Application of ICP-MS to Study the Rare Earth Element Characteristics and Sedimentary Environment of Black Shale in the Longmaxi Formation in the Southwestern Sichuan Basin
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摘要: 为深入研究盐源盆地龙马溪组黑色岩系的沉积环境、构造背景及物源属性，指导区域页岩气勘探及相关研究，本文利用电感耦合等离子体质谱技术研究了盐源盆地南缘龙马溪组黑色页岩的稀土元素地球化学特征和沉积环境。结果表明：该区稀土总量偏低，轻稀土富集重稀土亏损，轻重稀土分异较大；δCeN均值为0.87，显示弱的负异常，δEuN值为0.66~1.43，变化范围较大，说明源岩成分复杂；LaS/YbS均值为1.31，说明研究区构造背景主要为被动大陆边缘，源岩为古老沉积岩、花岗岩及中基性岩的混合物，推测康滇古陆为其提供了主要物源。δCeS为负异常，δEuS为正异常，La/Ce < 1等特征均指示沉积期水体为还原环境，推测为深水陆棚沉积，且受到一定程度热水沉积影响。深海热液带来丰富的营养元素有利于有机质形成，还原水体环境有利于有机质保存，使得研究区龙马溪组泥岩页富含有机质，且富有机质页岩段厚度较大，由此初步认为龙马溪组泥页岩为优质烃源岩储层。
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关键词:
- 盐源盆地 /
- 泥页岩 /
- 稀土元素 /
- 沉积环境 /
- 电感耦合等离子体质谱法
Abstract: In order to study the sedimentary environment, tectonic background, and source features of the Longmaxi Formation black shale in the Yanyuan Basin, southwest Sichuan and guide regional shale gas exploration and related research, the rare earth element (REE) composition of these rocks was analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in this study. The results show that ΣREEs are low with the enrichment of light rare earth and the depletion of heavy rare earth, and significant differentiation between them. These rocks have an average δCeN value of 0.87, indicating a weakly negative Ce anomaly. They have δEuN values of 0.66-1.43 with a large variation, indicating that the composition of the source rock is complex. Average (La/Yb)S value of 1.31 indicates that tectonic background of the study area is mainly passive continental margin. The source rocks are mixed ancient sedimentary rocks, granites, and intermediate-mafic rocks. Neagtive δCeS and positive δEuS anomalies, as well as La/Ce < 1 indicate that the sedimentary water body is a reducing environment formed by a deep-water shelf sedimentation and affected by a certain degree of hydrothermal sedimentation. Deep-sea hydrothermal fluids can bring rich nutrients, which favours the formation of organic matter. The reduction of a water environment is helpful for the preservation of organic matter, hence making the study area rich in organic matter with a large thickness. It is inferred that the black shale in Longmaxi Formation is a high-quality source rock reservoir. -
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