Vitrinite Reflectance as a Maturity Parameter - American Chemical

Chapter 6

Petrographic and Geochemical Anomalies Detected in Spanish Jurassic Jet 1

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I. Suárez-Ruiz , M . J . Iglesias , A. Jiménez , F. Laggoun-Défarge , and J . G. Prado Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: November 9, 1994 | doi: 10.1021/bk-1994-0570.ch006

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Instituto Nacional del Carbón (CSIC), La Corredoria, s/n. Apartado 73, 33080 Oviedo, Spain Unité de Recherche en Pétrologie Organique, Unité de Recherche Associée, 724 du Centre National de la Recherche Scientifique, Université d'Orléans, 45067 Orléans, Cedex 2, France 2

The Kimmeridgian Spanish jet was studied using petrographic and geochemical methods. Rank parameters obtained from chemical analyses could not be correlated with those determined by petrographic measurements. Explanation of the anomalies could be related to differential absorption in early diagenetic stages, of petroleums generated from the Pliensbachian infraadjacent source-rocks. These oils would only be retained inside ulminite micropores, affecting its reflectance and would give special properties to this coal.

From the point of view of rank studies, vitrinite reflectance (R ) is one of the parameters most widely used due to its quick and simple obtention and application for determining the organic evolution stages. Moreover it is, in general, recognized as one of the most useful methods for evaluating changes in the organic substances through their geological history. Although temperature, duration of heating and pressure (the latter in a different way) are the main factors affecting vitrinite reflectance during the coalification process, in the last ten years other factors have been also observed and recently reviewed by Barker (7). These factors may eventually affect the reflectance values and modify the normal growth of reflectance. For this reason, in some cases the suitability of vitrinite reflectance as a rank parameter may be limited and perhaps can not be considered an indicator of organic maturity. The anomalies in reflectance usually manifest themselves in a reduction of its values when they are compared to those found in profiles or normal evolution series. This is also the case when they are compared to the reflectance values obtained in adjacent sediments with the same diagenetic history. In other cases anomalous values are detected when for the same material vitrinite reflectance disagrees with other rank parameters. The reduction of reflectance just mentioned is known as reflectance suppression (7). 0

0097-6156/94/0570-0076$08.00/0 © 1994 American Chemical Society

In Vitrinite Reflectance as a Maturity Parameter; Mukhopadhyay, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Anomalies Detected in Spanish Jurassic Jet

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In this paper, the relationships between the absorption of oil or petroleum-like substances in the microporosity of huminite (ulminite) and the suppression of its reflectance are studied. Furthermore, the influence of this absorption on coal composition and on other evolution parameters is considered. The coal chosen, Spanish jet , is known due to its special properties, particularly for polishing, and also because it remains unaltered after long exposure to air. This coal has been utilized since Xllth and Xlllth centuries for gem and ornamental purposes. The deposits are found in precise locations in Asturias (Northern Spain), Figure 1, and their geological age corresponds to the Malm (Kimmeridgian) inside detrital facies (4,5). These coals have been formed from drift wood carried away by streams (6) and deposited in a transitional marine-continental sedimentary environment. Previous studies on this coal are scarce and they only refer to its physical characteristics (5,7). On the other hand and taking into account the location area of Spanish jet (the sediments belong to catagenetic stage of evolution), a very low reflectance for this coal was found (6).

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: November 9, 1994 | doi: 10.1021/bk-1994-0570.ch006

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Analytical Procedures The samples were obtained from an underground mine and prepared for pétrographie and geoehemieal analyses. A pétrographie study was carried out on a M P V Π Leitz apparatus by means of reflected white light using oil immersion objectives (32x) for the reflectance measurements in accordance with ISO 7404/5 (8) procedure. The maceral composition was obtained following the procedures described in ISO 7404/3 (9). The fluorescence characterization was performed on a M P V ΠΙ Leitz in water immersion and blue light. Spectral analysis and alteration measurements (after 30 min of exposure) were made by means of U.V. light following the procedure described by Martinez et al. (10). The spectral parameters considered in this work were the quotients QF-535, Q650/500 and the emission flux F. The organic phase organization, which compliments the pétrographie analysis, was carried out by X Ray Diffraction Analysis. The mineral matter of this coal was analyzed by means of S T E M (Scanning Transmission Electron Microscopy) and S E M (Scanning Electron Microscopy) coupled to an EDS (Energy Dispersive Spectrometer). For these analyses, the size of the grains was 2-5 mm. Moreover, the crystallized phases associated with this vitrain were determined by X-Ray Diffraction in a powdered sample. The chemical characterization (Proximate and Ultimate Analyses, Sulfur Forms and Calorific Value) was made in accordance with the international standard procedures, ISO-589, ISO-1171, ISO-562, ISO-925, ISO-157, ISO-1928 (11-16). The C, Η, Ν and S content was determined using a L E C O C H N 600 and L E C O SC 132 to(al

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"Jet is formed from drifted wood which has been secondarily impregnated with bitumen from the surrounding environment leading to an abnormally low reflectance, strongfluorescenceand uniform physical properties" (2). "Jet is drift wood (stems and branches), embedded in Jurassic oil shales, which is impregnated with secondarily bitumen. They outcrop in England, France and Southern Germany" (3).


VITRINITE R E F L E C T A N C E AS A MATURITY PARAMETER


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Figure 1. Location of the jet mine on the map, showing the distribution of the rank of the organic matter from the Jurassic according to the vitrinite reflectance data (6). Strat-column of Jurassic sediments from Asturias (Northern Spain) (38).



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Anomalies Detected in Spanish Jurassic Jet

apparatus, and the oxygen was calculated by difference. The oil and tar yields of the raw material were obtained through pyrolysis techniques: Gray-King Assay -ISO 502 (77)-, Rock-Eval -performed in accordance with Espitalié (18,19) and thermogravimetry in the conditions described in a previous paper (20). Furthermore, parameters affecting type, composition and evolution of this coal, obtained from Rock-Eval pyrolysis, were also considered. The functional groups of Spanish jet and its oil -obtained through pyrolysis- were studied by Fourier Transform Infrared Spectroscopy (FTIR). The solid samples were prepared using the traditional procedure (21,22), the ratio being coal:KBr of 1:100 and more than one pellet was prepared. The oil was dissolved in dichloromethane and dropped gently on a KBr pellet and the solvent evaporated. All absorbance spectra were recorded on a Perkin Elmer 1750 spectrometer co-adding 25 interferograms obtained at a resolution of 4 cm" . Each spectrum was corrected for scattering and then calibrated to 1 mg (daf)/cm . The soluble organic fraction was obtained according with the extraction procedure described by Blanco et al (23). Thus, a weighed amount of ground coal (

Vitrinite Reflectance as a Maturity Parameter - American Chemical

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