Molecular structure and hydrocarbon generation characteristics of kerogen in low-maturity shales: A case of the Paleocene Shahejie Formation in Shuguang area, Liaohe Depression, Bohai Bay Basin

Exploring the structure and hydrocarbon generation characteristics of kerogen at a molecular scale and revealing its reaction pathway for hydrocarbon generation and generation model holds great significance for the study on the hydrocarbon generation of kerogen in low-maturity shales and shale oil exploration. A combination of analytical techniques including ultimate analysis, solid-state ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), is applied to investigate the heteroatom morphology, carbon skeleton structure, and aliphatic and aromatic functional groups of kerogen in low-maturity shales from the 4th member of the Paleocene Shahejie Formation in the Shuguang area, Western Sag, Liaohe Depression. Accordingly, a two-dimensional average molecular structure model of kerogen is established with C₁₈₈H₃₁₀O₁₄N₄S, which boasts a high proportion (73.40 %) of aliphatics, a low proportion of aromatics, and long aliphatic chains (methylene chain carbon number: 5.04). The results of the reactive force field molecular dynamics (ReaxFF MD) simulation reveal that the mass fractions of gaseous hydrocarbons (C₁—C₄), light oil components (C₅—C₁₃), and heavy oil components (C₁₄—C₃₉) reach up to 41.32 % at 3500 K, 20.75 % at 3300 K, and 30.22 % at 2800 K, respectively, with a conversion rate of kerogen pyrolysis up to 61.67 %. The hydrocarbon generation process of kerogen in the shales progresses through multiple stages, including structural transformation, weak bond breaking, strong bond breaking, secondary cracking, and polycondensation reaction sequentially. During these stages, kerogen molecules undergo the bond breaking of heteroatoms and carbon-hydrogen atoms in both aliphatics and aromatics, as well as dehydrogenation-induced polycondensation reactions of aromatic rings.