Geochemical characteristics of rare earth elements in petroleum and their responses to mantle-derived fluid: an example from the Dongying Depression, East China

This paper studies the characteristics of rare earth elements (REEs) in petroleum, in order to investigate the influence of mantle-derived fluid on petroleum. Oil and gas samples were collected from the petroliferous Dongying Depression, East China. Basic igneous rocks and mantle-derivcd fluid have been found along the fault belts that cut to the basement in the active areas of this depression. The mudstone-normalized REE patterns of petroleum from regions that have not been affected by mantle-derived fluids are evidently characterized by enrichments in a) Nd, Eu, Tb, and Ho, the middle REEs (MREEs) that contain an even number of f-electrons, and b) Er, Tm, Yb and Lu, the heavy REEs (HREEs). The introduction of mantle-derived fluids into the basin is supported by helium isotope composition of the natural gas from the region. MREE anormalies and HREE enrichment are diminished in these affected areas, as compared to oils from the unaffected areas. Mudstone-normalized Eu/Eu* and Lu/Er were selected to describe this diminishment. The correlation between observed changes in REE patterns in the oils and helium isotope ratios in co-produced natural gases suggests that the changes were likely caused by the mantle-derived fluid and that REEs in petroleum may potentially act as a tracer for understanding the influence of the mantle-derived fluid on oil, when combined with 3He/4He. Further more, the relationship between mudstone-normalized Eu/Eu* and Lu/Er values REEs in oil may help to identify whether the mantle-derived fluids influence oil oil or on hydrocarbon generation.