Evolution of Heteroatomic Compounds and Hydrocarbons from Kerogen Pyrolysis: Potential Implications for Shale Oil Mobility

Abstract

Liquid products released from kerogen cracking usually exhibit significant compositional differences at various stages, which may greatly impact the viscosity, density, and fluidity of shale oil. Previous studies have mostly focused on hydrocarbons generated during kerogen pyrolysis, but the formation and transformation of heteroatomic compounds (such as oxygen-containing and nitrogen-containing polar components) during thermal maturation, as well as their quantitative relationship with generated hydrocarbons, are not yet fully elucidated. This has limited the in-depth understanding of changes in composition and physical properties of shale oil during hydrocarbon generation. Heteroatomic compound concentration and composition can significantly affect shale oil mobility. It is therefore important to investigate their relationship with hydrocarbons formed at each stage of kerogen pyrolysis. The shale of the third member of the Shahejie Formation (E2s3) in the Bohai Bay Basin has high shale oil potential and contains predominantly type I kerogen. In this study, a low-maturity kerogen sample from the E2s3 of the Bozhong Sag, the Bohai Bay Basin, was used for gold tube closed-system thermal simulations (340–445 °C), then gas chromatograph (GC) and ESI FT-ICR MS techniques were used to study the pyrolysis products at various temperature points. The evolutionary pathways of polar heteroatomic compounds (O1–O4, N1 species) and hydrocarbons (mainly n-alkanes) were explored. The results show that with increasing thermal maturity, the relative concentrations of oxygen-containing CHO compounds generally decreased in the order of O4 > O3 > O2 > O1, while the relative abundance of N1 species increased. With increasing maturity (Easy%Ro = 0.589–1.336), continuous decarboxylation and degradation of fatty acids led to decreased relative concentrations and a significantly narrowed carbon number distribution, with a synchronous increase in n-alkane yield. At the peak hydrocarbon generation stage (Easy%Ro = 0.892–1.169), fatty acids degraded fast, while the corresponding n-alkane yield gradually reached the maximum. Furthermore, within the Easy%Ro range of 1.169–1.336, high-carbon-number (C14+) n-alkanes slightly decreased due to secondary cracking, while low-carbon-number (C13–) n-alkanes kept increasing. This indicates that the generated shale oil became significantly lighter within the Easy%Ro range of 1.169–1.336, which may potentially contribute to improved shale oil mobility.