Trace and rare-earth element characteristics of fine-grained sediments in the Upper Triassic Xujiahe Formation in the we
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ORIGINAL ARTICLE
Trace and rare‑earth element characteristics of fine‑grained sediments in the Upper Triassic Xujiahe Formation in the western Sichuan Basin, SW China: implications for the provenance and depositional environment Yu Yu1,2 · Liangbiao Lin1,2 · Hongli Nan3 Accepted: 6 November 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Xujiahe fine-grained sediments (FGSs) are regarded not only as important shale gas targets but also as significant regional hydrocarbon source rocks. To analyze the provenance and depositional conditions of the Xujiahe FGSs, trace elements and rare-earth elements (REEs) from two wells in the western Sichuan Basin were investigated. The results indicate that the Xujiahe FGSs mainly originated from the upper continental crust (UCC), the source rocks gradually changed from mixed influence to relatively pure sedimentary rocks from the second member of Xujiahe Formation (Xu2 member) to the fourth member of Xujiahe Formation (Xu4 member), which was related to the uplift of Longmen Mountain. La–Th–Sc and Th– Co–Zr/10 diagrams indicate an active continental margin environment. The redox condition indices (V/Cr and Th/U) suggest that the Xujiahe FGSs were deposited in an oxic environment, while the paleoclimate indices suggest differences among the members. The Xu3 member shows a stable warm-humid climate, and the Xu2 and Xu4 members alternate between humid and arid climates. The combination of climate and tectonics is the key factor in the formation of the Xujiahe FGSs. The warm and humid climate result in high productivity in the basin, tectonic movement influence the substance supply and create a high sedimentary rate for the Xujiahe Formation; thus, the organic matter in the Xujiahe FGSs can still be effectively preserved even under oxidation conditions. Keywords Provenance · Depositional environment · Fine-grained sediments · Sichuan basin · Upper triassic
Introduction
* Liangbiao Lin [email protected] Yu Yu [email protected] Hongli Nan [email protected] 1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
2
Institute of Sedimentary Geology, Chengdu University of Technology, 1#, Dongsanlu, Erxianqiao, Chengdu 610059, China
3
Exploration and Production Research Institute, Southwest Branch of SINOPEC, Chengdu 610000, China
The Sichuan Basin has become a favorable area for unconventional resources; numerous major shale gas exploration breakthroughs have been made in Paleozoic marine shale since the last decade (Huang et al. 2012; Li et al. 2016; Luo et al. 2016). Fine-grained sediments (FGSs), such as shale, mudstone, and siltstone, are widely developed in the Xujiahe Formation of the Upper Triassic, which is not only the important regional hydrocarbon source rocks for the Xujiahe tight gas sandstone (Yu et al. 2019), but also become a significant shale gas exploration target in recent years (Jiang et al. 2017; Yang et al. 2019). Although numerous schol
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