Formation of 1-Methylphenanthrene from Diterpenoid-Derived Precursors during Early Thermal Alteration of Conifer-Derived Organic Matter

Abstract Methylphenanthrenes are widely used as molecular indicators of the thermal maturity of sedimentary organic matter; however, the reaction pathways controlling their isomer distributions at immature stages remain poorly constrained. In this study, we investigate the formation pathways of methylphenanthrenes from diterpenoid-derived precursors using heating experiments of abietic acid as a model compound, together with heating experiments and compositional analyses of immature coals. The results demonstrate that 1-methylphenanthrene is preferentially formed during the early stages of thermal alteration via degradation of abietane-type precursors, likely involving dealkylation and aromatization processes. In contrast, progressive thermal maturation promotes isomerization toward thermodynamically more stable β-substituted methylphenanthrenes, resulting in a shift toward equilibrium-controlled distributions. Analyses of immature coals show a strong positive correlation between retene, a key intermediate derived from abietic acid and related diterpenoids, and the relative abundance of 1-methylphenanthrene, supporting a precursor-driven reaction pathway. These findings indicate that methylphenanthrene compositions at low maturity primarily reflect kinetically controlled transformation processes rather than thermodynamic equilibration. The predominance of 1-methylphenanthrene thus provides insight into the reaction mechanisms governing the early-stage formation of alkylated aromatic hydrocarbons.