Dull Coal Research Articles

Candiota coalfield: a world class Brazilian coal deposit

The Candiota coalfield is situated at the southwestern margin of the Sul-Riograndense shield in the State of Rio Grande do Sul, south Brazil. The coal seams and associated sedimentary rocks occur in the Rio Bonito Formation, Guatá Group, of Artinskian/Kungurian age (Lower Permian). The palynological assemblage found in the Candiota coals is characterized by the predominance of pteridophytic spores. Pollen grains from gymnospermic vegetation, as well as algae-related elements, are minor elements in the assemblage. Coal resources in the basin reach more than 12 billion t and the production of run-of-mine coal was about 1,500,000 t in 1989. Several coal beds occur in the basin. However, only the Candiota seam is exploited in an open-cast mine. The macroscopic seam section of the Candiota seam shows a predominance of banded and dull coal, which characterizes a mineral-rich and vitrain-poor coal. Vitrinite never reaches 50% and minerals vary from 36% to 50% under microscopic examination. The coal is poor in inertinite when compared with other Brazilian coals and the content of exinite is about 7%. The composition of samples collected in various areas of the basin is very uniform, as revealed by proximate analysis. The coal is very high in ash yield and even the hand picked vitrain present values of about 12%. The calorific value is very low (3,300 kcal/kg) for samples of run-of-mine coal with 52% ash. Based on data from both moisture and carbon contents of vitrain, the Candiota coal seam can be classified as equivalent to subbituminous A coal in the American Standards for Testing and Materials classification.

Peat accumulation in coastal-plain mires: a model for coals of the Fruitland Formation (Upper Cretaceous) of southern Colorado, USA

In the northwestern part of the San Juan basin, Colorado, thick high-volatile B bituminous coal deposits in the Upper Cretaceous Fruitland Formation are associated with nearshore marine sandstones of the Pictured Cliffs Sandstone. Detailed work along the outcrop and examination of drill cores, revealed two coal-bearing zones in the lower 60 m of the Fruitland Formation. Each zone is up to 13 m thick and consists of interbedded bright and dull coal (average ash values of 17 and 34% on a moisture-free basis, respectively), thin fine-grained clastic partings and abundant altered volcanic ash partings. Isopachs of the interval between the top of the Pictured Cliffs Sandstone and a marker bed (Huerfanito Bentonite Bed) in the underlying Lewis Shale show linear zones where the interval abruptly thickens. These zones, which trend northwest to southeast, represent areas where the shoreline paused during an overall migration to the northeast. Isopach maps of coal in the lower part of the Fruitland Formation and subsurface correlation of shoreface sandstones with coal zones show that the thickest accumulation of coal is 20–25 km landward of these coeval shorelines. The Fruitland coals may be compared to the high-ash peats of the Dismal Swamp in the southeastern U.S.A., which form in pocosin mires about 20 km inland from the Atlantic coast. Clastic deposirion, resulting from coastal processes, precludes the formation of peat in low-lying mires adjacent to the shoreline. The high ash yield, numerous partings and the relationship with the coeval shoreline suggest that the coals in the lower part of the Fruitland Formation accumulated in mires that were transitional from low-lying to raised.

Coal seams of the Walbrzych formation, Intrasudetic Basin, Poland: inferences on changing depositional environment

The petrological composition of coal seams of the Walbrzych Fm. was studied, and depositional conditions were reconstructed on the basis of both coal and clastic sediment analysis. Coal of this formation is of medium- to low-volatile bituminous rank; vitrinite reflectance R o max ranges from 0.8 to 2.0% and volatile matter content from 29 to 16%. The coal seams of the Walbrzych Formation contain mainly banded coal; bright and dull lithotypes occur sporadically. The amount of banded and banded dull coals increases towards the top of the formation at the expense of banded bright coal. Consequently, vitrinite and vitrite content tends to decrease upwards. Peat deposition in the lower part of the Walbrzych Formation occurred predominantly in a telmatic forest zone. The mires were located between interdistributary channels on a delta plain, and they were probably ombrotrophic. In the upper part of the Walbrzych Formation, swamps were the primary sites of peat deposition. Abandoned channels were favoured as peat deposition sites for seams where increases in seam thickness are associated with increases in coarse clastic content below seams. Continuous peat deposition in overbank zones is postulated for seams wherein decreases in seam thickness are associated with increases in coarse clastic content. Where no relationship between seam thickness and coarse clastic content beneath the seam exists, the swamp may have encroached into overbank and channel zones rapidly. Changes in the environment of peat deposition may be related to tectonic activity along the basin margins during the Namurian.

Paleoecological trends and petroleum potential of Upper Carboniferous coal seams of Western Germany

In northwestern Germany, the oldest coal-bearing strata occur in the Upper Namurian and extend into the uppermost Westphalian D. The cumulative thickness of this sedimentary unit is about 4000 m, including more than 100 coal seams. As their thickness rarely exceeds 2 m (equivalent to 15 m of uncompacted peat), coal seams contribute only 5% to the total volume of the sedimentary sequence. The remainder consists of siltstones and sandstones with various amounts of dispersed organic matter that were mainly deposited in a fluvio-deltaic regime. The coal seams investigated are of late Westphalian (B and C) age and between 0.4 and 2.4 m thick. Mean random vitrinite reflectances are in the range of 0.7–1.0%, thus indicating high-volatile bituminous A coals. Petrographic changes in maceral composition and mineral abundances from seat earths underlying the coal seams to the roof strata reveal that change of organic facies and paleodepositional environment are common. In general, the degree of degradation for plant material was high during the deposition of seat earths and basal sections of the seams. This can be deduced from the high content of detrital, mechanically broken macerals and from the relatively high inertinite/vitrinite ratio. Above the base of the seams, thick when vitrinite-rich sections are common and represent time when conditions ar favorable for the preservation of autochthonous organic particles. In the central and upper parts of the seams, dull coals rich in inertinite and sporinite are also abundant. These dull coals mainly occur in rather thick mineral-poor coal seams and may represent highly degraded and oxidized peats that accumulated at morphologically elevated areas. Subsequent deposition of clastic roof strata had only minor influence on organic facies of underlying coal. Coals lying directly mudstone partings or roof strata do not show any specific maceral composition but are generally enriched in early diagenetic pyrite. Vertical variations in maceral composition within the coal profiles are clearly different from seam to seam, thus revealing significant differences between depositional environments of mineral-rich and mineral-poor coal seams (Littke, 1987). Geochemical analyses of petrologically well-defined lithotypes provide additional data that may help us to understand deposttional environments of coal seams and their maturation. Hopanes are major contributions to the saturated fraction of coal extracts, indicating a major contribution of bacterial, microscopically unidentifiable material. The abundance of specific bacteria-derived biomarkers is, however, clearly different in a vitrinite-rich and in a sporinite-rich sample from the same coal seam. Geochemical analyses also provide a means to a better understanding of maturation processes by comparing data from coals of different maturity but similar, petrologically determined organic facies. On the basis of a material balance for the elemental hydrogen (80 wt.%) remains in the coals, party as newly formed bitumen. The transport of hydrocarbons and carbon dioxide out of the coals is nevertheless an efficient process because of the large amount of hydrogen and hy

Hydrocarbon distribution in coals and in dispersed organic matter of different maceral compositions and maturities

More than 60 samples from 5 coal seams and adjacent siltstones and sandstones were selected from a well through Westphalian B sediments from Northern Germany in order to study bitumen generation and migration. The cores were drilled between 1200 und 1530 metres depth. In this interval, vitrinite reflectance (Rr) is increasing from 0.70 to 0.92%. Results reveal high hydrocarbon generation potentials for all coal samples, but not for adjacent clastic rocks, although maceral compositions of both are quite similar. Yields and compositions of thermal extracts from the coals depend on maceral compositions and rank. However, the bulk of the already generated bitumen has not been removed during natural coalification and is not easily releasable by thermovaporisation. A mass balance based on elemental composition of dull coals indicates that in the rather narrow rank interval (0.70–0.92% Rr) a total mass loss of 20% may have occurred by migration. This is evidenced by a loss of about 20% of the original hydrogen, the rest being stored in the coals.

Coal reviewed: depositional controls, modern analogues and ancient climates

Summary Ancient economic coals usually contain more than 90% organic matter and less than 1% sulphur. Sedimentological reconstructions show the interbedded clastics to be fluvial-plain, deltaic, barrier coastline, estuarine or alluvial-fan deposits. However, bases of coal beds unconformably transgress these various facies, and coals only rarely pass laterally into clastic facies. Thus coal-forming peats post-dated the underlying clastic sedimentation and most coals did not form in deltas. Local coal thicknesses can be controlled by compaction of underlying sediment, syndepositional faulting, or basement structure. Coals with more than 1%–2% sulphur are usually overlain within 10 m by marine sediments which provided SO 4 2− by diffusion. However, most coals contain less sulphur and must be freshwater in origin. Lithotype and spore profiles vertically through coals indicate a characteristic change of peat floras. This resulted from equisites-dominated seat earths, followed by diverse tree-sized vegetation which formed bright coal and lastly by low diversity stunted vegetation which formed dull coal. These features compare well with peats formed in modern ombrogenous blanket or raised bogs. The ancient mires formed in rainwaters (low sulphur), excluded clastic sediments (low ash), had a lateral floral zonation, usually post-dated clastic deposition and occurred inland from the shoreline. Modern ombrogenous mires are abundant, especially in humid temperate climates. These peats are forming today during a period of long-term climatic cooling in areas with a frequent reliable rainfall. Ancient coals may also have formed during long-term climatic cooling epochs when volcanic CO 2 was being removed from the atmosphere. This could be regarded as a Gaian biosphere maintaining Earth temperatures similar to those of the present day.

Petrography and depositional conditions of the 64/65 coal seam in the Intrasudetic Basin, SW Poland

Variation in petrological composition of the coal of the 64/65 seam has been studied. Two types of vertical lithotype succession were distinguished. The first type is represented by bright lithotypes interlayered with clastic partings, while the second one shows a succession of lithotypes from bright to dull between clastic partings. The occurrence of these two types is believed to be the result of a different rate of subsidence. A trend of upward-decreasing vitrinite content has been observed in the seam with the second type of the vertical succession. This pattern was frequently interrupted by clastic partings. This tendency was not developed in the seam of the first succession type. On the basis of microlithotype and maceral composition it is found that the 64/65 seam represents a wide range of peat-forming environments from telmatic forest moor to open moor. Almost all bright-banded and banded coals were deposited in telmatic forest moor facies, while dull-banded coal, depending upon its petrographic composition, represents telmatic forest moor, telmatic reed moor, limno-telmatic forest moor and limno-telmatic reed moor. Dull coal represents only an open moor environment. Facial analysis of clastic sediments surrounding the seam revealed that there was a relationship between coal and clastic sedimentation. The features of the seam are strongly related to sub-peat topography, distance from active distributaries, and stability of the area. Sulfur and ash contents as well as character of clastic partings and petrology of the seam suggest conditions typical for groundwater and surface dominated swamps with pH a little higher than 4.

Water-soluble chlorine and associated major cations from the coal and mudrocks of the Cannock and North Staffordshire coalfields

Samples of high-Cl coals and their immediate roof, floor and intraseam dirt bands were analysed for major water-soluble ions in order to predict Cl leachability. In organic-poor mudrocks the total CI is as NaCI groundwater solution which is 100% water-soluble. In the coals the absolute water-soluble CI levels are higher, but even with ultrafine grinding 100% extraction cannot be achieved. The relation of the water-soluble Cl to Na in the coarse fractions and Ca in the ultrafine samples suggests different mechanisms of Cl holding and release. Dull coal has a higher water-soluble Cl content that bright coal, which may be explained by the greater size of dull coal pores and greater accessibility of these pores to the groundwaters. The groundwater Cl is Na-associated and leached with relative ease from coarse samples; it accounts for approximately one-third of the total Cl. For ultrafine samples there is a close correlation between water-soluble Cl −, Ca 2+, HCO − 3 and pH. This Cl − is thought to be combined with the organic matter and released by ion exchange (OH − for Cl −), the acidity created being balanced by carbonate solution. Fine grinding is essential to create a maximum surface area for exchange reactions to take place. If the groundwater Cl concentrations are high enough, they may override the total Cl-coal type relationship, but if the water-soluble Cl is leached from coal thus affected, the ‘normal’ relationship of highest total Cl in bright coal and lowest in dull coal holds true.

Total chlorine in coal seam profiles from the South Staffordshire (Cannock) coalfield

In a coalfield of noted high-chlorine coals, seam profiles were taken from two collieries. Three seam profiles from each colliery were analysed for total chlorine to try to correlate chlorine level with position in the seam. Moisture contents and ash yields were determined for four of the seams. Total chlorine is highest in the coals, reaching 1 wt%, and low in the mudrocks, down to 0.08 wt%. Intermediate chlorine values are associated with dirty coals and carbonaceous mudrocks. Water centrifuged from an intraseam dirt band showed that the moisture held the total chlorine content. Similar element ratios were achieved from water centrifuged from coal. These results, together with National Coal Board groundwater data, suggest that the majority of chlorine in coal has been introduced by chloride-rich groundwaters which evolved during diagenesis, as has been suggested for Illinois and Nottinghamshire coals. There is a very significant positive correlation between coal moisture and total chlorine contents. Total chlorine levels are thus thought to be controlled by groundwater concentration levels and coal porosity, hence the positive relationship between organic carbon and chlorine concentrations. Total chlorine levels vary through the seams, but there is not a consistent change with position in the seam. There exists a correlation between coal type and total chlorine content. Bright coals hold significantly more chlorine than dull coals, owing to different porosities of the coal macerals. Departures from this relationship with coal type may be explained by mineral matter dilution or the ingress of recent groundwaters, overriding the original coal type-chlorine relationship.

Distribution of water-soluble chlorine in coals using stains and acetate peels

A technique has been developed to show the distribution of readily leachable chlorine in high chlorine coals using silver nitrate as a stain. Acetate peels may be taken of the precipitated silver chloride for closer investigation of the relations between the water-soluble chlorine and the coal lithotypes. High chlorine coals from Yorkshire Main Colliery (Barnsley Seam) and Thoresby Colliery (Top Hard Seam), showed that the durain fraction of coal will rapidly yield at least some of its water-soluble chlorine. However, vitrinite is the major host of water-soluble chlorine and the predominant component of bright coal. Bright coal shows no water-soluble chlorine component. This apparent discrepancy is explained by the size and distribution of pores and the permeability of different coal macerals. The water-soluble chlorine component is believed to be a NaCl solution held within coal pores. Pores found within the dull coal are much larger than those found in bright coal especially where the former has a high detrital mineral matter component. Bedding planes within durain bands which are coated by fossil remains or thin fusain layers are seen to be the major contributory fractions of the dull coal.

Petrography and depositional environment of the liddell seam, upper hunter valley, New South Wales

The Liddell Seam occurs in the lower part of the Permian Foybrook Formation of the Sydney Basin, eastern Australia. The seam profile is dominated by banded lithotypes (clarain). Maceral and microlithotype analyses of lithotypes indicate a limited degree of uniformity in their composition. A comparison of whole-seam petrographic analyses with compositions calculated on the basis of the seam lithotype profile and lithotype petrography suggests that the latter method may provide for a rapid assessment of petrographic variation in operating mines. Petrographic analyses form the basis of an interpretation of Liddell peat-swamp facies. The seam contains few clastic layers which are associated with relatively dull coals whose petrography reflects elevated water levels during deposition in the limno-telmatic, reed-moor and open-moor facies. Between these zones the seam profile is dominated by relatively bright coals deposited between periods of inundation, in the telmatic forest-moor and reed-moor zones. The lithotype profile and the high vitrite and clarite content indicate the significant influence of arborescent vegetation, a feature typical of seams deposited in tectonically unstable areas.

Petrology of Some Holocene Peat Sediments from the Okefenokee Swamp-Marsh Complex of Southern Georgia

Ten or more samples of surface peat were obtained from each of six peat-forming environments in the Okefenokee swamp-marsh complex of southern Georgia. These peat-forming environments were chosen for study because they were found from core studies to have been the most important peat producers throughout much of the history of peat formation in the swamp. The peat samples were dehydrated in alcohol, embedded in paraffin (using the methods of Cohen and Spackman, 1972), and sectioned (to 15 µ) with a sliding microtome. The microscopic features found to be most useful in characterizing the peats and relating them to their original depositional and vegetational environments were (1) their color (at 30×); (2) the ratio of framework to matrix; (3) the ratio of nonsedimentary to sedimentary constituents; (4) the proportions of remnants of plants assignable to particular plant species; and (5) the percentages of various accessory ingredients, such as fungal remains, fecal pellets, diatoms, insect parts, sponge spicules, quartz, and charcoal. The peat-forming environments investigated were (1) the Nymphaea environment (open-water marshes dominated by floating aquatic plants); (2) the Carex , Panicum , and Woodwardia environments (glades or island fringes dominated by emergent aquatic plants); and (3) Cyrilla and Taxodium environments (tree islands and swamps). The peat sediments deposited in each of these regions were found to have distinctively different micropetrographic characteristics. Aside from the qualitative and quantitative data collected for the purpose of defining the peat types, data were also collected in a form which could be used to determine the most likely eventual coal compositions of the peats. The herbaceous peats were found to have low percentages of pre-resinites (cell fillings and secretions), pre-sclerotinites (fungal remains), and fusinite (charcoal), and high percentages of pre-micrinites (fine granular debris). On the other hand, peats derived from tree vegetation had higher percentages of pre-resinites, pre-sclerotinites, and fusinites, and had relatively smaller amounts of pre-micrinitic materials. The former would probably result in a massive (unlaminated) dull coal, and the latter would probably become a somewhat brighter laminated type.

Fusinized resins in Gondwana (Permian) coals of India

The micro-constituents identified previously as fungal sclerotia in lower Gondwana coals of India are concluded on detailed study including their re-examination to be in fact fusinized resins. They occur as discrete, medium to highly reflecting bodies of variable shape, size, and form showing optical features that developed on devolatilization during the fusinization of plant resins. They are common to very common in dull coals and rare in bright banded coals.

Pollen from deeply buried coal measures, Taranaki, New Zealand—No. 1

Abstract Five well samples from depths ranging from 10,700 to 13,040 ft were examined for pollen and spore content. The material graded from dull black coal or coaly mudstone to bright black coals below 12,450 ft. All yielded workable pollen floras, though there were fewer grains and species in the lower three samples. Pollen from all five samples indicates that the coal measures~are of Late Eocene age, Runangan down to about 11,000 ft and Kaiatan below, to the bottom of the well at 13,040 ft.

Petrology and preparation of certain (Permian) coal seams of India

The petrological characteristics of many of the Permian coal seams of India have a profound and particular influence upon their preparation and utilization properties. Relatively high proportions of ash are largely derived from the finely dispersed sedimentary mineral matter generally present in all coal types. The proportions of the clastic minerals generally increase with those of the inert seam constituents and are thus generally greatest in the common dull coals. The intimate association of fine mineral matter and inert organic constituents in both macroscopic and microscopic bands makes the problems of preparation more difficult. Reduction in the proportions of clastic mineral matter inevitably lowers the content of the inerts and thus increases those of the coking constituents. Consequently the characteristics of the coke produced vary considerably with the degree of cleaning achieved on the coal. Substantial reduction in the clastic mineral content of the coal charge could be accompanied by significant deterioration in the mechanical strength of the coke produced.

Petrographic examination of washed, screened and crushed samples of coal from the Sydney and Saint Rose coalfields, Nova Scotia

The coal seams of the Sydney coalfield are in several ways extraordinary, since no great differences occur either in rank or petrographic composition. Over a stratigraphic interval of 3,800 feet the volatile matter content varies between 36 and 40 percent (mineral matter free). All 12 seams, with the exception of one, may be classed as very bright coal containing between 73 and 84 percent vitrain, plus bright clarain. However, each seam contains dull coal horizons high in opaque matter and/or exinite. Concentration of these dull parts by washing, screening and crushing was the objective of this study. Coals were examined from the Emery and Harbour seams of the Sydney coalfield, and from No. 5 seam of the St. Rose field.The petrographic analyses were carried out with an integrating stage, using polished specimens of ground coal, mounted in lucite. Accuracy tests on this method were highly satisfactory.Results obtained are: 1. Washed coal is lower in opaque matter than non-washed coal, since opaque matter together with shaly impurities concentrates in the fractions of higher density. 2. Concentration of opaque matter by screening alone depends on the amount and distribution of this material in the seam. Fusain is always higher in the finest fraction. 3. A notable concentration of dull coal is obtained by pounding and subsequent screening. Grinding instead of pounding does not give results. Only by taking advantage of the natural differences in strength of the coal components can concentrations be obtained.

XIII.—The Formation of Rock Joints and the Cleat of Coal

The occurrence in coal seams of planes of division, generally approximately at right angles to the stratification, has been known for centuries. Owing to the important part these planes play in the lay-out of workings, miners have found it necessary carefully to pay heed to their direction and character. They long ago observed, for example, that, in the majority of seams, there are main divisions or “cleats” and secondary divisions or “cross-cleats,” and that these systems intersect at an angle of generally 80 to 90 degrees. It was also seen that cleats are always best developed in the bright parts of bituminous coal, and that they are less distinct, more widely spaced, or even non-apparent in durain and in cannel. When the dull and bright parts are finely laminated, however, the cleat planes pass through the mass without interruption. Sparry inclusions (ankerites) are very common in the cleats of bright coal and equally rare in dull coal; occasionally the inclusions extend a little way into the roof, and then the existence of associated cleat in the roof is particularly easy to detect.

The Nomenclature of the Banded Constituents of Coal

IN view of the wide acceptance in Great Britain of the terms vitrain, clarain, durain, and fusain devised by Dr. Stopes to describe the banded constituents of British bituminous coal, and the introduction of the further terms anthraxylon and attritus by Dr. R. Thiessen, it may be of interest to define the relation between them. At a recent symposium of the Coal Research Club, at which both Dr. Stopes and Dr. Thiessen were present, it appeared to be agreed that the two systems have entirely different bases and that each has its validity and use. It was the important service of Dr. Stopes (Proc. Roy. Soc., 1919) to replace the vague terms bright and dull coal by others capable of exact definition. The basis of her system is a lithological one. There are two kinds of bright coal. Vitrain is not in itself banded, and has a glassy lustre and conchoidal fracture. Clarain is inherently banded or striated, and consequently scatters light, and has a silky lustre, and does not break with a conchoidal fracture. These purely lithological characters are sufficient to define the terms, without resort to the microscope, a feature essential for practical purposes to retain. Dr. Stopes correlated them with the microscopic and chemical characters, but these are under further investigation.

X.—Investigation of the Banded Structure of a Fifeshire Coal Seam

It has long been known that a bituminous coal seam is not a homogeneous bed, but is made up of a number of parallel bands. For example, Karsten in 1826 observed and described three banded constituents of such a seam, viz. Faserkohle, Matt Coal, and Glanz Coal, corresponding respectively to mineral charcoal, dull coal, and bright coal, and he gave some chemical details of their composition. Bowman, 1841, stated “there is a peculiarity in the texture of coal which has not received the attention it deserves. If it be clearly examined it will be found to consist of a series of parallel horizontal layers varying in thickness, these laminæ being best observed in the cross fracture, and often exhibit considerable difference in compactness and lustre. This structure, though in coal seams indistinct, is so general that it seems to point to some law which the chemical geologist might advantageously investigate.” Again, Wethered in 1884 observed that a coal seam is separated by partings into many beds differing in quality as in structure while in 1885 he pointed out the advisability of studying the different bands of a seam, and of recording details of its dull and bright portions.