Volatile displacement of Meghalaya coals — A pointer to explore low sulphur coals

Volatile displacement, which measures the difference between calculated and experimental volatile matter, is indicative of abnormality in coals which may be related to petrological or chemical parameters. The volatile displacement (δυ) values of Meghalaya coals were calculated from their chemical analyses. Correlations of volatile displacement (δυ) with parameters such as carbon, hydrogen, moisture, oxygen, oxygen plus sulphate sulphur, oxygen plus pyritic sulphur, oxygen plus organic sulphur and total sulphur were studied. An approximately linear relationship exists only between δυ and moisture, and δυ and total sulphur, and not between other parameters. Plots on Seyler’s chart indicate the coals as perhydrous to orthohydrous. The linear relationship with total sulphur indicates that the coals may have become abnormal mainly due to the marine environment of deposition and weathering.     

Modelling of hydrocarbon generation in the Cenozoic Song Hong Basin,Vietnam: a highly prospective basin

The Cenozoic Song Hong Basin, situated on the northern part of the Vietnamese shelf, has been only sporadically explored for hydrocarbons. A review of the results of the exploration efforts so far shows that the distribution of potential source rocks and their time of hydrocarbon generation are the critical risks for finding commercial amounts of hydrocarbons. In the Song Hong Basin, including the Hanoi Trough, the rocks most likely to have source potential are: (1) oil-prone Eocene–Lower Oligocene lacustrine mudstones and coals, (2) oil- and gas-prone Middle Miocene coal beds, (3) gas-prone Upper Oligocene–Lower Miocene coals, and (4) gas- and oil-prone Miocene marine mudstones. To assess the time of hydrocarbon generation from these units, relative to the formation of traps, the generation history was modelled at 32 well and pseudo-well locations. The modelling demonstrates that the two first-mentioned source rock units are especially important. In the northern and northeastern part of the basin and along its western margin traps may have been charged by Eocene–Lower Oligocene source rocks. In the Hanoi Trough, the excellent Middle Miocene coal beds have probably generated hydrocarbons within the last few million years. Thus the huge and still underexplored Song Hong Basin provides attractive areas for further exploration.     

Regional geology, depositional environment and maturity of organic matter of Early to Middle Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides, NE Turkey

Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides in NE Turkey have been investigated using microscopical, petrophysical and detailed organic geochemical methods in order to determine their catagenetic stage, to reveal the composition of the organic matter and to discuss the depositional environment. The Liassic–Dogger period in the Eastern Pontides was characterised by the presence of a rift system which resulted in rock units of very variable lithology and facies. Coal seams, ranging from a few centimetres to several decimetres and intercalated with shales, claystones and sandstones occur within the basal part (the Aggi Formation) as well as in the uppermost part (the Hamurkesen Formation) of the rift deposits. All coal seams investigated are at a catagenetic maturation stage corresponding to subbituminous B up to low volatile bituminous ranks. They represent a depositional environment of short-lived swamp areas with intense aerobic reworking of the higher plant detritus by heterotrophic bacteria, but with possible anaerobic microenvironments at deeper levels. At least some coal seams were influenced temporarily by marine ingressions. Most samples are impure humic coals with highly variable chemical compositions as indicated by the broad range of their hydrogen contents. This variation in hydrogen content is partly attributed to variable contributions of algal material. On the other side, considering several analytical results, the hydrogen variation is primarily due to bacterial reworking that affected the composition of the organic matter to variable extent and resulted especially in an enrichment of bacterial lipids. Bacterial reworking by an active microbial community within the upper layer of the peat swamp is inferred from elevated concentrations of iso-alkanes even exceeding those of the corresponding n-alkanes in several samples which, to the best of our knowledge, have not been observed with coals before.     

Origin and distribution of biomarkers in the sulphur rich Utrillas coal basin – Teruel mining district – Spain

The Utrillas coal facies are located in the Maestrazgo basin in NE Spain. This mining district of Teruel contains sub-bituminous deposits from the Middle Albian (Lower Cretaceous 105 Ma) in areas near a delta estuary with abundant sulphur. The high sulphur content is due to an influx of sulphate caused by the geological recycling of Triassic gypsum from the catchment area into the delta estuary. In some outcrops, the weathered coal reveals leonardite deposits. The depositional environment of the basin originated coals, some of which are currently mined. The organic matter of the coals has been the object of scattered reports. Studies have focused on bulk pyrolysis parameters and microscopic observation in Utrillas samples, as well as the inorganic and insoluble organic fraction.We analysed the organic soluble extract of the Utrillas coals using GC–MS in order to characterize their aliphatic, aromatic and organosulphur compounds. The biomarker distribution allowed us to recognize different inputs, assess their depositional palaeoenvironment and finally determine their degree of maturity. In particular, homologous series of hopanes related to eubacteria were present. Biomarkers characteristic of higher plant inputs were also widely distributed (e.g. phyllocladane or C₂₉ steranes). The presence of linear alkylbenzenes allowed us to recognize the palaeodepositional reducing environments where they were deposited. Specifically, thienylhopanes were associated with sulphur-reducing environments. Finally, the abundance of unsaturated biomarkers such as diacholestenes indicated low-maturity coals. Various aromatic ratios such as the methylphenanthrene index also suggested diagenesis in the initial stage.     

Quality of selected coal seams from Indiana: implications for carbonization

The chemical properties of two high-volatile bituminous coals, the Danville Coal Member of the Dugger Formation and the Lower Block Coal Member of the Brazil Formation from southern Indiana, were compared to understand the differences in their coking behavior. It was determined that of the two, the Lower Block has better characteristics for coking. Observed factors that contribute to the differences in the coking behavior of the coals include carbon content, organic sulfur content, and oxygen/carbon (O/C) ratios. The Lower Block coal has greater carbon content than the Danville coal, leading to a lower O/C ratio, which is more favorable for coking. Organic sulfur content is higher in the Lower Block coal, and a strong correlation was found between organic sulfur and plasticity. The majority of the data for both seams plot in the Type III zone on a van Krevelen diagram, and several samples from the Lower Block coal plot into the Type II zone, suggesting a perhydrous character for those samples. This divergence in properties between the Lower Block and Danville coals may account for the superior coking behavior of the Lower Block coal.     

准噶尔盆地侏罗纪煤成油研究

本文以有机岩石学和有机地球化学研究为基础，结合模拟实验，对中国西北地区准噶尔盆地侏罗纪煤成油问题进行了详细讨论，结果表明，准噶尔盆地侏罗纪煤含有丰富的类脂成分，壳质组特别是角质体含量高可能导致了本区高蜡石油的生成，壳质组和基质镜质体的含量决定了煤的氢指数和热解烃产值，它们共同构成了盆地内煤成油的母质。模拟排油实验证明了煤孔隙的吸附能力是有限的，在一定的压力条件下，煤中液态烃达到一定数量后就可以较好地排出。为了便于生产勘探，文中还讨论了煤系地层的沉积有机相，依据沉积相，有机地球化学和有机岩石学特征将煤系划分成四种沉积有机相:分别是高位沼泽有机相，森林沼泽有机相，流水沼泽有机相和开阔水体有机相，其中，流水沼泽有机相生成液态烃的潜力最大，以含有大量的角质体为特征；森林沼泽有机相的生烃潜力次之，以基质镜质体为主要成分；高位沼泽有机相生烃潜力最差，以惰性组和镜质组为主要的有机组分；而开阔水体有机相不是煤成烃研究的理想场所     

河南省平顶山煤田晚石炭世太原组沉积环境和聚煤沉积特征

平顶山煤田的太原组属于混合型的碳酸盐浅海和陆源碎屑海岸沉积。下部和上部灰岩段主要形成于滨海潮间带和浅海中,并在其中发育行风暴浊流沉积。中部碎屑岩段为障壁岛-泻湖-潮坪体系沉积。太原组煤的显微组分为微镜惰煤,煤质属于低灰高硫煤。     

Palaeoenvironmental control on coal formation, distribution and quality in the Permian Vryheid Formation, East Witbank Coalfield, South Africa

The No. 1 and No. 2 coal seams from the Permian Vryheid Formation in the east Witbank Coalfield, South Africa are described with respect to their distribution, thickness and quality. These two coal seams accumulated in a postglacial climatic environment and peat accumulation was closely associated with and influenced by deposition in a braided river system. The fluvial channels that were syndepositional with peat accumulation have resulted in thinning of coal below and above channel axes and pinch-out of coal adjacent to channel margins. Low-ash coal originated from peat which accumulated in areas away from the influence of clastic sedimentation. In contrast, higher-ash coals are situated adjacent and parallel to channel margins where interbedded channel sand and silt contaminated the peat.The lower No. 1 seam peat originated under near-optimum conditions in a lacustrine swamp which blanketed an underlying platform of glaciofluvial braided river sediment. This peat swamp was not subjected to syndepositional clastic contamination and as a result is of superior quality (lower ash/higher calorific value and volatile matter) than the overlying No. 2 coal seam. The No. 2 seam is split by a clastic parting produced by a braided fluvial channel which transected the swamp midway through peat accumulation. This fluvial clastic parting deleteriously affected coal thickness and quality.A comparison of the Gondwanan Permian peat-forming conditions with those from Carboniferous northern hemisphere counterparts suggests that the differences in coal characteristics between these two regions are probably related to different palaeoclimatic conditions and basin tectonics. Cool-temperate climatic conditions which prevailed over the Permian peat swamps resulted in less species diversification of vegetation at these high-latitude settings than the diverse floral assemblages of the Carboniferous swamps. A stable intracronic basin platform caused lateral dispersion of sedimentary facies rather than the stacking of vertical facies which occurred in rapidly subsiding depositories. Partial exposure of the Permian peat swamps during peat accumulation may account for the relatively higher inertinite content of the coals.     

Tertiary coal geology and stratigraphy of the Port Phillip Basin,Victoria

Results from coal‐exploration drilling in the onshore part of the Port Phillip Basin, Victoria, have established stratigraphic and age correlations of the Lower Miocene Werribee Formation brown coal deposits at Bacchus Marsh to similar brown coals at Altona. The coal deposits occur in a northwest‐southeast structural depression (the Parwan Trough) that appears to be a southeast continuation of the Ballan Graben. Recent drilling for potential coal‐bed methane in the trough has provided new data on the deeper stratigraphy not penetrated by earlier drilling, including recognition of an Upper Cretaceous to Eocene Yaloak Formation coal‐bearing interval, similar to the Anglesea area, Ballan Graben and Lal Lal Basin. Up to 200 m of coal‐bearing sediment and minor volcanics underlie the Miocene coal measures. A marine facies transition takes place between the Miocene coal swamps of the Parwan Trough, through barrier sands west of Werribee, to carbonate facies near Geelong. To the south beneath Port Phillip Bay, a similar transition probably occurs between coal swamps of the Parwan Trough and fully marine carbonate environments of the contiguous Sorrento Graben. The palaeogeographical reconstructions suggest a similar coal‐to‐carbonate facies transition as in the adjacent onshore Gippsland Basin.     

Coalification patterns of the Pittsburgh coal: their origin and bearing on hydrocarbon maturation

The coalification pattern of the Pittsburgh coal as established by isoreflectance contours has an overall trend which intersect with the prevailing structural trend of the Dunkard basin. Reflectance values increase from 0.53% in southeastern Ohio to 1.57% in the Maryland panhandle. Divergences of the reflectance contour pattern from the overall trend coincide in part with the present areas of high geothermal gradient. Crustal radiogenic heat, or regional geothermal heating, was probably the dominant heat source responsible for the coalification of the Pittsburgh coal in the Dunkard basin.A time-temperature, Lopatin-type diagram, which was constructed for the Dunkard basin near Wheeling, West Virginia, delineates conditions on maturation of sediments and provides clues to the times of petroleum migration and to the possible source beds. Silurian strata matured during subsidence and burial prior to the orogeny. The Mississippian and Pennsylvanian strata matured during or after the orogeny. The Pennsylvanian strata west of the Dunkard basin, as suggested by vitrinite reflectance values, are apparently still immature.     

大别山构造带东段杨山组煤的变质演化

采用地史分析与数值模拟相结合的研究方法,对下石炭统杨山组煤的变质演化历程进行了定性和定量研究。结果表明:杨山组煤的变质作用经历了海西旋回中期和燕山旋回中-晚期两个演化阶段,煤级在第一阶段异常低热古地热场中深成变质作用下达到焦煤,第二阶段却在异常高热古地热场导致的热变质作用下剧增至超无烟煤。在包括研究区在内的大别山构造带北麓,海西旋回中期可能具有冷盆的特征,而燕山旋回中-晚期却表现出热盆的性质。      

Simulation of temporal variation for reference evapotranspiration under arid climate

Interpretation of 2500 km of newly processed 2D seismic and regional potential field data has led to the identification of two new Miocene–Oligocene sedimentary basins in the northern part of offshore Egypt. The western part of the basin is deeper and has thick sedimentary cover. The upper part of the Miocene section is tested in its southern part through two wells, where the eastern subbasin is still virgin and is located at the intersection between Temsah basins and southeastern end of the Levantine basin. The main sedimentary unit of the basin is the Miocene–Oligocene sedimentary section, which was proven by the exploration and production activities in Nile Delta to be one of the hydrocarbon-generating facies. This fact is supported by 1D basin analysis using the average parameter values, which shows a hydrocarbon window, extending from lower Miocene to upper Miocene. The basin dimensions are more than 2000 km² area and more than 4000 m of thickness, with variable petroleum system parameters’ potentialities, starting from Tortonian–Serravallian to lower Oligocene section.

     

Geological controls on the sulphur content of coal seams in the Northumberland Coalfield, Northeast England

The sulphur content of coal is an important consideration when developing reserves for exploitation, driven by emission limits from power stations becoming more stringent. Variations in the sulphur content of Westphalian A and B coals from the predominantly freshwater Northumberland Coalfield, Northeast England, were studied according to their regional, stratigraphic and in-seam location. The observed variation in sulphur content spatially increases towards the source area away from more marine influenced areas, with increased sulphur content through time linked to changes in the general depositional environment as conditions became more marine-influenced. A model of basinal surface water and groundwater flow driven by post-depositional source area tectonism is thought to have played only a minor role in contributing secondary sulphur to the coal. However, the isotopic composition of coal pyrite shows a similar range in composition to that of pyrite and other sulphides from the North Pennine Orefield along the southern margin of the coalfield, suggesting an additional potential source of secondary sulphur, as sulphur-rich fluids were expelled northwards through the coal measures during early Permian Variscan transpression from the south. The Westphalian A and B are interpreted as third-order depositional sequences, defined by third-order maximum flooding surfaces. Each sequence is made up of several coal-bearing fourth-order parasequences, which tend to be more brackish to marine in character, on either side of the third-order maximum flooding surfaces when base level was relatively high. The lowest sulphur coals are confined to the lower to middle, relative low stand part of the Westphalian A third-order base level curve and the lowest part of the Westphalian B third-order base level curve. This difference is attributed to a more rapid rise of base level in the Westphalian B. The stratigraphic and spatial distribution of coal sulphur has been used as a guide to prediction of reserve identification for surface mining operations.     

Distribution of sulphur in the tertiary coals of Meghalaya,India

The distribution pattern of the different forms of sulphur in the coal seams of Meghalaya has been worked out. In general, the total sulphur content is found to increase from the base to the top of a seam. There is also a general trend of upward increase of sulphur content in the seam-wise sequence, i.e. the upper seam is found to contain more sulphur than the seam lying below. Pyritic sulphur shows an upward increasing tendency both intraseam- and interseam-wise. Variation in organic sulphur shows an apparent parallelism with that of total sulphur. A gradual decrease in total sulphur content is also seen in the coals of Meghalaya from east to west within the basin of deposition. This regional lateral variation in sulphur content was due to the more marine nature of the peat-forming swamps of the Khasi and Jaintia Hills (eastern Meghalaya) as compared to that of the Garo Hills (western Maghalaya).     

Organic petrology of Paleocene Marcelina Formation coals, Paso Diablo mine, western Venezuela: Tectonic controls on coal type

About 7 Mt of high volatile bituminous coal are produced annually from the four coal zones of the Upper Paleocene Marcelina Formation at the Paso Diablo open-pit mine of western Venezuela. As part of an ongoing coal quality study, we have characterized twenty-two coal channel samples from the mine using organic petrology techniques. Samples also were analyzed for proximate–ultimate parameters, forms of sulfur, free swelling index, ash fusion temperatures, and calorific value.Six of the samples represent incremental benches across the 12–13 m thick No. 4 bed, the stratigraphically lowest mined coal, which is also mined at the 10 km distant Mina Norte open-pit. Organic content of the No. 4 bed indicates an upward increase of woody vegetation and/or greater preservation of organic material throughout the life of the original mire(s). An upward increase in telovitrinite and corresponding decrease in detrovitrinite and inertinite illustrate this trend. In contrast, stratigraphically higher coal groups generally exhibit a ‘dulling upward’ trend.The generally high inertinite content, and low ash yield and sulfur content, suggest that the Paso Diablo coals were deposited in rain-fed raised mires, protected from clastic input and subjected to frequent oxidation and/or moisture stress. However, the two thinnest coal beds (both 0.7 m thick) are each characterized by lower inertinite and higher telovitrinite content relative to the rest of Paso Diablo coal beds, indicative of less well-established raised mire environments prior to drowning.Foreland basin Paleocene coals of western Venezuela, including the Paso Diablo deposit and time-correlative coal deposits of the Táchira and Mérida Andes, are characterized by high inertinite and consistently lower ash and sulfur relative to Eocene and younger coals of the area. We interpret these age-delimited coal quality characteristics to be due to water availability as a function of the tectonic control of subsidence rate. It is postulated that slower subsidence rates dominated during the Paleocene while greater foreland basin subsidence rates during the Eocene–Miocene resulted from the loading of nappe thrust sheets as part of the main construction phases of the Andean orogen. South-southeastward advance and emplacement of the Lara nappes during the oblique transpressive collision of the Caribbean and South American tectonic plates in the Paleocene was further removed from the sites of peat deposition, resulting in slower subsidence rates. Slower subsidence in the Paleocene may have favored the growth of raised mires, generating higher inertinite concentrations through more frequent moisture stress. Consistently low ash yield and sulfur content would be due to the protection from clastic input in raised mires, in addition to the leaching of mineral matter by rainfall and the development of acidic conditions preventing fixation of sulfur. In contrast, peat mires of Eocene–Miocene age encountered rapid subsidence due to the proximity of nappe emplacement, resulting in lower inertinite content, higher and more variable sulfur content, and higher ash yield.     

Petrographic characteristics and depositional conditions of Permian coals of Pench, Kanhan, and Tawa Valley Coalfields of Satpura Basin, Madhya Pradesh, India

This paper attempts to characterize the coals of Satpura Gondwana basin using a large number of pillar coal samples drawn from the working coal mines of Pench, Kanhan, and Tawa (Pathakhera) Valley Coalfields of this basin. This westernmost Gondwana basin of Peninsular India is graben/half-graben type and occupies an area of 12 000 km² with sedimentary fills (>5000 m) ranging in age from Permian to Cretaceous. The Barakar Formation (Permian) is exclusively coal-bearing with a total coal reserve of nearly 2000 Mt. The results show that the coals of this basin are equally rich in inertinite (22.8–58.7%, 24.5–62.0% mmf basis) and vitrinite (24.4–52.4%, 24.4–56.0% mmf basis). The concentration of liptinite ranges from 8.8% to 23.2% (9.0–26.0% mmf basis). The dominant microlithotypes of these coals are inertite and vitrite with comparatively low concentrations of vitrinertite and clarite. The vitrinite reflectance (Rom% values) suggests that the Pench Valley (0.30–0.58%) coals are subbituminous C to high volatile C bituminous in rank, while the Kanhan and Tawa Valley coals (0.52–0.92%) are subbituminous A to high volatile A bituminous in rank. The localized enhancement of rank in the latter two basins has been attributed to the extraneous heat flow from deep-seated igneous intrusions in the basin. The microlithotype composition of these coals is suggestive of their evolution in limno-telmatic zones, under fluvio-lacustrine control with the development of upper deltaic and lower deltaic conditions near the fresh water lacustrines. The floral input is characteristic of forest swamps with intermittent floods, leading to the development of reed moor and open moor facies, particularly in the Pench Valley basin. The Gelification Index (GI) and Tissue Preservation Index (TPI) are suggestive of terrestrial origin with high tree density. Further, moderately high GI and exceedingly high telovitrinite based TPI along with high ash content, particularly for the coals of Kanhan and Tawa Valley Coalfields, are indicative of the recurrence of drier conditions in the forested swamps. Furthermore, lateral variation in TPI values is indicative of increase in the rate of subsidence vis-à-vis depth of the basin from east to west (Pench to Tawa Valley Coalfield). The Ground Water Index (GWI) suggests that these coals have evolved in mires under ombotrophic to mesotrophic hydrological conditions. The Vegetation Index (VI) values are indicative of the dominance of herbaceous plants in the formation of Pench Valley coals and comparatively better forest input in the formation of Kanhan and Tawa Valley coals.     

The structural evolution of organic matter during maturation of coals and its impact on petroleum potential and feedstock for the deep biosphere

The structural evolution of coals during coalification from peat to the end of the high volatile bituminous coal rank (VR_r = 0.22–0.81%) has been studied using a natural maturity series from New Zealand. Samples were studied using a range of standard coal analyses, Rock–Eval analysis, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and pyrolysis gas chromatography (Py-GC). The structural evolution of coal during diagenesis and moderate catagenesis is dominated by defunctionalisation reactions leading to the release of significant amounts of oxygen and thereby to an enrichment of aromatic as well as aliphatic structures within the residual organic matter. Based on the evolution of pyrolysis yields and elemental compositions with maturity it can be demonstrated that oxygen loss is the major cause for increasing Hydrogen Index values or hydrocarbon generating potentials of coals at such maturity levels. For the first time, the loss of oxygen in form of CO₂ has been quantified. During maturation from peat to high volatile bituminous coal ranks ∼10–105 mg CO₂/g TOC has been released. This is equivalent to 2.50E−4 to 1.25E−3 mg CO₂ generated from every litre of sediment per year falling into the range of deep biosphere utilisation rates. Immature coals, here New Zealand coals, therefore manifest the potential to feed deep terrestrial microbial life, in contrast to more mature coals (VR_r > ∼0.81%) for which defunctionalisation processes become less important.     

Synsedimentary tectonics of the Palaeozoic of the Drahany Upland (Sudeticum, Moravia, Czechoslovakia)

Southern parts of the Moravian Palaeozoic afford a good example of the complicated interrelationship between geosynclinal sedimentation and tectonics. The function of transverse deep-seated synsedimentary faults, separating the basin into big transverse blocks, is described in detail. The mobility of the basement of the individual transverse blocks influence sediment development, thickness, and the intensity of their deformation. The flysch and molasse facies complexes display a close relationship with the lifting of intermountain blocks and subsidence of basins. The clastic material was transported transverse to the basin axis. Where transverse axial depressions occur coarse clastic material accumulated as alluvial fans. This material was reworked by lateral currents and distributed parallel to the basin axis, within the limits of the zone of maximum subsidence. This zone of maximum subsidence shifted from the intermountain block toward the foreland (geosynclinal polariry in the sense of Aubouin, 1965). The transverse blocks with maximum sediment thickness (mainly those belonging to pre-flysch and flysch facies complexes) display the most distinct polarity and most intense sediment deformation. By contrast, transverse blocks with sedimentary sequences of small thickness (where carbonate, and eventually molasse facies complexes prevail) lack the pronounced polarity and are less deformed. Our observations lead to the conclusion that dilation stage of geosyncline development did not exist in the course of the deposition of pre-flysch facies complex.     

华南晚二叠世层序—古地理及聚煤规律*

晚二叠世是地质历史中重要的成煤时期,在中国南方多个省区都形成了可采煤层。受海平面变化的影响,华南地区晚二叠世发育一套完整的陆相—海陆过渡相—海相含煤岩系。通过对野外露头及钻孔剖面资料分析,识别出了区域不整合面、下切谷底部冲刷面、河道间古土壤层和沉积相转换面等关键层序地层界面,以及依据海相石灰岩标志层横向分布规律反映的区域最大海泛面,同时结合前人关于华南地区地层对比及煤层对比方面的成果,将华南上二叠统划分为3个三级层序及9个体系域。利用单因素分析多因素综合作图法恢复了各层序中岩相古地理及煤层分布规律,结果表明:层序Ⅰ中煤层主要形成于潮控下三角洲平原以及潟湖潮坪环境,厚煤层的展布受这些沉积相带控制;层序Ⅱ中三角洲平原环境中煤层最厚,开阔台地煤层最薄,平面上煤层厚度表现为西部厚、中部次之、东部无煤的阶梯式递变;层序Ⅲ中煤层主要形成于三角洲平原环境,此时聚煤作用主要发生于华南西部的康滇古陆东侧,在华南中部及东部大部地区发育的浅海台地和深水盆地均不利于聚煤作用发生。     

Coal rank variation in the Intrasudetic Basin,SW Poland

The Upper Carboniferous, coal-bearing sequence of the Intrasudetic Basin (SW Poland) includes coals ranging from high-volatile bituminous to anthracitic rank. The lowest values of reflectance are recorded around the basin margins (0.6% R₀ max), the highest ones appear in the center of the basin (exceeding 4% R₀ max). Reflectance gradients are very high, reaching 0.6%/100 m in the centre of the basin.A comparison of the isoreflectance maps for three lithostratigraphical units—the Walbrzych, Bialy Kamien and Zacler Formations, with the present-day burial depth and the depth of burial during the Westphalian B/C—indicates that there is a strong relationship between reflectance and the sediment cover during the Westphalian B/C, particularly in the vicinity of Walbrzych and Lubawka. This suggests that the increase in coal rank is related to the increase in cover which permitted the temperature to build-up to high values.In the eastern and central parts of the basin and the Nowa Ruda area, higher reflectance than that derived from burial depth is observed which is believed to result from higher heat flow from the basement. The volcanic rocks of the Intrasudetic Basin appear to have little effect on coal rank and are not considered to be a significant contributor to the heat flow of the region.During coalification, the oldest Westphalian coal seams were buried to about 700 m and the youngest seams of the Walbrzych Formation to 900 m. Around the basin margins the coals had reached their present-day rank by the Westphalian B/C and in the central part probably by the end of the Stephanian. Most effective coalification took place during the Westphalian A,B occupying a period of less than 20 million years. The coalification temperature is calculated to be 160–170°C with a geothermal gradient of 8–10°C/100 m. These geothermal conditions support the suggestion of a volcanic origin for the Intrasudetic Basin.