柴北缘鱼卡地区中侏罗统石门沟组页岩段稀土元素地球化学特征与地质意义

对柴达木盆地北缘(简称柴北缘)鱼卡地区鱼油页1井(YYY-1)页岩段稀土元素地球化学分析,揭示了其对古环境、古物源和古气候的响应,并探讨了对油页岩成矿的影响。分析结果表明:石门沟组页岩段稀土元素总量(ΣREE)为(139.16~342.59)×10~(-6),平均值为236.66×10~(-6),高于北美页岩(NASC)和后太古宙页岩(PASS)。ΣLREE/ΣHREE和(La/Yb)_N等地球化学参数表明,轻重稀土元素分异明显;分布模式为轻稀土元素富集、重稀土元素相对亏损。地球化学参数以及La/Yb-ΣREE图解表明,研究区物源主要为上地壳沉积岩和火山岩的混合来源。元素Eu具有明显的负异常,指示沉积水体为缺氧的还原环境,为油页岩中有机质的富集提供良好的保存条件。根据高含量的ΣREE值以及δEu异常值,结合前人对该地区的孢粉研究成果,石门沟组页岩段形成于暖湿的气候条件,为油页岩的形成提供良好的物质来源和保存条件。

Rare earth element geochemical characteristics and geological significance of shale member in Middle Jurassic Shimengou Formation in Yuqia area of northern margin of Qaidam Basin

In this paper, the geochemical characteristics of REE from shale member of Shimengou Formation in Well YYY-1 have been analyzed, which illustrated the significance of paleoenvironment, paleoprovenance and paleoclimate, and revealed the impact on oil shale mineralization. The results show that the total content of REE(ΣREE)of shale member of Shimengou Formation varied from 139.16×10-6 to 342.59×10-6 with an average at 236.66×10-6, which is higher than the content of NASC and PASS. Geochemical parameters such as the ratios of ΣLREE/ΣHREE, (La/Yb)N indicate an obvious differentiation of ΣLREE and ΣHREE. The REE distribution patterns show that LREE is obviously richer than HREE. Geochemical parameters and La/Yb-ΣREE diagram suggest that the paleoprovenance was a mixture of sedimentary and volcanic rocks from the upper crust. Eu is obviously negative abnormality, indicating that the sediment was formed in anoxic environment suggesting that anoxic environment is in favour of the organic matter of oil shale enrichment. According to the high levels of ΣREE and δEu abnormal value, combined with previous research results based on sporopollen, the shale member of Shimengou Formation formed in the warm and humid climate conditions, which provided a good source of material and preservation conditions for the oil shale.