Department of Geology, Liaoning Technical University, Fuxin, Liaoning 123000, China.
Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.
J Nanosci Nanotechnol. 2021 Jan 1;21(1):195-211. doi: 10.1166/jnn.2021.18752.
Understanding the controls on composition changes and porosity evolution in the critical zone of shale remains a major challenge. The aim of the present study is to develop a model of the changes in mineral compositions, chemical compositions and nanopore formation in shale during the initial weathering stage. To understand these processes, we selected a Silurian shale profile rich in pyrite and organic matter located in South China. Based on X-ray diffraction (XRD) and bulk elemental data, the variations in mineralogical and chemical compositions with depth were studied. To characterize the full pore size spectrum and to gain insight into the nature of secondary pores and their relationship with weathering, nuclear magnetic resonance (NMR) measurements and petrographic observations were combined with scanning electron microscopy (SEM) imaging. The results show that Al and K are enriched slightly, while Ca and Na are depleted in the upper part of the weathering profile. Si, Mn and Ti are relatively stable from the bottom to the top of the profile. Quartz, feldspar, mica, illite and chlorite are the main minerals in the parent rock, and they are relatively stable along the profile. The rock density gradually decreases from 2.6 g/cm³ to 2.1 g/cm³ from the bottom to the top, and the color of the shales changes from black to grayish yellow, but no secondary minerals are detected. The chemical weathering of black shale is dominated by the oxidation of pyrite and organic matter, giving rise to color variation and nanopore formation. The increase in interparticle pores at the nanometer-micron scale is initiated by the dissolution of easily weathered components such as organic matter and pyrite. The removal of clay minerals and tiny particles by groundwater seepage may be the main cause of porosity enhancement during the initial weathering stage. This study suggests that nanoporosity may play an important role in the process of fluid-rock interaction within black shale during the initial weathering stage.
了解泥页岩关键带中组成变化和孔隙演化的控制因素仍然是一个主要挑战。本研究旨在建立一个泥页岩在初始风化阶段矿物组成、化学成分和纳米孔形成变化的模型。为了理解这些过程，我们选择了中国南方富含黄铁矿和有机质的志留系泥页岩剖面。基于 X 射线衍射（XRD）和全岩元素数据，研究了矿物学和化学成分随深度的变化。为了描述全孔径谱特征，并深入了解次生孔的性质及其与风化的关系，将核磁共振（NMR）测量和岩相观察与扫描电子显微镜（SEM）成像相结合。结果表明，风化剖面的上部 Al 和 K 略有富集，而 Ca 和 Na 则有所亏损。Si、Mn 和 Ti 从剖面底部到顶部相对稳定。石英、长石、云母、伊利石和绿泥石是原岩的主要矿物，它们在剖面上相对稳定。岩石密度从底部到顶部逐渐从 2.6 g/cm³降至 2.1 g/cm³，页岩颜色从黑色变为灰黄色，但未检测到次生矿物。黑色页岩的化学风化主要由黄铁矿和有机质的氧化作用控制，导致颜色变化和纳米孔形成。易于风化的有机质和黄铁矿等成分的溶解，导致纳米级到微米级颗粒间孔隙的增加。地下水渗流去除粘土矿物和微小颗粒可能是初始风化阶段孔隙度增强的主要原因。本研究表明，纳米孔隙度可能在初始风化阶段黑色页岩中流体-岩石相互作用过程中发挥重要作用。
