Generation of abnormal distributions of inter- and intramolecular δ13C compositions in hydrocarbons from gold tube pyrolysis of an Australian torbanite

Abstract

Abnormal stable carbon isotopic (δ¹³C) compositions, deviating from the conventional order of δ¹³C₁ < δ¹³C₂ < δ¹³C₃, are frequently observed in natural gas reservoirs. For thermogenic gas, these anomalies, such as δ¹³C₁ < δ¹³C₃ < δ¹³C₂ or δ¹³C₁ > δ¹³C₂ > δ¹³C₃, have multiple formation mechanisms including gas mixing in conventional systems and desorption processes of gaseous hydrocarbons in unconventional shale gas systems due to their inconsistency with Rayleigh fractionation processes. Considering distinct reaction pathways (e.g., C₁ polymerized to C₂), these aberrant δ¹³C signatures are often construed as intrinsic hallmarks of extensively evolved natural gas. However, on the basis of gas generation simulation, not all findings exhibit abnormal δ¹³C values, hinting at multifaceted and intricate mechanisms governing the isotopic fractionation of alkane gas components. This study conducted gold tube pyrolysis of an Australian torbanite, revealing four distinct types of δ¹³C anomalies in hydrocarbon classes. Polycyclic aromatic hydrocarbons (PAHs) exhibited δ¹³C values more negative than co-occurring n-alkanes. δ¹³C₃ displayed a negative trend shift from EasyRo = 3.5 %, resulting in a partially δ¹³C-reversed gas (δ¹³C₁ < δ¹³C₃ < δ₁₃C₂) formed at EasyRo ≈ 4.1 %. Moreover, intramolecular δ¹³C₃ (both terminal and central carbons, termed δ¹³Ca and δ¹³Cb, respectively) reversed alongside the overall δ¹³C₃ trend. Additionally, the evolution of site preference in δ¹³C₃ (termed ‰SP = δ¹³Ca – δ¹³Cb) transitioned from progressively negative to positive. The results of this study demonstrate that the potential conversion between saturated hydrocarbons, aromatic hydrocarbons, and alkane gases is at least partially responsible for δ¹³C anomalies, but also that gaseous hydrocarbons formed from other fractions at high maturity cannot be ruled out, such as kerogen and pyrobitumen.