Nitrogen Isotope Composition Characteristics And Geological Significance Of Organic Matter In Typical Sedimentary Environment

The element of nitrogen is indispensable in living creature and widely distributed in nature.Due to the low contents of nitrogen in sedimentary organic matter,and the experimental methods of nitrogen isotope is strictly required,the experiment researches on nitrogen isotopes have been limited and fell behind for a long time.At present,the study on typical marine sedimentary environments is very limited,especially on the nitrogen isotopic composition of hydrocarbon source rocks and crude oils with low nitrogen content in marine sedimentary environments.In this paper,according to the Dumas combustion method,we adopted EA-IRMS technology and optimized experimental analysis method,tested the nitrogen isotope distribution of hydrocarbon source rock and crude oil samples from Chongqing region and Tarim Basin.Meanwhile,the paper combined conventional geochemical parameters for comparative analysis and discussed the distribution characteristics and geological significance of nitrogen isotope.We obtained the following knowledge:The nitrogen isotope distributions of three formation typical organic hydrocarbon source rocks in Chongqing region are significantly different.The Longmaxi Formation is in a weakly oxidized sedimentary environment with a heavier nitrogen isotope distribution.The nitrogen isotope distribution of Wufeng Formation is in the middle as a whole.The Niutitang Formation is in a sulfide reduction sedimentary environment and the distribution of nitrogen isotopes is lighter.The nitrogen isotope distribution range of the hydrocarbon source rocks of Yuertusi Formation in Tarim Basin is large,the bottom section in an oxygen-rich sedimentary environment,and the distribution of nitrogen isotopes is light.When transition to the upper-middle section,it is in an anoxic-hypoxic sedimentary environment and the nitrogen isotopes have positive excursion.The nitrogen isotope distribution of crude oil samples in different sedimentary environments of Tarim Basin is significantly different,that of terrestrial crude oil samples is heavier,and that of marine crude oil samples is lighter.The non-hydrocarbon nitrogen isotopes of marine crude oil are generally heavier than those of asphaltene.The nitrogen isotopic composition of different polar nitrogen-containing components in crude oil has obvious difference,the distribution of nitrogen isotopes in non-alkaline nitrogenous compounds is the heaviest,while that of alkaline nitrogen compounds is the lightest.The light or heavy nitrogen loss caused by hydrocarbon source rocks during the post-depositional processes,will not significantly change the nitrogen isotope distribution characteristics of the original sedimentary environment.There is no significant correlation between the maturity of crude oil and nitrogen isotope values.But,the sedimentary environment and the distribution of nitrogen isotopes have a significant correlation,the effects of the sedimentary environment on nitrogen isotopes are mainly about the influence of redox conditions in sedimentary environment on the nitrogen cycle pathway.During the anaerobic-anoxic nitrogen cycle stage,the nitrogen cycle is dominated by anaerobic nitrogen fixation,thus,the nitrogen isotope values are lighter.Under hypoxic-oxygenated condition,the nitrogen cycle included the processes of nitrification,denitrification,and ammoxidation,which led to a relatively strong isotopic fractionation,causing the?¹⁵N values exhibit a positive excursion.During the oxygen-enriched nitrogen cycle stage,the denitrification and anaerobic ammonia oxidation are suppressed,only occurs nitrification and the distribution of nitrogen isotopes is lighter.Therefore,nitrogen isotope is a reliable indicator of sedimentary paleo-environment.