Abstract Due to the complex structure of the brittle formations like shale, their microcracking and mechanical deformation behavior are not well known. To understand the fundamental factors at the micron scale, Marcellus shales were cored both parallel and perpendicular to the bedding orientation and mechanically tested through triaxial creep test under constant stress. The parallel-bedded shales exhibited higher level of permanent strain in the radial direction, which increased with increasing time. This also confirmed their anisotropic nature due to the bedding. Their advanced characterization via X-ray diffraction (XRD) and Raman spectroscopy techniques before and after the creep tests revealed the mineralogical heterogeneity, and the presence of an organic matter (kerogen) concentrated in the microcrack areas. Besides, the XRD peak shifts and changes in peak shape indicated that there was a certain level of macrostrain and microstrain in the shale. Further investigation of the XRD peak shape (integral breadth) changes using the Williamson-Hall method demonstrated a higher concentration of lattice defects and associated microstrain (inhomogeneous lattice strain) present in the calcite phase compared to the quartz. The parallel-bedded shales also revealed a higher level of microstrain with respect to the perpendicular-bedding. The results proved that the microcracking (initiation and propagation) and associated mechanical deformation of the Marcellus shales were mostly influenced by the presence of organic matter, microstrain level (defect concentration) in the calcite mineral, and the bedding orientation.

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Marcellus 页岩的矿物学特征和宏观/微观应变分析

摘要 由于页岩等脆性地层结构复杂，其微裂纹和力学变形行为尚不清楚。为了了解微米尺度的基本因素，马塞勒斯页岩平行和垂直于层理方向取芯，并在恒定应力下通过三轴蠕变试验进行机械测试。平行层状页岩在径向上表现出更高水平的永久应变，随着时间的增加而增加。这也证实了它们由于层理而具有的各向异性性质。他们在蠕变试验前后通过 X 射线衍射 (XRD) 和拉曼光谱技术进行的高级表征揭示了矿物学的非均质性，并且存在集中在微裂纹区域的有机物质（干酪根）。除了，XRD 峰移和峰形变化表明页岩中存在一定程度的宏观应变和微观应变。使用 Williamson-Hall 方法对 XRD 峰形（积分宽度）变化的进一步研究表明，与石英相比，方解石相中存在更高浓度的晶格缺陷和相关微应变（不均匀晶格应变）。平行层理的页岩也显示出相对于垂直层理的更高水平的微应变。结果证明，马塞勒斯页岩的微裂纹（起始和扩展）和相关的机械变形主要受有机质的存在、方解石矿物中的微应变水平（缺陷浓度）和层理取向的影响。