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Journal ArticleDOI

Petrography and metamorphism of the lower Gondwana (Permian) coal of Rangit Valley, eastern Himalays, India

01 Sep 1997-International Journal of Coal Geology (Elsevier)-Vol. 33, Iss: 4, pp 351-368

AbstractA triangular outlier of coal-bearing lower Gondwana rocks comprises a tectonic window within the Precambrian metamorphic terrain of the Rangit Valley, in the eastern Himalays of India. Due to a series of tectonic events which took place during the Mesozoic and Tertiary the succession has become reversed and the coal has undergone severe physical, chemical, microstructural and optical changes. Studies reveal that (1) the rank of the coal prior to tectonism was low, (2) the coal underwent oxidation in the peat forming stage and during subsequent orogenic stages but effect of oxidation on the properties of coal was insignificant, (3) the coal reached its present abnormally high rank mainly due to heating under tectonic pressure and (4) the heat which was generated due to tectonic pressure, affected all the coal seams of this belt. more

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Abstract: Coal weathering and laboratory (artificial) oxidation have been topics of much discussion and extensive research since over a century ago, as they cause a profound effect on the chemistry of coal and its technological properties and as they induce distinct and irreversible changes to coal macromolecular structure. This article focuses on the effect of coal weathering and laboratory (artificial) oxidation on the microscopic appearance of coal, its microstructure as well as coal optical properties. The coal petrographic effects of in situ weathering and laboratory oxidation have been a topic of research within the International Committee for Coal and Organic Petrology (ICCP) for years and involve discoloration of macerals, formation of oxidation rims, and changes in polishing relief with formation of micropores, microcracks, and microfissures. Regarding oxidation rims, their presence as a result of in situ weathering and laboratory oxidation is petrographically well documented. Vitrinite reflectance measured on weathered or artificially oxidised coals of different ranks exhibits a differential trend, depending on conditions such as temperature and duration of exposure . Also, the decline of fluorescence intensity has been reported for these coals. In addition to the overview of coal petrographic oxidation features of weathered and laboratory oxidised coals at ambient to low-temperature conditions, the influence of these processes on coal chemical, physical, and technological properties has been discussed. Previous investigations have shown that coal weathering and laboratory (artificial) oxidation affect the suitability of coal for technological uses and provide self-heating or spontaneous combustion risk to fossil resources in coal deposits, in open-air storage facilities and discard dumps, silos, and bunkers. Oxidation also affects coal transport facilities, preparation and processing techniques, as well as, the prediction of coal combustion and carbonisation behaviour, changing cokemaking requirements. It has been similarly shown in numerous research programs and investigations that it poses a detrimental effect on coal liquefaction, gasification and manufacturing of coal-based carbon materials. In addition, low-temperature oxidation is considered a primary source of heat, leading to self-heating and uncontrolled spontaneous combustion. Further, the review provides a broad overview of practical importance of coal weathering and coal oxidation research to coal conversion technologies and final coal utilization products, supported by numerous laboratory investigations. The article presents also a short summary of optical microscopy and non-optical microscopy methods applied to identify coal weathering and laboratory (artificial) oxidation. It is hoped that issues raised in the review article will enhance further debates and novel ideas in coal and coal-related research.

38 citations

Journal ArticleDOI
Abstract: Petrological and palynological analyses of coals from two bore-holes, intruded by an igneous body (dolerite dyke), reveal characteristic changes due to thermal alteration The unaffected coals are high volatile bituminous (Rr: 050–061%) The coals recovered from samples located close to the dyke had vitrinite with devolatization microvacuoles, higher reflectance ranging from 09% to 30% and liptinite with none to very low fluorescence There is an increasing trend in reflectance and a decreasing trend in fluorescence in the samples occurring closer to the dyke Presence of injected mineral matter and pyrite in the macerals further indicate the impact of thermal metamorphism on the coals occurring near the igneous body The palynological investigation of unaffected coals shows the presence of a number of well-identified spores and pollen However, very few palynomorphs could be recovered from coals located close to the intrusive as the pollen/spores were found to be charred beyond recognition

26 citations

Journal ArticleDOI
Abstract: Geological investigations in the offshore area of eastern Russian Arctic attracted an increased interest in the recent years. Data concerning organic and coal petrology is scarce for the shelf and offshore area of the eastern Russian Arctic. Consequently, thermal maturity and depositional environments of sedimentary facies remain less described and examined. A detailed characterisation of organic matter type and origin as well as determination of thermal maturity have been carried out on siliciclastic and carbonate sedimentary samples from Paleozoic, Mesozoic and Cenozoic outcrops of the New Siberian Islands, eastern Russian Arctic. The acquired organic and coal petrological data contribute to the overall assessment of petroleum potential of the studied offshore area and the neighboring shelf area. The investigated organic matter of Silurian and Middle Devonian mudstones and siltstones contain distinct, multi-staged solid bitumen (“dead oil”) preserved either as fracture-, fissure-, and vein-fillings, indicating activation of migrations pathways for re-occurring crude oil-bearing fluids or in form of “streaks”. The examined dispersed organic matter of Triassic deposits is predominantly composed of terrestrially derived macerals followed by alginite macerals ( Tasmanite and Leiosphaeridia prasinophytes). The analysed Cretaceous and Tertiary deposits reveal (i) Aptian/Albian inertinite-rich sub-bituminous coal and lignites formed in fresh water lakes and swamps to brackish near-shore depositional environments with phases of wildfires as well as (ii) Cenomanian/Turonian lignites with pollen grains and carbonised debris (pyrofusinite), indicating stages of forest/moor paleofires. The (iii) Paleocene and Eocene deposits bear mineral-rich and xylite-rich lignites characterised by the lowest coal rank stage, containing different fossil conifer species. The thermal maturity of organic matter in Silurian to Tertiary deposits obtained from random vitrinite and bitumen reflectance measurements is an important parameter in determining the petroleum generation stage at the studied localities. The calculated thermal maturities of Silurian to Middle Devonian deposits acquired from random reflectance measurements of bitumen indicate condensate-wet gas to early dry gas generation window. The thermal maturities of sedimentary organic matter in samples collected from Tertiary to Triassic successions ranges from immature to main phase of oil generation. The composition of the organic matter and thermal maturity of the analysed Triassic siliciclastic deposits suggesting mature stage for oil generation can be used among others to evaluate their hydrocarbon potential. The Cretaceous and Tertiary deposits can be described as thermally immature. In two localities, where the thermal maturity is anomalously high, the examined organic matter has been thermally altered beyond the dry gas preservation limit.

9 citations

Journal ArticleDOI
Abstract: The present study focuses on the investigation of the optical anisotropy, optical sign, textural heterogeneity and deformational features of the maceral grains along with the Raman spectral characteristics of the seven coal samples collected from the Himalayan fold-thrust belts of Sikkim State, India. The coal samples were extremely fragile and pulverized due to the intense tectonic deformation. The maceral composition revealed the dominance of semifusinite over collotelinite grains. The calculated maximum vitrinite reflectance (5.94-8.66%) and mean random reflectance (4.11-5.36%) suggest anthracite A rank of the coal samples following ISO 11760:2005. The proximity of the intermediate reflectance axis value (R-INT) to maximum reflectance axis value (R-MAX) as well as the range of Reflectance Indicating Surface (RIS) style (R-st) values (-9.98 to -19.37) indicates the biaxial negative optical texture of the vitrinite grains. The augmented bireflectance values due to enhancement of the R-MAX associated with strong decline in R-MIN may suggest the commencement of pregraphitization. In addition, the strong linear correlation (r = 0.94) of the RIS-anisotropy (R-am) parameter with the bireflectance values may imply the role of tectonic stress on the optical transformations of the samples. The range of the peak temperature (334.94-369.01 degrees C) calculated from mean random vitrinite reflectance may suggest the effect of thermo-stress coupling on the metamorphism of these coal samples. Microlithotype study combined with deformational aspects of macerals shows the presence of "deformed", "sheared" and "smashed" grains within each sample, which may, additionally, document the tectonic stress influence on the coal particles. Moreover, relatively, larger area of 'defect band 1 (D-1)' than that of 'graphitic band (G)' along with the broad G band in the first order Raman spectra may indicate the considerable presence of structural dislocations and aromatic compounds with disordered bond angle within the microstructure induced by the tectonic deformation. The lowest intensity of the 'defect band 4 (D-4)' may suggest the preferential removal of aliphatic compounds from the samples in response to the tectonic stress degradation. In addition, the relative area ratio calculated from the D-1 and the G bands (AD(1)/(AD(1) + AG)) may indicate that the studied anthracite samples would have attained the metamorphic temperature ranging from 325.12-387.89 degrees C.

8 citations

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Abstract: Summary For simple models of the movement on a fault during an earthquake the temperature on the fault plane is shown to reach the melting temperature for short periods if the slip is sufficiently large. The displacement required to produce melting depends on the magnitude of the regional stress and also on the friction stress. If both stresses are one kilobar melting can occur for displacements as small as one millimetre. If, however, both stresses are ten bars melting is impossible for any observed displacement. It is estimated that the width of the melted zone could be between 2 mm and lcm. Melting on fault planes during earthquakes is not in disagreement with geological and geophysical observations, and suggests a new explanation for some mylonites. Melting may be important because the molten film rock formed on the fault may act as a lubricant and in the case of large earthquakes allow the release of almost all the elastic strain in the region of the shock. This would provide an important constraint in estimating the state of stress along a fault.

335 citations

01 Jan 1981
Abstract: Petroleum exploration is an expensive and increasingly difficult but necessary operation. It requires the use of sophisticated technology and an understanding of the various chemical and geological processes involved in the generation of petroleum from the accumulated organic matter in the source rocks, via organic maturation and migration to the ultimate accumulation of hydrocarbons in the reservoir rocks. During the last decade there has been particular progress made in the study of hydrocarbon source rocks, orqanic maturation and the generation, occurrence and properties of petroleum. These advances and applications are reflected in the present volume. The main aim of Organic Maturation Studies and Fossil Fuel Exploration is to provide a platform for current research, developments, applications and discussion on organic maturation of sedimentary organic matter presented against the ever increasing requirements of petroleum exploration. The book contains papers by geologists, geochemists, chemists and palvnologists, presented at an international symposium held at the University of Cambridge in July 1980. An interdisciplinary approach was adopted and the book contains papers by acknowledged acadernic and industrial experts concerned with organic maturation and petroleum exploration.

160 citations

Journal ArticleDOI
Abstract: Measurement of the levels of organic maturation below and within shear zones of thrust faults in the Rocky Mountains has revealed no general thermal metamorphism that can be attributed to faulting, with the exception of very localized areas. With few exceptions the vitrinite reflectance values obtained are in the range expected if the maximum level of organic maturation was produced as a result of increasing temperature during progressive burial accompanying sedimentation. Only at Marias Pass evidence has been found to suggest additional maturation as a result of post-orogenic burial below the thrust sheets. Anomalously high vitrinite reflectances obtained from the Lewis thrust, McConnell thrust, Coleman thrust and two unnamed thrusts, are restricted to very narrow films immediatly adjacent to, or within the shear zone which, considering any reasonable thermal conductivity, indicates elevated temperatures were very short lived. The anomalously high vitrinite reflectances within these films, when compared to laboratory heated coals, suggest temperatures in the order of 350°-650°C were locally generated during faulting. Such high temperatures are considered to have been generated during stick-slip faulting at macro-asperities or at ramps on the fault plane where local, and possibly transient, high frictional stresses existed. The absence of evidence for extensive frictional heating supports previous arguments that stable-sliding and/or low frictional stress must exist during thrusting.

80 citations

Journal ArticleDOI
01 Jan 1976-Geology
Abstract: Mesozoic sedimentary rocks overridden by thrust plates in the disturbed belt of northwestern Montana have been metamorphosed by burial beneath these plates. Bentonite in the Cretaceous section has been converted to potash bentonite by this metamorphism, allowing radiometric dating of the emplacement of the thrust plates by the K-Ar method. Ages determined thus far range from 72 to 56 m.y. B.P. The oldest ages coincide with estimates for the beginning of thrusting from stratigraphic and structural evidence. Field evidence does not allow a reliable estimate for the end of thrusting. Thus, the younger ages, which indicate completion of thrusting at the end of Paleocene time, represent additional data of interest to the structural history of the disturbed belt.

68 citations

Journal ArticleDOI
Abstract: The anisotropic “reflectance fabric” of coal is used as a finite strain marker in the structural analysis of deformed strata in the Broad Top Coal Field of central Pennsylvania. Oriented blocks of coal were collected at various field locations. The reflectances of three orthogonal surfaces were used to estimate the magnitudes and orientations of the three principal reflectance axes. Broad Top coals are shown to have three principal reflectance values, in contrast with most bituminous coals, which have only two. Reflectance values can be represented by a biaxial indicatrix that may be either prolate or oblate. The oblate indicatrices, showing flattening in the bedding plane, are associated with gently deformed strata. The prolate indicatrices, elongated parallel to the fold axes (N32°E), are associated with more intensely folded strata and show the influence of strong lateral compression superimposed upon gravitational loading. The reflectance indicatrices maintain a constant spatial orientation, regardless of the inclination of the enveloping bedding. The minimum principal reflectance axis plunges steeply toward the west, indicating that during deformation the direction of maximum compression was plunging toward the northwest. These results suggest that in the Broad Top field maximum coalification and, therefore, maximum burial were penecontemporaneous with the later stages of deformation, postdating most of the folding.

67 citations