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Journal of Geological Research
Volume 2012 (2012), Article ID 526016, 16 pages
Applications of Vitrinite Anisotropy in the Tectonics: A Case Study of Huaibei Coalfield, Southern North China
1MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China
2Key Lab of Computational Geodynamics of Chinese Academy of Sciences and College of Earth Science, Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
3Key Laboratory of Coal Resources, China University of Mining and Technology, Beijing 100083, China
4Key laboratory of Basin Structure and Petroleum Accumulation, CNPC and PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
Received 28 February 2012; Accepted 6 April 2012
Academic Editor: Hongyuan Zhang
Copyright © 2012 Yudong Wu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
29 oriented and 10 nonoriented coal samples are collected in the study from three different regions of the Huaibei coalfield, eastern China, and their vitrinite reflectance indicating surface (RIS) parameters are systematically calculated and analyzed. Using the available methods, Kilby’s transformations and RIS triaxial orientations are obtained. The magnitudes and orientations of the RIS axes of the three regions were respectively projected on the horizontal planes and vertical sections. The results show that the samples in high deformed region have significant anisotropy magnitudes (higher and values) with a biaxial negative style, whereas the samples in the slightly deformed area have unimpressive anisotropy magnitudes with a biaxial negative style. Thermal metamorphism superposed might enhance the complication and variation of RIS style. RIS projection analysis deduced that the RIS orientation is mainly controlled by regional tectonic stress, and likely influenced by deformation mechanisms of coal.
During the coalification, tectonic-thermal events could influence the vitrinite reflectance of coal [1, 2]. Effects of temperature and stress on modifying vitrinite reflectance anisotropy (VRA) were proved by high temperature-pressure experiments on bituminous anthracite [3–13]. Patterns of VRA [14, 15] in coal are usually represented by a three-dimensional graphical form resembling an ellipsoid and termed as RIS (reflectance indicating surface).
The investigation of the RIS for vitrinite in coals is performed by means of sections cut from oriented samples, and the magnitudes and orientations of the ellipsoid parameters are obtained by calculating the data measured from three mutually perpendicular surfaces. This method has been reported  and verified by high temperature-pressure experiments  and tectonic stress field analysis [11, 15, 17–20]. The anisotropy of RIS in this method is quantified by a function of rank , including in terms of bireflectance ratio , anisotropy ratio  and bireflectance .
The RIS styles may also be obtained by nonoriented sections from crushed coal samples. The method requires measurement of the maximum and minimum apparent reflectance from a series of randomly oriented vitrinite particles, and the magnitudes of RIS principal reflectance axes were estimated by using the reflectance crossplots [23, 24]. A series of parameters named as Kilby’s transformations in the method are proposed more effectively to estimate the anisotropy magnitude and shape of RIS ellipsoid, including RIS style (), anisotropy magnitude (), and reflectance of equivalent volume isotropic RIS (). Compared with Levine’s method, basing on the application of vitrinite maximum reflectance versus vitrinite random reflectance , less time consuming is found during Kilby’s data processing [13, 23, 26–30]. In the nonoriented sections, however, the orientations of three principal reflectance axes are missing.
Previous works suggested that the anisotropic strains developed by lithostatic pressure, especially by tectonic strains, including oriented pressures and shears. High rank coals and anthracites show evidence of the presence of biaxial negative materials [15, 19, 23, 31, 32]. As the result of tectonic dislocations, in many cases associated with fractures in the plane normal to the coal bedding, it is possible to find microtextures responsible for the biaxial positive character [29, 30]. It is found that minimum reflectance develops incrementally parallel to the direction of maximum compressive stress during coalification, and maximum reflectance develops incrementally parallel to the direction of minimum compressive stress [1, 14]. These viewpoints are verified by simple shear experiments in high temperature-pressure environment [28, 33, 34]. On this basis, the RIS may approximate to finite strain ellipsoid , and the orientations and magnitudes of RIS principal reflectance axes may correspondingly have further usages in the tectonic stress field analysis.
The Huaibei Coalfield in Anhui Province of China has abundant coal resources. Thermal model research showed that the coal experienced low temperature (<230°C) after being buried during the coalification process  and, therefore, thermal function was limited in the coalification history. Although the coalification process was influenced by other geological agencies such as tectonic stress, the RIS parameters may provide semiquantitative information about the magnitudes and orientations of tectonic deformation.
In this paper, 29 oriented coal samples from the working face of the underground mines and 10 nonoriented coal samples from the core drilling were collected. In order to estimate the magnitudes and orientations of the three principal reflectance axes, the RIS was built on the oriented coal sample blocks. The orientation of RIS was obtained by calculating the data measured from three mutually perpendicular surfaces. Meanwhile, the anisotropy magnitude and shape were analyzed with the parameters such as bireflectance ratio, Flinn’s diagram, and Kilby’s transformations. The effects of tectonic stress and thermal evolution on coal are therefore expected to be evaluated.
2. Geology of the Study Area
The Huaibei Coalfield in the southeast area of North China Craton was influenced by the Tancheng–Lujiang sinistral strike-slip fault in Mesozoic [39–42] and a related arcuate thrust system . Many researchers believed that the arcuate thrust system was a west-thrust imbricate fan with the arc-top near to Huaibei City [37, 43]. A syncline formed in the margin of the thrust system had a certain influence on the burial depth of coal seams and the level of coal rank . Coal-bearing strata mainly occur in Permo-Carboniferous age.
The Huaibei Coalfield is divided into three regions representing different stress-thermal environments. Vitrinite reflectance of coal samples collected from these regions was measured and sampling locations are shown in Figure 1(a).(1)Region A, including the coal mines of Taoyuan (label 3), Qinan (label 4), Qidong (label 5), and Xutuan (label 9). The strata were likely influenced less by magmatism and tectonic agency and more by burial metamorphism. (2)Region B, including the coal mines of Haizi (label 6), Linhuan (label 7), Tongting (label 8) and Baishan (label 10). Both Mesozoic deformation and magmatism affected deeply the strata and likely impacted on the samples collected in the region.(3)Region C, including the coal mines of Luling (label 1), Zhuxianzhuang (label 2), Shitai (label 11) and Yangzhuang (label 12). The strata in the region strongly deformed, and samples are probably highly influenced by effects of tectonic deformation and thermal-shearing.(4)The lithostratigraphic sequence of the study area is shown in Figure 1(b). The major coal seams occur in the lower Permian system () and minor coal seams locally occur in the upper Permian System (). Southward marine regression during early Permian time led to deposition in an island-lagoon environment and followed by terrestrial delta-plain deposits that contained extensive and minable coal seams . The deltaic environment became unstable in early Late Permian, and coal seams became thinned, discontinuous and unminable. The alluvial deposits in Late Permian marked the end of coal accumulation in the Huaibei coalfield . All the oriented samples collected from the working face in coal seams of 3rd, 7th, 8th and 10th and the nonoriented samples collected from the well-drills belonging to the Permian strata.
3. Experimental Section
Coal macerals of samples were determined by spot checks before the reflectance measurement, and the reflectance was measured by means of the method described in GB/T 8899-1998 (equated with ISO7404/3 standard). At least 500 points for each sample were measured on the same polished specimen. The contents of vitrinite, inertinite, liptinite, and mineral matter were obtained and are shown in Table 1. Coal lithotype in the Huaibei coalfield is dominated by clarain, with secondary durian and vitrain, and coal maceral is dominated by vitrinite (40.6~92.5%), which is followed by inertinite and liptinite (Table 1).
3.1. Sample Preparation
The 29 oriented samples were prepared following the rules described in GB/T 16773-2008 (equated with ISO7404/2 standard) and cut into cubes with approximately 3-4 cm in dimensions (Figure 2). One pair of planes are paralleling to the horizontal plane and the other two mutually perpendicular pairs normal to the horizontal one. Each sample, therefore, has three pairs of mutually perpendicular surfaces (, , and as shown in Figure 2) under the process of polishing surface.
The 11 nonoriented samples were prepared by a modified procedure of GB/T 16773-2008 (equated with ISO 7404/02 standard). These samples were also used in our previous work for thermal simulation of maturation for coal .
3.2. Optical Microscopic Measurements
The measurements were performed for each oriented sample using an MPV-3 Combi (Leitz) microscope, reflected light, and oil immersion objectives. The maximum and minimum apparent reflectance ( and ) on each polished surface were measured in polarized light () and by rotating the microscope stage through and following the analytical method described in GB/T 6948-2008 (equated with ISO7404/5 standard). at least 20 points on each surface of each sample were measured to obtain the mean statistical values and azimuths of and .
As for nonoriented samples, at least 20 points were measured with the same analytical method, and the mean statistical values of and were obtained.
3.3. Data Processing
On the basis of measured mean values, the values and occurrences of maximum reflectance (), intermediate reflectance (), and minimum reflectance () axes may be obtained by solving ellipsoid equation in analytic geometry of space , which is the magnitudes and orientations of RIS. The computing method is in Figure 3 as follows: where
This ellipsoid equation has three eigenvalues, corresponding with the values of three RIS ellipsoid axes ():
The occurrence of three RIS ellipsoid axis is calculated according to the formula where and represent the direction and dip angle of the axis in the coordinate system, respectively.
The geological occurrence of RIS ellipsoid axis (), however, needs a data conversion as follows: where , ; where represents the geological direction of arrowhead in Figure 2.
Therefore, the reflectance anisotropy (bireflectance and coefficient values) and RIS shapes (such as , coefficient value, Flinn’s parameters, etc.) can be determined. The details of the parameters are given as follows.(1) coefficient value , representing bireflectance value, is calculated according to the formula.  represents the bireflectance ratio.(2) Flinn’s parameter was firstly proposed for examining the geometry of three-dimensional homogeneous strain [35, 45]. Formula of Ramsay was chosen here where (3) coefficient value [23, 24], representing the RIS anisotropy magnitude, is calculated according to the formula where This value is the distance between the plotted position of RIS and the position of an isotropic RIS, a single measure of anisotropy for all styles of RIS .Isotropic constituent of coal is characterized by . The higher coefficient value is, the stronger optical anisotropy will be. Typical values for raw vitrinites are from 0.03 to 0.05 and the value of 0.1 represents strong anisotropy of a constituent.(4) coefficient value [23, 24], representing RIS style, is calculated according to the formula where and are determined in the same way as shown above. values range from (−30) to () and the RIS shape is described as : uniaxial negative; : uniaxial positive; : biaxial negative; : biaxial positive; : biaxial neutral.(5) coefficient value [23, 24], representing the reflectance of equivalent volume of isotropic RIS, is calculated according to the formula
4.1. Anisotropic Properties of Nonoriented Samples
and of 11 non-oriented samples were measured and the anisotropic properties of the material such as and / were calculated as shown in Table 2.
4.2. Magnitude and Orientation of RIS
The magnitudes and orientations of RIS for 29 oriented samples were calculated by measuring the values and azimuths of the and . Many of the coal samples suffered from tectonic deformation during coalification. The deformation types were identified by the proposed tectonic coals standard  and divided into three series: brittle-ductile transition, ductile shear, and brittle crush as shown in Table 3.
4.3. Anisotropic Property of Oriented Samples
Based on magnitudes and orientations of RIS, Levine’s rank and Kilby’s transformations are obtained as shown in Table 4.
5.1. Major Facts Affecting VRA
Both temperature and stress have influences on modifying VRA. It is therefore necessary to estimate roughly the thermal effects during coalification before analyzing RIS.
In previous works, the mean random vitrinite reflectance ( %) shows a strong correlation (, ) with maximum burial temperature ( in °C). These data are modeled by the linear regression equation :
The conversion between and is following the analytical method described in GB/T 6948-2008 (equated with ISO7404/5 standard). On the basis of correlation given above, the maximum temperatures sustained during burial are estimated.
As shown in Table 5, the measured reflectance values range from 0.74% to 3.01, indicating conversion of medium-volatile bituminous coal to anthracite coal. Most of the studied samples experienced a maximum temperature between 120~20°C, less than 290°C The available high temperature-pressure experiments on bituminous anthracite show that there is little change in vitrinite reflectance anisotropy under the temperature less than 400°C [47, 48]. It is reasonable to believe that the vitrinite reflectance anisotropy here was mainly caused by the tectonic differential stress.
5.2. Magnitudes of RIS
The three sampling regions represent different stress-thermal environments and, therefore, the corresponding coal samples experienced different effects of deformation and metamorphism. The parameters of reflectance anisotropy (bireflectance and coefficient values) and RIS shapes (such as and coefficient values, Flinn’s parameters) are analyzed as follows.
The bireflectance increases progressively with upgrading of coal rank under the heat treatment without differential stress involved. In that case, the bireflectance for the coal ranking from medium-volatile bituminous to anthracite coal should be at the same level and the coefficient, representing bireflectance ratio, should be suitable for the bireflectance analysis among various coal ranks.
Relationships between and / are shown in Figure 4. The range of value is 0.81~1.21% in Region A (Taoyuan mine, Qidong mine, and Xutuan mine) and 1.21~3.13% in Region B (Linhuan mine and Haizi mine). This obvious difference above indicates that samples from Region B was dominated by the magmatic thermal activity. However, compared with Region A (Luling mine, Shitai mine, and Yangzhuang mine), samples in Region C where experienced extra effects of tectonic deformation and shearing heat, are manifested by the various ranges of 0.83~1.72%.
In contrast to Region B, the value in Region C and Region A simultaneously increased with the and indicates that the reflectance in the two regions is dominated by the tectonic deformation and shearing heat.
The 95% confidence intervals of value are in Region A and in Region C. The difference between two regions represents the effects of tectonic stress. The same indictor in Region B is . The coal samples in the region were partly influenced by Mesozoic magmatism, and represent polarization of value. Meanwhile, some strata underwent stronger deformation with increasing values.
5.2.2. Reflectance of Equivalent Volume Isotropic RIS
RIS parameter , the reflectance of the isotropic RIS of equivalent volume, as a result of the chemical structure ordering during heating, is suggested to characterize the basic structure unites (BSU) .
As shown in Figure 5, positive linear regression relationships between and for Regions A, B, and C are 0.956, 0.980, and 0.965, respectively. The correlations between values and the hydrogen contents () have been verified that the depends on the chemical composition of anthracites rather than the three-dimensional BSU . Since the chemical composition and structure mainly evolved under the effect of temperature and coincided with the results, the linear- regression slope of Region B seems to be mainly influenced by the thermal effect.
5.2.3. RIS Style and Anisotropy Magnitude
The coefficient values of and construct polar scatter plots proposed by , and the three-dimensional style and anisotropy magnitude of RIS can be analyzed together. As shown in Figure 6(a), samples from Region A and C are characterized by substantial RIS style of biaxial negative ( ranges between −30~15). The anisotropy magnitude in Region C ( ranges between 0.04~0.08), however, is higher than that in Region A ( ranges between 0.02~0.04), which coincides with the distribution of bireflectance ratio. Conversely, as shown in Figure 6(b), the samples from Region B are characterized by significant biaxial positive character or unconspicuous biaxial negative ( ranges between −15~30) and the anisotropy magnitude () is various, ranging from 0.02 to 0.06.
5.2.4. RIS-Logarithmic Flinn Diagrams
For a more complete characterization of the optical properties of both raw and carbonized anthracites, the modified Flinn’s parameters , related to the anisotropy as well as the optical character, are also calculated and shown in. The samples from Region C and A are characterized by evolvement of RIS style from constriction types to flattening types ( shown in Figure 7(a)). However, RIS styles of the samples from Region B are complicated and various (as shown in Figure 7(b)), corresponding to plane strain in finite strain analysis.
5.2.5. RIS Anisotropy Evolution Stages
The deformation path of finite strain analyses proposed by Ramsay suggested that the cleavage is developed by the route of “sphere types → uniaxial oblate types → uniaxial prolate types →uniaxial oblate types → flattening types”. The anisotropic coal samples may have the same evolution process. A related evolutionary path of RIS style during coalification was reported by Levine  and the stages are given as follows.
The first stage was after the deposition. The peat was subjected to mild geothermal process due to broad regional subsidence and burial of overlying rocks. During this period, the ambient geologic stresses were nontectonic and only due to vertical static pressure loading by the overlying strata. Owing to the vertical downward lithostatic pressure, RIS would show the style of uniaxial negative. At the second stage, the anisotropic strain developed by tectonic differential stress (in a lateral direction) and lithostatic pressure (in the vertical direction) and the coal seams would suffer from extrusion in two directions and the anisotropic RIS represents the style of biaxial positive. At the third stage, with the increase of buried depth (more than 1000 m), the pressure on the coal seams from the overlying strata may approximately represent as isotropic hydrostatic pressure (Heim’s hypothesis). The tectonic differential stress is upgraded with the enhancement of tectonic agency. All of these factors would make the anisotropic RIS style as biaxial negative.
As the suggested route given above, the style of uniaxial negative might take place either in the third stage or in the transition between the first and the second stages. More evidence is needed to constrain the actual process of physical deformation and metamorphism. During coalification, the coal samples collected from three regions likely belong to different evolution stages.(1)Samples of Region C experienced more effects of tectonic deformation and shearing heat. With significant anisotropy magnitude (higher and value) and biaxial negative style, they have more likely reached into the third stage.(2)Samples of Region A experienced more effect of burial metamorphism and less influenced by magmatism and tectonic agency. With unimpressive anisotropy magnitude (lower and value) and biaxial negative style, they more likely belong to the transition between the first to the second stage.(3)Samples of Region B influenced by the Mesozoic magmatism, representing polarized anisotropy magnitude (diversified value of , and ). With complicated and various RIS style, they probably belong to the transition between the second and the third stage. Scanning electron microscopic (SEM) photos of coals in the study area indicated that some coal seams have strong deformation (Figure 8). The local stress field was presumably influenced by the complicated volcanic activity such as granite pegmatite and diorite porphyrite during the Yanshanian orogeny .
5.3. Implications of RIS-Orientations
As mentioned in Section 1, previous works suggest that minimum reflectance develops incrementally parallel to and maximum reflectance develops incrementally parallel to during coalification [1, 14]. These viewpoints were verified by simple shear experiments under high temperature-pressure environment [28, 33, 34]. On this basis, the vitrinite reflectance may provide important information about tectonic stress in each period during the coalification, and the -direction could be indicated by the minimum reflectance axis.
Eastern North China Craton experienced an important tectonic inversion during Mesozoic. The EW-trending tectonic grain was transformed to NE-NNE-trending and the contractional regime to an extensional regime during Jurassic-Middle Cretaceous . The Huaibei Coalfield lies in the southeastern margin of North China Craton, and influenced by multistage and complicated tectonic events, which could be represented by the triaxial rotation of RIS.
5.3.1. Horizontal Projection
The horizontal projections are based on the stereographic polar method. The radius of base circle represents the value of Rint. Based on the feature that minimum reflectance develops incrementally parallel to [1, 14], the cumulative effect of multistage tectonism on each sample is indicated by the orientation of axis.
The samples collected from different working faces have different RIS orientation, even though they are in the same coal mine. The samples of Region C are fan-shaped distributing on the thrust arc, and their Rmin axial directions are mainly pointing to the center of the circle such as, from north to south in order, NW-SW or NNW-SSE direction in the Luling mine), NEE-SWW direction in Yangzhuang mine, SSE-NNW or NNE-SSW direction in the Shitai mine. In other words, the axial direction was parallel to the thrust- direction.
The samples of Region A and B are distributing in the front of the thrust arc and a series of NNW-NE-trending folds. Their axial directions are mainly perpendicular to the corresponding fold-hinge as shown in Figure 9. These features suggest that the RIS-orientation in these areas was primarily controlled by the thrust-fold system.
5.3.2. Vertical Projections
The cross-section (A-B-C) is about perpendicular to the hinges of the major folds and its position is shown in Figure 9. The vertical projections on the cross-section are based on RIS-analyses of the samples from five coal mines. Since most of the axial directions are located in the cross-section, the projection provides information on RIS-dip angles.
Samples of the Luling mine belong to Region C, and their RIS orientations were following the direction of thrust. Number 8 coal seam has similar RIS dips as Number 10. However, samples of the Taoyuan mine, Xutuan mine, Linhuan mine and Haizi mine seem to be more influenced by the regional tectonic stress, the distinctions between samples of different coal seam are obvious.
Combining with the deformation type of coal samples (as listed in Table 3), No. 7, No. 8, and No. 9 coal seams are mainly characterized as ductile-type or brittle-ductile-type, with lower axial dip angles. In contrast, N0. 10 coal seam is mainly characterized as brittle-type or brittle-ductile type, and their RIS axial dip angles are higher. The contrastive fact indicates that the coal samples in different deformation types have remarkably different RIS orientation, although they belong to the same structural unit.
Our results indicate that:(1)there are obvious relationships between the anisotropic parameters and tectonic stress action. Based on the tectonic situation, the Huaibei Coalfield is divided into three regions. Samples of each region have special RIS-shape and magnitude. Samples of Region C have significant anisotropy magnitude (higher and value) and biaxial negative style. By contrast, samples of the Region A have unimpressive anisotropy magnitude and biaxial negative style. Samples of Region B represent polarized anisotropy magnitude (diversified value of , and ) with complicated and various RIS styles,(2)orientation of RIS is mainly controlled by regional tectonic stress: either parallel to the direction of thrusting or perpendicular to the hinge of folds. It is likely influenced by deformation mechanisms of coal.
These research results are part of a key project carried out in 2006–2011 and financially supported by The national Natural Science Foundation of China (Grant no. 41030422; 40940014; 40772135) and National Basic Research Program of China (Grant no. 2009CB219601). The authors are grateful to Dai Jiming and Ai Tianjie at China University of Mining and Technology (Beijing) for their assistance in the experiments. Many thanks to Zheng Yadong, Wang Guiliang, Jin Weijun and Liu Qinfu, Liu Dameng, and three reviewers for their help with improving the paper.
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