Kerogen aromatization and late hydrocarbon generation: Evidence from position-specific isotope composition of propane

Natural gas accumulation involves complex multi-source mixing and post-genetic alterations that can lead to carbon isotopic reversals. Here, we report for the first time the co-occurrence of bulk and propane position-specific (PS) isotope reversals in conventional overmature gas reservoirs. We propose an aromatization-stabilization mechanism to explain these anomalous isotopic patterns. The Longdong area of the Ordos Basin provides a unique natural laboratory due to its well-defined thermal maturity gradient, the presence of multiple source rocks, and the coexistence of gases derived from different kerogen types. Integrated geochemical analyses of Cambrian-Carboniferous gases from six wells, reveal two key findings. First, the natural gases are in the overmature stage and exhibit both bulk carbon isotope reversals (δ¹³C₁ > δ¹³C₂ > δ¹³C₃ or δ¹³C₁ > δ¹³C₂ < δ¹³C₃), and Position-specific carbon isotope of propane anomalies (Δ_C-T = δ¹³C_center−δ¹³C_terminal<0‰; δ¹³C_center: isotopic composition of the central carbon in propane, δ¹³C_terminal: isotopic composition of the terminal carbon in propane). The propane in natural gas exhibits 25%–50% contribution from the isopropyl pathway. This is a formation mechanism theoretically restricted to low-maturity systems. This indicates preservation of early-formed branched precursors through cyclization/aromatization processes. We establish an “early-aromatization, delayed-cracking” model. This work highlights position-specific isotope analysis as a powerful tool for identifying post-genetic modifications in deep conventional and unconventional reservoirs.