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

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 δCe_N value of 0.87, indicating a weakly negative Ce anomaly. They have δEu_N 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 δCe_S and positive δEu_S 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.