鄂尔多斯盆地奥陶系马家沟组五段富铁白云石成因

doi:10.11743/ogg20170414

摘要/Abstract

摘要： 鄂尔多斯盆地马家沟组五段白云岩，按照晶粒可分为两种类型：晶粒较细的M型微晶白云岩和晶粒相对较粗的F型粉细晶白云岩。这两类白云岩都具有较高的Fe含量，但Fe并不是均一分布的。在M型微晶白云岩中，基质白云石的铁含量为583×10^-6~3 811×10^-6，富铁白云石主要以膏溶铸模孔的孔隙充填物的形式存在，粗晶白云石充填物铁含量29 112×10^-6~47 148×10^-6，均匀分布于晶体之中；而粉晶白云石充填物，铁含量最可达81 752×10^-6，仅富集于白云石的边缘。在F型粉-细晶白云岩中，其基质白云石的铁含量为233×10^-6~2 007×10^-6；裂缝和孔隙内充填的白云石的铁含量较高，可达9 178×10^-6，均匀分布。铁的富集可能与孔隙的发育有一定联系。孔隙越发育，容易富集铁。M型微晶白云石的δ¹³C（V-PDB）值为-3.5‰~1.4‰，δ¹⁸O（V-PDB）值为-6.5‰~-8.0‰；孔隙充填物粉晶白云石δ¹³C（V-PDB）值为-0.7‰~-2.7‰，δ¹⁸O（V-PDB）值在-9.8‰~-11.5‰；粗晶白云石的δ¹³C（V-PDB）值范围为-1.7‰~-2.9‰，δ¹⁸O（V-PDB）值范围为-9.9‰~-11.3‰，可能受到过较高温度流体的影响。F型粉细晶白云岩的δ¹³C（V-PDB）值为-0.4‰~-0.8‰，δ¹⁸O（V-PDB）值为-5.3‰~-6.6‰，与海水较为接近。两类白云岩的Fe可能来源于上覆的铁质粘土岩。白云岩形成后，后经历了漫长的风化暴露剥蚀，形成了大量的储集空间，并残存形成了上覆铁质粘土岩。当再次进入埋藏期后，铁质粘土岩中富铁的流体在重力的作用下，向下部的地层运移，且到一定深度时，氧化态Fe³⁺转化为还原态的Fe²⁺，且已具有相对较高的地温，使得Fe²⁺更容易进入白云石晶格，形成了富铁的白云岩层系。

关键词: 孔隙, 富铁白云岩, 马家沟组, 奥陶系, 鄂尔多斯盆地

Abstract: According to the crystalline form and size,dolostones from the 5^th 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

中图分类号:

TE121.3

张军涛,何治亮,岳小娟,等. 鄂尔多斯盆地奥陶系马家沟组五段富铁白云石成因[J]. 石油与天然气地质, 2017, 38(4): 776-783. doi: 10.11743/ogg20170414 Zhang Juntao,He Zhiliang,Yue Xiaojuan,et al. Genesis of iron-rich dolostones in the 5th member of the Majiagou Formation of the Ordovician in Ordos Basin[J]. Oil & Gas Geology, 2017, 38(4): 776-783. doi: 10.11743/ogg20170414

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