Position-specific carbon isotopes of propane in coal systems in China

Abstract

Evaluating the origin and fate of hydrocarbons holds significance in many fields, such as energy, geology, astrobiology, biosphere, and environment. However, challenges arise in many cases owing to the limitations of conventional methods. Intramolecular isotope analysis of propane is a new technique that offers the potential to provide insights into gas formation mechanisms; however, a comprehensive understanding of the method remains limited. Specifically, there is little knowledge about its ability to distinguish gases generated by source rocks deposited in similar sedimentary environments such as coal and coaly mudstone. Therefore, this study was undertaken to investigate position-specific carbon isotopes of 40 propane samples from 15 coal-type gas accumulations across four basins in China (Sichuan, Ordos, Qaidam, and Songliao); additionally, clumped isotopologues of methane samples were also analyzed. These coal systems cover a wide range of thermal maturity (from marginally mature to over-mature) at various age strata from the Permian, Triassic, and Jurassic periods, to the Cretaceous period. Our results revealed that propane generated from coal and coaly mudstone differed greatly in intramolecular isotope compositions despite having similar bulk δ13C3 values. Propane generated from coaly mudstone at a wide range of maturity had relatively more stable δ13C values in both the central (δ13Ccental) and terminal carbons (δ13Cterminal), near the theory generation line of chain-alkane cracking in the plot of δ13Cterminal vs. δ13Ccental. However, propane generated from coal had higher stable δ13Cterminal values and lower δ13Ccentral values that progressively increased with maturity. Under cooling-down conditions, propane from over-mature natural gases became extremely 13C-depleted in both the terminal and central positions, with Δ13Ccentral (δ13Ccentral minus δ13Cterminal) values being as low as –10‰, suggesting a partial origin of methane polymerization. Two propane samples from Cretaceous brown sandstone in the Songliao Basin exhibited an increase in both central and terminal carbons, suggesting that they may have surpassed chemical oxidation owing to high-valence Fe(Mn) oxides. These results indicate that intramolecular isotopes of propane can effectively differentiate dominant gas sources in a coal system with coal or coaly mudstone, as well as easily identify isotopic fractionation caused by post-generation processes, such as methane polymerization, mixing and chemical oxidation.