Hydrocarbon and Non-Hydrocarbon Generation Potential of Deltaic and Lagoonal Coals from Bohai Bay Basin

Coal-derived gases are important natural gas resources, but the evolution of hydrocarbon products during coal thermal transformation remains incompletely understood. Here, two low-maturity coals from deltaic and lagoonal settings (Ro = 0.57% and 0.58%, respectively) were investigated using closed-system gold-tube pyrolysis at 300–600 °C to characterize their hydrocarbon-generation sequences, gas compositions, and carbon isotope evolution. Both coals showed a similar hydrocarbon generation sequence, comprising an oil-dominated interval at 300–350 °C, an intermediate interval characterized by light hydrocarbons and heavy hydrocarbon gases at 350–475 °C, and a methane-dominated interval above 475 °C. Maximum oil yields were 81.94 and 85.08 mg/g TOC, and maximum light hydrocarbon yields were 9.64 and 9.97 mg/g TOC for C-1 and C-2, respectively. At 600 °C, hydrocarbon gas (C1–5) yields were 163.07 and 189.69 mL/g TOC, respectively. CO2 was the dominant non-hydrocarbon gas, with yields of 24.93 and 32.12 mL/g TOC at 600 °C. The δ13C values of methane, ethane, and propane became lighter up to approximately 425 °C and then progressively heavier, while maintaining the normal isotope sequence of δ13C1 < δ13C2 < δ13C3. These results delineate a common hydrocarbon generation trajectory for the two coals, with differences between the samples expressed primarily in gas generation intensity rather than in the sequence of product evolution.