Nitrogen isotope systematics of petroleum fractions of differing polarity – Neutral versus basic compounds

Organic nitrogen in living organisms occurs overwhelmingly in the amide functionality of proteins and peptides. In contrast, nitrogen in petroleum is dominated by pyrrolic and pyridinic functional groups in aromatic heterocycles. The origin of these compounds is unclear. The nitrogen isotope composition of the neutral low and high polarity fractions, as well as the basic fractions, of petroleum isolated from reservoir core samples collected from the Liaohe Basin in NE China was investigated using liquid chromatography and bulk isotopic ratio mass spectrometry. The basic fractions were depleted significantly in 15N compared to the neutral fractions by more than 5‰. These results suggest that the source of pyridine-type N differs significantly from that of pyrrole-type N-compounds in petroleum and/or that the mechanism of formation of these two types of compound is quite different.