Oil & Gas Geology ›› 2017, Vol. 38 ›› Issue (4): 776-783.doi: 10.11743/ogg20170414

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Genesis of iron-rich dolostones in the 5th member of the Majiagou Formation of the Ordovician in Ordos Basin

Zhang Juntao1, He Zhiliang1, Yue Xiaojuan2, Sun Yipu1, Jin Xiaohui1, Chen Xia1   

  1. 1. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China;
    2. School of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China
  • Received:2016-08-15 Revised:2017-07-05 Online:2017-08-28 Published:2017-09-09

Abstract: According to the crystalline form and size,dolostones from the 5th member of the Ordovician Majiagou Formation,Ordos Basin,can be divided into two types:M-type microcrystalline dolostone and F-type silt-to-fine-sized crystalline dolostone. Both types have a high Fe content. However,Fe is not uniformly distributed in the two types of dolostonees. In the M-type dolostone,the matrix dolomite has a Fe content of 583×10-6 to 3 811×10-6 and the iron-rich dolomite mainly occurs as filling material in dissolved mold pores,the coarse crystalline pore-filling dolomites have a Fe content of 29 112×10-6 to 47 148×10-6 and the Fe is evenly distributed in the crystalline,and the silt-sized crystalline pore-filling dolomites have a Fe content up to 81 752×10-6,and the Fe concentrate only at the edge of the dolomites. In the F-type dolostones,the matrix has a Fe content of 233×10-6 to 2 007×10-6,dolomites filled up fractures and pores have an evenly distributed Fe with a content up to 9 178×10-6. There might be a correlation between the Fe enrichment and pore development. Well-developed pores generally mean higher concentration of Fe. The value ranges of δ13C(V-PDB) and δ18O(V-PDB) of the M-type microcrystalline dolomites are -3.5‰ to 1.4‰ and-6.5‰ to -8.0‰,respectively. Those of silt-sized crystalline pore-filling dolomites are -0.7‰ to -2.7‰ and -9.8‰ to -11.5‰,and those of coarse crystalline pore-filling dolomites are -1.7‰ to -2.9‰ and -9.9‰ to -11.3‰,indicating possible influences of hydrothemal fluids. The value ranges of δ13C(V-PDB) and δ18O(V-PDB) of the F-type are -0.4‰ to -0.8‰ and -5.3‰ to -6.6‰.The two types may have their Fe derived from the overlying ferruginous clay rocks. After long exposure to weathering and erosion,the dolomites harbored a large number of pores with some residual overburden of ferruginous clay rocks,from which Fe-rich fluids migrated downward by gravity during burial process,and Fe3+ was transformed into F2+ when the fluid reaches a certain temperature and depth making it easier for F2+ to enter into dolomite lattice and forming iron-rich dolomites.

Key words: pore, iron-rich dolomite, Majiagou Formation, Ordovician, Ordos Basin

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