Characteristics of authigenic carbonate minerals in carbonate concretions of the Chang 7 oil-bearing interval of Upper Triassic Yanchang Formation in Weibei area, Ordos Basin
Li Yun1,2, Hu Zuo-Wei1,2, Liu Can1,2, Dong Jie1,2, He Jing3, Yuan Xiao-Qi3, Wei Yang1,2
1 State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China; 2 Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China; 3 Exploration and Development Research Institute of Changqing Oil Field Branch,PetroChina, Xi’an 710018,China
Abstract The early carbonate concretions would undergo the complex cementation of various carbonate mineral phases during the long-term burial process. The detailed petrology studies are the key to explore the genetic mechanism of the carbonate concretations. In this paper,the characteristics of authigenic carbonate minerals constituting the calcite and dolomite concretions that are rich in the shale of Chang 7 oil-bearing interval of Yanchang Formation in Weibei area of southern Ordos Basin,is studied through detailed observation in the field,together with the petrology studies and cathodoluminescence technology. The authigenic carbonate minerals constituting the concretions that may form in the early diagenesis,have marked features as follows: (1)the calcite spheres are composed of fibrous or flaky calcites. The crystalline calcites are filled between the calcite spheres,or the spheres are closely contacted because of compaction. Under the cathodoluminescence condition,the fibrous calcites show red orange light and dark red light,and the flaky calcites show dark light;(2)the calcites developed in fine-grained crystalline concretions show irregular shapes,containing many organism inclusions or residual fibrous textures. It shows that the natural bitumen and residual fiberous calcites appear between the calcite crystals,which presents dark red light under the cathodeluminescence;(3)the dolomite concretions are characterized as micrite and fine-grained crystalline dolomites. Authigenic fine-grained crystalline dolomite concretions usually contain large amounts of argillaceous bands. The fiber columnar dolomites are common in the argillaceous bands or organic matter enriched bands;(4)Fibrous and fiber columnar calcites and dolomites that present dark red color or colorless under the cathodeluminescence,usually grow along the cracks. The calcite and dolomite minerals developed in the carbonate concretions of Chang 7 oil-bearing interval,have different genetic types and complex cementation history. Spheroidal calcites and micritic dolomites produce at the beginning of the cementation,indicating the early genesis of concretions. Fine-grained crystalline calcites and dolomites indicate metasomatism. Crack-filled fibrous,radial and columnar shaped calcite and dolomite aggregates indicate strong compaction after the formation of concretions.
Fund:Co-funded by the National Natural Science Foundation of China(Nos. 41772100,41302085,41372113)and the National Science and Technology Major Project of China(No.2016ZX05050)
Corresponding Authors:
About the corresponding author Hu Zuo-Wei,born in 1981,is a professor of Chengdu University of Technology. He is mainly engaged in teaching and research of sedimentary geology. E-mail:huzuowei@foxmail.com.
About author: About the first author Li Yun,born in 1983, is a senior laboratory technician. She is mainly engaged in researches of petrology and reservoir sedimentology. E-mail: liycdut@foxmail.com.
Cite this article:
Li Yun,Hu Zuo-Wei,Liu Can et al. Characteristics of authigenic carbonate minerals in carbonate concretions of the Chang 7 oil-bearing interval of Upper Triassic Yanchang Formation in Weibei area, Ordos Basin[J]. JOPC, 2019, 21(4): 577-588.
Li Yun,Hu Zuo-Wei,Liu Can et al. Characteristics of authigenic carbonate minerals in carbonate concretions of the Chang 7 oil-bearing interval of Upper Triassic Yanchang Formation in Weibei area, Ordos Basin[J]. JOPC, 2019, 21(4): 577-588.
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