Distribution Characteristics Of Nitrogen Isotope And Paleoenvironment Of Shale From Wufeng Longmaxi Formation In Middle And Upper Yangtze Region

The Late Ordovician-Early Silurian is an important period in the history of the Earth.During this period,the Hernantian glaciation and two mass extinctions occurred,accompanied by changes in paleoproductivity,sea level and redox environment.The Paleoenvironment is particularly complex.This study focused on the distribution characteristics of nitrogen isotopes of source rocks,which is of great significance to the analysis of the evolution process of marine environment and climate at the Ordovician Silurian transition.This paper mainly tests the nitrogen isotope values and related geochemical indexes of the source rocks from the Wufeng and Longmaxi formations in the Xiuye-1 and Gongdi-1 wells in the middle and upper Yangtze area,and analyzes in detail the paleoenvironmental characteristics during the Wufeng-Longmaxi periods.This paper discusses the correlation between nitrogen isotope and sedimentary environment,and further studies the effects of geological events on the nitrogen isotope valuesThe nitrogen isotope compositions of the source rocks from the Wufeng and Longmaxi formations are different.The?¹⁵N values of the Wufeng Formation are generally low.From the bottom to the top,there is a first increasing trend of the?¹⁵N values,followed by a decreasing and then a increasing trend.On the contrary,the?¹⁵N values of the Longmaxi Formation are generally heavy and fluctuate frequently.The source rocks at the bottom of the Wufeng Formation were probably deposited in oxygen-containing environment,and there is an alternating change of oxygen-containing and anoxic upward;However,the sedimentary environment of Longmaxi formation source rock is relatively single,and the whole environment is weak oxygen environment.Wufeng Formation and Longmaxi formation have similar continental margin sedimentary environment,terrigenous detrital source and high marine primary productivity.The redox environment of sedimentary water has a profound influence on the nitrogen cycle path and nitrogen isotope value of source rocks.In the oxygen rich environment,biological nitrogen fixation and nitrification are the main processes,and the nitrogen isotope fractionation is small and the nitrogen isotope value is light;In the stage of poor oxygen,the nitrogen cycle mainly includes nitrification,denitrification and ANAMMOX,with large nitrogen isotope fractionation and heavy nitrogen isotope value;In anoxic environment,biological nitrogen fixation was dominant,and the distribution of nitrogen isotope value was light.The hernantian glacial event in the late Ordovician has a great influence on the nitrogen isotope of source rocks.During the glacial period,along with the tectonic uplift and large-scale regression,the sedimentary environment changed from the oxygen bearing environment of the open basin to the oxygen poor environment of the limited basin,with large nitrogen isotope fractionation and positive isotope value;After the end of the glacial period,the temperature warmed up,the sea level rose,the sedimentary environment was stratified and anoxic,the oxygen content of the whole water body increased,the nitrogen cycle was dominated by nitrogen fixation,and the nitrogen isotope value decreased.