基于扫描电镜和CT成像技术的碳酸盐岩溶蚀作用微观结构和变化规律研究

邓宇林; 郭绪磊; 罗明明; 陈祥勇; 况野; 周宏

doi:10.11932/karst20220503

基于扫描电镜和CT成像技术的碳酸盐岩溶蚀作用微观结构和变化规律研究

doi: 10.11932/karst20220503

1.
中国地质大学(武汉)地质调查研究院, 湖北武汉 430074
2.
中国地质大学(武汉)环境学院, 湖北武汉 430074

基金项目: 武汉市多要素城市地质调查示范项目“武汉市地下水系统补充调查与集成研究”（WHDYS-2021-006）；中国地质调查局项目“清江流域水文地质调查”（DD20190824）

详细信息

作者简介:
邓宇林（1993－），男，硕士研究生，主要研究方向为岩溶水文地质。E-mail:348956332@qq.com

中图分类号: P642.25
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出版历程
- 收稿日期: 2021-12-15
- 刊出日期: 2022-12-02

Study on the microstructure and variation law of carbonate rock dissolution based on scanning electron microscopy and CT imaging technology

1.
Institute of Geological Survey, China University of Geosciences(Wuhan), Wuhan, Hubei 430074, China
2.
School of Environmental Studies, China University of Geosciences(Wuhan), Wuhan, Hubei 430074,China

摘要

摘要: 为研究溶蚀作用下碳酸盐岩孔隙的演变规律及控制作用，文章选取三峡地区4种类型碳酸盐岩开展溶蚀实验。同时结合扫描电镜、CT成像对实验前后岩石的溶蚀特征及孔隙结构进行测试。结果表明：溶蚀总发生在低晶格能的矿物处且沿矿物晶体的菱形解理面以及薄弱部位发育，表现为对矿物和孔隙等结构的选择性溶解；碳酸盐岩的孔隙度对溶蚀过程影响较小，岩石的孔径大小是影响碳酸盐岩溶蚀速率的重要因素；小孔径岩石的溶蚀多在样品表面发育小型溶孔，大孔径岩石的溶蚀主要发生在孔隙隙壁且有向岩石内部溶蚀的痕迹；经溶蚀改造，孔喉半径和连通性均呈现出增长趋势。本研究对碳酸盐岩差异性岩溶作用机理及岩溶发育规律的认识有一定指导意义。
- 碳酸盐岩 /
- 溶蚀实验 /
- 溶蚀特征 /
- 孔隙结构 /
- X射线显微层析
Abstract: The dissolution of carbonate rocks is the most important process of karst development, and it is also the basis for the processes affecting karst geomorphological evolution, hydrochemistry of karst water, karst water system development, karst carbon sinks and global carbon cycle. Scholars have explored the dissolution mechanism of carbonate rocks and the factors that affect the development of carbonate karst through indoor simulated dissolution and in-situ experiments, but they mostly focus on the effect of chemical components of rocks on karst processes. In field surveys, it was found that not only limestone formations, but also some dolomite formations have developed karst. It is not comprehensive enough to explain the solubility of carbonate rocks only by chemical composition. The pore structure of the rock also plays an important role,but the influence mechanism of rock pore structure on karst development is still unclear. In order to study the variation law and control effect of carbonate rock pores under dissolution experiment,in this paper, we selects four types of carbonate rocks (Sinian Dengying Formation-medium fine-grained dolomite, Cambrian Tianheban Formation-oolitic limestone, fine-grained limestone, and Qinjiamiao Formation-dolomitic breccia)in the Three Gorges area to carry out dissolution experiments. At the same time, scanning electron microscopy and CT imaging methods were used to test and analyze the dissolution characteristics and pore structure of rocks before and after dissolution reaction. The results show that, (1)The dissolution of carbonate rocks always occurs in soluble minerals with low lattice energy and develops along the rhombohedral cleavage planes and weak parts of mineral crystals,showing the selective dissolution of soluble minerals and pore structures. Dissolution preferentially dissolves soluble minerals, and insoluble components either fall off or protrude from the rock surface.Due to the control of rhombic cleavage on the surface of dolomite crystals, the dissolution will develop into dissolution fractures along the cleavage, and at the same time, dissolution pores appear on the surface of dolomite crystals, showing a "honeycomb" dissolution characteristic; Lattice defects, cleavage, and the weaker plasmonic and molecular contact forces at the edge of the crystal are the places where the dissolution preferentially occurs, and the dissolution strength at the rock pores is slightly higher than that at other parts. (2) The pore size of carbonate rocks controls the development of dissolved pores and the evolution of dissolved pores during rock dissolution.For oolitic limestone and fine-grained limestone samples, because their pore diameters are mainly concentrated in 18-80 μm and the pore distribution is uniform,after the dissolution reaction, the small pores further increased, only a small amount of pores were enlarged by dissolution, and the area with increased pore size was mainly concentrated in 40-50 μm,and the pores were mostly developed on the surface of the rock. The pores on the surface of the medium-fine-grained dolomite are relatively developed and the pore size is large, about 20% of which are larger than 100 μm, after the dissolution transformation, the proportion of pores with a pore size of more than 50 μm in the rock increased to 78%. It is shown that the dissolution of rocks with small pore size (40-60 μm) mainly occurs on the surface of the sample, and small dissolved pores are often developed. The dissolution of large pore size (>80 μm) rocks mainly occurs in the pore walls and there are signs of dissolution into the rock. After dissolution transformation, the pore throat radius and connectivity of medium-fine-grained dolomite show a certain increase. (3) The dissolution process of carbonate rocks is jointly affected by chemical composition and pore structure, and is less affected by rock porosity. The pore size distribution is the key factor affecting the dissolution rate of carbonate rocks. The porosity of the samples before dissolution is between 1.97 and 10%. Under the experimental conditions (20 ℃, 1 atm, 0.1 mol·L⁻¹ HCl solution), the dissolution rate of carbonate rocks is between 0.497 g·d⁻¹ and 0.598 g·d⁻¹. The mass loss of dolomitic breccia through dissolution is the largest due to the influence of cement shedding; the medium-fine crystal dolomite with more developed surface pores and larger pore size is second;fine-grained limestone with dense grains and small pores has the slowest dissolution rate.Samples with smaller pore diameters are greatly affected by the stagnant boundary layer, and the dissolution medium is likely to form a local saturation state inside the pores; the samples with larger pore diameters can expand into the rock during the dissolution process of the "water-rock" reaction, which is affected by the stagnant boundary, but the effect of the layer is less and the water flow conditions are better than the small pore size samples, so the dissolution rate is higher. This study has certain guiding significance for the understanding of the differential karstification mechanism and karst development law of carbonate rocks.
- carbonate rocks /
- dissolution experiments /
- dissolution characteristics /
- pore structure /
- X-ray microtomography

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图 1 采样点位置图

Figure 1. Locations of sampling points

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图 2 碳酸盐岩的溶蚀特征

注：A-D. 白云质角砾岩 E-F. 鲕粒灰岩 A-B. 实验前的样品，晶体形状完整，部分黏土矿物附着在白云石晶体上，晶面光滑、晶间缝隙清晰，无明显溶蚀痕迹 C. 溶蚀6 h后，白云石晶体上发育有溶缝、溶孔 D.溶蚀24 h，白云石晶体沿表面向内部逐层溶解，局部溶蚀强烈位置呈现出“蜂窝状”溶蚀特征 E.实验前的样品，样品表面较为平整 F.溶蚀24 h，鲕粒处发生明显溶蚀并向内凹陷，鲕粒周围的晶粒无晶形。

Figure 2. Dissolution characteristics of carbonate rocks

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图 3 鲕粒灰岩的溶蚀特征及EDS能谱图

Figure 3. Dissolution characteristics and EDS spectrum of oolitic limestone

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图 4 碳酸盐岩的选择性溶蚀特征

注：A-D. 盐酸溶液，溶蚀24 h的岩样 A-B. 中－细晶白云岩，溶解分别发生在白云石晶体边缘处及白云石晶体表面 C-D. 试样编号，细晶灰岩，溶解分别发生在方解石孔隙边缘及微裂隙处。

Figure 4. Selective dissolution characteristics of carbonate rocks

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图 5 样品溶蚀前后孔隙相形态图

注：A1-1、A2-1、A3-1分别表示溶蚀反应前鲕粒灰岩、细晶灰岩和中－细晶白云岩的孔隙相 A1-2、A2-2、A3-2分别表示溶蚀24 h后鲕粒灰岩、细晶灰岩和中－细晶白云岩的孔隙相

Figure 5. Morphology of the pore phase before and after the sample dissolution

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图 6 溶蚀前后碳酸盐岩孔径分布曲线

Figure 6. Pore size distribution curve of carbonate rocks before and after dissolution

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图 7 A3样品溶蚀前后的喉道特征图

Figure 7. Throat characteristic map of A3 sample before and after dissolution

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表 1 样品信息及实验结果

Table 1. Sample information and experimental results

编号	岩性	层位	矿物组成/%			孔隙度/%		比溶蚀率
编号	岩性	层位	方解石	白云石	其他	溶蚀前	溶蚀后	比溶蚀率
A1	鲕粒灰岩	Є₁t	85.04	1.88	13.07	2.77	3.47	0.95
A2	细晶灰岩	Є₁t	87.04	11.23	1.73	2.55	2.76	0.86
A3	中-细晶白云岩	Z₂dn³	27.66	70.12	2.22	1.97	2.16	1.03
A4	白云质角砾岩	Є₂q	10.59	27.27	62.14			1.16

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