Quantitative fluorescence results from sample exchange studies

Org. Geochem.Vol. 12, No. 4, pp. 323-332, 1988 Printed in Great Britain. All rights reserved

0146-6380/88 $3.00+0.00 Copyright ~ 1988 Pergamon Press plc

Quantitative fluorescence results from sample exchange studies CAROLYN L. T~tOMPSON-RIZEgj, ROGER A. WOODSt and KARL OTTENJANN: tConoco Inc., P.O. Box 1267, Ponca City, OK 74603, U.S.A. 2Geologisches Landesamt Nordrehein-Wesffalen, De-Greiff-Str. 195, D-4150 Krefeld, F.R.G. (Received I I September 1987; accepted 16 March 1988) AbstractmMany workers during the past 20 years have demonstrated the ability of quantitatively measuring the fluorescence color and intensity of organic particles in both coals and petroleum source rocks (shales). Those workers have also shown the use of fluorescence data in coal rank or petroleum source rock maturity determinations, maceral identification, and bitumen generation studies. However, today problems with the fluorescence technique persist, particularly standardization problems, which prevent the widespread use of quantitative fluorescence. The ultimate goal of exchanging samples for fluorescence measurements is to finalize a suitable method for obtaining repeatable and comparable values for spectral distribution, intensity, and alteration. The results from three different comparison studies show that many laboratories around the world are measuring fluorescence and that there is some agreement in the reported values. One study, initiated by Conoco, included both coal and shale samples. The second study shows a comparison between published values for 33 Kentucky coal samples and those measured at Conoco. The third study, of Plexiglas and coal samples, was conducted by the International Committee for Coal Petrology (ICCP) during 1984-1985. By examining the results from studies like these, a standarization of methods may emerge. Key words: fluorescence, coal, shale, rank, maturity, vitrinite reflectance

INTRODUCTION The measurement of the fluorescence of coals and organic-rich shales is becoming useful in the determination of the rank or maturation level as well as providing information on secondary bituminous products. Liptinitic particles (sporinite, alginite, cutinite, resinite, some amorphous kerogens, etc.) emit light of different wavelengths when excited with near-ultraviolet light. The color of the emitted light is measured using a monochromator and a photomultiplier detector. Several papers have been published describing the instrumentation and techniques for measuring and reporting microspectrofluorescence (in particular seen Jacob, 1972; Van Gijzel, 1979; Ottenjann, 1981/1982). Three kinds of fluorescence can be measured for organic geological samples: (l) The intensity of a sample, at any wavelength, is measured with reference to a fluorescence intensity standard which is either arbitrarily set at 1 or 100, or has its intensity independently determined. (2) The alteration (positive or negative) of a sample, when exposed to near-ultraviolet light, for specific periods of time is measured relative to the initial intensity value. (3) The spectral distribution of fluorescence intensity is usually related to the wavelength of maximum intensity (2max) and ratios of specific wavelengths. Both alteration and spectral distribution can be related to the intensity standard. In order to properly measure spectral distribution, it is desirable to optimize the entire measuring system by correcting for changes introduced by the optical and photometric components of the system. The necessary corrections include the use of a standard

lamp (known as a grey body radiator) which has been calibrated by the manufacturer or a national physical institute (such as the National Bureau of Standards in the U.S.A.) and has a known spectral emission. Properly measured fluorescence spectral distributions also require the subtraction of "background", as measured on a nonfluorescing particle, which helps to remove the effects of electronic noise, stray light, etc. By comparing results from the same samples measured in different laboratories we can learn about the quality of various instruments and various measuring techniques. Sample exchange studies are simply compilations and examinations of data measured by separate laboratories on the same or similar samples. The coordinator of a sample exchange mails samples to interested people and asks for specific types of measurements and results. It is possible that the "correct" data will come from several different methods. Eventually an agreement may be made in which a certain method of measurement or style of reporting data may be recommended for future work. This paper is an attempt to publicize the results of three sample exchange studies of quantitative fluorescence measurements. Three different sample exchange studies are discussed. The study initiated by Conoco included four coal samples and two shale samples. The goals of that study were to determine if differences in equipment, listed in Table 1, affect fluorescence data and to learn how other laboratories measure and report fluorescence. Nine laboratories participated, including one with two different instruments. It was decided that a more free exchange of results would take place

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Quantitative fluorescence results from sample exchange studies if the identities of the participating laboratories were kept confidential. The laboratories included: two universities, four oil companies, and three state or national institutions in the United States, Federal Republic of Germany, and Canada. The second laboratory comparison consisted of Conoco measuring the same set of samples for which results were published by Poe and Hower (1983). The third study was conducted by the ICCP working group on the standardization of fluorescence measurements. A set of three colored Plexiglas samples and one alginiterich coal (torbanite) sample were exchanged. Twenty laboratories from all over the world took part in the ICCP study, including: universities, oil companies, and state or national institutes. Ottenjann discussed the results at the ICCP meeting in Dubrovnik, Yugoslavia (September 1985) which are given in this paper without identifying the specific laboratories. MATERIALS AND METHODS

325

objective lens type, monochromator type, etc.) which is shown in Table 1. A photomicrograph was requested for each sample to illustrate the kind of particle for which fluorescence was measured.

The comparison between Poe and Hower (1983) and Conoco Poe and Hower (1983) published both vitdnite reflectance and sporinite fluorescence values for 43 samples of coal from western Kentucky. In 1984 they supplied to Conoco 37 of the polished mounts used in their study and six bulk samples of the remainder. The polished mounts were repolished (1200 silica carbide and one micron alumina) at Conoco, and the bulk samples were made into polished mounts. The Poe and Hower (1983) study employed a Leitz MPVCompact detection system and a 50 x oil immersion objective lens. At Conoco the fluorescence was measured using a modified Zeiss Zonax system and a 40 x dry objective lens.

The Conoco initiated study

The [CCP study

Four Ohio coal samples were made available to Conoco by W. KneUer of the University of Toledo Organic Carbon Facility (UTOC). The samples are: UTOC 268 from the Waynesburg No. 11 seam (Belmont County), Pennsylvanian age; UTOC 285 from the Pittsburg No. 8 seam (Belmont County), Pennsyivanian age; UTOC 289 from the Sharon seam, Pennsylvanian age; and UTOC 298 from the Washington No. 12 seam (Belmont County), Pennsylvanian-Permian age. The crushed samples were packaged in individual paper envelopes containing approximately 50 g each. The participants prepared their own polished mounts of the crushed coals. Two shale samples were made available to Conoco for distribution to participants. L. Barton of the Kentucky Energy Cabinet Laboratory provided a crushed composite sample of the Cleveland member of the Upper Devonian Ohio Shale from Lewis County, Kentucky. Mark Fan of Mobil Corp. supplied numerous 2 cm cubes of the Woodford shale from an outcrop block in the Arbuckle Mountains, Carter County, Oklahoma. Since the two shale samples are nearly stratigraphically equivalent, they contain similar organic particles of both alginite and sporinite. A 50 g envelope of the Ohio shale and one cube of the Woodford shale were mailed to the labs that had returned coal data. The shale samples were to be prepared by any method preferred by the laboratory. Most laboratories polished the Woodford shale cube and most made polished mounts of the Ohio shale, except one lab which made a visual kerogen strewn slide following acid (HCI, HF) removal of the minerals. Vitrinite reflectance and a variety of fluorescence parameters were requested for each sample. The labs were also asked to supply information about their equipment (such as microscope brand and model,

A set of three different colored Plexiglas samples was mailed to the participants. No polishing or sample preparation was needed on these samples. They are strong, nearly monochromatic fluorescence emitters and were chosen for the laboratory comparison study because of their suitablility to check the following equipment properties: wavelength adjustment, calibration and correction factors, and background or noise. A polished mount of alginite-rich coal, torbanite, from Yorkshire, England (probably Carboniferous Westphalian A), was distributed for the second part of the comparison study. A photomicrograph marked with the kind of alginite to be measured accompanied each sample.

RESULTS AND DISCUSSION

The Conoco initiated study Table 2 lists both the vitrinite reflectance and fluorescence data measured by the participants in the Conoco study of four coals and two shales. The statistical mean and standard deviation are given for all four coal samples since they are of nearly the same reflectance. It should be noted that the mean values do not necessarily represent the "correct" values. No statistical information is given for the shale samples due to the large variation in the data. All four coal samples are close to 0.60% random vitdnite reflectance (Ro). The wavelengths of maximum fluorescence intensity of the coals are between 570 and 600 nm, when the runaway values are excluded. The value of the ratios of the intensities in the red region (650 nm) to those in the green region (500 nm), called Q, for the coals are predominantly 1.1-1.3 (excluding runaway values). The Commission Internationale de l'Eclairage chromaticity coordinates (C.I.E. x,y) are variable among laboratories with

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three labs not measuring them. The mean CTE. values are reasonable for this reflectance level. The relative or normalized fluorescence intensity value at 490 nm has been found to correlate with maturity (Thompson-Rizer and Woods, 1987) and should be between 60 and 70% for this rank of coal. Various forms of sporinite macerals were measured by most of the laboratories. All laboratories reported that background measurements were subtracted from the spectral data, except for labs A and E. Overall, there are no distinct differences in the results that can be attributed to the differences in equipment used (Table l). There is considerable variation in the shale sample results, shown in Table 2, for both reflectance and fluorescence values. The fact that only four laboratories returned data (one company has two lab instruments, and one lab reported two sets of measurements) suggests that there are more difficulties in preparing and measuring shale samples than coals. Some of the differences in data may be due to the kinds of grains measured (alginite, sporinite) or the sample preparation of the Cleveland shale. Lab C reported alteration (Ottenjann, 1985) trends for the brightly and weakly fluorescing alginite grains and the bituminous mineral matrix which suggest that the Cleveland shale is entering the oil window (Ro = 0.5-0.6%) and is less mature than the Woodford shale. Conoco's organic geochemical analyses (Rock Eval pyrolysis) for the Cleveland and Woodford shales respectively indicate immaturity (Tmax= 425, 427°C, PI =0.02, 0.03) and excellent petroleum source rock potential ( T O C = 13, l 1%; HI = 482, 544; RCI = 49, 56). Solvent extraction of the Cleveland shale resulted in 1,165ppm of Cl5 + hydrocarbons, of which 74% are resins, asphaltenes, nitrogen, sulfur, and oxygen-containing compounds possibly indicative of immaturity. The comparison between Poe and Hower (1983) and Conoco

Figure 1 illustrates the very different, but consistent, trends of sporinite fluorescence versus vitrinite reflectance between measurements made on the same set of samples by Poe and Hower (1983) and by Conoco two years later. The wavelength of maximum intensity and Q fluorescence data are compared between the labs. Ten of the original 43 samples were omitted from the comparison because the vitrinite reflectance values for a sample disagreed by more than 0.1% R0, no reflectance value was supplied and/or no fluorescence values could be determined. Poe and Hower (1983) reported maximum vitrinite reflectance values, whereas Conoco measures random vitrinite reflectance. The small differences between the two kinds of vitrinite reflectance measurements for these high volatile bituminous coals do not account for the consistently different fluorescence trends between the two laboratories. The trends for 2max from both laboratories are approximately linear. The

trend for Q data from Conoco is linear (the correlation coefficient does not significantly improve when the quadratic equation is fit to the data) while the trend from Poe and Hower (1983) is best expressed by a quadraatic curve. It is interesting to note that neither of these trends is the same as either of those established by Ottenjann (1981/82) for random T a b l e 3. R e s u l t s f r o m t h e I C C P id. °

Lab

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I1 12 14 15 17 18 3 20 8 Mean

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1 2 4 5 6 7 9 10 I1 12 14 15 17 18 3 20 8 Mean

575 589 579 575 575 570 580 596 570 580 590 560 580 580 580 576 580 579 (8)

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study of three Plexiglas samples

QC Alteration Yellow Plexiglas

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bAverage wavelength of maximum intensity (ram). CAverage red/green quotient ( 6 5 0 / 5 0 0 n m ) . ' P e r c e n t o f a l t e r a t i o n ( O t t e n j a n n , 1985). L a b 15 u s e d 16 s o f l a s e r ( 3 2 5 r i m ) e x c i t a t i o n . L a b 16 u s e d 3 0 r a i n o f b l u e l i g h t excitation. W A = without alteration. ' I n t e n s i t y a t 5 4 6 n m ( J a c o b , 1973, m e t h o d ) . L a b I u s e d 4 0 / I . 3 0 l e n s . L a b 12 u s e d 1 6 / 0 . 4 0 a n d 2 5 / 0 . 5 0 l e n s e s . () Standard deviation.

Quantitative fluorescence results from sample exchange studies

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"/.Ro Fig. 1. Fluorescence data (Y axis) are shown compared to vitrinite reflectance data (X axis) from two different laboratories measuring the same samples. The solid Conoco regression line for ,;.max has the equation y = 520 + 133x with a correlation coefficient (r) of 0.74; for Q the equation is y = 0.20 + 1.98x and r = 0.75. The broken line represents the linear regression analysis of the Poe and Hower (1983) data for ~.max and has the equation y=515+71x with r=0.80; for Q the relationship is nonlinear y = 1.51 - 3.92x +4.26x 2 with r =0.84.

vitrinite reflectance and the total sporinit¢ fluorescence in samples measured for that publication. In general, the Conoco trends lines are slightly above Ottenjann's while the Poe and Hower lines are significantly below Ottenjann's lines. This example demonstrates the frustrations which workers often encounter with quantitative fluorescence. The trends shown in Fig. 1 suggests that the two labs may have some physical systematic differences due to measuring technique or correction factors, etc. It is also possible that the samples have oxidized or otherwise changed during the years between measurement (probably only a small part of the difference between

labs). Thompson-Rizer and Woods (1987) suggest a practical solution to laboratories which get consistently different fluorescence data, that is the correlation of vitrinite reflectance to the individual lab fluorescence values. This sort of correlation may help more laboratories to develop and apply fluorescence data to geological problems; however, we hope that most laboratories can find the source of systematic deviations and arrive at the more "correct" fluorescence values.

The ICCP study The fluorescence data from the Plexiglas samples

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Quantitative fluorescence results from sample exchange studies are listed in Table 3. These samples emit strong fluorescence in narrow regions, so detection of the wavelength of maximum intensity should not depend on the sensitivity of the equipment. The values for the wavelengths of maximum intensity show good agreement. Those widely deviating from the mean values (approximately 500 nm for the yellow sample, 550 nm for the orange sample, and 575 nm for the red sample) may be indicative of a monochromator out of adjustment, improper correction factors, or stray light. The small standard deviations for :.max (Table 3) are noteworthy. The Q values for these samples are more difficult to measure and to reproduce due to the low intensities at either 500 or 650 nm; for that reason, it is meaningless to calculate the statistics for Q in Table 3. For example, the yellow Plexiglas has little or no intensity at 650 nm and almost 100% at 500 nm, so Q should be very small. Any light leakage, noise, or improper correction factor will significantly change the ratio of red to green light. Several labs attempted to measure alteration in the Plexiglas samples which were found to be fairly free of alteration. Intensity measurements after Jacob's (1973) method were reported by some of the labs and show that there are several problems with this parameter such as the thickness of objects, standards, measuring aperture, and field stop sizes. Despite the problems, most labs found a similar sequence of intensities among the three samples (the yellow sample is the brightest, the red the lowest). The results from the exchange study of the torbanite sample are shown in Table 4. Even though a photomicrograph accompanied each polished sample with arrows showing the kind of alginite grain to be measured, participants reported fluorescence measurements for all three kinds of alginite (bright, medium, and weak fluorescing) present in the sample. Ottenjann sorted out the data into the categories shown in Table 4. In general, there is good agreement among labs on the spectral (2max and Q) values for the medium aiginite grains in this coal when the outliers are excluded. The alteration trends for the three kinds of alginite have final values which are negative for the bright, less negative for the medium, and positive for the weak alginites. The results of intensity measurements show the need for improvement in this technique. The C.I.E. chromaticity coordinates were reported by three labs for either the Plexiglas or torbanite samples and are shown in Table 5. There appears to be an agreement in the values, however, too few results are given.

331

Table 5. C.I.E. coordinates for samples in the 1CCP study Lab id.

5 17 5 17 5 17

C.I.E. x

Plexiglas--yellow 0.1660 0.1807 Plexiglas---orange 0.3973 0.4135 Plexiglas--red 0.5024 0.5152

C.I.E. y

0.4449 0.4669 0.5896 0.5746 0.4873 0.4751

Torbanite--bright 5 5 17 ~

17~ 19

0.3335

0.3921

Torbanite--medium 0.3790 0.4136 0.4108 0.4195 0.4029 0.423 I 0.3800 0.4000

Torbanite--weak 19

0.4600

0.4500

"Two different instruments.

initiated study and for the medium fluorescing alginites in the ICCP torbanite study are reasonable. The ICCP Plexiglas :.max study helps to show us that most laboratories have good monochromators, so we can eliminate that possible source of error. The data presented for the two shale samples in the Conoco initiated study illustrate that the fluorescence analysis of dispersed organics in sedimentary rocks is in need of much more work. The variability in Q data from the ICCP Plexiglas study suggest that several laboratories may have problems with improper correction factors or noise in their detection systems. The wide variations in the measurement of intensity and alteration are probably results of system instabilities during prolonged irradiation and must be further investigated. The results of these studies point out the need for standardized procedures for measuring fluorescence. We feel that good, reproducible results can be achieved with: properly adjusted instruments, good quality attachments, objective lenses of low fluorescence (preferably not oil immersion), good excitation filters, background subtraction, and "true" correction factors. The ICCP working group on the standardization of fluorescence measurements is currently writing new guidelines on both intensity and spectral measuring techniques. At this time, we feel that more instruction on how to correctly use the equipment with respect to physical laws is necessary. Later on, after the improvement of instruments and methods, new exchange studies with unequivocal coal and shale samples should be made.

CONCLUSIONS

Overall, we are encouraged by the results from many laboratories all over the world which are successfully measuring fluorescence in some geological samples. We feel that the majority of the data supplied for the four coal samples in the Conoco

Acknowledgements--We thank Conoco Inc. and Dr A. V. H. Smith (ICCP, President of Commission I) for permission to publish these results. We also thank Dr A. Davis and Dr W. D. Kalkreuth for their helpful reviews of this manuscript.

332

CAROLYNL. THOMPSON-RIZERet al. REFERENCES

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proben aus stratigraphisch jungen Schichten des Oberkarbons. Fortschr. Geol. Rheiald. u. West/. 33, 169-195. Poe S. H. and Hower J. C. 0983) Petrographic characterization of Kentucky coals. Department of Energy Report No. DE83016511, 68 pp. National Technical Information Service, Springfield, VA. Thompson-Rizer C. L. and Woods R. A. (1987) Microspectrofluorescence measurements of coals and petroleum source rocks. Int. J. Coal Geol. 7, 85-104. Van Gijzel P. (1979) Manual of the techniques and some geological applications of fluorescence microscopy. Workshop sponsored by American Assoc. Stratigr. Palynol., 55 pp. Core Laboratories Inc., Dallas.