A Preliminary Electron Microprobe Study of Green River and Devonian

13 A Preliminary Electron Microprobe Study of Green River and Devonian Oil Shales

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: December 1, 1978 | doi: 10.1021/ba-1978-0170.ch013

E. A. HAKKILA, Ν. E. ELLIOTT, J. M. WILLIAMS, and E. M. WEWERKA Los Alamos Scientific Laboratory, Los Alamos, NM 87545 One sample each of Green River shale from Colorado and Devonian shale from West Virginia was examined with the electron microprobe. These studies demonstrated that the identities and distributions of many shale minerals can be inferred by correlating elemental associations with x-ray mineralogical data. Dark bands in both samples were com posed primarily of organic matter associated with quartz, feldspars, clays, and carbonates. The carbonates present in the dark band were identified as calcite and dolomite which contained small amounts of iron. Dolomite, iron-rich dolo mites and magnesites, and small amounts of siderite pre dominated in the light organic-poor bands in these speci mens. Organically associated sulfur was observed only in the Green River shale sample.

Hphe energy crisis, brought about by dwindling oil and gas reserves, has prompted our nation to seek new sources of these strategic com modities. Among the possible oil and gas sources are the Green River shales of the Rocky Mountains and the Devonian shales in the eastern United States. The organic matter (kerogen) in the Green River shale is the potential source of over 4 trillion barrels of oil or petroleum feed stock (I). In addition to the kerogen entrapped within the Devonian shale there is a huge resource (over 500 quadrillion cu ft) of natural gas (2). Unfortunately, the resources in both these deposits are con tained in rock matrices, and extraction of the oil or gas from either of these shale beds is costly and very difficult. While some efforts are underway to increase production efficiency, others have been directed at characterizing the rock matrices to deter mine how the resources are contained. This knowledge will be useful 0-8412-0395-4/78/3-170-181$05.00/l © 1978 American Chemical Society In Analytical Chemistry of Liquid Fuel Sources; Uden, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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in deciding how the oil or gas can be produced. The purpose of this study is to demonstrate the utility of the electron microprobe in characterizing energy bearing shales. For this purpose, we have chosen a sample from each of the major U.S. shale areas. This initial study, although involving only two samples, still allows some comments to be made about the compositional similarities and differences between the two shale types.


Instrumentation and Procedure One sample each of Green River and Devonian shales was obtained from the Laramie and Morgantown Energy Research Centers, respectively. The Green River shale was taken from the Mahogany Zone in the Piceance Basin of Colorado; the Devonian specimen was taken from the Marcellus layer in Jackson County, West Virginia. Each sample was sectioned as a slab, 2.5-3.0-cm long and 1.2-cm wide by 0.6-cm thick, with the long direction parallel to the vertical axis of the core. The Devonian sample was pressure-impregnated with epoxy resin to preserve its structure during sample polishing. The samples were polished to reveal the microstructure and then coated with a 100-Â layer of carbon to provide electrical conductivity. The x-ray analyses were obtained using powder camera and diffractometer techniques. In the former, powdered shale was loaded into a glass capillary which then was exposed to 20 hr of C u ( K « ) radiation. The x-ray pattern was taken with a standard Norelco 114.6-mm diameter powder camera equipped with a nickel filter. For the diffraction technique, —325-mesh shale powders were packed into the 7/8-in. diameter by 1/16-in. deep cavity of a 1-in. diameter aluminum holder and pressed smooth with a glass plate. A NorelcoPhillips diffractometer equipped with a C u ( K « ) source was used to analyze the samples. The diffractometer was driven at l°/min and the intensity response of the nonrotating sample recorded on a strip chart recorder. Peak assignments in both cases were made by visual comparisons with standards. The electron microprobe used in this study was an Applied Research Laboratories Model E M X - S M , equipped with three wavelength dispersive x-ray spectrometers of differing wavelength capabilities and with a silicon (lithium) (Si (Li)) energy dispersive spectrometer. This arrangement allowed all elements heavier than beryllium (Be) to be studied. The spectrometers, stage motion (x- or y-direction ), and sealertimer readout were controlled manually or through a PDP-8E computer. The energy dispersive spectrometer was used for rapid identification of elements in areas or grains; however, the detailed studies were performed with the wavelength dispersive spectrometers which give higher sensi-

In Analytical Chemistry of Liquid Fuel Sources; Uden, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


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tivity. A secondary electron detector permitted scanning electron photo micrographs (SEM) to be taken. These S E M photographs allowed the two-dimensional x-ray images of elemental distributions to be correlated with surface features. Two techniques were used to obtain information on sample compo sition. In the first, the electron beam was rastered rapidly over a specified area (here 80 X 100 /xm) and the characteristic x-rays of the elements sought were monitored. An x-ray intensity ratio for each element in the entire area was determined from the intensity of the measured x-rays vs. those from a standard material: pure iron for iron, pyrite for -sulfur, pure aluminum for aluminum, a grossularite for calcium, and graphite for carbon. By sweeping many adjacent areas, detailed information about the microscopic changes in elemental and mineral composition along the total length of a sample could be obtained. In the second technique, the electron beam was rastered slowly across a selected area of the sample, and the intensities of the elements of interest were recorded photographi cally from an oscilloscope screen. This method provided point-by-point information about the elements present and rough estimates of their rela tive concentrations and distributions. Elemental data from the microprobe analyses were correlated with the x-ray mineralogical analyses to provide the basis for inferring the micromineralogy of these two samples. Results and

Discussion

The Green River oil shale is an organic-rich marlstone that is thought to have been deposited in a lacustrine environment (3,4). These shales are composed of distinct pairs of light and dark bands, called varves, that reflect seasonal variations in the organic content of the shale (5). The varves in the Green River shale were shown by Smith to average about 20 μΐη in thickness (3). Dolomite, quartz, and analcite were the major minerals found in our shale sample. Lesser amounts of calcite, albite, K-feldspars, K-clays, and mica-clays were observed as well as trace levels of siderite and either pyrite or marcasite. This mineralological composition is similar to that reported elsewhere for the Green River shales (3,4). The banded structure of the Green River shale studied here is shown in the photomicrograph of Figure 1. Using the rapid rastering microprobe technique, the entire 3-cm length of the sample was analyzed at 100-jLim intervals for Fe, S, Ca, Al, and C. The ratios of the measured x-ray intensities to those for the standards for these elements are plotted in Figure 1. The dark shale areas are enriched considerably in organic matter as indicated by the higher intensity C x-rays. The elemental analyses indicate that the light bands, on the other hand, are composed


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Figure I. Distributions of Fe, Al, S, Ca, and C along length of Mahogany zone shale. Letters on photomicrograph refer to areas described in text.


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primarily of carbonates, and they are seen to contain less S and A l and more Ca and Fe than the darker C-rich bands. The sharp spikes in the S intensities are associated with areas of high Fe (presumably pyrite or marcasite) concentrations. These pyritic areas are associated mainly with the organic-rich bands. A notable exception to the Fe/S association is the S spike at approximately 16,000 /nu which is not accompanied by a corresponding rise in Fe intensity and which indicates the presence of nonpyritic S. These observations are consistent with patterns of geo chemical genesis proposed by Smith for the formation of Green River shale (3). From the areas of this Green River shale sample that were examined in detail using the slow rastering microprobe technique, we have chosen three for discussion. Area A was a dark band located near the 6000-/xm position in Figure 1. Area Β was in the wide, lighter colored area located at ~ 13,000 μΐη, and area C included a black, organic strip at ~ 16,000 μΤΠ.

A scanning electron photomicrograph (SEM) and corresponding element x-ray intensity for area A are shown in Figure 2. This area contains numerous particles, which appear as light, often angular, crys tals in the S E M . These particles contain only S and Fe in detectable concentrations, and in accordance with the x-ray analysis they are prob ably pyrite or marcasite. At least three other minerals are also present in this area. A phase that appears fine-grained in the S E M contains pre dominately Ca, magnesium (Mg), C, and oxygen (O) and small amounts of Fe, and is most likely a dolomite. The large, circular grains in the SEM contain predominately Al, silicon (Si), sodium (Na), and Ο and is most likely analcite, but may be albite, which was shown also by x-ray analysis to be a constituent in this shale. Lastly, potassium (K) is associated with Al in afine-grainedsilicate phase which may be a potas sium clay or feldspar. Area Β was observed under the microscope as a tan-colored strip located on either side of the crack at ~ 13,000 μχη on the photomicro graph in Figure 1. This area is characterized by a significantly higher Fe concentration than in the darker areas of the sample. The Fe is asso ciated with Mg, C, and Ο and lesser amounts of Ca, and occurs probably as Fe-rich dolomite or magnesite. Between grains of this material is a mineral that contains Al, Si, K, and Ο and may be a K-clay or feldspar. Another mineral phase contains Al, Si, and O, but no Κ or Na, and is possibly kaolinite although this mineral was not observed by x-ray dif fraction. Quartz (Si0 ) and titanium dioxide ( T i 0 ) are also present in minor amounts in this area. X-ray diffusion studies did not identify the T i 0 mineral. 2

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Area C contains a black strip which is predominantly C, but also with



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significant amounts of Na, S, and O. The S is probably organic, as indi cated by the shape of the Κβ x-ray bands (7). A dolomitic region, con taining less Fe and Mg than found in area B, and a silicate phase, con taining Al and K, are distributed heterogeneously around the black strip. Small inclusions, generally less than 5 μτη in diameter, contain T i (tita nium) and Ο and appear outside of the black strip. Smith has suggested that the Green River shales were formed at p H levels between 8.5 and 10 (3). The carbonate types typically encoun tered under these conditions are calcite, dolomite, magnesite-siderite co-crystals and siderite. All these species were observed with the microprobe or by x-ray diffraction in the carbonate phases of the shale sample that we studied. Smith also reasoned that the presence of organic matter in the Green River shales should enhance the precipitation of silicate minerals. Our data is in accord with this postulation. Thus, we find that the organic-rich bands in this shale sample have higher concentrations of siliceous minerals than do the organic-deficient bands. These siliceous minerals occur as quartz and aluminosilicates. Relatively large negative E h values (reducing conditions) also have been predicted for the formation of the Green River shales (3). The observation that none of the sulfur present in shale sample studied is observed as sulfate agrees with this. The S is observed generally as FeS particles less than 50 μχη in diameter, most often observed in or adjacent to the organic-rich areas. Some organic S is observed in organic material. 2

Several other minerals were observed in this sample of Green River shale, include several phosphorus (P)-containing minerals, possibly in the form of apatite or fluoroapatite. Certain rare earth elements—cerium (Ce), lanthanum (La), praseodymium (Pr)—were found in some of the calcium phosphate regions. Phosphorus has ben detected as a trace con stituent of Green River shales, and has been postulated to exist as apatite although it has not been identified by x-ray diffraction (8). In contrast to the Green River shale, the Devonian shale is regarded generally as being elastic in origin. Mineralogically, our sample was composed primarily of quartz and K-clays or micas. Minor amounts of pyrite and alumina were present as well as trace amounts of siderite, kaolinite, dolomite, and marcasite. The siderite content was higher in this Devonian shale sample than in the Green River shale sample studied. The Devonian shale sample generally had a darker appearance with fewer and less distinct bands than the Green River shale sample, and the organic matter appeared to be more evenly distributed over the 2.5-cm sample. Several areas of this sample were examined using the slow rastering microprobe techniques to determine the elemental distribution in this material. The observations made in two of the areas are presented below.


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Figure 2. Scanning electron micrograph (SEM) and S, Fe, Co, Mg, C. Si, Al, Na, and Κ x-ray intensity distributions in dark band of Mahogany zone shale


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Figure 3.

SEM and C, Fe, Ca, Mg, Si, ΑΙ, Κ, Na, and Ti x-ray intensity distnbutions in dark band of Devonian shah



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One of the areas studied is shown in Figure 3 and is located in a darker, organic-rich band. The matrix of this area contains Si, primarily as quartz, but also as aluminosilicates with low concentrations of K. Two distinct carbonate regions can be differentiated in this sample. The first, shown in the upper right of the SEM, contains predominantly Fe with lesser amounts of Mg and even smaller amounts of Ca. Based on the x-ray analysis, this region probably is composed of siderite or Fe-rich magnesite. The second carbonate region located in the lower right of the S E M contains predominantly Ca and lesser amounts of Fe and Mg, and most likely is impure calcite although this mineral was not observed in the x-ray diffraction analysis. Several irregularly shaped particles shown in the lower left and center of the S E M contain T i and O. Another area studied in the Devonian shale sample is located also in a darker part of the shale structure .(Figure 4). This area contained numerous, metallic-appearing particles as well as a black, irregularly shaped, carbon-rich region. The metallic particles contained only Fe and S and are mainly pyrite. Pyrite has been reported as the primary heavy mineral identified petrographically in a series of core samples from the Devonian shales of West Virginia (6). The black, irregularly shaped, organic region is surrounded generally by siliceous material consisting of quartz and aluminosilicates that contain only minor concentrations of K. As with the Green River shale, aluminosilicates containing Κ and Na associated together were not observed. Calcium (Ca), Mg, or Fe were not found in the aluminosilicates. This could suggest authigenic forma tion, but confirmation would require further studies, such as observation of cathodoluminescence (9). After polishing, this sample was covered with a cap. When the cap was removed, small droplets of liquid were observed on the surface. Upon exposure to dry air, the liquid disappeared leaving small cubic crystals containing only Na and chlorine (CI). Presumably, Na and CI ions were leached from the interior of the sample and crystallized as NaCl on the surface as the liquid evaporated. Similar results were not observed with the Green River shale, suggesting that NaCl was not pres ent in significant amounts. This study has shown that the sample of Devonian shale is composed primarily of silicates with much lower amounts of carbonate minerals. Here, as with the Green River shale sample, the silicate minerals were associated primarily with the organic-rich areas of the shale, and when present, the carbonate minerals were found mainly in the organic-poor areas. The siliceous minerals of this Devonian shale sample were found to be quartz, illite, and muscovite, with trace amounts of kaolinite. Calcite, dolomite, Fe-rich dolomite or magnesite, and siderite were ob served in the carbonate regions of the Devonian shale. However, the


Green River and Devonian Oil Shahs

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Figure 4. SEM and Fe, S, Ti, and C x-ray intensity distributions in dark area of Devonian shale. Large Ti particle at upper right contains C; small Ti particle near center contains O.


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Devonian shale had regions with much higher concentrations of siderite than were found in the Green River shale sample. As in the Green River shale, the FeS particles were less than 50 /mi in diameter and in most instances were observed only in the organic-rich areas of the samples. 2


Summary This paper is not intended to be a definitive microprobe comparison of the two shales, but, rather, is a prehminary study intended to demon strate to those not familiar with the electron microprobe the utility of using this technique in studying energy-bearing carbonaceous shales. We have attempted to show that the microprobe can produce a large amount of information about the elemental associations and micromineralogy of these shales in a relatively short period of time. The elemental analyses reported here from this study are qualitative, but methods for quantitative analyses are available to aid in the identification of specific minerals (10). Acknowledgment We thank the Laramie ( L E R C ) and Morgantown ( M E R C ) Energy Research Centers for providing us with shale samples, as well as J. W . Smith of L E R C for pointing out the need for these characterization studies and for his encouragement and helpful discussions. We also thank R. B. Roof of L A S L for his help in determining the x-ray identifi cation of the minerals in this study, and R. D. Reiswig of L A S L for preparing the samples for microprobe analyses. The work upon which this chapter is based was performed under the auspices of the U S D O E . Literature Cited 1. Atwood, M. T., Chem. Technol. (1973) 617-621. 2. Overbey, W. K., Morgantown Energy Research Center, personal communi cation (1975).

3. Smith, J. W., Rocky Mountain Association of Geologists, 1974 Guidebook, pp. 71-79.

4. Robb, W. Α., Smith, J. W., Rocky Mountain Association of Geologists, 1974 Guidebook, pp. 91-100.

5. Bradley, W. H., U. S. Geol. Survey (1929) Prof. Paper 158-E, 87-110. 6. Larese, R. E., Heald, M. T., Morgantown Energy Research Center report MERC/CR-77/6

(1977).

7. Hurley, R. G., White, E. W., Anal. Chem. ( 1974) 46, 2234-2237. 8. Desborough, G. Α., Pitman, J. K., Huffman, G., Chem. Geol. (1976) 17, 13-26.

9. Kastner, M., Am. Mineral. (1971) 56, 1403-1442.

10. Bence, A. E., Albee, A. L., J. Geol. (1968) 76, 382-403. RECEIVED August 5, 1977.


A Preliminary Electron Microprobe Study of Green River and Devonian

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