Types and origin of pore-fillings from the 5<sup>th</sup>member of the Ordovician Majiagou Formation in Ordos Basin

Zhang Juntao; Jin Xiaohui; Li Shujun; Li Wei; Sun Yipu

doi:10.11743/ogg20160508

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Types and origin of pore-fillings from the 5^thmember of the Ordovician Majiagou Formation in Ordos Basin
Vol. 37, Issue 5, Pages: 684-690(2016)
Online First：08 October 2016，

Published：2016
DOI： 10.11743/ogg20160508
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DOI：

Zhang Juntao, Jin Xiaohui, Li Shujun, et al. Types and origin of pore-fillings from the 5^thmember of the Ordovician Majiagou Formation in Ordos Basin[J]. 2016, 37(5): 684-690. DOI: 10.11743/ogg20160508. DOI：

摘要

鄂尔多斯盆地马五段主要孔隙类型为硬石膏结核溶模孔，充填矿物有方解石、粉晶白云石、粗晶白云石，以及少量的石英、硬石膏、高岭石等。方解石多为粗晶，阴极发光下为黄色，具有较低的Fe含量、中等Mn含量，明显偏负的碳、氧同位素，说明其形成于大气降水环境中。粉晶白云石多为半充填，矿物学特征与基质白云石类似，与基质白云石具有同源性，但具有两种阴极发光情况，一类为橘色，另一类与基质白云石相近，为暗红色，电子探针背散射下，可分为外边和内核两部分，内核的Fe和Mn的含量与基质白云石相近，外边具有较高的Fe和Mn含量。粉晶白云石较基质白云石碳、氧同位素偏负，这些差异反映了成岩流体的变化。粗晶白云石染色后呈蓝色，具有较高的Fe和Mn含量，较负的碳、氧同位素，其可能形成于后期的热流体活动。因此，不同类型的充填物是不同流体活动的结果。方解石形成于表生期的大气降水充填作用，与岩溶古地貌有一定的联系，多分布于流体相对充足地势相对低洼的区域；而粉晶白云石的含量则与沉积环境相关，后经历过热流体改造；粗晶白云石的形成于与裂缝相关的热流体活动。

Abstract

Mold pore of anhydrite nodule is the major pore type in the 5

member of Ordovician Majiagou Formation

Ordos Basin.The fillings of the pore consist mainly of calcite

very fine crystalline dolomite

and coarse crystalline dolomite

as well as a small amount of quartz

anhydrite

kaolinite

etc.The calcite

fully filling the pores

is mostly coarse crystalline and shows yellow color under cathodoluminescence.Its moderate Mn content

a relatively low Fe content and the obviously negative δ

O values indicate that the calcite deposited in atmospheric environment.The very fine crystalline dolomite

usually partly filling the pores

is similar to matix dolomite in mineralogy and origin.It shows two color types (one is light orange and the other is dark red similar to matix dolomite) under cathodoluminescence.The very finecrystalline dolomite can be divided into two parts under electron probe backscatter:the core and the cortex.The Fe and Mn content of the core are close to that of the matrix dolomite

while the cotex has the highest Fe and Mn content in carbonate minerals.Furthermore

its δ

O and δ

C values are more negative than that of the matrix dolomite.These features illustrate the change of diagenetic fluids.The coarse crystalline dolomite

blue after dyeing

has higher Mn and Fe content

relatively negative δ

O and δ

C values

suggesting a possble late hydrothermal fluid origin.Different pore-fillings in the reservoir are the results of different fluids.The calcite is possibly the result of meteoric water filling in hypergenic stage and has certain relatonship with karst paleogeomorphology.It commonly occurs in low-lying areas with surficient fluids.In constrast

the very fine crystalline dolomite is related to sedimentary environments and alterated by hydrothermal fluids.The fromation of coarse crystalline dolomite as a result of hydrothermal fluids associated with fractures.