生物降解作用对原油稀土元素的响应——以渤海湾盆地渤中坳陷庙西凹陷为例

Response of Rare Earth Elements in Crude Oil to Biodegradation:A case from the Miaoxi Sag,Bohai Bay Basin

【目的】原油中的稀土元素(Rare Earth Element,REE)被广泛地应用于油—油(源)对比和沉积环境重建的工作,然而生物降解作用对原油REE浓度和分配模式仍不明确。因此,迫切需要进一步明确REE在油气成藏过程中的地球化学行为,探讨REE所蕴含的地球化学意义。【方法】选取渤海湾盆地渤中坳陷庙西凹陷地区成熟度类似、受运移作用影响小,仅在生物降解程度上具有差异性的原油,采用GC-MS和ICP-MS对该地区14个原油样品,进行了分子标志物和无机元素的测试。【结果】通过正构烷烃完整性及25-降藿烷含量对原油生物降解程度进行了判识,发现研究区原油样品从PM0~PM7(PM指数代表生物降解等级)均有分布,具有较好的生物降解梯度。在高硫油中检测到高含量伽马蜡烷和高C_(35)升藿烷,认为其主要来自还原性更强、盐度更高的沙四段烃源岩的贡献;低硫油表现为低伽马蜡烷、低C_(35)升藿烷、高C_(27)重排甾烷、高四甲基甾烷的特征,推测主要来自沙三段烃源岩的贡献。原油的ΣREE浓度、La_(N)/Yb_(N)与C_(35)H/C_(34)H、Ts/(Ts+Tm)、ββ/(ββ+αα)-C29RS不具备明显的相关性,表明成熟度对其影响有限。随着生物降解程度增加,ΣLREE/ΣHREE和La_(N)/Yb_(N)先降低后增加,在轻微生物降解阶段(PM<4),ΣLREE/ΣHREE和La_(N)/Yb_(N)显著下降,在严重—强烈生物降解阶段(PM>4),ΣLREE/ΣHREE和La_(N)/Yb_(N)表现为增加的趋势。Sm/Nd、δEu和Pr/Pr^(*)随生物降解程度的增加呈现为降低的趋势,δCe表现为微弱增加的趋势。【结论】对比发现陆相成因原油普遍比海相成因原油的REE浓度高,可以利用REE浓度和分布模式区分海相和陆相成因原油。母质沉积环境及成熟度对原油的REE浓度与配分模式影响有限,生物降解作用对REE浓度与配分模式具有较大影响。原油中的REE浓度随生物降解程度增加不断升高,REE模式和La_(N)/Yb_(N)、δEu和δCe等指标也随生物降解程度的增加而发生规律性变化。REE浓度能够作为标志物应用于轻微到极端程度和各种类型含油气系统的生物降解程度评价。

[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 biodegradation on REE concentrations and patterns remains unclear.[Methods]Crude oil in the Miaoxi Sag of the Bozhong 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 chronmatography-Mass spectrometry(GC-MS)and Inductively coupled plasma-Mass spectrometry(ICP-MS).The degree of biodegradation 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 C_(35) 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 C_(35) hopanes,high C_(27) disteranes and high 4-methyl steranes,mainly derived from Es3 source rocks.[Conclusions]Comparison revealed that lacustrine crude oils generally have higher REE concentrations 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 distribution 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.,La_(N)/Yb_(N),δEu,δCe)also change regularly with the degree of biodegradation.In summary,REE concentrations are useful markers for evaluating the extent of biodegradation over a slight to extreme range and in various petroleum systems.