The primary migration of petroleum has been recently described in detail for a thermally mature core of the Barnett Shale with almost nonvariant organofacies and maturity. Here, we use samples from the same well to provide new insights into the fractionation of pyrrolic nitrogen compounds during primary migration. Using gas chromatography-mass spectrometry (GC-MS), a decrease in concentration of carbazoles and benzocarbazoles was observed which correlated with migration distance. However, a preferential removal of individual isomers like benzocarbazole[a] relative to benzocarbazole[c] could not be detected. Enlarging the analytical window, we studied the effect of primary migration on high molecular weight nitrogen-containing compounds using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) combined with electrospray ionization (ESI) in the negative ion mode. Compounds with one or two nitrogen atoms are most abundant. Among the N-1 compounds, those with 12, 15, 18, 20, and 23 double bond equivalents (DBEs) were dominating classes representing carbazole-type compounds (among others) with one to three ortho-annelated benzene rings (12, 15, and 18) and four to five ortho- and peri-annelated benzene rings (20, 23). In comparison, the N-2 compound class mainly consists of compounds with 12, 15, 17, 20, and 23 DBEs representing (among other compounds) biindoles with zero to one ortho-annelated benzene ring (12, 15) and carbazolocarbazoles with zero to two ortho-annelated benzene rings (17, 20, and 23). A relative enrichment of biindole-type compounds relative to carbazolocarbazoles in migrated petroleum was shown. This might indicate that aromatic compounds with separated ring systems like biindoles are less strong retained in the source rock than fully annelated polycyclic aromatic compounds like carbazolocarbazoles. Within individual DBE classes of N-1 and N-2 compounds, very similar carbon number (CN) distributions were illustrated for nonmigrated and migrated fluids, i.e., a predominance of C0-5 alkylated homologues maximizing at C-2 or C-3 substitutes. Regardless of fractionation, the overall similar distributions in DBE and CN suggest that pyrrolic nitrogen-containing compounds have restricted precursors and common mechanisms of formation. Nonfluorescent chlorophyll catabolites (NCCs), the final breakdown products of chlorophyll, were tentatively suggested as possible precursors.