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Sketch tectonic map of the North China Craton [modified after Liu and You (2015)] and the location of the studied coal mine.
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This work investigated the mineralogy and geochemistry of the No. 3 coal seam in Permian Shanxi Formation in Guotun Mine, Juye Coalfield, north China, in order to understand the genesis of the minerals and the enrichment of trace elements. Approaches used were optical microscopy and electron probe microanalysis for minerals, X-ray fluorescence analy...
Citations
... The enrichment of trace elements in coals is affected by numerous geological factors. Trace elements are generally derived from various sources Dai et al., 2012a), including source rocks (Sun et al., 2016;Zhang et al., 2019;Zhou et al., 2021;Zhao et al., 2022), seawater (Chou, 2012;Zhao et al., 2019a,b;Dai et al., 2020;, volcanic ash input (Creech, 2002;Bohor and Triplehorn, 1993;Spears, 2012;Dai et al., 2011Dai et al., , 2017aDai et al., , 2017bZhao et al., 2019b;Wang et al., 2022), groundwater (Seredin and Finkelman, 2008;Dai et al., 2018;Guo et al., 2019), and hydrothermal fluids (Dai et al., 2012a). Hydrothermal fluids can be categorized into three types: magmatic hydrothermal fluids (Dai et al., 2003a(Dai et al., , 2015aDai and Ren, 2007;Wang et al., 1999;Rimmer et al., 2009;Zhao et al., 2019b), low-temperature-hydrothermal fluids (Dai et al., 2004;Xie et al., 2017;Pei et al., 2017), and submarine exhalation (Dai et al., 2008a). ...
This study investigated the elevated concentrations and modes of occurrence of trace elements such as Cu, As, and Se and sulfide minerals in the Pennsylvanian No. 9 coal seam from the Yunjialing Mine, the Handan Coalfield, north China. The techniques used for the investigation included electron probe micro-analysis, X-ray spectrometry, X-ray powder diffraction, atomic fluorescence spectrometry, and inductively-coupled plasma mass spectrometry. The coal in Yunjialing Mine is anthracite and is characterized by a high total sulfur content (0.61%-7.12%, dry basis). Minerals identified in the Yunjialing coal mainly include illite, quartz, calcite, and kaolinite and trace amounts of anatase, chalcopyrite, chalcocite, and chamosite. Furthermore, the studied coal samples are enriched in Cu content (431 μg/g on average, whole coal basis, the same below), with up to 1,095 μg/g in an individual coal ply. Moreover, the coals are slightly enriched in As (20.9 μg/g on average) and Se (4.27 μg/g on average). In this study, high proportions of albite and illite and almost all the quartz were derived from the sediment source region in the north of the basin. The high contents of sulfur and pyrite in the coal seam indicate the occurrence of seawater transgression during peat accumulation. However, the highly elevated concentrations of elements (especially S, Cu, and Ca) are found only in the upper part of the coal seam, along with ring-banded sulfide minerals (e.g., chalcopyrite and chalcocite), cleat-or fracture-filling calcite, and cavity-filling chamosite. These minerals were probably precipitated from the Ca-S-Fe-Mg-Cu-rich hydrothermal fluids generated by the igneous intrusions.
... The microscopic morphologies (Zhang et al., 2019) of fracture surface are shown in Figure 10. ...
In this work, a series of intensive laboratory tests are conducted to measure the material constituents, mechanical properties, and to examine macro-micro-failure modes of various types of rocks from tight gas reservoirs in the Da Qing oilfield in China. A set of key parameters are experimentally determined, including porosity, mineralogical compositions, microstructure, Young’s modulus, Poisson’s ratio, triaxial compressive strength, as well as macro- and micro-morphology failure modes. The relationships of these parameters are systematically analyzed, and the effects of the material constituents and microstructure characteristics such as cementation type, porosity, and mineral composition on rock mechanical properties are revealed as well as the patterns of micro- and macro-failures in types of rocks are investigated. The result shows that the micro-failure mainly exhibits features of transgranular and intergranular porous polymer fracture, and the macro-failure modes are mainly three types: shear-dominated, mixed shear–tensile and mixed tensile–shear. The mixed tensile–shear failure has mainly tensile fractures with branch fractures crossing each other, which forms a complex system fracture network. These findings are of importance for “sweet pot” evaluations, wellbore stability analysis, and hydraulic fracturing design for oil and gas production in tight gas reservoirs.
In order to ascertain the kaolinite crystallinity of Carboniferous Permian coal-measure kaolinite rocks, seven groups of fresh samples were collected from below the ground in the Xiaoyu mine, Datong coalfield. Microscopy, X-ray diffraction (XRD), differential thermal analysis (DTA), infrared (IR) spectroscopy and X-ray fluorescence (XRF) spectrometry methods were applied to the samples. The petrographic analysis results show that the kaolinite rocks are characterized as compact, phaneritic, clastic, sand-bearing, sandy and silty types; the kaolinite content in the Shanxi formation and upper Taiyuan formations was more than 95%, while it was 60–90% in the middle and lower Taiyuan formations. Based on the Hinckley index and the features of XRD, DTA and IR of kaolinites, crystallinity was classified as having three grades: ordered, slightly disordered and disordered. The kaolinites’ SiO2 /Al2O3 molar ratio was about 1.9–5.7, with a chemical index of alteration (CIA) of about 95.4–99.5. This research suggests that the kaolinite crystallinity correlates positively to its clay mineral content, purity and particle size, which are also related to the SiO2/Al2O3 molar ratio and CIA. The original sedimentary environment and weathering have a direct influence on kaolinite crystallinity, and the existence of organic matter is conducive to the stable existence of kaolinite. The study results have significance for the extraction and utilization of coal-measure kaolinite and the development of kaolinite crystallography and mineralogy.
Historically coal mining wastes have been viewed as heterogenous and hazard-prone geomaterials. Given that failures of colliery tips and tailing dams are reported on a regular basis, reclamation of coal mining wastes from storage facilities is increasingly being considered. There is a resistance to the use of coal mining waste in construction industry despite scattered but growing reports of successful applications. As the construction industry around the globe seeks to reduce its carbon emissions by looking for supplements for cement, the voluminous amount of coal mining wastes currently stored in spoil heaps and impoundment facilities present a potential source of raw materials. This article reviews the literature on the geochemical, geotechnical and structural engineering properties of coal mining waste geomaterials to assess their suitability as replacement for both aggregates and binders in concrete and cementitious composites (as opposed to reviewing the properties of those products themselves). It is found that coal mining wastes are indeed good candidates (as raw materials) for the uptake and process into higher level construction purposes. Geochemically, the key to a successful upcycling operation is the knowledge of their mineral contents (which is typically diverse and varies from one mine to another) and the processes they undergo while being transformed into constituents of new materials. The few studies on concretes made with coal mining wastes indicate that the mineralogical and mechanical characterisation of the wastes to obtain a mix featuring strength and durability performance that meets specification is important to a successful utilisation. In the geotechnical literature, coal mining wastes are known to be highly heterogeneous and may host expandable minerals with potential durability problems. However, this review also found that simple geotechnical index tests can be conducted to yield useful information for the initial screening of coal mining wastes into a construction product. The state-dependent properties of coal mining wastes (e.g., water retention, hydraulic conductivity, shear strength) are found to be governed by complex factors such as coal content, particle size and shape, pore size and shape, and the presence and interaction of pore air and pore water in the void space, some of these are well-studied but much of these are to be further researched.
The Xihu Sag in the East China Sea Shelf Basin contains abundant oil and gas reserves and is a focus for hydrocarbon exploration and development. Source rocks are mainly coals and coal-measures mudstones in the Paleogene Pinghu and Huagang formations. Samples from the Pinghu Formation in the Xihu Sag were collected for petrology, total organic carbon, and Rock-Eval analysis for the purpose of investigating macerals component and their contributions to hydrocarbon generation potential. The coaly source rocks from the Pinghu Formation are dominated by vitrinite (average 86.18%) but have an obviously elevated content of liptinite (average 12.59%) and a much lower amount of inertinite (average 1.23%). Liptinite of the samples is mainly composed of resinite, with a small amount of cutinite, sporinite and alginate in descending order. TOC values are 37.55%–65.58% (average 49.16%). Effective HI values are 167–281 mg HC/g TOC (average 223.5 mg HC/g TOC), suggesting the organic matter is type II kerogen. Relatively high HI values and macerals components suggest that the coaly source rocks can generate both oil and gas. Although the liptinite in the coaly source rocks has a content lower than vitrinite values, it makes a significant contribution to both total hydrocarbon and liquid hydrocarbon generation. The contributions of vitrinite, liptinite and inertinite to the total hydrocarbon generation approximately are 63.21%, 36.46% and 0.33%, respectively. The contributions of vitrinite and liptinite to the liquid hydrocarbon generation are approximately 40.95% and 59.05%, respectively. These results demonstrate that the coaly source rocks are dominated by vitrinite macerals with a relatively higher content of liptinite macerals, especially resinite, and these source rocks are more prone to both total hydrocarbon and liquid hydrocarbon generation. Paleogene coaly source rocks from other parts of the world should be considered for their oil-prone nature.
