Nuclear magnetic resonance spectroscopy of crude oil as proxies for oil source and thermal maturity based on H-1 and C-13 spectra

Nuclear magnetic resonance (NMR) approach has unique advantages in the geochemical analysis of crude oil because it can provide information on chemical functional groups. Previous studies have focused on marine crude oils and investigated the interrelationship between the H-1 spectra and physical properties of the oil. However, the organic geochemical significance of the comprehensive NMR information and its effectiveness for studying non-marine crude oil are still not clear. Here we investigated crude oil from the Eocene Shahejie Formation in the lacustrine Dongpu Sag, Bohai Bay Basin, China, with NMR techniques. Our results show that the NMR spectra of these crude oils provide important information on organic matter functional groups, including aliphatic hydrocarbons, cycloalkanes, aromatic hydrocarbons, and other structural types. The NMR parameters of the studied crude oils reliably reflect the source rock sedimentary environment, and organic matter source and thermal maturity. Comparatively, H-1 and C-13 NMR spectra are useful for evaluating organic matter source and thermal maturity, respectively. It was found that the organic matter that produced crude oil in the northern part of the basin is of better quality, and deposited in a higher-salinity and more reducing sedimentary environment, than in the south. The sedimentary environment in the central part of the basin reflects transitional characteristics with large variation. This complements our understanding of traditional organic (biomarker) geochemistry. Our data show that NMR parameters robustly reflect the chemistry of crude oil, particularly the composition of functional groups, and its geochemical application to studies of crude oil can be further expanded, from marine to non-marine, from H-1 spectroscopy to C-13 spectroscopy, and from physical property to geochemical characterization.