Geochemical Characteristics of Rare Earth Elements in Petroleum and Their Responses to Mantle-Derived Fluid: An Example from the Dongying Depression, East China

Abstract

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-derived 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-normalizd 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 coproduced 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 mudstonenormalized Eu/Eu* and Lu/Er values REEs in oil may help to identify whether the mantle-derived fluids influence on oil or on hydrocarbon generation.

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

“…The concentrations of REE collected from site 3 are the highest among all collected crude oils, while site 1 has the lowest REE concentrations. Although the concentrations of REE have wide variations among each site, such a variation can be caused by different abundances of functional groups in crude oils (Zhang et al, 2009). All the crude oil samples showed negative Eu anomalies, whereas no distinct Ce anomalies were found.…”

“…Total REE abundances reported by Akinlua et al (2008) and Takeda and Arikawa (2005) are less than 3 ng/g. Recently, Zhang et al (2009) reported REE patterns for crude oils, which show a zigzag structure with enrichments of Nd, Eu, Tb, Ho, Tm and Lu in all their crude oil samples except one. They suggested that the enrichment of MREE with an even number of f electrons and the enrichment of HREE in crude oils result from higher stability constants of organic complexes with these elements.…”

“…Our report of complete REE patterns of crude oils is thus important, since resolution of the fine structure of REE patterns allows us to estimate the REE species in the samples. Total REE abundances of the crude oil samples showing zigzag patterns in Zhang et al (2009) (Table 1).…”

“…These properties enables REE pattern to be a useful effective geochemical tracer (Henderson, 1984). Recently, Zhang et al (2009) suggested that REE patterns of crude oils could provide genetic information on the oils, while Akinlua et al (2008) noted that REE contents of oils are useful in the classification of oils. However, the existing quantum of REE data of crude oil samples is still insufficient.…”

Origins and Fates of CO2 in the Dongying Depression of the Bohai Bay Basin

“…The source rocks are dark mudstones and shales in the upper part of the 4th member of the Paleogene Shahejie Formation (Es 4 U ) and the lower part of the 3rd member of the Paleogene Shahejie Formation (Es 3 L ) ( Fig. 2; Zhang et al, 2009;Sun et al, 2006). Hydrocarbons migrated and accumulated in reservoirs twice: 1.)…”