Energy & Fuels 1990,4, 705-709
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Practical Considerations for Using Reflectance Spectroscopy as a Screening Tool for Geoporphyrinst Peter N. Holden* and Michael J. Gaffey Department of Geology, Rensselaer Polytechnic Institute, Troy, N e w York 12181 Received M a y 29, 1990. Revised Manuscript Received J u l y 30, 1990
Reflectance spectroscopy of powdered shale samples can be used as an effective screening technique for geoporphyrins. The porphyrins are identified by their characteristic absorption bands which are superimposed on a sloping continuum that results primarily from intervalence electronic transitions in iron. The porphyrin absorptions are a composite of the spectral contributions of all porphyrin species in the bitumen and kerogen fractions. This composite spectrum can be interpreted in terms of porphyrin geochemical parameters provided uncertainties introduced by the sample preparation, spectrum measurement, and data reduction steps are minimized.
Introduction Porphyrin heterocyclic compounds were first identified in organic-rich shale samples by Triebs,' who attributed their presence to diagenetic modification of chlorophyll and heme structures after deposition. Today, over 50 years after Triebs' discovery, there is considerable interest in geologic porphyrins as biomarkers of depositional envir ~ n m e n t ,thermal ~,~ h i ~ t o r y ,and ~ , ~original organic input into the sediment c01umn.~J Although the porphyrins should function well in this capacity, their practical application has been limited by the complexity and expense of the analytical techniques involved and by incomplete or inadequately tested models of porphyrin behavior during diagenesis and catagenesis. One approach to reducing the analytical time and expense involved is to develop quick, reliable screening techniques. An effective screening procedure could yield sufficient information about the porphyrin constituents of samples so that the number of more exhaustive analyses that need to be performed can be reduced. For example, the ubiquitous first step of current techniques for analysis of geologic porphyrins is extraction of the organic matter from the rock or sediment sample. This is frequently the most time-consuming step in the procedure and introduces a bias from the onset in that only those porphyrins in the soluble bitumen fraction will be analysed. Considerable time savings could be realized were it possible to measure geochemically relevant characteristics of the porphyrins in each sample prior to extraction. This would allow the investigator to focus on those samples of particular interest and to compare the results of the more detailed extraction studies with those for the whole rock. A screening technique based on whole-rock reflectance spectroscopy is being developed that permits some porphyrin analyses to be made prior to extraction. Sample preparation consists of powdering the sample followed by sieving to obtain fine particle size fractions. Reflectance spectra of the various particle size fractions are measured and digitally recorded for computer processing. Porphyrins are identified by characteristic absorption bands in the visible and near-ultraviolet region of the spectrum. These absorption bands appear to be a composite of the spectral contributions of all porphyrin compounds present in the bitumen and kerogen fractions. The composite spectrum 'Presented a t t h e Symposium on Porphyrin Geochemistry, 199th National Meeting of t h e American Chemical Society, Boston, MA, April 22-27, 1990.
can be interpreted in order to obtain geochemically relevant information by relating the spectral data to chemical characteristics of the porphyrin species in each sample. Previous studies have indicated that spectra of samples which contain a large porportion of nickel porphyrin exhibit porphyrin bands which are shifted to shorter wavelengths as compared to equivalent absorption bands observed in spectra of samples containing a large proportion of vanadyl porphyrin.8 There also appears to be a correlation between the intensity of the porphyrin bands and the concentration of porphyrin species in the sample. These observations suggest that reflectance spectroscopy can be used to estimate the overall porphyrin concentration in shale samples as well as the porphyrinic vanadium to nickel ratio, a geochemical parameter thought to be related to the alkalinity and redox conditions of the environment in which the shale was deposited.2 Before these parameters can be accurately determined by using reflectance spectroscopy, there are a number of tasks that must be accomplished. First, sources of error in the sample preparation and spectrum measurement steps must be identified and minimized. Second, accurate, precise, and reproducible techniques for measuring any systematic changes observed in the spectra must be developed. Third, the source or sources of the systematic variations observed in the spectra must be established. Fourth and last, an empirical calibration or interpretive model must be developed that permits the relevant geochemical information to be extracted from the spectral measurements. Elements of tasks one, two, and three are treated in this paper. Sources of systematic error by the sample preparation procedures and the spectrum measurement steps are discussed, and methods of minimizing their effects are suggested. A method of measuring spectral parameters of the composite Soret band is presented. The spectral (1)Triebs, A. Angew. Chem. 1936,49,682-686. (2)Lewan, M. D.;Maynard, J. B. Geochim. Cosmochim. Acta 1982, 46, 2547-2559. (3)Moldowan, J. M.; Sundararaman, P.; Schoell, M. Org. Geochem. 1986,10, 915-926. (4)Sundararaman, P.;Biggs, W.; Reynolds, J. G.; Fetzer, Acta C. Ceochim. Cosmochim. Acta 1988,52, 2337-2341. (5)Barwise, A. J. G.; Roberts, I. Org. Geochem. 1984,167-176. (6) Ocampo, R.; Callot, H. J.; Albrecht, P. Metal Complexes in Fossil Fuek; Filby, Branthaven, Eds.; American Chemical Society: Washington, DC, 1987. (7) Kaur, S.; Chicarelli, M. J.; Maxwell, J. R. J. Am. Chem. SOC.1986, 108, 137-138. (8)Holden, P.N. Master's Thesis, Rensselaer Polytechnic Institute, 1988.
0887-0624/90/ 2504-0705$02.50/0 0 1990 American Chemical Society
Holden and Gaffer
706 Energy & Fuels, Vol. 4, No. 6, I990
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Figure 1. Effect of different sieving techniques on percent total organic carbon in each sieve fraction. The open squares are dry-sieved fractions. The black triangles are wet-sieved fractions. The precision of the total organic carbon measurements is estimated to he *0.2%. variability measured by use of this technique is compared to that which is expected to result from natural variations in the vanadium to nickel ratio. Descriptions of empirical calibrations for specific applications are left for follow-up papers so that both the calibrations and subsequent interpretations can be described together.
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Figure 2. Typical reflectance spectrum of an organic-rich shale containing geologic porphyrins. The porphyrin absorption bands occur at approximately 400,580, and 540 nm in order of decreasing intensity. Using the nomenclature designated for metalloporphyrins these correspond to the Soret, a, and 0 bands, respectively. The sloping continuum results primarily from the broad, shallow absorptions of intervalence electronic transitions in iron.
Results Any chemical alteration or fractionation of a sample that may result from powdering and sieving was investigated by measuring total organic carbon of particle size fractions produced by different preparation techniques. Specifically, alteration produced by mechanical and hand grinding as well as wet and dry sieving were investigated. Total organic carbon was found to be approximately 20.0% lower in the mechanically ground fractions versus the hand ground fractions. In addition, a systematic drop in total organic carbon for decreasing particle size fractions is observed for both wet- and dry-sieved fractions (Figure 1). The wet-sieved fractions appear to have experienced a loss in organic carbon as compared to equivalent dry-sieved fractions. An exception is the finest sieve fraction which shows a comparatively high percentage of organic carbon relative to the other wet-sieved fractions. This fraction also contains a high proportion of a buoyant material that appeared at the water surface of each sieve. Systematic sources of noise introduced during measurement of the spectra were also investigated. Recurring, spurious bands were noted in reflectance spectra of shales at approximately 411 nm, nearly coincident with the position of the most intensely absorbing porphyrin band (Figure 2). The spurious feature consists of three or four data points whose reflectance values fluctuate with an amplitude less than or equal to f0.15'70 reflectance. The background noise level increases from *0.01% reflectance at wavelengths longer than 411 nm to *0.05% reflectance at wavelengths shorter than 411 nm. Sources of the noise and methods of minimizing the errors that can result from it are considered in the Discussion section. Variability in the shape and position of the Soret band has been noted in spectra of shale samples with different porphyrinic vanadium to nickel ratios. This variability was investigated in an attempt to determine whether it results from differences in vanadium to nickel ratio or results from random variations produced by errors introduced at different steps in the technique. Differences between Soret bands can be seen in spectra of samples of the New Albany shale, Green River shale, and Bakken shale. Previous analyses of these shale formations indicate that the porphyrin compounds in the Bakken shale are predominantly vanadyl species: the
400 600 WAVELENGTH (NRNOHETERSI
Figure 3. Porphyrin absorption bands from reflectance spectra of the New Albany (A), Green River (B),and Bakken (C) shales. The bands have been normalized by dividing out a linear continuum.
porphyrin compounds in the Green River shale are predominantly nickel species: and those porphyrins in the New Albany shale are made up of both species with a slightly higher proportion of the nickel species.'O The porphyrin bands present in the reflectance spectrum of each shale were isolated by dividing out a linear continuum and smoothed by using a three-point running mean. They are shown in Figure 3. The Soret band minimum of the Bakken shale spectrum is shifted approximately 10 nm to longer wavelengths as compared to the Soret bands of the New Albany and Green River shale spectra. The Soret bands of the Bakken shale and Green River shale spectra are symmetric about their respective band minima while the New Albany shale spectrum is asymmetric with a shoulder on the longwavelength side. The differences observed between the Soret bands are similar to those reported for absorption spectra of individual vanadyl and nickel porphyrin structures. For equivalent structures, the Soret band of the vanadyl species is shifted approximately 5 to 10 nm to longer wavelengths than the Soret band of the nickel species." The composite Soret bands observed in reflectance spectra behave as though each consists of two overlapping bands (9) Louda, W.; Baker, E. Written communication, 1987. (IO) Van Berkel, G. J.; Quirke, J. M. E.; Filby, R. H.Org. Geochem. 1989, 14, 119-128.
(11) Hodgson, G. W.; Baker, B. L. Chem. Geol. 1967,2, 187-198.
Reflectance Spectroscopy for Geoporphyrins h
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Figure 4. First derivative of the Woodford shale reflectance spectrum shown in Figure ,2. Both smoothed and unsmoothed data are shown. The long- and short-wavelergth inflection points are defined as the relative maximum and minimum points associated with the Soret band.
whose band minima are separated by a distance of approximately 10 nm. The relative intensities of these bands appear to be governed by the relative concentrations of nickel and vanadyl species in each sample. Changes in vanadium to nickel ratio are indicated by changes in symmetry and band minimum position. If this model is correct, spectra of samples containing either solely vanadyl porphyrin or solely nickel porphyrin should exhibit symmetric Soret bands whose band minima are located at either the vanadyl or nickel position. Spectra of samples containing more vanadyl than nickel species should exhibit a Soret band with the vanadyl band minimum position and a shoulder on the short-wavelength side. Similarly, spectra of samples with more nickel than vanadyl species should exhibit a Soret band with the nickel band minimum position and a shoulder on the longwavelength side. An intermediate band minimum position should result for samples containing nearly, but not exactly, equal concentrations of vanadium and nickel porphyrin due to slight differences in the extinction coefficients of the respective Soret bands. Ideally, this model would be tested by correlating any spectral variability that is observed for a set of standard shales with the porphyrinic vanadium to nickel ratio of those shales. Unfortunately, there are very few samples available that have been previously characterized for porphyrinic vanadium to nickel ratio and, for those that have, the characterizations seldom account for those porphyrin species in the insoluble kerogen fraction. Another problem, that of accurately measuring the differences between each Soret band, must also be considered. Changes in absorption band position and symmetry can be monitored by measuring geometric parameters of absorption bands such as band minima and inflection points. Three spectral parameters were chosen to monitor changes in the Soret band. These are the band minimum as determined after continuum removal, the long-wavelength inflection point as determined from the first-derivative spectrum and the short-wavelength inflection point also determined from the first-derivative spectrum (see Figure 4). These are not the only methods available for determining these parameters12J3 but are straightforward techniques that serve the purpose of this study. The problem created by the limited number of characterized samples is treated by tabulating the wavelength
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Figure 5. Core sample (top) and theoretical (bottom) histograms of the wavelength positions of inflection points and band minima. Black shading indicates long-wavelength inflections. Diagonal shading indicates band minima. Grey stipple shading indicates short-wavelength inflection points. The theoretical histogram was constructed as described in the Experimental Section.
positions of the spectral parameters for a set of uncharacterized shale samples. The resulting distribution of the wavelength positions of these parameters can then be examined to see if it is consistent with that which is expected to result from natural variability in vanadium to nickel ratio. If the results are consistent, it would suggest that the model of spectral variation is correct and that the method of measuring the spectral variation is successful. The results for core samples taken from various organic-rich shales are shown in the form of a histogram a t the top of Figure 5. Peaks in the histogram occur a t approximately 426,416,411,406, and 396 nm. There is a broad distribution of short-wavelength inflection points in the 392-406-nm region. These results can be compared with a theoretical histogram of band minima and inflection points shown a t the bottom of Figure 5. This histogram shows results expected based on our model of spectral variation and was constructed as described in the Experimental Section. A comparison between the core-sample histogram and the theoretical histogram reveals a correlation between the major peaks with the exception of a peak in band minima located a t 411 nm and a broad distribution of short-wavelength inflection points between 392 and 406 nm. These are present only in the core-sample histogram. The implications of these results are considered below. Discussion Sample Preparation. For a sample with a given porphyrin concentration, the intensity of the porphyrin bands
708 Energy & Fuels, Vol. 4, No. 6, 1990
Holden and Gaffey
observed in the reflectance spectrum is a function of particle size, absorbing efficiency of the sample's nonporphyrin constituents, and the extinction coefficients of the specific porphyrin species in the sample. Particle size can be constrained through powdering and sieving of the sample. Spectra of the smallest particle size fractions exhibit the strongest porphyrin absorption bands. Measurements of total organic carbon made on particle size fractions demonstrate that the powdering and sieving procedures chemically alter and/or fractionate the sample. The large organic carbon loss in mechanically ground samples most likely results from volatilization due to excessive heating. This does not come as a great surprise as the rotating disk grinder used has no cooling mechanism to minimize internal temperatures. The organic carbon loss is so extreme, however, that this form of alteration should be considered in any mechanical grinding system. The loss of organic carbon through volatilization is best avoided by hand grinding all samples. As this may be impractical in many cases, a mechanical grinding system that minimizes heating of the sample by operating at slow speeds or by circulating cooled liquids is recommended. The efficiency of heat minimization should be checked periodically by comparing results produced by splits of samples that have been hand ground and mechanically ground. Also interesting is the apparent systematic loss of organic carbon with sieving and the lower levels of organic carbon in wet-sieved fractions as compared to the dry-sieved fractions (Figure 1). One possible explanation for this behavior is a concentration of organic matter in extremely fine particle sizes. These fine particlese may be inclusions in larger particles that are liberated when the larger grains are broken down in the powdering process. This would account for the appearance of a buoyant material during wet sieving as well as for the high percentage of organic carbon in the particle size fraction that contains most of the buoyant material. The generally higher levels of organic carbon among the dry-sieved fractions is consistent with electrostatic adhesion of the particulate organic matter to other particles. The existence of very fine particulate organic matter could also help explain Ferguson's observation that very fine particle size fractions are necessary to optimize bitumen extra~tion.'~ Of the two sieving methods investigated, wet sieving appears to be more efficient for obtaining the fine particle sizes that are necessary in order to produce reflectance spectra with high spectral ~ 0 n t r a s t . l ~In addition, electrostatic attractions that will affect the spectral results are eliminated as a result of the sieving process. There is always the risk of losing soluble material with wet sieving. Should the buoyant material observed in this study turn out to be particulate organic matter, however, wet sieving may be an efficient way of concentrating the organic matter before measuring the reflectance spectrum. Spectrum Measurement. Instrumental noise was increased at wavelengths shorter than 412 nm in the spectra used in this study. While the absolute amplitude of the noise is very low, the relative amplitude is significant. The anomalous points in the vicinity of 411 nm appear to be related to a filter change from a broad- to narrow-pass filter. The narrow-pass filter is a more efficient screen than the broad-pass filter resulting in a lower photon flux and an increased background noise level at wavelengths shorter than 411 nm. Many commercial instruments are likely to produce noise of this amplitude or higher. The specific
wavelengths affected will vary. The amplitude of the noise observed in the spectra in this study is sufficient to interfere with attempts to interpret the spectra in terms of porphyrin chemistry. Frequency filtering does not eliminate the noise because it is not continuous throughout the wavelength interval of interest. Smoothing reduces the amplitude of the noise at wavelengths shorter than 411 nm but spreads the region affected by the filter change over a wider range of data points. These smoothed data points can easily be misinterpreted to be individual absorption bands or shoulders on existing bands. If the investigator is unaware of the anomalies produced by the filter change, the data are likely to be misinterpreted. The best approach to dealing with noise such as this for the investigator to be aware of its presence and how it is likely to affect data reduction. In some cases, however, it may be possible to move the location of a filter change by a few nanometers. This is advantageous in case where the anomalous points fall on or close to an absorption band. One difficulty with this approach is that the filter changes are often located a t points where the photon flux passing the filters falls precipitously. Considerably longer integration times may be required to record the same data points with the desired noise level. The Soret Band and the Vanadium to Nickel Ratio. The histogram of inflection points and band minima taken from core-sample spectra is consistent with the theoretical histogram. The peak in band minima a t 411 nm seen in the core-sample histogram most likely results from inability to discriminate the filter change anomaly and the Soret band minimum for low Soret band intensities. The broad distribution of short-wavelength inflection points in the 392-406-nm region results from the increased instrumental noise in this region of the spectrum. Other than these two features, there is a good correlation between the major peaks in the core-sample histogram and the theoretical histogram. The relative peak heights in the two histograms would probably be similar were those spectral parameters affected by instrumental noise placed at their appropriate wavelength positions. On the basis of the similarities between the two histograms, it can be concluded that the observed variations are consistent with natural variations in porphyrinic vanadium to nickel ratio. This implies that the model of Soret band variation resulting from changes in vanadium to nickel ratio is correct and that the method of measuring the spectral variation faithfully records that variation. Despite this, it would be premature to suggest that the down-hole variations in vanadium to nickel ratio can be modeled by the suite of samples used to construct the theoretical histogram (see Experimental Section). The results are insufficiently rigorous at this time to determine to what degree the suite of samples represent a unique solution. Although the Soret band analysis is intended to be qualitative, further studies with characterized standards should make more refined determinations of vanadium to nickel ratio possible. There are implicit assumptions in this approach that should also be considered. Vanadyl and nickel complexes are assumed to be the only porphyrin species present in each sample. While these are the dominant species reported in lithified sediments,16very little is known about those porphyrins in the insoluble kerogen fraction. Occasionally, anomalous absorption bands are noted in shale spectra that appear to correlate with known porphyrin
(14) Ferguson, W.S.AAPG Bull. 1962,46, 1613-1620. (15) Clark, R. N.; Roush, T. J . Geophys. Res. 1984,84, 6329-6340,
(16) Baker, E.;Louda. W. Biological Markers in the Sedimentary Record; Elsevier: Amsterdam, 1986.
Reflectance Spectroscopy for Geoporphyrins
bands. These may represent as yet unaccounted for species in the kerogen fraction. The presence of unknown species could affect the vanadium to nickel determinations depending on the spectral properties of the unknown species. It is also assumed that the nickel and vanadyl porphyrins i n each s a m p l e consist of equivalent structures with similar spectral properties. To produce the observed results, s o m e degree of similarity between t h e spectral properties of t h e nickel a n d vanadyl structures m u s t exist. If t h e r e are n a t u r a l samples for which the spectral properties of the nickel and vanadyl structures a r e grossly different, inaccurate estimates of vanadium to nickel ratio m a y result using t h i s technique. Final Note. The Kubelka-Munk function should not be used to e s t i m a t e the overall porphyrin concentration from diffuse reflectance spectra of shales. The KubelkaM u n k function has been shown to deviate significantly from reality for reflectance levels below 62.O%.l5J7 T h i s constraint effectively negates its application for organicrich shales whose background reflectance level seldom exceeds 15.0% reflectance. The best a p p r o a c h to estimating concentration is to develop a n empirical calibration that t a k e s i n t o account each of the factors affecting the observed intensity of absorption bands in reflectance. T h i s a p p r o a c h has been taken b y previous investigators in mineral spectroscopy studies.1s-20
Experimental Section All reflectance spectra used in this study were measured and digitally recorded at the RELAB facility (Reflectance Experiment Laboratory) of Brown University. RELAB is supported by NASA as a multiuser facility. The RELAB instrument measures bidirectional reflectance relative to a Halon standard with a spectral resolution of 2 nm in the 350-850-nm range. The light sources are a Jarrel Ash monochromator and a quartz halogen lamp. A photomultiplier tube is used as the detector for the visible region. Computer control is provided by an LSI 1103 computer. The RELAB facility has been described in detail by Pieters.21 The shales used in this study come from cores of Miocene beds of the Monteray Formation and cores of Pennsylvanian beds of the Denver and Powder River basins. Samples of the Bakken, Green River, New Albany, and Woodford shales were also used. All shales, except those used to compare preparation techniques, were hand ground with ceramic mortar and pestel. Both the mortar and pestel were thoroughly washed and rinsed with acetone (17)Wendlandt, W.; Hecht, H. Reflectance Spectroscopy; Interscience: New York, 1966. (18)Cloutis, E.; Gaffey, M. J.; Jackowski, T. L.; Reed, K. L. J . Geophys. Res. 1986,91,11641-11653. (19)McFadden, L. A.; Gaffey, M. J. Meteoritics 1978, 13, 556-557. (20)Roush, T. L.; Singer, R. B. J . Geophys. Res. 1986, 91, 10301-10308. (21)Pieters, C. M.J . Geophys. Res. 1983,88,9354-9544.
Energy & Fuels, Vol. 4, No. 6, 1990 709 between samples. The samples were wet sieved with water using Tyler standard 3-in.-diameter sieves. The sieve fractions were dried on petre dishes a t a constant temperature of 40 OC. The samples are currently stored in air tight glass containers. Measurements of total organic carbon were performed by Huffman Laboratories of Golden, C 0 . 2 2 The precision of these measurements is estimated to be h 0 . 2 7 ~total organic carbon. First-derivative spectra were calculated by using simple numerical approximations programmed on spreadsheet software. All other data reduction was performed by use of a specialized spectral processing program (SPECPR),23 adapted in this version to run on IBM personal computers. The theoretical histogram shown in Figure 5 was constructed based on the following observations from studies of characterized samples: The long-wavelength inflection point of the Soret band occurs a t approximately 426 nm for samples containing mostly vanadyl porphyrin. The band minimum for these samples occurs at approximately 416 nm. The Soret band minimum for samples containing mostly nickel porphyrin occurs a t approximately 406 nm. Spectral parameters for other cases could not be determined due to the limited number of standards and due to instrumental noise. The following assumptions were made to complete the histogram: For samples containing solely vanadyl porphyrin, the inflection points will be symmetric about the band minimum a t 426 and 416 nm. For samples containing solely nickel porphyrin, the inflection points will be symmetric about the band minimum at 416 and 396 nm. For samples containing mostly vanadyl and some nickel porphyrin, the inflections will occur at 426 and 396 nm and the band minimum at 416 nm. Similarly, for samples containing mostly nickel and some vanadyl porphyrin, the inflections will occur a t 426 and 396 nm and the band minimum a t 406 nm. The theoretical histogram is based on a suite of samples made up of 10 samples containing solely vanadyl porphyrin, 6 samples containing higher concentrations of vanadyl than nickel porphyrin, 4 samples containing higher concentrations of nickel than vanadyl porphyrin, and no samples containing exclusively nickel porphyrin. The suite is chosen to reflect the relative rarity of nickel porphyrin as compared to vanadyl porphyrin in natural samples.
Acknowledgment, We t h a n k Chevron Oil Field Research Center and Amoco Production Company Research Center for providing shale samples and for permitting us to publish these results. We a r e also i n d e b t e d to David Freeman of The University of Maryland, Earl Baker and William Louda of Florida Atlantic University, and J e r r y Clayton of the U.S. Geological Survey for providing samples and for helpful suggestions. We t h a n k Steven Pratt of Brown University for his h e l p with the spectrometer. Revisions suggested b y t w o anonymous reviewers helped improve the manuscript. T h i s research is s u p p o r t e d b y NASA Grant NAGW-642. (22) Huffman Laboratories, 4630 Indiana Avenue, Golden, CO, 80403. Pac. 1980,92, 221-224. (23)Clark, R. N. Publ. Astron. SOC.
