Determination Methods and Distribution Characteristics of Nitrogen Isotopes in Different Nitrogenous Components of Crude Oil

The distribution of nitrogen isotopes has important significance in paleoclimate, paleo-sedimentary and evolutionary characteristics. Nitrogen is one of the important heteroatomic elements in crude oil. The nitrogen content of crude oil is generally 0.1% to 2%, mainly in the non-hydrocarbon and asphaltene fractions, and can be divided into basic and non-basic nitrogenous fractions according to polarity. Due to the high carbon/nitrogen ratio of crude oils, accurate nitrogen isotope analysis results cannot be obtained from direct testing for most crude oils. As a result, the research on nitrogen isotopes in crude oil has been developed slowly, and the available data are few, while not being well applied in practice.

To determine the nitrogen isotopic distribution characteristics of different nitrogen-containing components in crude oil, and to address the problem of low nitrogen content in crude oil, which makes the direct determination of nitrogen isotopes less stable and reproducible.

A two-stage separation pre-treatment method was utilized to successively separate the non-hydrocarbon and asphaltene fractions of crude oil, and the basic and neutral nitrogen fractions were separated from the non-hydrocarbon fraction. The characteristics of nitrogen isotope distribution in different nitrogenous components of crude oil were determined by the Dumas combustion method. Multiple stable nitrogen isotope standards (USGS61, USGS62, USGS63) were selected for quality control of nitrogen isotope results.

The nitrogen isotope data obtained by this method for each component of crude oil were of good quality, with the uncertainties less than ±0.4‰. By separating and measuring crude oil samples from marine and continental sedimentary environments, the results showed that the nitrogen isotope of crude oil samples in continental sedimentary environment was heavier than that in a marine sedimentary environment. The distribution of nitrogen isotopes of asphaltene fractions was heavier than that of non-hydrocarbon fractions. The distribution characteristics of the nitrogen isotopes of the different polar nitrogen-containing compounds also differed significantly.

The technical dilemma that crude oil samples cannot accurately produce stable nitrogen isotopes due to the disparity in carbon to nitrogen ratios is solved and technical support for the practical application of a crude oil nitrogen isotope index is also obtained.