Influence Of Thermal Evolution Degree On Nitrogen Isotopes In Source Rocks

Nitrogen is another important element in the source rock and crude oil except carbon and hydrogen.The nitrogen content of the source rock is relatively low relative to carbon.The analysis of the nitrogen isotope requires high-quality experimental instruments and methods.The current research results show that the nitrogen isotope composition of the source rocks is significantly correlated with the sedimentary environment and organic matter sources,while the thermal evolution degree of the source rocks has little effect on the isotope distribution characteristics.In this paper,the high temperature and high pressure thermal simulation experiments of gold tubes were used to simulate the different thermal evolution of source rocks.The nitrogen and carbon isotopic compositions were determined by the combination of Thomas combustion method and EA-IRMS.The hydrocarbon generation characteristics of five samples in Dongpu Depression were obtained,and the distribution characteristics of nitrogen isotope of source rocks of different organic matter types were determined.The characteristics of nitrogen isotope distribution in actual underground source rocks and simulation experiments were compared and analyzed.The results show that the source rock of Hu 7-18 well has high organic matter abundance,large total organic carbon content and highest gaseous hydrocarbon yield,followed by Wen 13-54.The Bai 24 well has a low gaseous hydrocarbon yield and a low percentage of methane,mainly composed of CO₂.With the increase of thermal evolution,the carbon isotope in the source rocks is basically unchanged.The nitrogen isotope value of the Wen 13-54 source rock?type??increases in the thermal simulation temperature range of 300-400°C?Easy Ro<0.92%?,and the trend is more obvious after the simulated temperature is 400°C.With the increase of the simulated temperature,the nitrogen isotope of the type II₁ source rock represented by Bai24 and Hu 7-18 is more concentrated.However,with the increase of thermal simulation temperature,the thermal evolution degree of Wei 20?type II₂?becomes larger,while the nitrogen isotope change has no obvious law,which fluctuates between 2.4‰and 6.1‰.The Xin 8 sample?type III?increased with the degree of thermal evolution,and the nitrogen isotope composition characteristics remained basically unchanged.Compared with the nitrogen isotope composition of source rocks with different thermal evolution degrees in actual underground,it is found that the actual formation source rocks have an increasing degree of thermal evolution,and their nitrogen isotope distribution characteristics have no obvious regular changes.This is mainly because the cycle is short under laboratory conditions,the reaction rate is fast and sufficient,and no other geological effects are combined.