不同沉积环境下原油氮同位素的地球化学特征

氮是原油中的重要组成元素,但是由于原油含氮量较低,碳氮比高,造成氮同位素样品制备困难。为揭示不同沉积环境原油的氮同位素分布特征及其主要影响因素,依据杜马斯燃烧法原理,利用EA-IRMS连用技术测定几个典型沉积盆地原油的氮、碳同位素组成。结果表明:不同沉积环境原油氮同位素组成具有明显的差异,海相沉积环境原油氮同位素明显轻于陆相沉积环境的原油;陆相沉积环境中原油氮同位素组成特征受沉积环境的盐度及氧化还原程度的影响,源于弱氧化-弱还原环境的原油氮同位素组成明显偏重,这是该沉积环境有利于反硝化作用的进行所致;原油的氮同位素组成可以有效用于原油的油源分析和对比,原油的氮同位素组成可能是油气地球化学中一项重要指标。

Geochemical characteristics of nitrogen isotope of crude oils in different depositional environments

Nitrogen is an important element in crude oil. Due to the low nitrogen content and high C/N ratio of crude oils, the isotope samples are difficult to be prepared, resulting in the few studies and applications using the isotope. In this work, a series of crude oil samples from several typical sedimentary basins in China were determined to reveal the distribution characteristics and the main influence factors of nitrogen isotope in different sedimentary environments. To measure the carbon and nitrogen isotope values of crude oil samples, the research uses an elemental analyzer (EA) coupled directly to an isotope ratio mass spectrometer (IRMS) , based on Dumas combustion method. The results show that there are obvious differences between nitrogen isotopes of crude oil in different sedimentary environments. The values of the marine sedimentary environment are significantly lighter than the continental sedimentary environment. In the continental sedimentary environment, the composition characteristics of nitrogen isotope in crude oil are related to the salinity and redox condition of the sedimentary environment. The higher nitrogen isotope values appeared in suboxic/dysoxic and slightly saltish water column conditions, which is beneficial to denitrification and thus resulting in heavy nitrogen isotope values in oil.