Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases
Nivedita Thiagarajan, Hao Xie, Camilo Ponton, Nami Kitchen, Brian K. Peterson, Michael Lawson, Michael J Formolo, Yitian Xiao, John M. Eiler
Abstract
Significance The mechanisms of natural gas formation are important to the carbon cycle and predicting where economical amounts of natural gas form. However, the formation mechanisms of natural gas are not clear, with hypotheses including both irreversible chemical processes such as thermal cracking of long-chain hydrocarbons and thermodynamic equilibrium processes such as transition metal catalysis. Here we show that hydrocarbon species in natural gases are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the H and C isotopic distributions within and among coexisting light n -alkanes approach thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage. Our finding has significant implications for natural gas exploration.