W. Kalkreuth

The relationship between coal petrography and depositional environments from selected coal basins in Canada

Abstract This paper reviews current research in Canada relating to the sedimentology of coal-bearing sequences and the petrographic character of contained seams from selected intermontane and foreland basin settings. The intermontane Cumberland and Stellarton basins of Nova Scotia contain Westphalian coal-bearing strata of different character. Coal seams that formed in fluvial-dominated depositional environments of the Springhill Coalfield, Cumberland Basin, commonly brighten upward and laterally display distinct areal zones: a piedmont zone formed by interfingering of mire and distal fan sheetflow deposits; an inner mire zone largely devoid of partings and a riverine zone with large-scale seam splits about multistorey sandstone bodies. Maceral-based groundwater influence and vegetation indices, combined with miospore analyses, indicate that the peat precursors of the No. 3 seam of the inner mire zone accumulated from a groundwater-influenced (rheotrophic to mesotrophic) forest mire (swamp, fen). Both the gelification of lignin-derived maceral precursors and the domination of mire vegetation by lycopsid trees reflect this groundwater influence. Coal that formed from the frequently flooded paleomires of the Stellarton Basin is generally dull, microbanded and mineral-rich. The location of mires within the basin was largely determined by basin subsidence rates. When subsidence rates were high, mires developed around the margin of a central basin lake. Coal seams that formed from these mires (e.g. McLeod seam) thicken toward basin margins and grade into sapropelic shale near the basin centre. Petrographic analyses of the McLeod seam suggest development from limnic to wet forest swamp conditions and a return to a limnic environment at the top of the seam. Thick coal (e.g. Foord seam) developed near the basin centre during periods of lower subsidence rates and grades into red beds and paleosols toward the basin margin. The Foord seam of the basin centre is characterized by low mineral matter contents and a high proportion of degraded vitrinite macerals in the central parts of the seam, interpreted to reflect prolonged peat accumulation with very limited clastic input. The top and the base of the seam, in contrast, were subjected to significant clastic input, as indicated by microbanding associated with significant amounts of detrital minerals and macerals. Coal seams of the Lower Cretaceous Gates Formation in the Western Canada Basin formed in depositional settings ranging from coastal swamps (strandplains) to the upper delta plain. Strandplain coal is characterized by great lateral continuity and substantial thickness, whereas coal of the upper delta plain is thin and discontinous. The strandplain coal is characterized by variable lithotype sequences (oscillatory, brightening-up, dulling-up). Maceral-based facies parameters indicate a wide range of mire facies for these lithotype successions: open moor, forest moor, herbaceous and/or shrubby marshes and slightly raised mires. Results from megafossil and phyteral analyses suggest that the coastal plain mires were dominated by taxodiaceous conifers, whereas ferns, angiosperms and herbaceous lycopods are thought to have occupied the more marshy and/or marginal (stream/lake side) environments.

Lithotype (maceral) composition and variation as correlated with paleo-wetland environments, Gates formation, northeastern British Columbia, Canada

Abstract Lithotype samples collected from mid-Albian Gates Formation coal seams in northeastern British Columbia, were analysed in order to gain a better understanding of coal facies variation. Compositional boundaries between lithotypes are gradational. From bright to dull coals, there is a progressive decrease in vitrinite and increase in inertinite. Liptinite is negligible (

Sequence stratigraphy of paralic coal-bearing strata: an overview

Abstract Sequence stratigraphy arose in the late 1980s to fundamentally change the science of stratigraphy. Former practice of labeling formations and erecting stratigraphic columns gave place to a dynamic genetic stratigraphic analysis, where the main concern is about understanding the history of sedimentation and to establish models able to predict facies. Born and mainly applied to an environment of petroleum prospecting and exploration, sequence stratigraphy has gained entrance to other branches of sedimentary geology. The present paper gives a short introduction to sequence stratigraphic concepts and shows an overview of its application on coal-bearing strata. Two case studies, one from the Early Permian coals of the Parana Basin, Brazil, and one from the Lower Cretaceous coals of the Western Canada Sedimentary Basin illustrate the concepts.

Coal facies interpretations based on lithotype and maceral variations in Lower Cretaceous (Gates Formation) coals of Western Canada

Abstract Maceral analyses of lithotype samples from Lower Cretaceous coal seams have been used to define compositional characteristics of the various lithotypes and to assess coal facies changes within the seams based on maceral and lithotype variations. The mean petrographic compositions of the lithotypes show a general trend to decreasing vitrinite, increasing inertinite and increasing mineral matter contents from the bright to dull varieties. The bright lithotypes are considered to have been formed in wet and densely forested swamps, whereas the banded lithotypes were formed under somewhat drier conditions. Within the dull lithotype two distinct types were recognized based on differences in petrographic composition and the occurrence or absence of facies diagnostic macerals. It is suggested that these differences are due to formation in distinct mire facies, under the influence of open moor or raised mire conditions. The lithotype variations within the seams, together with the data on their petrographic composition, were used to define facies changes, which occurred during accumulation of the peats. Three types of seam development are recognized: (a) seams containing a high proportion of clastic beds indicative of frequent floodings of the swamps. These seams are characterized by the dominance of oscillatory sequences of bright lithotypes and dulling upward sequences below clastic beds. The brighter lithotypes were formed in forested swamps and fens, whereas for the duller sequences influences of open moor conditions are indicated; (b) seams containing only minor thin clastic beds indicating relative stability of water levels during formation of the peat. These seams are characterized by the predominance of duller lithotypes with common repetitions of dulling upward sequences. The dulling up sequences indicate transitions from wet and very wet forest swamps to drier conditions with lower tree density; (c) transitional seams characterized by a few clastic beds. These seams formed under intermediate conditions of stability in respect to flooding events. The seams are characterized by the predominance of banded lithotypes, dull lithotypes near partings and brightening and dulling up sequences. The brightening up sequences represent transitions from dry forest swamp to wet and very wet forest swamps, while a reverse trend is indicated for the dulling up sequences. The overall petrographic characteristics of full seam sections indicate that the seams are in general relatively low in vitrinite and relatively rich in inertinite macerals. Liptinite content is negligible and mineral matter contents are low. Based on petrographic indices obtained from facies diagnostic macerals, an accumulation of the ancient peats under prevailing wet to very wet forest swamps is suggested. Vicinity to more open moor conditions is indicated for one seam by the higher input of dispersed macerals.

The geology and coal petrology of a Miocene lignite profile at Meliadi Mine, katerini, Greece

Abstract The present study describes vertical variations in petrographic composition of a Miocene lignitebearing succession from the Moschopotamus basin, Greece. The lignite beds are described megascopically in terms of lithotypes and microscopically by maceral contents. Examination of lithotype variations within the seams and the distribution of facies critical macerals such as humotelinite and humodetrinite and the relatively low inertinite contents suggest a formation of the Miocene lignites as reed marsh to wet forest-type swamps in a lower delta plain depositional setting. Reflectances measured on the maceral type eu-ulminite B range from 0.36 to 0.42% R random placing the Meliadi coals into a transition zone from lignite to subbituminous coal.

Petrologic and geochemical characteristics of Seam 9-3 and Seam 2, Xingtai Coalfield, Northern China

Abstract Six samples of Carboniferous (Mississippian–Pennsylvanian) coal (Seam 9-3) and 11 samples of Permian coal (Seam 2) from the Xingtai Coalfield were studied by petrological and organic geochemical methods. Both seams show different petrological and geochemical compositions. In Seam 9-3 of the Carboniferous age, the predominant maceral is vitrinite (83%) whereas in Seam 2 of Permian age, inertinite predominates (45%). “Barkinite” was found with an average content 1% only in Seam 2. Sixty-four different aromatic compounds were identified by gas chromatography (GC)/mass spectrometry (MS) analysis of solvent extracts (Extr) of both seams. Abundant polyaromatic sulfur hydrocarbons (PASH) were determined in coal samples from Seam 9-3, while they are very low in samples from Seam 2. 1,2,5-Trimethylnaphthalene and 1,2,5,6-tetramethylnaphthalene contents are much higher in Seam 2, while 2-methylfluorene contents are higher in Seam 9-3. Cadalene was found in Seam 2 with a high content of 94 mg/kg coal but was not detected in samples from Seam 9-3. This might indicate a different floral contribution to the sedimentary organic matter. All petrologic and geochemical results indicate that Seam 2 formed in a more oxidized environment compared with Seam 9-3.

Sedimentological and petrographical characteristics of Cretaceous strandplain coals: a model for coal accumulation from the North American Western Interior Seaway

Abstract Coal seams formed on many Cretaceous wave-dominated strandplain sediments in North America are characterized by great lateral continuity (tens to hundreds of kilometres), substantial thicknesses (up to 12 m), relatively low ash and sulphur contents. The coals formed behind an active shoreline in areas undergoing subsidence due to shale compaction and dewatering. The zone of peat accumulation was remote from the shoreline and storm/tidal inundations and generally protected from fluvial flooding. If the rate of subsidence was too great, lakes formed and peat did not accumulate. Statistical evaluation of petrographic properties, by correspondence analysis, of the Lower Cretaceous coals show that the strandplain coals form distinctive petrographic groups that are characterized by relatively low vitrinite contents and high inertinite contents. Semifusinite and inertodetrinite dominate in the inertinite maceral group. Liptinite contents are negligible. Tissue Preservation Indices and Gelification Indices indicate for the strandplain coals a forest-type paleodepositional environment in which a relatively low water table allowed the accumulation of oxidized and partly oxidized components (fusinite and semifusinite). Significant amounts of detrital components, such as inertodetrinite and vitrinite B, are diagnostic that some transportation of the organic material took place prior to deposition. Comparison of coal facies and depositional environments from Permian coals of Australian show that the Lower Cretaceous strandplain coals have petrographic similarities to coals that were formed under regressive back-barrier conditions in the Permian. Due to differences in nomenclature, previously interpreted regressive back-barrier environments may be similar to the strandplain environments discussed here.

Physicochemical transformations of coal particles during pyrolysis and combustion.

Abstract The pyrolysis and combustion behaviour of a set of 11 Canadian coals with different ranks (lignite to low volatile bituminous) and maceral compositions has been investigated by TGA techniques. Temperature programmed heating of the coals was carried out both under nitrogen and under air, and the weight loss rates occurring in the two experimental conditions were compared in the whole temperature range studied (100–1000°C). Results showed that the pyrolysis curves of the coals do not match at all with any specific feature of the corresponding combustion profiles, and that the temperatures of initiation of both processes are very different in the low-rank end (higher initiation temperatures for pyrolysis), to become similar only for coal ranks of ∼0.8% vitrinite reflectance and above. This contradicts existing theories which state that coal combustion under TGA conditions is a three-stage process, namely volatiles release, vitrinite combustion and inertinite combustion. The processes leading to the weight loss rates occurring in the early stages of combustion were also investigated, with special emphasis in the temperature interval where no substantial weight losses had occurred yet in either a combustion or a pyrolysis experiment. This was done by heating the coals to 300°C in the TGA under air, and then switching the gas flow to nitrogen and allowing the sample to further devolatilise until 1000°C. Also, partly burnt and/or pyrolysed samples were obtained from the TGA and characterised by optical microscopy techniques. It was observed that the volatile yields of all the coals were substantially reduced as a consequence of their initial heating under air. Besides, evidences of melting and thermal annealing in the inner core of burning coal particles were noticed to occur at lower temperatures than in pyrolysing particles. This was attributed to a sealing effect of the oxidation rim formed in the early stages of combustion, which might give rise to a higher pressure build-up in the inner part of burning particles, thus enhancing the likelihood for condensation reactions to take place in the newly formed metaplast. As combustion profiles are commonly used to infer about combustion behaviour of coals, much care should be exercised in interpreting them, since even in a pure vitrain, two rather than one single material will be involved in the measured weight losses, and, more, these materials will often display fairly different reactivities.

The petrology of Greek brown coals

Abstract Results are given in the petrography of Greek coals collected from most of the major coal-bearing basins in Greece. Rank was determined by measuring reflectances on the maceral varieties eu-ulminite A and eu-ulminite B and on the maceral textinite. Reflectances obtained from these components indicate a coalification stage of brown coal for all samples. Within this group of samples there is, however, a fairly wide scatter of reflectance values indicating for some of them the transition zone from peat to brown coal and for others a coalification stage close to the transition into bituminous coals. Reflectances obtained from eu-ulminite A and eu-ulminite B were found to correlate well with chemical rank parameters such as volatile matter and calorific value. Composition was determined by maceral analysis. The coals are in general characterized by low amounts of macerals of the inertinite group, low to intermediate amounts of macerals of the liptinite group and high amounts of macerals of the huminite group. Within the latter group densinite, attrinite, eu-ulminite and textinite make up the bulk of the samples. Typical macerals observed in the coals are illustrated by two black and white and three colour plates. Cluster analysis based on maceral distribution, mineral matter and reflectance indicates that the samples studied can be divided into three major groups. The first one is dominated by eu-ulminite and densinite with relatively high reflectances. The second is dominated by attrinite, textinite and texto-ulminite with somewhat lower reflectances. The third is represented by a single sample in which textinite and resinite are the most abundant macerals. This sample also has the lowest reflectance.

Burial History and Thermal Maturity, Rocky Mountain Front Ranges, Foothills, and Foreland, East-Central British Columbia and Adjacent Alberta, Canada

The regional pattern of maturation of Cretaceous strata in the study area was determined from vitrinite-reflectance measurements. Maturation increases from west to east across the Foothills to a maximum near the eastern limit of Foothills deformation and decreases farther east. Maturation along the eastern limit of deformation also decreases northward significantly. Reflectance measurements from Carboniferous strata exposed in the Front Ranges are much lower than values from the Lower Cretaceous near the eastern limit of deformation. Modeling using burial history curves indicates the regional maturation pattern largely reflects variations in the depth and/or duration of burial beneath Maastrichtian-Eocene foredeep deposits. However, differential vertical movements associa ed with the Peace River arch/embayment in the Carboniferous, Triassic, Early Cretaceous and Maastrichtian-Eocene had an important effect on the maturation pattern. Determined and estimated maturation levels for reservoir strata are consistent with the known occurrences of gas fields and oil pools, except along the relatively unexplored western margin of the study area. There, moderate maturation levels indicate a potential for wet-gas or oil preservation in shallow structures containing Triassic and Lower Carboniferous carbonates in the south. In the north, structures in the western Foothills deforming Triassic strata with lower levels of maturation are breached.