International Journal of Coal Geology最新文献

{"title":"Aspects of Germanium and Nickel accumulation in low-ash coal: Examples from the Blue Gem coal, eastern Kentucky, USA","authors":"James C. Hower ,&nbsp;Susan M. Rimmer ,&nbsp;Darrell N. Taulbee ,&nbsp;Debora Berti ,&nbsp;Robert B. Finkelman ,&nbsp;David French ,&nbsp;Ian T. Graham ,&nbsp;Harold H. Schobert ,&nbsp;Lei Zhao ,&nbsp;Shifeng Dai","doi":"10.1016/j.coal.2025.104927","DOIUrl":"10.1016/j.coal.2025.104927","url":null,"abstract":"<div><div>The high volatile A bituminous Pennsylvanian (Duckmantian substage) Blue Gem coal, southeastern Kentucky, provides an opportunity to examine the concentration of minor elements in high-vitrinite, low-ash specific gravity fractions of the basal and middle lithotypes and the whole coal. For this study, the distributions of Ge and Ni are emphasized. While previous studies have suggested that Ge may be in an organic association, at least at ranks lower than the high volatile bituminous coal in this study, the studies generally relied upon indirect methods. Nickel, an element with known hyperaccumulation tendencies, is not correlated with the ash yield in the +90 %-vitrinite specific gravity fractions. Among the +90 %-vitrinite samples, Ni, Ge, and vitrinite decrease and Fe concentration increases with a decrease in density. While the absence of a correlation between element concentration and ash content might be an indirect indicator of an organic association, a previous transmission electron microscopy study demonstrated that Ni and Ge were present in a NiSn mineral, with the Ge possibly substituting for Sn. The association of Ge with minerals in high volatile bituminous coal does not necessarily imply that the peat through the low-rank precursors of that coal did not have organic associations. The functional groups responsible for binding inorganics in low-rank coals are lost in the metamorphic passage to bituminous coal and the previously organic elements may be incorporated into clays or other minerals, precipitated as oxides, or lost to the coal system. In the case of the Blue Gem coal, one factor in the coal metamorphism was the flow of hydrothermal brines coincident with the emplacement of the Pine Mountain thrust sheet. New elements introduced to the coal, at the time at a lower rank than the present high volatile A bituminous, could have interacted with elements in organic association, resulting in an episode of mineralization.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"314 ","pages":"Article 104927"},"PeriodicalIF":5.7,"publicationDate":"2026-01-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145796308","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Evolution of maceral composition with thermal maturation of kerogen type II: Example from Silurian marine shales, Baltic Basin","authors":"Grzegorz P. Lis ,&nbsp;Tomasz Topór ,&nbsp;Maria Mastalerz","doi":"10.1016/j.coal.2025.104935","DOIUrl":"10.1016/j.coal.2025.104935","url":null,"abstract":"<div><div>Organic matter (OM) quantity, type, and thermal maturity are fundamental parameters in the evaluation of unconventional shale reservoirs. While numerous prior studies have addressed the chemical evolution of OM, detailed petrographic analyses tracing evolution of maceral composition across a full maturity range are less common, especially for pre-Devonian shale formations. This study examines maceral composition in 93 Lower Silurian shale samples from the Baltic Basin, Poland, spanning a maturity range from immature to overmature (0.50–4.19 % VRE). The results show a systematic evolution of maceral composition with increasing thermal maturity. Immature samples are dominated by macerals of the liptinite group, with bituminite being the most common maceral in the majority of the samples. The exception are organic-lean samples, where alginite and liptodetrinite dominate, possibly due to preferential degradation of bituminite. With increasing maturity, these primary macerals progressively convert to secondary solid bitumen, which becomes the dominant OM component above 0.8 % VRE. The study identifies distinct populations of solid bitumen, including diagenetic, initial-oil, primary-oil, and late-oil varieties, often coexisting, which complicates the use of solid bitumen reflectance as a robust maturity indicator. Graptolites, present in most samples, do not show significant morphological change across maturity but serve as a reliable maturity indicator. The maceral composition, characterized by an abundance of liptinite macerals at lower maturity, indicates that the formation had significant hydrocarbon potential, and that its OM quality is not responsible for the current low gas content. This research provides a quantitative model for OM transformation in a pre-Devonian source rock, bridging the gap between geochemical and petrographic studies and providing a crucial reference for interpreting ancient marine shales.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"314 ","pages":"Article 104935"},"PeriodicalIF":5.7,"publicationDate":"2026-01-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145845305","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Emeishan large igneous province (middle-late Permian) and implications for source rock development in South China (Sichuan Basin)","authors":"Ahmed Mansour ,&nbsp;Lamia A. Abdelhalim ,&nbsp;Wei Hengye ,&nbsp;Thomas Gentzis ,&nbsp;Sameh S. Tahoun ,&nbsp;Daniel Campos","doi":"10.1016/j.coal.2026.104936","DOIUrl":"10.1016/j.coal.2026.104936","url":null,"abstract":"<div><div>The middle-late Permian was an interval with global paleoenvironmental and paleoeceanographic instabilities, including the end-Guadalupian or the Guadlupian-Lopingian major extinction event. It was controlled by massive eruptions of the Emeishan Large Igneous Province (LIP) in South China, leading to the release of significant quantities of greenhouse gases and toxic elements into the ocean-atmosphere ecosystem. Here, the middle-upper Permian strata at the Xibeixiang section and well Daye 1 (DY1) from the western and northern depression of the Sichuan Basin were analyzed for palynological data, total organic carbon (TOC), Rock-Eval pyrolysis, and organic petrography. The identified palynomorphs are dominated by significantly high concentrations of acritarch, mainly of <em>Leiosphaeridia</em> and <em>Lophosphaeridium</em>, along with moderate to low abundances of prasinophytes, mainly <em>Dictyotidium</em> with some <em>Cymatiosphaera</em>. These palynomorphs reflect deposition of the Gufeng and Wujiaping formations in a shallow marine shelf environment characterized by low-energy and reducing conditions, consistent with occasional occurrences of foraminiferal test linings, pollen, and spores. Elevated levels of phytoclasts and sparse spores and pollen in the Gufeng Formation reinforce high terrestrial influx, compared to low phytoclasts and absence of terrestrial palynomorphs in the Wujiaping Formation, which indicated low-energy, distal depositional conditions associated with subsidence of the Guangwang-Kaijiang–Liangping Trough. Source rock evaluation revealed fair to excellent organic matter richness in the Gufeng Formation, compared to poor to very good richness in the Wujiaping Formation. However, both formations exhibited poor present-day hydrocarbon generation potential of Type III–IV kerogen, with most samples have reached the dry gas stage. The back-calculated (original) S₂o and HIo values revealed good to excellent generative potential and Type II to mixed Type II/III kerogen. In the study region, the eruption of the Emeishan LIP resulted in high rates of greenhouse outgasing and isotopically light carbon resulting in global perturbations in the carbon cycle, elevated organic carbon export, and the development of anoxic-euxinic conditions as evidenced from framboidal pyrite size and redox-sensitive elements. Additionally, it controlled a spike in heat flow patterns throughout the Sichuan Basin and thus expedited the thermal evolution of organic matter. As a consequence, the Permian source rocks in the Sichuan Basin entered the peak oil window in the Early Triassic, while they reached the dry gas window by the Jurassic.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"314 ","pages":"Article 104936"},"PeriodicalIF":5.7,"publicationDate":"2026-01-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145893887","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Isotope and chemical alteration of coalbed gases during migration from underground coal mine into the atmosphere","authors":"Yaroslav Bezyk ,&nbsp;Dawid Szurgacz ,&nbsp;Maciej Górka ,&nbsp;Dariusz Strąpoć ,&nbsp;Łukasz Kruszewski ,&nbsp;Przemysław Wachniew ,&nbsp;Jarosław Nęcki ,&nbsp;Miroslaw Zimnoch ,&nbsp;Izabela Sówka ,&nbsp;Magdalena Modelska ,&nbsp;Wojciech Wołkowicz ,&nbsp;Carina van der Veen ,&nbsp;Thomas Röckmann","doi":"10.1016/j.coal.2025.104922","DOIUrl":"10.1016/j.coal.2025.104922","url":null,"abstract":"<div><div>Methane release into mine workings and the atmosphere poses a serious environmental and safety risks. This study examines the composition and stable isotope geochemistry of coalbed methane in a hard coal mine of the Upper Silesian Coal Basin (Poland), with emphasis on gas migration pathways from the coal seam through mine workings and ventilation shaft to the atmosphere. Methane concentrations decrease progressively with distance from the active excavation zone toward the exhaust shaft. In addition to CH₄ and CO₂, the coalbed gas contained notable amounts of nitrogen heterocyclic compounds, sulfur-bearing species, and chlorinated gases, depending on the sampling location and temporal window.</div><div>Geochemical results reveal a strong link between gas migration and isotopic variability in the underground environment. Zones of methane accumulation, commonly associated with bends in return airways, showed enrichment in ¹³C, consistent with diffusive fractionation. Variations in CH₄ and corresponding isotope effects during coal transport indicate multiple gas origins within the coal seam. Isotope signatures point to predominantly thermogenic CH₄ at the active mining level, whereas ¹³C-depleted methane at the exploited and flooded levels suggests secondary microbial generation. The microbial contribution is likely promoted by infiltrating meteoric waters supplying nutrients for methanogenic archaea, as supported by δ²H, δ¹⁸O signatures of mine water, and measurable tritium activity. The absence of enriched δ¹³C values in CO₂ and dissolved inorganic carbon (DIC) further implies mixed thermogenic-microbial gas origin in abandoned mine workings.</div><div>These findings highlight the combined influence of mining operations, diffusion-desorption processes, microbial activity, and gas mixing on the molecular and isotope composition of coalbed gases transported toward the exhaust shaft. Moreover, isotopic data from downwind emission plumes demonstrate the value of integrating underground measurements with atmospheric observations to trace and quantify methane emissions from coal mining operations.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104922"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145689431","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Origin and diagenetic evolution of claystones in the CP2 coal seam, Adaohai Mine, Daqingshan Coalfield: Evidence from mineralogy and geochemistry","authors":"Ni He ,&nbsp;Shangying Li ,&nbsp;Hongfei Cheng ,&nbsp;Zhenxiao Wu ,&nbsp;Yichen Wang ,&nbsp;Shaoxian Liang","doi":"10.1016/j.coal.2025.104913","DOIUrl":"10.1016/j.coal.2025.104913","url":null,"abstract":"<div><div>The Daqingshan Coalfield in Inner Mongolia takes the CP₂ coal section (Shuanmazhuang Formation) as its primary mineable layer, with the CP₄ coal section (Zahuaigou Formation) as a local mineable layer. These coal sections contain abundant claystone resources. This study presents a systematic analysis of claystone samples from the CP₂ seam in the Adaohai Mine, integrating mineralogical and geochemical data to elucidate sediment provenance and diagenetic evolution. The dominant clay minerals include kaolinite, illite (with K⁺ primarily occupying the interlayer sites), and NH₄⁺-bearing illite (with NH₄⁺ primarily occupying the interlayer sites), with minor non-clay phases such as quartz, pyrite, diaspore, anatase, and calcite. The provenance of the claystones in the study area mainly derives from the intermediate-felsic volcanic ash associated with the Late Paleozoic continental arc volcanic activities in the Inner Mongolia Uplift, which is located on the northern margin of the North China Block. During the Late Paleozoic, the Paleo-Asian Ocean Plate subducted beneath the North China Block triggered extensive volcanic activity in the Inner Mongolia Uplift along the northern margin of the North China Block. The erupted volcanic ash was deposited in the coeval peat swamp basins of the North China region. Kaolinite primarily formed through the alteration of volcanic ash in basins, with minor kaolinite originating from the recrystallization of aluminosilicate colloidal solutions. Numerous fine-grained illite and NH₄⁺-bearing illite exhibit textural characteristics of replacing kaolinite grains, indicating that illite and NH₄⁺-bearing illite were formed by the transformation of the precursor mineral kaolinite under hydrothermal fluids associated with thermal anomaly activity in the Late Jurassic-Early Cretaceous of North China. These fluids, enriched in Ca, Mg, Al, Si, and N, can form fracture fillings predominantly composed of silexite, dickite, calcite, diaspore, and NH₄⁺-bearing illite with fractures. Beyond providing thermal impetus for mineral alteration, the hydrothermal fluids also have provided the necessary elements supply for the compositional changes of the claystone, promoting mineral transformation. This work highlights the complex interplay between volcaniclastic provenance and post-depositional hydrothermal processes in shaping clay mineral assemblages in coal-bearing sequences.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104913"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145619780","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"In loving memory of Deolinda Flores (1958–2025)","authors":"Manuela Marques ,&nbsp;Isabel Suárez-Ruiz ,&nbsp;Lopo Vasconcelos ,&nbsp;Henrique Pinheiro ,&nbsp;João Graciano Mendonça Filho","doi":"10.1016/j.coal.2025.104923","DOIUrl":"10.1016/j.coal.2025.104923","url":null,"abstract":"","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104923"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145732135","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A deep learning system for sporomorph identification and Spore Coloration Index (SCI) calculation for evaluating source rocks' thermal maturity","authors":"Richard Bryan Magalhaes Santos ,&nbsp;Karen Soares Augusto ,&nbsp;Sidnei Paciornik ,&nbsp;Marcos Henrique de Pinho Maurício ,&nbsp;Tiago de Souza Mota ,&nbsp;Bruno Conceição do Nascimento ,&nbsp;Italo de Oliveira Matias ,&nbsp;Lauro Tiago Souza Santos ,&nbsp;Leandra Costa Lages ,&nbsp;Gil Marcio Avelino Silva ,&nbsp;André Luiz Durante Spigolon","doi":"10.1016/j.coal.2025.104921","DOIUrl":"10.1016/j.coal.2025.104921","url":null,"abstract":"<div><div>Determining the degree of maturation and thermal evolution in kerogen is vital to the hydrocarbons industry. This study evaluates the thermal maturation of source rock sections in sedimentary basins. Its determination makes it possible to estimate whether a sample from a given depth in an oil well falls within the window of oil or natural gas generation or remains immature. The Spore Coloration Index (SCI) can indicate the maturation of oil- and gas-prone material. The objective of this study is to automate this analysis, thereby achieving significant improvements in time efficiency and reliability. Traditionally, an operator visually identifies the presence of sporomorphs in slides using an optical microscope. When they encounter a sporomorph, the operator visually compares it with standard reference slides. The standard consists of 19 slides of spores of different degrees of maturity, ranging from the lightest to the darkest, from 1.0 to 10.0, in increments of 0.5. Comparison with the standard allows the operator to estimate the Spore Coloration Index (SCI) of a sporomorph. Various tests corroborated the literature's indication that the red channel best correlates with the thermal maturation index. A linear relationship was obtained between the average intensity of the red channel and the SCI (correlation coefficient R² = 0.97). Images of samples from various wells at different depths were acquired under conditions similar to those standardized for organopalynological slides. Deep Learning-based systems were trained to identify sporomorphs in the images. With the help of an expert operator, the objects were manually outlined in the images to create a reference database. This database was separated into a training set and a validation set, allowing the network to learn and then have its performance evaluated (accuracy ≈ 86 %). The captured images feed the system, which identifies the presence of sporomorphs, measures the SCI value of each one, and generates a histogram of SCI distribution for each case, allowing, for the first time, the direct calculation of the SCI without the need for visual analysis.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104921"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145689426","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Assessment and integration of different methodologies for the characterisation of carbon aromaticity and structure in biochar","authors":"K. Alami Sounni ,&nbsp;M. Camps-Arbestain ,&nbsp;J. Kaal ,&nbsp;C.J. Tighe ,&nbsp;M.M. Titirici ,&nbsp;G. Siavalas","doi":"10.1016/j.coal.2025.104925","DOIUrl":"10.1016/j.coal.2025.104925","url":null,"abstract":"<div><div>Carbon removal permanence refers to the duration over which carbon remains securely stored across various reservoirs without re-entering the atmosphere. Random reflectance (R_o) has been proposed as a proxy for assessing carbon permanence in biochar. In this study, R_o was measured for 25 plant-derived biochars and compared with (i) the molar H/C ratio, (ii) Raman spectroscopy parameters, and (iii) pyrolysis-GC–MS fingerprints. Four of the biochars showed bimodal R_o distributions, indicating two distinct reflectance populations. A strong relationship was found between mean R_o and H/C (r² = 0.88). Biochars with H/C ≤ 0.40 had mean R_o ≥ 2.3 %, with ≥83 % of the 500 measurement points per sample exceeding 2 % (IBR_o2), except one bimodal sample. This sample, despite mean R_o of 3.72 %, and H/C of 0.37, had only 65 % of measurements &gt; IBR_o2. Based on the fraction &gt; IBR_o2, the biochars were grouped as: Class 1: &lt; 50 %; Class 2: 50–80 %; Class 3: ≥ 80 %. For unimodal biochars, these classes corresponded to mean R_o &lt; 2.0 %, 2.0–2.3 %, and ≥ 2.3 %, and H/C ≥ 0.45, 0.40–0.45, and ≤ 0.40, respectively. Among Raman parameters, saddle intensity effectively distinguished the classes. Mean R_o inversely related to analytical pyrolysis yield (r² = 0.72). Chemical markers (methoxyphenols and (alkyl)phenols) helped explain transitions between classes. The strong correspondence between mean R_o and H/C in unimodal biochars suggests either proxy may suffice in most cases. However, the fraction &gt; IBR_o2 becomes critical for polymodal, Class 2, non-plant feedstock biochars, or when required by certification registries.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104925"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145732129","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Changes in optical properties and morphology of anthracites oxidized under different conditions: In HNO3, air and plasma oxygen","authors":"Sławomira Pusz ,&nbsp;Anna Hercog ,&nbsp;Karolina Olszowska","doi":"10.1016/j.coal.2025.104926","DOIUrl":"10.1016/j.coal.2025.104926","url":null,"abstract":"<div><div>Anthracites, due to their highly ordered internal structure, are chemically quite inert and thus difficult to process. However, for various applications of anthracites, including as precursors of carbon materials with specific properties, improving their processability is often necessary. For this purpose, various pretreatments are used, among which oxidation methods are of great importance. This work aimed to study the influence of oxidation using three different methods (in concentrated HNO₃, in air at 420 °C, in plasma oxygen) on optical properties and morphology of three anthracites of increasing rank (VIC &lt; MO &lt; DON) from the Lower Silesian Coal Basin and Upper Silesian Coal Basin (Poland) and from Donbass (Ukraine), respectively. Scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), Raman spectroscopy, and light microscopy (imaging and reflectance measurement) allow for following the changes in morphology and optical properties of anthracites after oxidation. In the case of the highest rank DON anthracite, its morphology and optical properties remained almost unchanged following oxidation by all the methods used, except for the narrow oxidation zone created by HNO₃. In the case of lower rank anthracites, MO and VIC, their morphology and optical properties changed significantly under oxidation in HNO₃ and air, but remained unchanged after plasma oxidation. The extent and intensity of the oxidation process were stronger for VIC anthracite than for MO.</div><div>Ultimately, it was shown that the level of anthracite oxidation in HNO₃ and thermal oxidation with air strongly depends on the rank of the anthracite. The nature of the changes in morphology and reflectance parameters also suggests that the liquid agent, HNO₃, acts more intensively but also more closely at the surface of anthracite grains, as it has a lower ability to penetrate deeper into the grains than air. The oxidation level under the influence of oxygen plasma was similar for all anthracites, regardless of their rank.</div><div>Optical properties are very sensitive indicators, which can reveal even very subtle structural changes in anthracites during oxidation, invisible to other research methods. The nature and intensity of changes of optical properties illustrate the extent and level of anthracites' oxidation and can be a useful complement to the results obtained using other research methods for determining anthracites' structural transformations. Due to the growing interest in using anthracite as a potential source of new carbon materials, thorough knowledge of their structural changes resulting from various technological processes, including oxidation, seems to be important from a scientific as well as technological point of view.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104926"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145753508","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sweet spot development in heterogeneous shales of lower Silurian","authors":"Yujie Yuan ,&nbsp;Songtao Wu ,&nbsp;Emad Al-Khdheeawi ,&nbsp;Zihao Lin ,&nbsp;Jun Wang ,&nbsp;Menglei Wang ,&nbsp;Yuhan Zhou ,&nbsp;Lai-Chang Zhang ,&nbsp;Zhuo Feng","doi":"10.1016/j.coal.2025.104902","DOIUrl":"10.1016/j.coal.2025.104902","url":null,"abstract":"<div><div>The identification of sweet spots in shale is crucial for ensuring energy security and the economic development of gas resources. However, the pronounced heterogeneity of laminae and lithofacies presents substantial challenges in the accurate identification of sweet spots, particularly in shale formations deposited across the Early Silurian, which was significantly affected by episodic volcanism. To achieve a high-resolution understanding of sweet spot development and accurately identify the economic intervals, this study conducted an integrated analysis of continuous borehole core samples mostly from the Longmaxi Formation. This involved high-resolution lithological, mineralogical, petrophysical, and geochemical characterizations, combined with in-situ gas content measurements. The results establish lamina-scale criteria for sweet spot identification, emphasizing the critical role of fine-scale heterogeneity. Our findings demonstrate that sweet spot development during the Early Silurian is strongly governed by the architecture of laminae and lithofacies, which significantly influence reservoir quality. Based on the proposed criteria, the optimal sweet spot interval (Class I) was identified in the lower Longmaxi Formation, specifically within the integrated sublayer of ‘upper Long-1₁² + Long-1₁³’. This interval is characterized by interbedded thick silty-thin muddy laminae and organic matter (OM)-rich mixed shale (ORMS). It exhibits the highest total organic carbon (TOC), excellent petrophysical and mechanical properties, well-developed nanopore systems, and the highest in-situ gas content. Additionally, Class II and III sweet spots were identified in the Long-1₁⁴ and the integrated ‘Long-1₁¹ + upper Long-1₁²’ sublayers, respectively. These intervals experienced a less intensive volcanism, exhibiting comparable laminae configurations and dominated by OM-moderate siliceous and mixed shale lithofacies, resulting in favorable reservoir quality and gas content, albeit relatively inferior compared to Class I. In contrast, no sweet spots are observed in the Long-1₂ and Long-2 intervals of the upper Longmaxi Formation, with trace volcanism evidence. These intervals are dominated by thick massive, muddy laminae and OM-poor argillaceous shale lithofacies and exhibit poor petrophysical and mechanical properties with low gas content. This study underscores the importance of laminae and lithofacies in heterogeneous shales and the controlling mechanisms of sweet spot development. The findings provide a scientific basis for more accurate resource evaluation and exploitation of shale gas resources.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"313 ","pages":"Article 104902"},"PeriodicalIF":5.7,"publicationDate":"2026-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145553671","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}