Nitrogen Evolution in Type I Kerogen and Derived Shale Oil during Thermal Maturation

Nitrogen heteroatoms in shale systems strongly influence hydrocarbon quality, catalyst performance, and the release of harmful byproducts, yet their transformation pathways during thermal maturation remain poorly constrained. In particular, investigations into the coevolutionary relationship between solid-phase (kerogen) and liquid-phase (shale oil) nitrogen species during thermal maturation of sedimentary organic matter remain scarce. In this study, we quantitatively evaluated the structure and hydrocarbon-generation behavior of lacustrine type I kerogen across the oil window (vitrinite reflectance, Ro = 0.5∼1.6%) to determine the compositional evolution of organic nitrogen functionalities within the kerogen matrix, the thermal transformation pathways of basic nitrogenous compounds in generated shale oils, and the coupled-evolutionary dynamics between kerogen-bound and free nitrogen species during thermal maturation. X-ray photoelectron spectroscopy N 1s spectra identified five organic nitrogen moieties in kerogen, while high-resolution mass spectrometry determined that basic N1 class compounds were the predominant heteroatomic species in shale oils. During prepeak oil generation, rising thermal maturity released pyrrolic nitrogen and nitro groups while forming protonated pyridinic nitrogen associated with hydroxyl/carboxyl groups. During postpeak oil generation, the breakdown of these protonated complexes released pyridinic nitrogen and hydroxyl/carboxyl groups, with liberated oxygen-containing groups facilitating the formation of nitro groups. Graphitic nitrogen emerged exclusively above 1.4% Easy%Ro. Quinoline derivatives in shale oils were primarily formed at the onset of the oil window and progressively decreased with increasing maturity. Pyridine derivatives in shale oil formed continuously up to 1.4% Easy%Ro through the release of pyridinic nitrogen from kerogen and partial cracking of quinoline derivatives. Subsequent thermal alteration generated low-molecular-weight pyridines and significantly increased the relative abundance of nitrogenous polycyclic aromatic hydrocarbons. These results establish a coevolution framework linking kerogen-bound nitrogen moieties to nitrogenous compounds in shale oil, offering mechanistic insights into the staged transformation and release of nitrogen species. This framework provides predictive value for shale oil quality assessment, refining challenges, and mitigation of nitrogen-related emissions during exploitation of lacustrine Type I kerogen.