Characteristics of REE and sedimentary environment of mud shale in Wufeng Formation-Gaojiabian Formation in Jurong area, Jiangsu Province

In order to explore the depositional and tectonic environment as well as provenance feature of the mud shale of Wufeng Formation-Gaojiabian Formation and guide regional shale gas exploration, the rare earth element composition (REE) of these rocks in Well SY1 from Jurong area was analyzed by Inductively Coupled-Mass Spectrometry (ICP-MS) in this study.The result shows that the total amount of rare earth elements of mud shale in Wufeng Formation is lower than that in North American shale while that in Gaojiabian Formation is reverse.Both Wufeng Formation and Gaojiabian Formation of mud shale are characterized by enrichment of the light rare earth elements and by commonly high fractionation which gradually increases from the bottom to the top. δCe_N value is 0.76~0.91, with an average value of 0.86, showing a weak negative anomaly.The δEu_N mean value of mud shale in Wufeng Formation is 1.11, while that of the first member and the second member of Gaojiabian Formation is 1.71 and 1.11, respectively, indicating that the first member of Gaojiabian Formation has the most obvious differentiation and a large range of variation.The ΣREE-La/Yb graphic shows that the provenance of mud shale in Wufeng Formation is relatively single, while the parent rocks of mud shale in Gaojiabian Formation have complex and mixed composition.The mean value of La/Yb indicates that the deposition rate of mud shale in Wufeng Formation is less than that of Gaojiabian Formation, and the abundance of organic matter is negatively correlated with the value of(La/Yb)_s to a certain extent. δEu, δCe, U/Th, V/Cr, Ni/Co and other parameters suggest that the seawater reducibility were characterized by oxygen-enriched, anaerobic and oxygen-enriched transition from base to top.The reduced water and relatively euxinic sedimentary environment in the Late Ordovician and Early Silurian were favorable for the preservation of organic matter, which resulted in the organic-rich mud shales in the Wufeng Formation-Gaojiabian Formation in Jurong area reaching a thickness of about 30 meters.Accordingly, the shale of Wufeng Formation and the first member of Gaojiabian Formation in the Jurong area represent a high-quality shale gas reservoir.