Application Of Nitrogen And Carbon Isotope As Proxies For Oil Source: A Research Into Deep Crude Oil In The Tarim Basin, NW China

Nitrogen is one of the main component of life which widely distributed in sedimentary organic matter.However,because of the low the content of nitrogen in sedimentary organic matter,tests of nitrogeoiin isotope needs more sophisticated experimental methods.Thus,the research on nitrogen isotope get restricted for a long time.Recent studies have proven that the nitrogen distribution of crude oil sediment in different environment varies systematically.So it can be deduced that nitrogen isotope of crude oil may become a useful proxy in judging oil source correlation.However,there is still few of research on analyzing nitrogen isotopic composition of light oil and marine derived oil.In this paper,the carbon and nitrogen isotope of crude oil were tested by EA-IRMS basing on Dumas combustion method.Carbon and nitrogen isotope distribution characteristics of marine crude oil were determined.The result were also compared by common geochemical parameters of these crude oil.Conclusions from this experiments as following:(1)Carbon isotope composition of crude oil in Tarim Basin is mainly controlled by the sedimentary environment,influence of maturity of crude oil and the biodegradation factors are minimal.(2)The distribution characteristics of the nitrogen isotopes between the crude oil and the non-hydrocarbon and the asphaltene is similar to that of the carbon isotope distribution,which is that the nitrogen isotope of non-hydrocarbon is heavier than that of the asphaltene.(3)Nitrogen isotope of non-hydrocarbon and asphaltene of deep crude of Tarim basin are controlled by sedimentary environment and maturity of crude oil,but not much by biodegradation intensity.(4)The effect of deposition environment mainly behave as: the nitrogen isotope of resin peaks when the environment is suboxic,and it decreases gradually as the environment become anoxic.(5)The effect of maturity mainly is not obvious in low maturity stage,but nitrogen isotope of non-hydrocarbon will increase as its maturity become higher.(6)Although nitrogen isotope of crude oil affected both by sedimentary environment and maturity,the change of environment will have much greater influence than that of maturity.Thus,the nitrogen isotope of crude oil can still be an effective index to distinguish crude oil from different source rocks.