Note Nitrogen isotope systematics of petroleum fractions of differing polarity – Neutral versus basic compounds T.B.P. Oldenburg a, * , S.R. Larter a,b , H. Huang a a PRG, Petroleum Reservoir Group, Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary AB, Canada T2N 1N4 b NRG, School of Civil Engineering and Geosciences, University of Newcastle, Newcastle NE1 7RU, United Kingdom Received 9 November 2006; received in revised form 29 May 2007; accepted 30 May 2007 Available online 10 June 2007 Abstract Organic nitrogen in living organisms occurs overwhelmingly in the amide functionality of proteins and peptides. In con- trast, 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 15 N compared to the neutral fractions by more than 5&. These results suggest that the source of pyri- dine-type N differs significantly from that of pyrrole-type N-compounds in petroleum and/or that the mechanism of for- mation of these two types of compound is quite different. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Nitrogen isotope studies are important for eval- uating the source and fate of nitrogen in the marine and terrestrial environment and for understanding microbial processes behind the deg- radation of petroleum. Sweeney and Kaplan (1980) used d 15 N values to distinguish between terrestrial (+2&) and marine (+10&) organic matter (OM) in near surface sediments of the Santa Barbara Basin, whereas the plankton and the dissolved ammonium in the pore water were enriched in 15 N (d 15 N from 6.5& to 12.5& and 8.3& to 12.5&, respectively). However, they didn’t take into account that the nitrogen isotope composition of OM can change rapidly due to biogeochemical processes, as reported in more recent studies (e.g. Montoya et al., 1991). During diagenesis biological and thermal degradation of OM results in the for- mation of ammonium (NH 4 ) which can be incor- porated for potassium in clay minerals. This nitrogen in the crystal lattice of clay minerals and micas is mainly derived from decomposing OM and thus has a very similar isotopic composition to the OM (Hoefs, 2004). In general, the d 15 N of OM depends on the source (marine OM isotopi- cally heavier in N than terrestrial OM), the dura- tion in the water column (slow sinking particulate 0146-6380/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2007.05.016 * Corresponding author. Tel.: +1 403 220 3260; fax: +1 403 284 0074. E-mail address: toldenbu@ucalgary.ca (T.B.P. Oldenburg). Available online at www.sciencedirect.com Organic Geochemistry 38 (2007) 1789–1794 www.elsevier.com/locate/orggeochem Organic Geochemistry