The stable carbon isotope values of coalbed methane range widely, and also are generally lighter than that of gases in normal coal-formed gas fields with similar coal rank. There exists strong carbon isotope fractionation in coalbed methane and it makes the carbon isotope value lighter. The correlation between the carbon isotope value and R o in coalbed methane is less obvious. The coaly source rock maturity cannot be judged by coalbed methane carbon isotope value. The carbon isotopes of coalbed methane become lighter in much different degree due to the hydrodynamics. The stronger the hydrodynamics is, the lighter the CBM carbon isotopic value becomes. Many previous investigations indicated that the desorption-diffusion effects make the carbon isotope value of coalbed methane lighter. However, the explanation has encountered many problems. The authors of this article suggest that the flowing groundwater dissolution to free methane in coal seams and the free methane exchange with absorbed one is the carbon isotope fractionation mechanism in coalbed methane. The flowing groundwater in coal can easily take more 13 CH 4 away from free gas and comparatively leave more 12 CH 4 . This will make 12 CH 4 density in free gas comparatively higher than that in absorbed gas. The remaining 12 CH 4 in free gas then exchanges with the adsorbed methane in coal matrix. Some absorbed 13 CH 4 can be replaced and become free gas. Some free 12 CH 4 can be absorbed again into coal matrix and become absorbed gas. Part of the newly replaced 13 CH 4 in free gas will also be taken away by water, leaving preferentially more 12 CH 4 . The remaining 12 CH 4 in free gas will exchange again with adsorbed methane in the coal matrix. These processes occur all the time. Through accumulative effect, the 12 CH 4 will be greatly concentrated in coal. Thus, the stable carbon isotope of coalbed methane becomes dramatically lighter. Through simulation experiment on water-dissolved methane, it had been proved that the flowing water could fractionate the carbon isotope of methane, and easily take heavy carbon isotope away through dissolution.Keywords: coalbed methane, methane stable carbon isotope, fractionation mechanism, accumulative effect.Distribution and fractionation mechanism of stable carbon isotope of coalbed methane 1253
Multiple sets of thick coal beds characterized by simple structure and shallow burial depth were developed in the Early and Middle Jurassic strata of the Ordos Basin, northwestern China. The huge reserves of this high quality coal have a high commercial value. We studied the coal's petrologic characteristics and its maceral distribution to determine the maceral's contribution to generation of oil and gas. The results show that the Jurassic coals in the Ordos Basin have special petrological features because of the Basin's unique depositional environment which was mainly a series of high-stand swamps in the upper fluvial system. These petrographic features are a result of the development of typical inland lakes where some sand bodies were formed by migrating rivers. After burial, the peat continued to undergo oxidizing conditions, this process generated extensive higher inertinite contents in the coals and the vitrinite components were altered to semi-vitrinite. The macroscopic petrographic types of these Jurassic coals are mainly semi-dull coal, dull coal, semilustrous and lustrous coal. The proportions of semi-dull coal and dull coal are higher in the basin margins, especially in the area near the northern margin. The numbers of semilustrous and lustrous coals increase southwards and towards the central basin. This situation indicates that different coal-forming swamp environments have major controlling effects on the coal components. Another observation is that in the Ordos' coal sequences, especially in the lower part, some sandstone beds are thick, up to 20 m with a coarse grain size. The higher fusinite GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(1) (2012) 85e95 content in the macerals accompanies a higher semi-vitrinite content with more complete and regular plant cell structure. The fusinite structure is clear and well preserved. After burial, the lithology of the roof and floor rocks can continue to affect the evolution of coal petrology. The sand bodies in the roof and floor exhibit good physical conditions so that pore water can maintain a long-term state of oxidation, circulation and connection to the coal. So coal components remain in an oxidation environment for a long time. Conversely, in the basin center, lacustrine facies developed and peat was rapidly covered by mudstone after burial and subsequent coal beds rapidly entered a reducing environment. As a result, abundant gelatification occurred and the vitrinite content increased. Exinite often accumulated in a specific position in the coal bed. Although the average exinite content is not high on the whole, it does significantly contribute to the total hydrocarbon generation. The exinite content has been underestimated, especially the amorphous bituminous fluid and its importance is emphasized here. The reason is that the fluid flows easily into fusinite which has strong rigidity, or flows into some fissures, where it is commonly neglected. ª 2011, China University of Geosciences (Beijing) and Peking Uni...
In China, the world's largest producer of coal, the mean value of uranium in coal is 3 mg/kg, which is near the published value for uranium in coal in the United States and the world coal average. There are a few examples of Chinese coal that have exceptionally high concentrations of uranium and may be considered as a uranium source. In Hubei province in southern China a special type of stonecoal from the lower Silurian has a uranium concentration of 180 to 280 mg/kg in the coal ash. A few samples from Jurassic coal in Xinjiang of northwest China have more than 200 mg/kg uranium. The uranium in coal is mainly associated with organic matter in low rank coal. In high rank coal the uranium probably occurs in the minerals. In nature, the uranium-rich deposit is rare, and usually small in scale. Uranium abundance and occurrence in different places and strata, especially in coal stone in the South China and in marine shale, have potential hazards on the environment.
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Instrumental Neutron Activation Analysis (MAA) was done to determine the abundances of rare earth elements (REE) of 58 samples of Late Palaeozoic Carboniferous-Permian coals and related rocks in North China. Detailed study of REE geochemistry shows that the ZREE of most coals studied in this paper is in a normal range between 30x 1 Od and SOX 1 Od with a mean of 56x 1 Od. The REE in the Taiyuan Formation in the northern part of North China are much richer than those in the southern part. This is due to the shorter distance to the source area in the north. Moreover, the CREE is in positive correlation to coal ash, especially closely related to the content of clay minerals <2 pm in size. This reveals that most REE were carried by terrigenous clastic materials, especially fine clay minerals. In the coals the light REE (LREE) are much richer than the heavy REE (HREE), and the LREE/HREE ratio in coals generally varies from 2 to 8. The LREE/HREE ratio of high-ash, low-sulphur coals is higher than that of lowash, high-sulphur coals, and is even higher in the roof and floor rocks, reaching 12 with the highest (up to 21) in pyrite concretions, which shows that the HREE have a stronger affinity to organic matter in a reducing environment influenced by seawater. Furthermore, Eu is generally depleted in coals. The REE chondrite-normalized distribution patterns are very similar in coals of the whole of North China. From the REE geochemical characteristics it can be concluded that during the formation of Late Palaeozoic Carboniferous-Permian coals in North China, the supply of terrigeneous materials remained quite stable. The CREE in low-ash, high-sulphur coals is relatively low and the REE are mainly carried by fine-grained minerals and organic matter and a certain percentage of REE are adsorbed by organic matter; while the I: REE in high-ash, low-sulphur coals is higher and the REE are mainly present in detritus. The 2 REE of magmatism-influenced coals is the highest, which suggests that the introduction of magmatic substances may increase the CREE, thus causing the REE distribution patterns to show an abnormal feature. Moreover, some harrnhl elements such as U, W and As usually increase when the coals are influenced by magmatism.
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