Abundances of rare earth elements in crude oils and their partitions in water

Patterns of the entire range of REE in crude oils and coexisting water, collected from mud volcanoes in Xinjiang Province of China, are reported here for the first time. Crude oils show light REE enriched patterns with flat or depleted patterns in heavy REE, when normalized to chondrite. The REE concentrations in crude oils are larger than those in coexisting water by a factor of more than one hundred. Considering the hydrophobicity of oil and the high ionic characteristics of REE, it is strongly suggested that REE form complexes with ligands present in the crude oils. Based on the C-13 NMR spectroscopy, it is found that small amounts of phenol and carboxyl groups are contained in the crude oil samples, which could possibly provide complexing sites for REE. REE patterns of crude oils are similar to those of coexisting mud samples collected from the same mud volcanoes, which suggests that the REE in crude oils are derived from rocks and sediments where crude oils were generated.