be explained by intense evaporation among two isotopes,and salinity index Z value ranges from 128 to 166,indicating that the dolomites in the Lucaogou Formation were formed in a more closed salty lacustrine basin and that oxygen isotopes have been affected by meteoric waters.Along with the increase of water depth,the values of carbon isotope added,and there are abnormal positive carbon isotope characteristics in the middle and upper parts.As the result of biological effects,the carbon isotope value of the Hongyanchi section is higher than that of the Shirengou section.The strong positive carbon isotopic excursion in the Hongyanchi section indicates that the precipitation of the dolomite may have close relationship with the metabolic activity of the anaerobic microbe in methanogenesis zone.The abnormal enrichment of carbon isotopes in dolomite may be the result of isotope fractionation caused by metabolic activity of methanogenesis archaea.In the formation of dolomite,a large number of microbes are involved,and the possibility of algal genesis is not excluded.The?¹³C value of the Lucaogou Formation is obviously higher than that of the Fengcheng Formation,and the value of?¹⁸O shows more intense negative isotopic excursion,which indicate they form at different environments.The Lucaogou Form--ation is formed in a saline lake with more anoxic and higher surface water productivity,which is conducive to the enrichment of carbon isotopes in water bodies.The dolomitic rocks of Fengcheng Formation are deposited in the alkali lake,and formation process is influenced by hydrothermal fluid.The hydrothermal action leads to the mixture of poor ¹³C fluid and lake water,and causes the lake water temperature rising sharply and reduces the carbon isotope value of the Fengcheng Formation.The dolomites in the Lucaogou Formation are not affected by the hydrothermal fluid,therefore they have higher carbon isotope values.