oa Authigenic Reversal of Stable Carbon Isotope Revealed by Gold-Tube Pyrolysis of an Australian Torbanite

Irreversible thermal cracking strongly affects the d¹³C distribution of gaseous hydrocarbons, with kinetic isotope effects (KIE) playing a central role. While thermogenic gases usually follow a normal d¹³C trend (d¹³C1 < d¹³C2 < d¹³C3), abnormal patterns like d¹³C1 < d¹³C3 < d¹³C2 or full reversals are common in high-maturity systems. These anomalies are often linked to secondary processes (e.g., mixing, desorption), but may also arise from intrinsic mechanisms like reverse KIE during ethane cracking or methane polymerization. Although d¹³C reversals are seen in lab pyrolysis of mature kerogen, replication is inconsistent, suggesting complex controls.To investigate, gold tube pyrolysis of torbanite (rich in aliphatic Botryococcus braunii biomass) was conducted. Results showed unusual d¹³C patterns: liquids more depleted than n-alkanes, partial gas reversals, and intramolecular d¹³C3 anomalies. These are likely due to the kerogen’s high aliphaticity and low sulfur content. Intramolecular d¹³C data for propane show greater depletion at central carbons (d¹³Cb) during the dry gas stage, likely from Rayleigh fractionation. In over-mature systems, low d¹³Cb values may indicate abiotic origins such as methane polymerization.