International Journal of Coal Geology最新文献

Industrial applications of coal rely on understanding its macromolecular structure, which is primarily controlled by coal type and rank. The present study assessed five (5) samples from different collieries extracting coal from the No. 4 Seam of the Highveld Coalfield and their float products, produced at relative densities (RD) of 1.7 and 1.9 g/cm³. The aim was to assess changes in maceral composition and coal quality following the density fractionation, and to use Raman Spectroscopy to compare differences in macromolecular structures between the parent samples and the float products. Raman parameters were also determined for specific macerals, i.e., semifusinite and collotelinite. Mean random vitrinite reflectance (%RoV) values for the studied coals range between 0.57 and 0.60% (medium rank D/C bituminous) and the parent coals are inertinite-rich (70.3 to 88.7 vol% mmf), enriched in semifusinite and inertodetrinite. Following density fractionation, reactive macerals (a combination of liptinite, vitrinite, and reactive semifusinite) are enriched in the float products (designated by “F”), specifically in the products obtained at the 1.7 RD. In comparison, the proportion of inert macerals is higher in the F1.9 samples. These differences in maceral composition are reflected in the Raman spectra and parameters. Although the G and D1 bands for the parent coals and F1.9 samples are similar, these bands are narrower than for the F1.7 samples, indicative of greater aromaticity. The G FWHM values for the F1.9 samples are comparable to those for the parent coal samples, and lower than for the F1.7 samples. This reflects larger differences in maceral composition between the parent coals and the F1.7 samples. In contrast, the D1 FWHM values for the float products, particularly the F1.7 samples, are slightly higher than the parent coals, reflecting a disordered aromatic character mainly related to the presence of aliphatic chains. The Raman spectra for the F1.7 samples are more like that for collotelinite. In contrast, the Raman spectra and parameters (G and D1 FWHM) for the F1.9 samples are more comparable to semifusinite. Thus, the increased aliphaticity for the F1.7 samples is attributed to the relative enrichment of reactive macerals, whereas higher aromaticity for the F1.9 samples reflects a larger proportion of inert macerals. Raman spectroscopy expanded on the petrographic data by interrogating the macromolecular structure of the isorank Highveld coals and their float products. This may assist in predicting the behaviour of the coals during industrial applications (i.e., liquefaction, gasification, combustion, and carbon fibre production).

Identifying the dominating oil-producing layer(s) within a shaly system, typically characterized by multiple layers with similar properties, is always a critical yet formidable task, as the oil component disparities among these closely adjacent layers are too minor to be resolved by the traditional geochemical fingerprints. This challenge is now likely addressed by high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), which can resolve thousands of non-hydrocarbons that could serve as new fingerprints. Taking a typical shaly system in the Ordos Basin in China as an example, the specific proportions of non-hydrocarbon compounds from the retained petroleum of different source rock layers (rock extracts) and the produced oil at the wellhead were identified by FT-ICR MS. Their compositional similarity was calculated using a multidimensional scaling (MDS) method, and the layers hosting the retained petroleum with higher compositional similarity to the produced oils are considered to be the main contributors to oil production. The results show that the main producing layers identified by FT-ICR MS differ from those proposed based on traditional fingerprints like the Rock-Eval parameters. A typical example is that the thick low-TOC shales, conventionally proposed to be unessential to oil production, seem to have a similar contribution, if not higher, to their thick silty counterparts. This divergence may be attributed to the fact that the conventional fingerprints primarily rely on the rock extracts from a vertical well section, which may only represent a limited lateral area, while the non-hydrocarbon similarity approach involving the produced oil appears to be more realistic, as it can consider the engineering processes, like the horizontal well track and the hydraulic fracturing effects. The current approach provides a novel route for identifying the dominating producing layer(s) in a shale oil system, which may have extensive potential for optimizing the production strategy of shale oil wells.

This paper describes the evaluation of petrographic textures in char and activated carbon derived from coal, coal by-products and biomass, formed during carbonization and activation processes. This work represents the results of interlaboratory exercises from 2016 to 2022 of the Microscopy of Carbon Materials Working Group in Commission III of the International Committee for Coal and Organic Petrology. The interlaboratory exercises were run on photomicrograph samples. For textural characterization of carbon materials, the existing American Society for Testing and Materials (ASTM) classification system for metallurgical coke was applied. Morphological differences were evaluated in 29 carbon material types, including 22 char samples, and 7 activated carbon (AC) samples obtained experimentally using conventional direct/indirect and microwave heating technologies. This approach gives an extended view on the identification of microporous carbons, and how a certain heat treatment develops a certain optical texture and structure in a raw material. The requested evaluation of carbon materials was related to their porosity, origin, extent, and characteristics, which are particular to each carbon material type. Because carbon matrices can form a wide range of optical textures during heat treatment it is important to demonstrate which carbon occurrences will have a crucial role in industrial applications dominated by adsorption phenomena.

The interlaboratory exercises included 17 participants from 14 laboratories. Four sets of digital black and white and colour photomicrographs were distributed, which in total comprised 184 fields of different types of carbon material. The results were evaluated based on four levels: (i) optical texture (isotropic/anisotropic), (ii) optical type and size (punctiform, mosaic, fiber, ribbon, domain), iii) morphology (porous, non-porous/massive), and (iv) particle origin (precursor type).

The statistical method applied to evaluate the results was based on “raw agreement indices”. Comparative analyses of the average values of the level of overall agreement showed homogeneity in the results, the mean value was 89%, with a minimum value of 87% and a maximum value of 91% for those who participated in at least three out of four exercises.

The coal matrix exhibits significant heterogeneity at the micro-scale. In this study, a heterogeneous mechanical model of coal matrix from different coal ranks was constructed by using nanoindentation. Additionally, a fluid-solid coupling model was developed to consider matrix shrinkage and stress sensitivity, enabling the incorporation of porosity changes during methane diffusion. The methane concentration and matrix deformation characteristics in the simulation area at different times was obtained. The effects of matrix shrinkage and stress sensitivity on porosity changes and matrix deformation were compared. And the deformation of the matrix under different micromechanical properties was compared. The findings indicate that the nanoindentation method effectively characterizes the micromechanical heterogeneity of coal matrices. Furthermore, during the diffusion process, we observed an initial increase followed by a decrease in matrix deformation, with matrix shrinkage leading to more pronounced deformation. Notably, harder coal matrices exhibited reduced deformation. This study enhances the understanding of the dynamic changes in the matrix during coalbed methane extraction.

The upper Serpukhovian Poruba Member (c. 325–324 Ma) is a coal-bearing “paralic” succession deposited in the Upper Silesian Coal Basin located along the eastern foreland of the Moravo-Silesian segment of the Variscan fold and thrust belt. The basin formed an >150 km long, tectonically controlled embayment open to the north and northeast, with estuarine circulation, predominance of fluvial discharge and a limited tidal influence. A high-accommodation depocentre (possibly up to 1100 m/Myr) was filled by shallowing-upward successions related to the progradation of river dominated (bay-head) deltas, with subordinate fluvial and marine sediments. These strata form two orders of transgressive-regressive cycles, or genetic sequences, both overlapping with the Milankovitch band. Intervals of maximum transgression, marked by marine or brackish faunal horizons, immediately overlie coal beds, suggesting non-accretionary transgression in a low energy setting. Six medium-term genetic sequences (cyclothems) are recognized, each consisting of 4 to 6 elementary sequences and a number of smaller scale units of possible autocyclic origin. The medium-term sequences are attributed to a combined influence of relative sea-level change and changes in sediment input, both possibly as a far field response to Gondwanan glaciation through glacioeustasy and attendant changes in climatic seasonality.

Niobium, Zr, REE, and Y are widely enriched in the Upper Permian coal-bearing strata in southwestern China and are regarded as a substantial resource. To better understand the occurrence modes and enrichment mechanisms of these rare metals, comprehensive heavy-mineral separation, Micro-Raman spectroscopy, EPMA, and LA-ICP-MS analyses of primary and secondary metal minerals were conducted in the polymetallic beds in western Guizhou. In addition, the XRD, XRF, and ICP-MS analyses of bulk rocks were carried out. Our results suggest that Nb is primarily hosted in secondary TiO₂ polymorphs which converted from primary Nb-rich rutile, and occurs as minor ilmenorutile and columbite-(Fe). The Zr is sequestered in both detrital zircon and secondary microcrystalline zircon. REE and Y (REY) are mainly incorporated into secondary florencite-(Ce), rhabdophane-(Ce), churchite-(Y), and zircon, also partially adsorbed by clay minerals. The assemblage of detrital zircon, Nb-rich rutile, and discrete Nb minerals reveals that rare metals are preconcentrated during the waning-stage Emeishan alkaline magmatisms. Complex zonings in rutile crystals may result from the diffusion of Nb⁵⁺ and Fe²⁺, and these grains are further overprinted by magmatic-hydrothermal and enveloped by ilmenorutile and columbite-(Fe). Furthermore, magmatic rutile is commonly converted into anatase, brookite, and leucoxene under supergene conditions, and then persisted as stable phases. Authigenic REY-rich (Al)-phosphates and zircon are sourced from the leaching, even decomposition of primary REY-phosphates and zircons. Their colloidal aggregates are precipitated from the acidic and oxidizing low-temperature fluids, which are superposed onto clay matrix. The mineralogical and geochemical features suggest that the polymetallic beds are enriched by both alkaline magmatic-hydrothermal processes in source rocks and supergene fluid alterations in sediments. This study gives new insights into the enrichment processes and conditions, and further extraction of critical metals in the coeval coal-bearing strata in southwestern China.

The Permian Pingdiquan Formation (P₂p) in Shishugou Sag, Junggar Basin, Northwest China, is a typical example of mixed sedimentary strata formed in a saline lacustrine environment, characterized by complex lithological assemblages and extensive distribution of organic-rich source rocks. To further investigate the differences in oil-source contribution and hydrocarbon generation characteristics of source rocks with different lithologies, studying the difference of their geochemical characteristics and the formation mechanism of organic-rich source rocks is the top priority. Based on a comprehensive analysis of lithologic types, geochemical characteristics, and hydrocarbon generation potential of source rocks using organic petrology, molecular geochemistry and element geochemistry, this paper reveals the formation background and organic matter enrichment mechanism of the P₂p organic-rich source rocks. The results show that the mixed sedimentary rocks of the P₂p were deposited under relatively arid and saline lacustrine conditions. Paleoclimatic conditions tended to be more arid and hydrothermal activity became more frequent during the sedimentation process from mudstone, dolomitic mudstone to argillaceous dolomite. Consequently, the water column salinity elevated, leading to an increased proportion of salt-tolerant green algae and slight salt-tolerant cyanobacteria compared to salt-intolerant algae. Frequent changes in paleoclimate and hydrothermal activity control the interbedding deposition of mudstone, dolomitic mudstone and argillaceous dolomite, and the resulting salinity fluctuations contribute to variations in the proportion of salt-tolerant and intolerant algae in source rocks with different lithologies. Furthermore, the source rocks within the mixed sedimentary strata (mudstone, dolomitic mudstone and argillaceous dolomite) all exhibit substantial hydrocarbon generation potential, and there exists heterogeneity among source rocks of the same lithology. The key factor influencing the development of these organic-rich source rocks is the enhancement of palaeoproductivity driven by volcanic activity. Additionally, the water stratification caused by saline water promotes anoxic water columns, also contributing to organic matter enrichment in the source rocks.

The majority of Middle Devonian coal samples from Wujing (WJ), Damo (DM), and Batang (BT) areas in the Luquan region of China have cutinite content exceeding 50%, classifying them as typical cutinitic liptobioliths. These coals are notable for their thinly-bedded texture, which allows them to split readily into leaf-like laminae. However, coals from these areas display distinct macroscopic, microscopic, and geochemical features. The split laminae from WJ and DM coals are black and ribbon-like, with those from WJ being notably longer and wider. In contrast, BT coal laminae are irregular, small fragments with golden or brown hues. Correspondingly, the cutinite in WJ coals is predominantly thick-walled, DM coals primarily contain a medium-walled type, and BT coals are characterized by a thin-walled type. Additionally, BT coals are rich in sporinite, with some classified as sporinite-rich durain or sporinitic liptobiolith. Abundant tetracyclic diterpenoids were detected in the maltene fraction of all the coal extracts. The concentration and composition of these diterpenoids are influenced by the content and type of cutinite present in the coals. The presence of tetracyclic diterpenoids, with beyerane, atisane and kaurane skeletons, had already evolved in early land plants. This implies that the precursors of these compounds were likely significant components of the cuticles of early land plants or, at least coexisted alongside plant cuticles. The spore assemblages in coals and the microscopic features of cutinite suggest that the coal-forming plants of the Luquan cutinitic liptobioliths were early land plants, predominantly herbaceous lycopsids, rhyniopsids, Orestovia and Spongiophyton. However, it is important to note that the specific types of these coal-forming plants may vary by areas. This variation leads to the distinctive macroscopic and microscopic characteristics observed in coals from different locations.

Cutinitic liptobiolith appears to have been exclusive to the Middle Devonian period, especially during the Givetian. The emergence of this distinct coal type is likely due to the unique structure of Devonian land plants. Specifically, the combination of well-developed cuticles and less developed woody tissues in these early land plants might have played a key role in the formation of cutinitic liptobioliths.

Coal seams are typically self-sourced reservoirs. However, in certain special cases, the coal seams also contain large amounts of external gas from other source rocks. Successful development strategies in relation to Jurassic coalbed methane (CBM) in recent years have confirmed the presence of large amounts of external gas within the coal seams of the Baijiahai Uplift, Junggar Basin, China, which has important exploration and development prospects. However, the genesis and source of Jurassic CBM in the area remain unclear, limiting our understanding of its formation and accumulation as well as the planning of further development strategies. Based on a comprehensive analysis of the geochemical characteristics of Jurassic CBM, combined with research on the burial and thermal evolution history of the Jurassic coal seams, this study discusses the production curves of typical wells as well as regional tectonic styles, genesis, source, and accumulation of Jurassic CBM in the Baijiahai Uplift. Depending on the genesis, Jurassic CBM in the study area is divided mainly into coal-type (humic-type) gas, oil-type (sapropelic-type) gas, and mixtures of them, all of which are thermogenic gases of medium to high maturity. There are two main sources of Jurassic CBM in the Baijiahai Uplift. On the one hand, it comes from the vertical migration of gas generated by Permian and Carboniferous source rocks in the Baijiahai Uplift directly. On the other hand, it comes from the regional lateral migration from the Dongdaohaizi and Fukang Sags. Gas generated by source rocks in the Dongdaohaizi and Fukang Sags is driven by pressure, migrates towards the Baijiahai Uplift through sandstone channels, and accumulates in the Jurassic coal seams through faults. Compared with common self-sourced coal seams, enrichment of the coal seams with large amounts of external gases requires two necessary conditions: (1) Other layers of gas-generating source rocks are present at levels deeper than those of the coal seams, constituting additional gas sources for the coal seams, and (2) an open fault system is developed regionally, providing pathways for gas from other layers to enter the coal seams.