Water-induced variations in micro characteristics of clay-bearing conglomerates at great depths in China

Abstract

The micro characteristics, including micro-structural features of clay minerals and micro-pore textures of rock, change after the rock–water interaction and lead directly to rock deformation and deterioration. To understand the mechanism of the weakening process of rocks at great depths after interaction with water, we performed X-ray diffraction analysis, scanning electron microscopy, and mercury intrusion porosimetry experiments to investigate the mineral compositions, microstructures, and porosity characteristics of four clay-bearing conglomerate samples collected from Northeast China at great depths. In addition, the chemical analysis of the aqueous solutions was carried out to examine the variances of chemical compositions before and after the water absorption tests. The results showed that the dominant mineral components in the conglomerate samples were quartz, potash feldspar, plagioclase, and clay. The contents of clay minerals in all samples were relatively high and ranged from 8.9% to 14.9%, mostly containing a chlorite/montmorillonite mixed layer. The correlation analysis suggested that ion exchange occurred in the reactions between the conglomerate samples and water. The mixed layer of chlorite/montmorillonite expanded, spread, and flocculated after water absorption, jammed the pores, and reduced the porosity. A certain number of calcite particles formed on the surface of grains or surrounded the micro-pores, resulting in the blockage of pores and inducing the decrease in rock porosity. In addition, various occurrence features of the chlorite/montmorillonite mixed layer changed differently after interacting with water, leading to varied influences on the fractal characteristics of rocks.