Position-specific carbon and hydrogen isotopic compositions of propane from natural gases with quantitative NMR

Abstract

Position-specific isotope compositions of light hydrocarbons are expected to provide valuable information on their formation and migration-degradation processes. Here we present a high-accuracy and high-precision (≤±10 and ≤±1‰ for 2H and 13C isotope compositions, respectively) method to determine position-specific hydrogen and carbon isotope compositions of propane from natural gases with quantitative NMR. Customized, light-weight high-pressure sapphire NMR cells were developed for liquefied propane samples. Precision and accuracy of our technique were demonstrated using 13C-labeled compounds, neat samples of C3-C5 with natural isotope abundances, and inter-laboratory comparison of a C7 sample. To determine position-specific isotope compositions of propane from natural gas samples, a method was developed to collect and purify large amount (~6.8 mmol) of propane, using a variable-temperature cold trap. A test with a synthetic sample of natural gas mixture indicates that little isotope fractionation occurred during the propane separation and purification from natural gas mixtures. For the first time, high-precision and high accuracy data are reported of position-specific carbon and hydrogen isotope compositions of propane from different sources, including conventional and unconventional petroleum reservoirs. Preliminary results show that position-specific isotope fractionations between the center and terminal sites of propane vary widely for the different sources. Position-specific isotope compositions of propane have the potential to improve our understanding on the origins, migration and degradation processes of natural gas and hydrocarbons in general.