Presence of Remanent Cell Structure in Vitrinite ... - ACS Publications

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Presence of Remanent Cell Structure in Vitrinite and Its Influence on Reflectance Properties D. F. Bensley and J. C. Crelling Department of Geology, Southern Illinois University, Carbondale, IL 62901

The interpretation of reflectance is dependent in part upon an assessment of the reflectance distribution. In general, as the distribution of reflectance values broaden, displaying an increase in standard deviation, the analytical interpretation becomes increasingly difficult. Data scatter would traditionally be attributed to recycled vitrinite, oxidized vitrinite, multiple vitrinite populations, misidentification, analytical problems, vitrinite reflectance suppression or similar sources of error. It is of interest, however, that a broad reflectance distribution is seldom attributed solely to the heterogeneity of the vitrinite macerals themselves. It is concluded that part of the scatter in reflectance values obtained for a given coal is due to the presence of remanent cell structure. This structure can not only be observed through etching but can be directly measured using reflectance. Sequential spatially orientated reflectance readings using rotational polarization reflectance has been used to document cell structures unobservable with conventional microscopic analysis.

The limitations inherent in analyzing samples processed using density gradient centrifugation, D G C , necessitated the development of new quantitative tools to permit accurate characterization of ultra-fine coal particles. Correlations between reflectance distributions obtained using random reflectance on both -20 mesh vitrinite and corresponding vitrinite separations from the same sample were considered marginal at best. In some samples the relationship between random reflectance on D G C vitrinite separations and the vitrinite samples from which separations were made could not be clearly ascertained. Because reflectance distributions using

0097-6156/94/0570-0039S08.00/0 © 1994 American Chemical Society

Mukhopadhyay and Dow; Vitrinite Reflectance as a Maturity Parameter ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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VITRINITE REFLECTANCE AS A MATURITY PARAMETER

maximum reflectance tend to show less variation than random reflectance, it was reasoned that a more accurate reflectance value could be obtained if maximum reflectance was measured. Equipment and analytical techniques were developed to measure maximum reflectance on particles less than 2μπι. During testing, multiple reflectance measurements on single, apparently homogeneous vitrinite macerals were observed to produce highly variable results. Reflectance variability, as expected, increased with decreasing measuring point size. This variability, however, could not be attributed solely to analytical error or equipment response. One possible explanation for the observed variability in measured reflectance is the presence of underlying remanent cell structure. Several studies have shown that much of the actual morphology present in vitrinite is obscured during polishing, (1,2,3,4,5,6). If this hidden cell structure possesses distinctive reflectance properties, then measurements made on vitrinite would vary depending upon the unobserved heterogeneity of the maceral. Experiments were carried out to determine reflectance variability in single vitrinite macerals and any possible relationship to underlying structure. Sequential measurements were made across apparently homogeneous telocollinite with both the maximum reflectance and the spatial orientation of the point stored within a data file. After the initial reflectance measurement, the pellet was then etched. An oxidizing solution was prepared as described in Stach, et al, (7) and Moore and Stanton, (8). For this experiment 25 grams of K M n 0 dissolved in 70 ml_ of H 0 and mixed with 5 grams H S 0 were prepared. Pellet etching was carried out by immersing polished pellets into the prepared solution. To facilitate etching the solution was heated to 50° C. Etching times were found to be rank dependent and varied from 40 minutes for high-volatile C bituminous coals to 5 hours for low-volatile bituminous samples. 4

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Equipment Reflectance data was obtained using a Leitz MPVIII compact microscope modified for rotational polarization reflectance, (9); (Bensley and Crelling, Fuel., in press). The principle alteration from a typical reflectance microscope is the adaptation of the polarizer to permit rotation through 360 degrees. All polarizer rotation, data acquisition and data processing functions are computer controlled using a 80386/20 mHz computer. Microscope interfacing is made via a gear coupled polarizer and stepping stage, stepping stage controller and IEEE bus. A separate A / D converter controls data acquisition. Optical correction is made on all measurements to remove the effect of residual polarization within the vertical illuminator and transmission optics.


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Results and Discussion Figures 1 and 2 illustrate the results obtained from sequential reflectance measurements. The area delineated with grids represent point locations where individual reflectance measurements were made. The reflectance distribution and average maximum reflectance values are given for each telocollinite maceral. It should be noted that in each example there is at least a 1 V-type range in reflectance values. Figure 1 illustrates the distribution of maximum reflectance readings from 346 sequential measurements. An overall average reflectance of 0.99% was obtained for this maceral with individual reflectance values ranging from 0.95% to 1.06%. In figure 2 two adjacent telocollinite macerals were evaluated. Both macerals possess similar overall reflectance properties with an average reflectance of 0.97%, (256 readings), and 0.94%, (366 readings), noted for each maceral. Both macerals illustrated in figure 2 possess a distinctive bimodal reflectance distribution which contrasts from the distribution observed in the telocollinite maceral illustrated in figure 1. In all three cases telocollinite macerals from the same coal, possessing similar average reflectance, 0.99%, 0.97% and 0.94% respectively, show marked differences in the distribution of reflectance values observed for each maceral. Figure 3 illustrates the effect of etching on the maceral analyzed in figure 1. Comparison between the etched structure and the reflectance values obtained on the polished maceral are illustrated in figure 4. Figure 4 uses gray scale imaging to facilitate comparison between reflectance values and morphology in the maceral itself. In figure 4 a reflectance range has been assigned to a gray scale value and positioned to the appropriate spacial orientation. Comparison of the white light images in figure 3 and the coded, mapped reflectance area of figure 4 demonstrate some measure of influence on reflectance of the remanent cell structure revealed through etching. The low reflectance values, noted by dark values, correspond with areas etched away by the oxidizing solution. This is particularly noticeable in the right section of the image and the center where low reflectance values tend to correspond to open cell structure. The highest reflectance values also correspond to the areas of higher reflectance preserved in the etched sample. Similar correlation of reflectance values to maceral structure in the unetched image is not evident. Figure 5 illustrates the effects of etching on the two telocollinite macerals shown in figure 2. In this case there are marked differences between the properties of the two macerals despite similar average reflectance. The right maceral is extensively etched by the solution while the maceral on the left is preserved very much intact. In figure 6 the reflectance distributions are shown against the gray scale image of the unetched and etched macerals shown in figure 5. As noted previously, areas of extreme etching correspond to regions of lower reflectance in the



Figure 1: Reflectance distribution as measured on a homogeneous telocollinite.

apparently


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Figure 2: Reflectance telocollinite macerals.

distributions

measured

on two

adjacent



Figure 3: Unetched, (top), and etched, (bottom), photomicrograph of a telocollinite maceral illustrated in figure 1.


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Figure 4: Gray scale images of white light photomicrographs illustrated in figure 3 superimposed with coded reflectance map illustrated in figure 1.


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Figure 5: Unetched, (top), and etched, (bottom), photomicrograph of the two telocollinite macerals illustrated in figure 2.


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Figure 6: Gray scale images of white light photomicrographs illustrated in figure 5 superimposed with coded reflectance maps illustrated in figure 2.


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maceral shown in the left field of the figure. Areas of higher reflectance correspond to the most resistant regions in the etched maceral. Perhaps the most striking feature of the image is that areas possessing reflectance of intermediate value correspond very well to regions undergoing minor alteration after etching. This is seen most easily in the left most area of the image where an arc of intermediate reflectance values runs between lower reflecting regions. The maceral positioned to the right of the image illustrated in figure 5 lacks the clear associations with remanent cell structure to reflectance values noted in the left maceral. The lower reflecting areas in the left most field of the right image do correspond to regions of extensive etching. Most of the maceral, however, has been severely etched and little cell structure remains evident. Correlation between either the unetched telocollinite or the etched structure is difficult to ascertain. Figure 7 shows the effect of variation in the measuring spot size on vitrinite reflectance. It is evident that as the area of measurement increases, the resultant reflectance distribution tightens. The observed increase in standard deviation noted with decreasing measuring spot size has generally been attributed to signal degradation. The interpretation from reflectance mapping suggests that heterogeneity within the vitrinite can account for much of the observed scatter in the reflectance distribution. Figure 8 illustrates the effect of the measuring point size. During stage rotation the measuring point will migrate across an area which can represent hidden morphology of variable reflectance. The resulting reflectance value represents an average of the area measured with extreme high or low values effectively averaged out. Similarly, when larger measuring areas are used, reflectance tends to converge on an intermediate value. The precision of the measurement is therefore controlled by the size of the measuring point and the ellipse of rotation or measuring point size. When a small measuring point is used, the odds of landing on a single remnant cell is increased, hence extreme high or low values are more readily observed within the reflectance distribution. This interpretation is consistent with the data collected and can readily explain the observed increase in standard deviation noted for random reflectance, which typically uses small measuring points, when compared with conventional maximum reflectance measurement.

Conclusions The variation in vitrinite reflectance observed in single macerals can approximate the variation in reflectance for the whole coal. This variability appears to be caused by remnant cell structure within the vitrinite maceral itself. This heterogeneity is not readily observed since polishing obscures the morphology. Etching the polished sample is the only practical way this hidden morphology can be visualized.


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Figure 7: The variation in reflectance distribution noted for various measuring point sizes. All readings were taken from the same vitrinite maceral. Mukhopadhyay and Dow; Vitrinite Reflectance as a Maturity Parameter ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Figure 8: Illustration of measuring point size as it relates to remnant cell structure.


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The size of the measuring point controls the reflectance distribution and the effect of remnant cell structure on the distribution. As the measuring point size increases, the macerals inherent heterogeneity converges on an average. The larger the area of measurement the tighter the observed reflectance distribution. Not all vitrinite macerals show structural influence but many do. The precise influence of remnant structure on a typical reflectance analysis is unknown at this time. Data suggests that an increase in the reflectance distribution around the reflectance average should occur. The mean reflectance should not, however, change significantly.

Literature Cited 1.

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Stach, E. and Kuhlwein, F. L ; Die mikroskopische Untersuchung feinkörniger Kohlenaufbereitungsprodukte in Kohlenreliefschliff. Glückauf 64, 1928, 841-845. Stach, E.; Lehrbuch der Kohlenmikroskopie. - Verlag Glückauf: Kettwig, 1949, 285 pp. Mackowsky, M.-TH.; Das Ätzen der Steinkohle, ein aufschluβreiches Hilfsmittel in der Kohlenmikroskopie. - C. r. 7. Congr. intern. Strat. Géol. Carbonifère 3, Krefeld, 1974, 375-383. Hacquebard, P. Α., Birmingham, T. F. and Donaldson, J . R.; Petrography of Canadian coals in relation to environment of deposition. - Symp. Science and Technol. of Coal, 1967, 87-97, Ottawa. Pierce, B. S., Stanton, R. W., and Eble, C. F.; Facies development in the Lower Freeport coal bead, west-central Pennsylvania, U.S.A. Int. J . Coal Geol. 1991, 18, 17-43. Pierce, B. S., Stanton, R. W., and Eble, C. F.; Comparison of the petrography, palynology and paleobotany of the Stockton coal bed, West Virginia and implications for paleoenvironmental interpretations. - Org. Geochem. 1993, 20, 2. 149-166. Stach, E., Mackowsky, M.TH., Teichmüller, M., Taylor, G . H., Chandra, D., Teichmüller, R.; Stach's Textbook of Coal Petrology. Gebrüder Borntraeger, 1982, 535 pp. Moore, Τ. Α., Stanton, R. W.; Coal petrographic laboratory procedures and safety manual. - U.S. Geological Survey Open-File Report 1985, 85-20. Bensley, D. F. and Crelling, J . C.; The use of rotational polarization reflectance in the characterization of fine particulate macerals: Proceedings of the Int. Conf. on Coal Science, 1993, v. 1., 578-581.

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Presence of Remanent Cell Structure in Vitrinite ... - ACS Publications

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