Response of Rare Earth Elements in Crude Oil to Biodegradation:A case from the Miaoxi Sag,Bohai Bay Basin
[Objective]Rare earth elements(REE)in crude oil contain valuable geochemical information and are widely used in oil-oil(source)comparisons and sedimentary environment reconstructions.However,the effect of bio-degradation on REE concentrations and patterns remains unclear.[Methods]Crude oil in the Miaoxi Sag of the Bo-zhong Depression in the Bohai Bay Basin has been subjected to varying degrees of biodegradation.In this study,14 crude oil samples from the area were tested for molecular markers and inorganic elements,using Gas chronmato-graphy-Mass spectrometry(GC-MS)and Inductively coupled plasma-Mass spectrometry(ICP-MS).The degree of bio-degradation in the crude oil was recognized by n-alkane intactness and 25-norbornane content.[Results]It was found that the crude oil samples in the study area were distributed from PM0 to PM7,with a high biodegradation gradient.Large gammacerane content and high levels of C35 hopanes were detected in the high-sulfur oil,which suggests they were derived from the contribution of more reducing and high salinity Es4 source rocks.The low-sulfur oil contained low levels of gammacerane and C35 hopanes,high C27 disteranes and high 4-methyl steranes,mainly derived from Es3 source rocks.[Conclusions]Comparison revealed that lacustrine crude oils generally have higher REE concentra-tions than marine crude oils.The REE concentrations and distribution patterns are useful for distinguishing lacustrine oils from marine oils.Sedimentary environment and maturity have limited influence on the REE concentration and dis-tribution pattern in crude oil,whereas biodegradation has a large influence on these factors.The REE concentration in crude oil increases during the biodegradation process,and the REE patterns and some indicators(e.g.,LaN/YbN,δEu,δCe)also change regularly with the degree of biodegradation.In summary,REE concentrations are useful mark-ers for evaluating the extent of biodegradation over a slight to extreme range and in various petroleum systems.
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