Unaltered Coal Research Articles

Enrichment of rare earth elements in epigenetic dolomite occurring in contact metamorphosed Witbank coals (South Africa)

The emplacement of igneous intrusions into coal alters both the organic and inorganic components, and may result in the addition of specific components, including rare earth elements (REEs), into the country rock. Previous studies on the intruded coals of the Main Karoo Basin (South Africa) have mainly focused on understanding changes to their organic fraction despite the likelihood that the inorganic composition is also affected, as demonstrated by others studying intruded coals elsewhere in the world. This study reports on the effects of a 0.4 m thick dolerite dyke on the mineralogy and geochemistry of Witbank coals, South Africa. Coal samples were collected at 0.25 m intervals on both sides of the dyke (west and east); in addition to the intruded coals, an unaltered coal from the same seam and a sample of the dolerite were included. The samples were assessed using petrography, X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy, and Inductively Coupled Plasma Mass Spectrometry following sequential leaching, focusing on REEs for the latter analysis. Inertinite is the main maceral group in the coal samples; as a result, the impact of the intrusion on the organic fraction is compared using mean random total reflectance (%RoTmr). Total reflectance increases from 1.65 %RoTmr (for the unaltered coal) to 4.17 %RoTmr closer to the dyke on the western side. In contrast, the eastern side reports lower values (max. 1.94 %RoTmr). Similar to reflectance, dolomite is higher in samples from the western side of the dyke. Dolomite is also found in the unaltered coal (4 wt%) and the dolerite sample (15 wt%). Syngenetic dolomite and a weak positive Gd anomaly for the unaltered coal (ΣREE = 83.7 μg/g) suggest the presence of acidic groundwaters during peatification. Subsequently, the coals were also affected by hydrothermal activity, related to the intrusion of the dolerite dyke. For the intruded coals, dolomite (fracture-infilling) and ΣREEs are higher on the western side, at a maximum of 24 wt% and 153.0 μg/g, respectively. Carbonate associated REEs are higher for the intruded coals than the total for the dolerite (ΣREE = 23.3 μg/g). This reflects the incompatibility of REEs with early-formed minerals in the dolerite, concentrating them in the residual melt. Increased reflectance and dolomite (and associated REEs) in coals from the western side suggest pronounced hydrothermal activity, related to the availability of accommodation space (i.e., fractures).

Effects of contact metamorphism on δ13C, δ15N, and XPS nitrogen functional forms of inertinite-rich Witbank coals (South Africa): Indications for possible alteration by hydrothermal fluids

In this study, δ13C, δ15N, and nitrogen functional forms were used to evaluate the effects of contact metamorphism on inertinite-rich Main Karoo Basin coals (South Africa). Eighteen samples were collected at 0.25 m intervals from the western and eastern sides of a ∼ 2.6 m thick dolerite dyke emplaced into the No. 2 Seam; an unaltered coal from the same seam was included as a control (Witbank Coalfield). Inertinite (semifusinite and inertodetrinite) dominated the organic composition of the samples. δ13C and δ15N for the unaltered coal were − 21.1‰ VPDB and 1.5‰ Air, respectively. For the intruded coals from the western side, δ13C ranged between −22.4 and −21.4‰ VPDB, and δ15N between 1.2 and 2.6‰ Air. The δ13C for samples from the eastern side ranged between −21.8 and −20.7‰ VPDB, and between 1.8 and 3.0‰ Air for δ15N. Differences in δ15N between the unaltered and the intruded coals were generally more significant than in δ13C, especially for samples from the eastern side. The positive shifts in δ15N may reflect reactions between organic nitrogen in coal and mobile nitrogen transported by hydrothermal fluids. The absence of a relationship between δ15N and distance from the dyke, may reflect secondary, localized contact metamorphism due to hydrothermal fluid migration within the intruded seam. The circulation of a fluid transporting 15N-rich moieties, apparently more active on the eastern side, was supported by the higher ash yields, δ15N, and Ntotal/Corganic ratios. The correlation between Ntotal/Corganic and ash yields suggests deposition of NH4+ − bearing inorganic components preferentially into the eastern side. Similar to the unaltered coal (N-5 = 47.24 at.%), N-pyrrolic was the dominant nitrogen functional form of the intruded coals, reflecting the developmental pathways for the inertinite macerals as opposed to the contact metamorphism. The low proportion of graphitic (quaternary) nitrogen, irrespective of sample distance from the dyke, suggests metamorphic temperatures lower than 900 °C.

Determination of total organic carbon content using Passey's method in coals of the central Kalahari Karoo Basin, Botswana

This paper focuses on determining total organic carbon (TOC) from boreholes in the Kalahari Basin, Botswana, using Passey's method. The Kalahari Karoo basin is one of several basins in southern Africa filled with Late Carboniferous to Jurassic sedimentary strata that host Permian age coal seams. Nine exploration boreholes (wells) drilled in the central Kalahari Karoo basin are used to determine the Total Organic Carbon potential. Vitrinite reflectance (Ro), proximate and ultimate analyses were conducted on cored coal intervals. Passey's ΔLogR method applied in this study employs resistivity and porosity logs to identify and quantify potential source rocks. Results of Passey's method compared with laboratory-measured carbon showed that Passey's method effectively identifies coal intervals. In terms of TOC calculations, the method works poorly in coal metamorphosed by dolerite intrusions. The heat affected coal samples had Ro from 0.77% to 5.53% and increased in maturity from primarily maceral controlled to high volatile bituminous and anthracite coal. Results from proximate analysis showed compositional changes in the coal were controlled by proximity to sill intrusion, with a decrease in Fixed Carbon and an increase in ash yield in the contact metamorphism zone (2–12 m from sill). For the unaltered coal that has undergone burial maturation displaying Ro of 0.44%–0.65%, the method works well. In unintruded boreholes, correlations between Carbon and calculated TOC indicate strong relationships. Passey's ΔLogR method proved to be a suitable method of estimating TOC on coal that has undergone burial maturation. This study has demonstrated that TOC calculated from the sonic log is more reliable in coal not affected by contact metamorphism than TOC calculated from the density log.

Macromolecular structural changes in contact metamorphosed inertinite-rich coals from the No. 2 Seam, Witbank Coalfield (South Africa): Insights from petrography, NMR and XRD

The response of inertinite-rich Witbank coals (South Africa) to rapid heating was evaluated using petrography, selected Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) structural parameters. A total of thirteen (13) coal samples were collected from the No. 2 Seam; a set of six (6) samples from the western and the eastern sides of a ~ 0.4 m thick dolerite dyke emplaced into the coal seam at 0.25 m intervals. An unaltered coal (Un-C; the 13th sample), collected away from the dyke but within the same seam at the mine, was included for comparison. All 13 coal samples were dominated by inertinite. Semifusinite was most abundant in the samples from the western side of the dyke, whereas the proportion of inertodetrinite was highest in samples from the eastern side. Due to very low vitrinite contents, mean random total reflectance (%RoTmr) was used to compare the maturity of the inertinite-rich coals. A decrease in volatile matter was correlated with an increase in mean random total reflectance (%RoTmr; measured on semifusinite, inertodetrinite, and vitrinite), the latter used as a potential maturity indicator for the thermally altered inertinite-rich coals. Mean random total reflectance rose from a background value of 1.52%RoTmr (for Un-C) to 2.83%RoTmr for the coal sample at the western coal-dyke contact (0.0 m). The corresponding reflectance values for the thermally altered coals from the eastern side were less consistent, decreasing closest to the dyke (1.93%RoTmr for the sample at 0.0 m); this trend is also reflected in the NMR and XRD parameters. Eight (8) samples were selected for the NMR analysis, representative of the maceral composition and maturities of the coals investigated. For Un-C, the fraction of aromatic carbons (fa) was 0.88 (average number of carbons per aromatic cluster (C) = 17), and the interplanar spacing (d002) was 3.48 Å (with a crystallite height of 22.09 Å). For the thermally altered coals from the western side, fa decreased from 0.96 at the coal-dyke contact (C = 26 for sample at 0.0 m) to 0.92 for the sample at 1.25 m from the dyke (C = 20). For samples from the eastern side, fa increased from 0.91 (C = 19 for the sample at 0.25 m) to 0.93 (C = 23 for sample at 0.5 m), before decreasing to 0.89 further from the dyke (C = 18 for sample at 1.25 m). In general, the d002 values for the thermally altered coal samples were larger than for graphite. Similarly, only marginal increases in crystallite heights were observed. Although the dyke induced coal maturation and the resultant increase in fa, the overall lack of graphitic order in the thermally altered coals was ascribed to the limited thermal energy provided by the relatively thin dyke; increased heat input would be required to effect significant structural changes in the inertinite-rich coals. Differences between the two sets of thermally altered coal samples were attributed to the emplacement dynamics of the igneous intrusion, which require further investigation.

Characterization of structure of kaolinite in tectonically deformed coal: evidence of mechanochemistry

ABSTRACT Coal and gas outbursts are one of the most serious disasters in coal mines and are closely related to the distribution of tectonically deformed coal. To explore the environment of formation and the mechanochemical genetics of tectonically deformed coal and investigate the influence of tectonic stress on the crystal structure parameters of kaolinite in coal, this paper identifies whether the occurrence of the advanced evolutionary phenomenon of tectonically deformed coal is dominated by thermal metamorphic temperatures or mechanochemical stress. Three pairs of unaltered coal and tectonically deformed coal samples were collected from three different coal seams at the No. 8 coal mine in Ping Dingshan Henan Province in China. After low-temperature ashing treatment, SEM, XRD and FTIR were used to characterize the kaolinite crystal morphology, crystallinity parameter (Hinckley index, HI), structural defects, and other characteristics. The results show that compared with the unaltered coal, the crystal morphology of kaolinite in tectonically deformed coal is more disordered, the crystal plane spacings, d(020), d(1–10), and d(11–1), increase, the crystallinity parameter HI decrease from 1.018, 1.104, 1.001 (more than 1, ordered) to 0.882, 0.784, 0.978 (less than 1, disordered), and the strength of the characteristic absorption band decreases, the differences in morphology and structural defects are mainly caused by the destruction of bonds, such as -OH, Al-O-Si, Al-O-Al, and Si-O-Si. Tectonic stress promotes the mechanochemical evolution of kaolinite in coal, resulting in lattice dislocations, an increase in crystal plane spacing, an increase in crystal defects and a decrease in the ordering degree. The experimental phenomenon of kaolinite converting to montmorillonite (microcrystalline kaolinite) and the mechanochemical evolution result of the crystallinity parameter decrease reflect that the crystal structure of kaolinite in tectonically deformed coal has not evolved to be ordered with the aid of temperature, which confirms that stress plays a dominant role in the evolution of advanced phenomenon of tectonically deformed coal structures.

Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus.

Macromolecular transformations for tectonically-deformed high volatile bituminous via HRTEM and XRD analyses

Coalbed methane reservoir properties of tectonically deformed coals (TDCs) vary greatly in comparison to unaltered coals. These differences are attributed, in part, to their macromolecular transformations. The structure of different TDCs at various distances from a fault plane was evaluated using image analysis of aromatic fringes from HRTEM micrographs, and from XRD data. The distribution of fringe length, orientation, stacking, and curvature, were quantified to establish the structural organization. From the fringe length, orientation, and XRD data, two deformation stages were identified a) a weak deformation stage (–cataclastic–schistose–scaly) and b) a strong deformation stage (–wrinkled–mylonitic). In the weak deformation stage, the fringe length distribution was consistent, however the strong deformation coals had longer fringes. The majority (94–98%) of the aromatic fringes were individual layers for both stages. There was a significant improvement in the structural alignments however with strong deformation. The common statement that tectonic stress enhances the stacking of the aromatic clusters (and thus perhaps the graphitization ability) was not observed here. Rather, the enhancement in order was limited to the strong deformation stage. The aromatic fringes of the strong deformation coals had greater curvature extents. The XRD data also supported increased organization (lower d002 values) with strong deformation. These observations indicate there was an improvement in the basic structural unit from poorly ordered to more ordered that occurred in the strong deformation stage. These transformations are likely to impact the coalbed methane production and methane behaviors.

Methane sorption behaviour of coals altered by igneous intrusion, South Sumatra Basin

A detailed understanding on how igneous intrusions alter coal properties is important for predicting coalbed methane generation and storage behavior. Twelve coal samples were collected from an area influenced by an andesitic igneous sill in the South Sumatra Basin, Indonesia to investigate the changes in coal properties, water uptake and high-pressure methane sorption behavior under dry and moist condition. Vitrinite reflectance is at about 0.55% for the unaltered samples and ranges from 1.4 to 5.2% for the igneous-affected coals. Those coals affected by the intrusion show higher aromaticity and a lower contribution of carbonyl/carboxyl functional groups as revealed by ATR-FTIR analysis. Water uptake is dramatically decreased in intrusion-altered coals, the water uptake being controlled both by abundance of oxygen-containing functional groups and pore properties within the coal matrix. The results show that methane sorption capacity of selected coals is mainly controlled by micropore volume, where sorption takes place. Intrusion altered and unaltered coals show a similar competitive sorption behavior between water and methane that can be described by a volumetric displacement where on average 1.5 water molecules displace a single methane molecule. Therefore, the significantly larger uptake of water in unaltered coals results in a stronger loss of methane sorption capacity compared to thermally metamorphosed coals.

Microhardness of Coal from Near-Fault Zones in Coal Seams Threatened with Gas-Geodynamic Phenomena, Upper Silesian Coal Basin, Poland

Near-fault coal displays some specific structural and textural features. As the distance to the fault diminishes, one can observe ever stronger, gradual degradation of coal, demonstrated by the emergence of structural distortions exogenic in their origin, visible under a microscope. The process of gradual degradation of coal—manifested by the appearance of structural distortions exogenic in their origin—takes place. This can be observed under a microscope. The measurements of the microhardness of structurally altered coal carried out using the Vickers hardness test. For the purpose of this research, a microhardness tester by the CSM Instruments was used. The microhardness of particular structural types of coal was measured. The procedure encompassed both structurally unaltered and altered coal. The tested objects were exogenically fractured fragments, cataclastic, and mylonitic structures. Each of the analyzed structural types displayed a different range of the microhardness, with the highest values confirmed for the structurally unaltered coal. In the case of fractured coal, the microhardness values were somewhat lower. Finally, the lowest values were ascertained for cataclastic coal. Mylonitic coal, in turn, displayed microhardness values similar to those found in the unaltered coal. It was also observed that, in the case of the unaltered, fractured, and cataclastic coal, cracks propagated in the manner typical of brittle materials, whereas the mylonitic coal revealed some degree of elasticity. The analyzed microhardness parameters expose the structural–textural features of coal, particularly when it comes to the degree and character of destruction of the rock’s original matrix. The specific structural–textural composition of particular types of near-fault creations influences both their sorption parameters and the compactness of coal in a seam.

Petrographic composition of coals and products of coal combustion from the selected combined heat and power plants (CHP) and heating plants in Upper Silesia, Poland

Coal samples and by-products resulting from the combustion process collected from seven combined heat and power (CHP) plants and heating plants located in Upper Silesia, southern Poland, were subjected to petrographic analysis. The coal used as a fuel in these plants was collected from mines of the Upper Silesian Coal Basin; it occurs in a wide range of coalification and has variable quality parameters. The coal is dominated by macerals from the vitrinite group; however, a high content of macerals from the inertinite group has also been observed. Based on petrographic analysis of ash and slags, a highly variable mineral matter content, confirming variable combustion efficiency, has been found. In the case of samples collected from large CHP plants with pulverized and fluidized bed boilers, the mineral matter content is high. The share of mineral matter in combustion products clearly decreases in the case of smaller power plants and heating plants, especially those using grate boilers. The increased content of unburned coal can be explained by the fact that coals of higher rank are often used as fuels in the mentioned plants.The greatest diversity of char forms can be observed in the case of samples collected from small CHP plants. In the case of three samples of slag, collected from small heating plants using grate boilers, additional presence of coke, thermally altered, and unaltered coal has also been found.

Coalification and coal alteration under mild thermal conditions

Coalification temperatures are often considered to be approximately 100–170 °C for bituminous coal and 170–275 °C for anthracite. However, our micropetrographic observations, solid state 27Al magic-angle spinning nuclear magnetic resonance measurements, interpretation of δ13C values for whewellite in pelosiderite concretions from Carboniferous sediments, and assessment of whewellite thermal stability show that coalification temperatures can be significantly lower. Also the temperatures of coal alteration may be substantially lower than is stated. Ordinarily, high-temperature alteration is reported, but microthermometric measurements of fluids temperatures and micropetrographic observations show that the coal alteration can take place at low temperatures. For this reason, coals from the Kladno–Rakovník Basin, part of Late Paleozoic continental basins of the Czech Republic, were analyzed. Regarding coalification, micropetrographic characterizations of unaltered coals, the presence of thermally unstable Al complexes in the coal organic mass documented using 27Al MAS NMR method, and proven occurrence of whewellite in pelosiderite concretions suggest a lower coalification temperature, max. ~ 70 °C. Regarding coal alteration, micropetrographic observations revealed (a) the weaker intensity of fluorescence of liptinite, (b) mylonitic structures and microbreccia with carbonate fluid penetration, and (c) high oxygen content in coals (37–38 wt.%). These phenomena are typical for thermal and oxidative alteration of coal. As the temperature of carbonate fluids inferred from fluid inclusion analysis was evaluated as ~ 100–113 °C, the temperature of coal alteration was suggested as ~ 113 °C; the alteration was caused by hot hydrothermal fluids.

Effects of igneous intrusions on the structure and spontaneous combustion propensity of coal: A case study of bituminous coal in Daxing Mine, China

An investigation of the effects of igneous intrusions on the physicochemical structure and spontaneous combustion characteristics of coal is important for safe production in coal mines. In this study, unaltered coal and thermally-altered coal affected by igneous intrusions were collected from Daxing Mine. The mineral and ash composition, surface functional groups, surface morphology and pore structure were characterized by XRD, XPS, SEM, N2 adsorption and mercury intrusion porosimetry. The spontaneous combustion propensity of coal samples was investigated using oxidation kinetics experiment. Mineral and ash composition analysis revealed that igneous intrusions increased the relative contents of calcite and CaO in the thermally-altered coal. The results of XPS tests indicated that the content of CC/CH slightly increased and an obvious π − π∗ shake-up satellite peak appeared in the thermally-altered coal, but that the contents of CO, CO, and COO showed a decreasing trend. Compared with the pore structure of the unaltered coal, higher porosity, larger pore volume, and larger average pore diameter were found in the thermally-altered coal. Moreover, more pores with diameter >20 nm were formed due to igneous intrusions, which would improve the transport capacity of O2 in pores to react with active sites on pore surfaces in the thermally-altered coal. Moreover, igneous intrusions increased the O2 consumption rate of the thermally-altered coal, and decreased its crossing point temperature and spontaneous combustion propensity evaluating index I. This study concluded that igneous intrusions improved the spontaneous combustion risk of Daxing bituminous coal.

Carbon isotope analysis of whole-coal and vitrinite from intruded coals from the Illinois Basin: No isotopic evidence for thermogenic methane generation

Igneous intrusions into coals and organic-rich rocks may have contributed to global warming in the geologic past through the release of greenhouse gases. Evidence for a large release of thermogenic CH4 from the organics would include significant δ13Corg enrichment in the residual organic matter (OM). However, δ13Corg values of thermally altered OM in coals and shales adjacent to intrusions often show negative trends or, in some cases, ambiguous or positive trends. Previous studies have evaluated the δ13Corg of whole-coal samples rather than that of individual organic components, or macerals. As different macerals have different isotopic compositions, maceral-specific trends may be masked by variations in maceral composition of the bulk samples. This study evaluates the hypothesis that, if a large-scale release of 13C-depleted thermogenic CH4 resulted from intrusion of the coal, then it should have produced 13C-enriched coal residuum and, specifically, vitrinite (the most abundant component of the coal) adjacent to the intrusion. This study reports geochemical and petrographic data for whole-coal samples and vitrinite macerals (separated via density-gradient centrifugation, DGC) from a transect of thermally altered Springfield (No. 5) Coal (Pennsylvanian) in the Illinois Basin.Approaching the dike contact, mean vitrinite reflectance (Rr) increases from background levels of 0.55% up to ~4.8% and liptinites become indistinguishable from vitrinite. Isotropic and fine-grained circular mosaic coke is observed in samples within ~1.3m of the coal/intrusion contact. Approaching the intrusion, volatile matter (VM) decreases and fixed carbon (FC) and %C increase, whereas %H decreases. Total organic carbon (TOC) for whole coal decreases from 77% to 34%, whereas TOC for separated vitrinite fractions increases from ~66% to 93% toward the coal/intrusion contact. Density of DGC-separated vitrinites ranges from 1.27g/mL in the unaltered coal to 1.52g/mL in the sample adjacent to the coal/intrusion contact. Vitrinite density increases with increased Rr and %C, and decreases with increased %H and H/C.Despite these marked geochemical and petrographic changes, no significant changes in δ13Corg values of the whole coal (−25.3‰ to −24.9‰) or the separated vitrinites (−25.3‰ to −25.0‰) occur proximal to the intrusion. Changes in the isotopic signatures are not of a magnitude that would be expected if significant quantities of isotopically depleted thermogenic CH4 had been generated by the intrusive event. Moreover, no petrographic evidence supports the occurrence of condensed or immobilized thermal products due to rapid pyrolysis (such as 12C-rich pyrolytic carbon) close to the intrusion that could have moderated any changes in δ13Corg. Geochemical and petrographic results suggest that only minimal loss of CH4 was associated with the rapid heating of the Springfield (No. 5) Coal by the intrusion. These findings have significant implications for models linking past global warming to the intrusion of coals and carbonaceous shales.

Influence of thermal metamorphism on CBM reservoir characteristics of low-rank bituminous coal

As mining depths increase, new challenges have emerged, such as abrupt changes in coalbed methane (CBM) reservoir rules and gas outbursts, particularly in coal/sill contact metamorphism zones. To investigate the influence of magmatic-contact metamorphism on CBM reservoirs, fifteen samples were selected from the N1708 coalface in the Daxing Mine at different distances from a sill in the Tiefa Coalfield, China. This sill-form intrusive body had a positive effect on the properties of the CBM reservoir. The thermal effect of the igneous sill remoulds the pore structures of thermally metamorphosed coal (TMC). More generally, the seepage pores (pores >100 nm) and the surface areas of the TMC samples were more developed than those of the unaltered samples, suggesting that contact metamorphism might have significant implications for the excellent gas flow/adsorption capacity of TMC. The Langmuir volume of TMC sample #3 was measured as 36.1 m3/t. TMC samples showed higher initial desorption rates and lower moisture; in sample 2, the proportion of gas desorbed (cumulative volume/total volume) reached almost 0.5 in only 90 s. The diffusion coefficients (D) for four TMC samples were within the range 1.41 × 10−11–2.59 × 10−10 m2/s. These results show that the high initial gas desorption rate and low moisture of the TMC have a positive influence on gas diffusion and CBM drainage, but lead to high gas outburst index. The gas content and gas pressure beneath the sill were obviously greater than those in the unaltered coals. The CH4 levels of TMC were slightly higher than those of the unaltered samples. However, approaching the sill, it is significant that CO2 levels showed an obvious increase. It is postulated that the higher CO2 levels of TMC are likely derived from the breakdown of calcium carbonate under high-temperature conditions. These results indicate that the TMC covers dual gas (CH4 and CO2) outburst potential, thus gas pre-drainage before mining is necessary in contact metamorphism zones.

Thermally Altered Coals from Bore Core EBM-1, East Bokaro Coal Field, Damodar Valley, India: A Petrographic Inference

Abstract: Petrographic analysis of the coals from bore core EBM-1 of East Bokaro Coalfield revealed characteristic changes as a result of thermal alteration by an intrusive body. The enhancement of ‘rank’ (increase in the VRr% values) is observed in the heat affected coal in the lower part of the bore core. The vitrinite in the coal at a depth of 984.95 m showed devolatilization vacuoles and initiation of mosaic structure due to heat alteration, dessication cracks and micropores. Meta-liptinite having reflectance value higher (1.67%) than that of the host vitrinite (1.09% - 1.36%) was also recorded. Bi-reflectance values of the un-altered coals were normal in the range of 0.06% - 0.09% where as the altered coal at a depth of 984.95 m showed an abnormal increase in the values (0.23%). Presence of secondary injected mineral matter and char particles further indicate the effect of thermal alteration. Palynological investigation of both altered and un-altered coal reveals that the palynomorphs recovered from the altered coal are dark and opaque due to the charring effect while the palynomorphs from un-altered coals exhibit a clear internal structure and can be easily identified.

An effect of igneous intrusion on the structure, texture and microtexture of coal from the Sośnica coal mine, Upper Silesian Coal Basin, Poland

Two coal samples from the Sośnica coal mine, Poland, were analysed in this study. One sample was the natural char collected at the contact of magmatic intrusion, and another sample was the raw coal that was pyrolised in a laboratory furnace. Temperature of pyrolysis was similar to that calculated for the intrusion. The obtained char was analysed to compare its features with those characterizing a natural char sample. Optical microscopy, transmission electron microscopy and Raman spectroscopy studies show that the char from unaltered coal is characterized by the least developed structure, texture and microtexture compared to the natural char. Hence, it can be concluded that geological pressure generated by both the intrusion and the overburden, strongly affects the process of molecular ordering that took place during the heating of coal. The textural, structural or microtextural parameters of coal cannot be used as a geo-thermometer, because they are strongly dependent not only on the temperature but also on other factors.

Variations of concentration and composition of polycyclic aromatic hydrocarbons in coals in response to dike intrusion in the Huainan coalfield in eastern China

Although extensive studies have been conducted on coals influenced by igneous intrusion, the changes of concentration and composition of polycyclic aromatic hydrocarbons (PAHs) in coals responding to this kind of geological rapid heating are largely unknown. In the present study, thirteen coal samples, including unaltered coal, thermally altered coal and a dike-coal contact sample were collected along a transect approaching an igneous dike intrusion from the No. 3 coal seam of the Zhuji coal mine in Huainan coalfield, China. These samples represent a broad range of coal ranks ranging from high volatile A bituminous to anthracite. Vitrinite reflectance (%Ro) profiles indicate a progressive increase in %Ro value from ambient levels of 0.87–1.02% to 5.16% and include an alteration occurring out to ∼3.6m from the dike. Bulk chemical composition from proximate and ultimate analyses shows the typical trend with distance from the dike. The total amount of extractable PAHs (ΣPAH35), US-EPA priority pollutant PAHs (ΣPAH16) and carcinogenic PAHs (ΣPAH8) range from 3.4–81.1μg/g, 2.5–28.7μg/g and 0.7–16.9μg/g, respectively. PAH concentrations vary regularly with coal rank, reaching a maximum in low volatile bituminous coal (1.62%Ro), located 3.5m from the dike. This rank is higher than reported for coals not affected by contact metamorphism. Contact metamorphism has a dramatic effect on the coalification process and results in a different relationship between the PAH maximum concentration and coal rank. The igneous dike intrusion also has a significant effect on PAH distributions, where the proportions of 4–6 ring PAHs decrease toward the coal-dike contact and PAHs are dominated by those with 2 and 3 rings. The thermally altered coals show a decrease in alkylation compared with that of the unaltered coals, and the dealkylation process is apparently accompanied by the cross linking reaction of PAHs to form a more compact structure of organic macromolecule.

An occurrence of coked bitumen, Raton Formation, Purgatoire River Valley, Colorado, U.S.A.

Numerous examples of coke produced by igneous intrusion into coal have been reported in the Spanish Peaks region of south central Colorado. However, in a recent study of an intruded section of the Raton Formation (Upper Cretaceous-Paleocene) along the Purgatoire River near Medina Plaza, CO, coked bitumen has been observed. This material occurs in “fingers” (hexagonally jointed bodies) in a shaley xenolith within a lamprophyre sill and in carbonaceous Type III shale directly below the sill. The coke fingers are characterized by a remarkable flow mosaic texture, high vitrinite/coke reflectance (average random reflectance between 8% and 9%, but with maximum readings around 14–15%), high anisotropy, abundant devolatilization vacuoles, and an absence of inertinite inclusions. Within the underlying shales, the coked bitumen occurs as pore, void, and fracture linings and fillings. Geochemically, the coke in the fingers has low S1 and S2 values (<0.2 and <3mg HC/g, respectively), and low HI and OI values (<6mg HC/g TOC and <3mg CO2/g TOC, respectively). Within the fingers, there is evidence for multiple stages of accumulation, including coarse-grained circular or ribbon coke frequently containing pores edged by vapor-deposited carbon (pyrolytic carbon), layers of pyrolytic carbon, clusters of spherulitic pyrolytic carbon, and layers of highly porous coke. This coked bitumen is quite different from coked coal from the same locality. The coked coal has a medium-grained circular mosaic texture that is consistent with the high volatile bituminous rank of unaltered coal in the area. The coal-derived coke has similar devolatilization vacuoles but also has numerous inclusions of inertinite macerals such as fusinite and secretinite. A previously reported coal “dike” that had flowed through a sill in the same area also showed circular mosaic texture.These observations suggest that the coke found in this study was not formed by the direct coking of coal, but from a mobile phase (bitumen or pitch) that was subsequently coked by the intrusion. This bitumen was either derived from the abundant coal within the section above the sill, or from the organic matter contained within the Type III carbonaceous shales, or both. Unaltered Type III shale sampled at this site has a HI of ~350mg HC/g TOC suggesting some, albeit limited, capacity to generate petroleum. We suggest that bitumen was generated prior to or during the time of intrusion and was subsequently coked by the high temperatures of the emplaced sill. As the bitumen pooled adjacent to the sill, rapid heating led to the development of mesophase, resulting in a highly anisotropic ribbon mosaic texture on cooling, and vaporization of gases that subsequently condensed along pores, vacuoles, and fractures as pyrolytic carbon and spherulitic pyrolytic carbon. Bitumen within the underlying silty shales was also coked, but to a lesser degree. The texture of this coked bitumen looks very much like commercially produced petroleum coke. This coked bitumen differs from other reports of coked bitumen in its mode of occurrence (coke fingers in a xenolith), ribbon mosaic structure, and extremely high anisotropy and reflectance.

Effects of igneous intrusion on microporosity and gas adsorption capacity of coals in the Haizi Mine, China.

This paper describes the effects of igneous intrusions on pore structure and adsorption capacity of the Permian coals in the Huaibei Coalfield, China. Twelve coal samples were obtained at different distances from a ~120 m extremely thick sill. Comparisons were made between unaltered and heat-affected coals using geochemical data, pore-fracture characteristics, and adsorption properties. Thermal alteration occurs down to ~1.3 × sill thickness. Approaching the sill, the vitrinite reflectance (R o) increased from 2.30% to 2.78%, forming devolatilization vacuoles and a fine mosaic texture. Volatile matter (VM) decreased from 17.6% to 10.0% and the moisture decreased from 3.0% to 1.6%. With decreasing distance to the sill, the micropore volumes initially increased from 0.0054 cm3/g to a maximum of 0.0146 cm3/g and then decreased to 0.0079 cm3/g. The results show that the thermal evolution of the sill obviously changed the coal geochemistry and increased the micropore volume and adsorption capacity of heat-affected coal (60–160 m from the sill) compared with the unaltered coals. The trap effect of the sill prevented the high-pressure gas from being released, forming gas pocket. Mining activities near the sill created a low pressure zone leading to the rapid accumulation of methane and gas outbursts in the Haizi Mine.

Dike intrusions into bituminous coal, Illinois Basin: H, C, N, O isotopic responses to rapid and brief heating

Unlike long-term heating in subsiding sedimentary basins, the near-instantaneous thermal maturation of sedimentary organic matter near magmatic intrusions is comparable to artificial thermal maturation in the laboratory in terms of short duration and limited extent. This study investigates chemical and H, C, N, O isotopic changes in high volatile bituminous coal near two Illinois dike contacts and compares observed patterns and trends with data from other published studies and from artificial maturation experiments. Our study pioneers in quantifying isotopically exchangeable hydrogen and measuring the D/H (i.e., 2H/ 1H) ratio of isotopically non-exchangeable organic hydrogen in kerogen near magmatic contacts. Thermal stress in coal caused a reduction of isotopically exchangeable hydrogen in kerogen from 5% to 6% in unaltered coal to 2–3% at contacts, mostly due to elimination of functional groups (e.g., OH, COOH, NH 2). In contrast to all previously published data on D/H in thermally matured organic matter, the more mature kerogen near the two dike contacts is D-depleted, which is attributed to (i) thermal elimination of D-enriched functional groups, and (ii) thermal drying of hydrologically isolated coal prior to the onset of cracking reactions, thereby precluding D-transfer from relatively D-enriched water into kerogen. Maxima in organic nitrogen concentration and in the atomic N/C ratio of kerogen at a distance of ∼2.5 to ∼3.5 m from the thicker dike indicate that reactive N-compounds had been pyrolytically liberated at high temperature closer to the contact, migrated through the coal seam, and recombined with coal kerogen in a zone of lower temperature. The same principle extends to organic carbon, because a strong δ 13C kerogen vs. δ 15N kerogen correlation across 5.5 m of coal adjacent to the thicker dike indicates that coal was functioning as a flow-through reactor along a dynamic thermal gradient facilitating back-reactions between mobile pyrolysis products from the hot zone as they encounter less hot kerogen. Vein and cell filling carbonate is most abundant in highest rank coals where carbonate δ 13C VPDB and δ 18O VSMOW values are consistent with thermal generation of 13C-depleted and 18O-enriched CO 2 from decarboxylation and pyrolysis of organic matter. Lower background concentrations of 13C-enriched carbonate in thermally unaffected coal may be linked to 13C-enrichment in residual CO 2 in the process of CO 2 reduction via microbial methanogenesis. Our compilation and comparison of available organic H, C, N isotopic findings on magmatic intrusions result in re-assessments of majors factors influencing isotopic shifts in kerogen during magmatic heating. (i) Thermally induced shifts in organic δD values of kerogen are primarily driven by the availability of water or steam. Hydrologic isolation (e.g., near Illinois dikes) results in organic D-depletion in kerogen, whereas more common hydrologic connectivity results in organic D-enrichment. (ii) Shifts in kerogen (or coal) δ 13C and δ 15N values are typically small and may follow sinusoidal patterns over short distances from magmatic contacts. Laterally limited sampling strategies may thus result in misleading and non-representative data. (iii) Fluid transport of chemically active, mobile carbon and nitrogen species and recombination reactions with kerogen result in isotopic changes in kerogen that are unrelated to the original, autochthonous part of kerogen.