Species and formation mechanism of apatites in the 7th member of Yanchang Formation organic-rich shale of Ordos Basin,China

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  • 1.State Key Laboratory of Petroleum Resource and Prospecting,China University of Petroleum,Beijing 102249,China;
    2.College of Geoscience,China University of Petroleum,Beijing 102249,China;
    3.PetroChina Changqing Oilfield Company,Xi’an 710021,China

Received date: 2015-12-14

  Revised date: 2016-03-02

  Online published: 2019-09-11

Abstract

The organic-rich shale in the 7th member (Chang 7)of Upper Triassic Yanchang Formation in Ordos Basin is rich in authigenic apatites and volcanic ash sediments.Through rock thin section,scanning electron microscopy (SEM)and energy spectrum analyses,two kinds of apatites are found in the Chang 7 organic-rich shale:one is formed by bioclastic (bioclastic apatite,BA),and the other is colloidal aggregation of apatite (colloidal apatite,CA).According to the degree of recrystallization of CA,it can be further subdivided into three types:(1)CA-Ⅰ:CA has non-recrystallization or very low degree of recrystallization;(2)CA-Ⅱ:CA has lower degree of recrystallization;(3)CA-Ⅲ:CA has higher degree of recrystallization.Elemental analysis shows that the phosphorus (P)mainly came from volcanic ash caused by volcanic eruption.The formation of apatites in Chang 7 organic-rich shale is closely related to the life activity of organisms.P provided by volcanic ash sediments was absorbed through the biological metabolism,and buried with organisms into sediments after they died.The P element preserved in organisms would be transformed into apatites during the process of diagenesis.It is thought that the formation mechanisms of apatites have two kinds:one is directly biological effect,and the other is indirectly biological effect.There is close relationship among volcanic ash sediments,organic-rich shale and apatites and we consider that the volcanic ash sediments promote the formation of apatites and control the distribution of P2O5 contents and organic-rich shale.

Cite this article

Yuan Wei, Liu Guang-di, Luo Wen-bin, Li Chao-zheng, Xu Li-ming, Niu Xiao-bing, Ai Jia-yi . Species and formation mechanism of apatites in the 7th member of Yanchang Formation organic-rich shale of Ordos Basin,China[J]. Natural Gas Geoscience, 2016 , 27(8) : 1399 -1408 . DOI: 10.11764/j.issn.1672-1926.2016.08.1399

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