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Geoscience ›› 2019, Vol. 33 ›› Issue (04): 872-882.DOI: 10.19657/j.geoscience.1000-8527.2019.04.18

• Petroleum Geology • Previous Articles     Next Articles

Geochemical Characteristics and Depositional Environment Implications of Sedimentary Rocks in the Chang 7 Member of Yanchang Formation in the Ordos Basin

MA Huanhuan1(), LIU Chiyang1, ZHANG Long2, ZHANG Dongdong1(), WANG Wenqing1, ZHAO Yan1, GAO Ming3, QUAN Xiaoyuan1   

  1. 1. State Key Laboratory of Continental Dynamics,Northwest University, Xi’an, Shaanxi 710069,China
    2. School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an, Shaanxi 710065, China
    3. Shaanxi Xintai Energy Co., Ltd., Xianyang, Shaanxi 715300, China
  • Received:2018-05-30 Revised:2019-04-28 Online:2019-08-20 Published:2019-09-05
  • Contact: ZHANG Dongdong

Abstract:

Geochemical characteristics of sediments can reflect the depositional environment. In order to study its paleoclimate, redox conditions, and paleosalinity in the deposition period of the Yanchang Formation, geochemical (major, trace and rare earth elements (REEs)) characteristics of 15 sandstone and shale samples from the Chang 7 Member of the Yanchang Formation in the southeastern Ordos Basin were studied. The Al2O3/SiO2 and K2O/Na2O values of fine-grained sedimentary rocks in the Chang 7 Member indicate similar compositional maturity and high clay contents. The average ∑REE value (132.65×10-6) is significantly lower than that of the NASC and the UCC. The rocks also display relatively high LREE contents, weakly positive Eu and no Ce anomalies. The “C value”, Sr/Cu, Rb/Sr, Mg/Ca indices suggest that the entire deposition period of the Yanchang Formation was in a warm humid climate. Samples in the study area are rich in certain trace elements, such as U, Sc and Cs (average 20.11×10 -6, 9.66×10-6 and 6.08×10-6, respectively). This result and the values of V, δU, Th/U, V/Cr, V/Sc and V/(V+Ni) altogether indicate that the deposition period of the Yanchang Formation was dominated by a reducing, strongly stratified subsurface water layer. The values of Li, Sr, Ni, Ga, Sr/Ba, CaO/(CaO+Fe) and (MgO/Al2O3)×100 indicate that the water body of the deposition comprised mainly freshwater-brackish water.

Key words: Ordos Basin, Triassic, Yanchang Formation, geochemical characteristic, depositional environment

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