Energy & Fuels 1993, 7, 406-410
406
Pyrite Removal from Kerogen without Altering Organic Matter: The Chromous Chloride Method Francis V. Acholla* Central Research Laboratory, Mobil Research and Development Corporation, P.O.Box 1025, Princeton, New Jersey 08540
Wilson L. Orrt Mobil Exploration and Production Technical Center (MEPTEC),13777 Midway Road, Dallas, Texas 75382 Received December 9, 1992. Revised Manuscript Received March 10, 1993
The standard hydrochloric/hydrofluoric acid treatment used to remove most minerals in kerogen preparation often leaves abundant pyrite. This pyrite is a major obstacle to establishing elemental composition and functional group distributions in kerogen by both chemical and spectroscopic techniques. Quantitative pyrite removal by other conventional means such as nitric acid, lithium aluminum hydride, and sodium borohydride has not been possible without objectionable alterations of the organicmatter. Experiments reported here demonstrate that acidic chromous chloride reduction can remove pyrite quantitatively without causing significant alteration of the organic matter. Lack of organic alteration is indicated by examination of samples before and after treatment by infrared (FTIR) and C-13 NMRspectra as well as by atomic ratios (H/C, O/C, N/C, and SOrg/C).An important requirement for quantitative removal is fine grinding and repeated treatments (if necessary) after regrinding to expose occluded pyrite grains. Efficiency of pyrite removal can be determined by both the H2S evolved and the iron content in the treated kerogen. The experimental technique for preparation of pyrite-free and chemically unaltered kerogen evaluated in this paper provides kerogen samples that can be better characterized chemically and physically by any technique where pyrite causes complications. A current interest is providing samples suitable for determining organically bound sulfur in kerogen by sulfur K-edge X-ray absorptionnear-edge structure (XANES)spectroscopy.
Introduction The conventional hydrochloric/hydrofluoric acid treatment is effective for removing most carbonate, oxide, silicate, and monosulfide minerals from sedimentary rocks and leaves kerogen and pyrite largely unaffected. Rocks differ greatly in relative amounts of kerogen and pyrite, resulting in variable but often high pyrite levels in “kerogen preparations” or “kerogen con~entrates”.~-~ Petroleum geochemists have long been interested in obtaining pyritefree and chemically unaltered kerogen concentrates in order to improve determinations of optical, physical, and chemical properties of kerogen.l-16 It is well-known that pyrite interferes with many of the spectroscopic and
* To whom correspondence
should be addressed. Present address: Earth & Energy -. Science Advisors, P.O. Box 3729, Dallas, TX 75208. (1)Forsman, J. P.; Hunt, J. M. In Habitat of Oil; Weeks, L. G., Ed.; AAPG Press: Tulsa, OK, 1958;pp 747-777. (2)Saxby, J. D. Chem. Geol. 1970,6,173-184. (3)Saxby, J. D. In Oil Shale; Yen, T. F., Chlingarian, G. V., Eds.; Elsevier Scientific: Amsterdam, 1976;pp 103-128. (4)Durand, B.; Nicaise, G. InKerogen, Insoluble OrganicMatterFrom Sedimentary Rocks; Durand, B., Eds.; Editions Technip: Paris, 1980;pp +
?L69 YV
YV.
(5)Combaz, A. In Kerogen, Insoluble Organic Matter From Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980;pp 55112. (6)Lawlor, D. L.; Fester, J. I.; Robinson, W. E. Fuel 1963,42,239-244. (7) Robinson, W. E. Organic Geochemistry, Methods and Results; Eglinton, G., Murphy, M. T. J., Eds.; Springer: Berlin, 1969 pp 181-195.
(8) Vitorovic, D.; Krsmanovic, V. D.; and Pfendt, P. A. Advances in Organic Geochemistry; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon Press: Oxford, 1979;pp 585-589. (9)Orr, W. L. Org. Geochem. 1986,10,499-516.
chemical methods of analysis that are needed for better characterization of kerogen. The objective of this study was to find an effective procedure for pyrite removal that would not alter the chemical composition of the organic matter. Until recently, quantitative removal of pyrite was not possible without serious alteration of the organic composition. The intimate association of pyrite with kerogen prevents effective physical or mechanical ~eparation.~ Commonly used chemical methods such as dilute nitric acid, lithium aluminum hydride, and sodium borohydride are known to cause substantial chemical changes of the organicmatter in the residual kerogen.l4$&l1For example, dilute HN03 (even at room temperature) causes measurable oxidation and nitration of kerogen during exposure times necessary to remove pyrite. Likewise, the effective LiA1H4and NaFiH4 reagents reduce carbonyl to hydroxyl (10)Orr, W. L.; Sinninghe Damst.6, J. S. Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429; American Chemical Society, Washington, DC, 1990 pp 1-29. (11)Vrvic, M. M.; Djordjevic, V.; Savkovic, 0.;Vucetic, J.; Vitorovic, D. Org. Geochem. 1988,13,1109-1114. (12)Reaves, C. M. Ph.D. Dissertation, Yale University, 1984; 413 pp. (13)Canfield,D.E.;Raiswell,R.; Westrich,J.T.;Reaves,C. M.;Berner, R. A. Chem. Geol. 1986,54, 149-155. (14)Tuttle, M. L.;Goldhaber, M. B.; Williamson, D. L. Talanta 1986, 33,953-961. (15)Tuttle, M.L.,Ed. U.S.Geol. Survey Bull. 1991, 1973-A-G. (16)Tuttle, M. L.; Rice, C. A.; Goldhaber, M. B. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429;American Chemical Society, Washington, DC, 1990 pp 114148.
0887-0624/93/ 2501-0406$04.OO/0 0 1993 American Chemical Society
Energy & Fuels, Vol. 7, No. 3, 1993 407
Pyrite Removal from Kerogen
Table I. Rock Samples Used for Kerogen Preparationa TOC,b kerogen atomic ratiosn sample sample geologic no. type geographic area formation (age) w t % H/C O/C So& 6656 quarry Yorkshire, UK Kimmeridge 7.73 1.11 0.088 0.016 (Jurassic) 6648 outcrop Naples Beach, coastal Calif. Monterey 22.20 1.19 1.146 0.049 (Miocene) 6653 composite well core South Elwood, offshore Calif. Monterey 8.46 1.30 0.061 0.072 (Miocene) 1675-1 outcrop Trinity River, Dallas, TX Eagle Ford 3.93 1.23 0.157 0.029 (Cretaceous)
Rock-Eva1 datab kerogen HI 01 T,,, typec 470 16 425 11-111 380
28
395
II-S
514
16
411
II-S
395
37
424
11-111
Kerogen preparations from these rocks were made previously for hydrous pyrolysis studies (unpublished) except 1675-1. Previous analyses are given in Table I1 with suffix-A for comparison with new samples reported here with suffix-B. Measured after extraction with chloroform to remove bitumens and elemental S. Based on elemental atomic ratios. Table 11. Elemental Analysis of Kerogen Preparations before and after Chromous Chloride Treatment raw analytical data samplen ?' 6 Fe % ash %C %H %0 %N % Stot % Sovb % Soreb % C1 6.34 11.45 66.56 6.21 7.78 1.78 9.92 7.00 2.92 NA 6656-A 4.98 11.14 67.09 6.32 8.62 1.84 8.87 5.72 3.15 0.74 6656-B 3.81 76.01 7.24 7.57 1.99 3.07 6656-C 0.067 0.077 2.99 1.58 1.95 4.59 63.17 6.29 12.28 3.30 11.36 6648-A 3.06 8.30 NA 1.54 4.11 6.17 15.46 6648-B 62.58 3.42 9.97 1.77 8.20 0.75 0.004 1.45 66.68 6.64 15.08 3.37 7.84 0.005 7.84 0.31 66484 11.67 14.86 9.28 21.76 48.25 5.26 3.89 1.65 24.14 NA 6653-A 12.93 26.95 46.41 5.23 4.92 1.59 24.35 14.84 9.51 6653-B 0.97 2.21 1.64 10.69 1.43 8.05 66.31 6.89 7.55 12.33 0.31 6653-C 4.84 1675-1-B 13.68 1.64 15.70 3.62 23.33 46.99 9.84 19.32 0.34 1.08 4.25 1675-14 0.94 2.23 2.43 66.95 7.17 13.28 5.33 0.79
% Cr
NA 0.00 0.19 NA 0.01 0.23 NA 0.01 0.23 0.01 0.34 -A = Prior kerogen preparation & analysis from same rock batch as for -B. -B = Current kerogen preparation before chromous chloride treatment. -C = Kerogen residue from -B samples after two chromous chloride treatments, except 6553-C which received four treatments. Spyis calculated as 1.148 times % Fe for samples -B and -C and Sorgis taken as Stat - Spy For the -A samples, the more laborous method described by Orr (1986) was used. Comparison of So& ratios for -A and -B samples shows the approximation from Fe to be satisfactory.
in ketones, esters, and carboxylic acids. Therefore, these commonly used procedures are not useful for kerogen studies. Recently,a microbiological oxid ative-leachingprocedure employing Thiobacillus ferrooxidans was reported for removing pyrite.'l This procedure may oxidize kerogen in addition to requiring long leaching times. In this paper we describe amethod for the depyritization of kerogen preparations from typical petroleum source rocks. The method uses acidic chromous chloride in aqueous solution and is an adaptation of the method as reported by Reaves12 in 1984, and Canfield et al.13 and Tuttle et al.14 in 1986. During recent years, this method has become a favored analytical method for determination of the pyrite content in sediments and sedimentary rocks15J6J8but it has not been examined previously as a kerogen purification technique. The reduction of pyrite by acidic chromous chloride may be represented by the following reaction. FeS2(s)+ 2CrClJaq)
-
+ 4HCl(aq) FeC12(aq)+ 2CrC13(aq) + 2H2S(g)
The formation of Cr3+(t2p3)species which are more molecular orbital stabilized than Cr2+(t2,3eg1)is the most The reaction likely driving force for the above rea~ti0n.l~ is conducted under nitrogen with gas flow carrying the H2S to a trap where it is recovered as Ag2S. Excess acid and acid soluble salts are removed from the residual kerogen by water washing. (17) Kolthoff, I. M., Sandell, E.B., Eds. Textbook of Quantitative Inorganic Analysis; Macmillan: New York, 1963; p 759. (18) Zaback, D. A.; Pratt, M. L. Geochim. Cosmochim. Acta 1992,56, 763-774. (19) Luther, G. W. Geochim. Cosmochim. Acta 1987,51, 3193-3199.
Experimental Section Reagents. Analytical grade reagents were used without further purification. Rock and Kerogen Samples. Table I lists the samples and a summary of geochemical information commonly used in evaluating petroleum source rocks. Rocks were finely pulverized and extracted with chloroform to remove bitumens and elemental sulfur before the hydrochloric/hydrofluoric acid treatment. Core TX) performed the HCl/HF treatment using Laboratories (Dallas, instructions that included hot hydrochloric acid leach to remove any neofluorides and reextraction with chloroform to remove released bitumens and elemental sulfur. Kerogen samples were dried at 80 "C under vacuum before analysis. All elemental analyses were by Huffman Laboratories (Golden, CO). R.aw analytical data are listed in Table 11. Iron ( % Fe) in isolated kerogens is considered to exist entirely as pyrite and the pyrite sulfur (1.148 times % Fe) is subtracted from total sulfur to give organic sulfur (see notes to Table 11). The kerogen preparations differ greatly in initial pyrite content. The Naples Beach Monterey outcrop sample (6648-B) has only about 3% pyrite (sum of Fe and Spy) whereas the South Elwood subsurface Monterey (6653-B)and the Eagle Ford Shale (1675-1-B) samples have 28-29% pyrite. The Kimmeridge Shale sample (6656-B) is intermediate with 11% pyrite. The difference in pyrite concentration was partly the basis for selecting the samples for this study. Atomic ratios from these data are listed in Table 111. The atomic H/C, O/C, and Sorg/C ratios as well as visual kerogen evaluation are the basis of the inferred kerogen type listed in Table I.9J0 All samples are immature with respect to oil generation. Preparation of Acidic CrClzReagent. Fresh reagent must be prepared every few days and stored under nitrogen to prevent oxidation. A stock solution made to be 2.0 M in CrC13 and 0.5 M in HC1 was passed through a Jones reductor column under Nz pressure at a rate of 100 mL/min. The Jones reductor was a 2.5 X 50 cm column packed with amalgamated zinc granules as
-
408 Energy & Fuels, VoE. 7,No. 3, 1993
Acholla a n d Orr
Table 111. Kerogen ComDosition by Atomic Ratioss kerogen atomic ratios sample 6656-A 6656-B 6656-C 6648-A 6648-B 66484 6653-A 6653-B 6653-C 1675-1-B 1675-1-C
H/C
OK
N/C
1.11 1.12 1.13 1.19 1.17 1.19 1.30 1.34 1.24 1.23 1.28
0.088 0.096 0.075 0.146 0.185 0.170 0.061 0.080 0.085 0.157 0.149
0.023 0.024 0.023 0.045 0.047 0.044 0.030 0.030 0.029 0.030 0.029
SordC 0.016 0.018 0.015 0.049 0.049 0.044 0.072 0.077 0.060 0.029 0.024
From data in Table 11. directed by Kolthoff and Sandell.17 The green chromic chloride solution changes to dark blue upon reduction to chromous chloride. The reagent can be stored under nitrogen in agroundglass stoppered bottle to prevent atmospheric oxidation for several days. Pyrite Removal Experiments. The reactor setup consisted of a 250-mL three-necked flask equipped with a condenser, an addition funnel, a magnetic stirring bar, and a nitrogen inlet bubbler. The condenser was fitted with a Teflon adapter and tubing to facilitate H2Stransfer from the reactor to the HzS trap. The H2S trap consisted of 140 mL of 3% AgN03 in a 250-mL round bottom flask equipped with a drawn glass tip gas inlet. The AgzS precipitate was recovered and used to estimate pyrite removal. I t can be used for measuring the sulfur isotope ratio of the evolved HzS (634Sof pyrite S). The kerogen residue can be used for determining P4Sof organic sulfur if remaining pyrite is negligible. In a typical experiment, -2 g of the kerogen preparation and 20 mL of ethanol were added to the reactor flask with 80 mL of CrClz reagent and 40 mL concentrated HCI contained in the addition funnel. After the system was flushed with nitrogen, the acidic chromous chloride solution was added dropwise over a period of 20 min while stirring and with Nz flowing a t a rate of -5 bubbles/s. The flask was then heated to reflux for about 2 h (approximately the time required for the HzS trap to be clear) with Nz flow continuing to carry evolved HzS to the HzS’trap. After the reaction was judged to be complete by cessation of AgZS precipitation in the trap, the digestion flask was cooled to room temperature and the Nz bubbler disconnected. The residual kerogen was collected on an ashless No. 40 Whatman filter paper in a Buchner funnel and rinsed with distilled HzO until the filtrate was neutral (pH 7) and gave a negative test for C1- (with AgN03 solution). The residual kerogen was then dried under vacuum at 80 OC for -15 h and submitted for analysis. The efficiency of pyrite removal was judged from the decrease in iron content which, of course, should agree with the decrease in sulfur content and the amount of AgZS collected. If pyrite removal was not sufficiently complete, the sample was reground to expose new pyrite surfaces and the entire procedure repeated. In early experimenta (and data listed in Table II), two treatments gave near-quantitative pyrite removal (>98%) for two samples and about 93% for one sample. Pyrite removal proved more difficult for one preparation, being only 89% after four treatments (see discussion). Instrumentation. Infrared spectra (FT-IR) for CH and functional group analysis were obtained on KBr pellets using a Nicolet System 800 Fourier transform infrared spectrometer. Qualitative X-ray diffraction (XRD) analyses of the crystalline components were carried out by J. T. Edwards (MEPTEC). 13C nuclear magnetic resonance (NMR) spectra (magic angle spinning-cross polarization and dipolar dephase) for aromatic and aliphatic carbon distribution were obtained a t 25.09 MHz using a Chemagnetics MAS broadband probe on a JEOL FX-200 spectrometer by S. E. Schra”.
Table IV. Pyrite Removal by Chromous Chloride Treatment sample 6656-B 6656-C 6648-B 66484 6653-B 6653-C 1675-1-B 1675-14
pyrite, wt%
10.70 0.14 3.31 0.01 27.77 3.07 29.38 2.02
Py-S/org-S ratio 1.82 0.03 0.22 0.00 1.56 0.15 3.64 0.25
% pyrite
removal
no. of treatments
98.7
2
99.7
2
88.9
4
93.1
2
Results and Discussion The data in Table I1 show the changes in the kerogen samples before and after the acidic CrCl2 treatment (-B and -C samples, respectively). The efficiency of pyrite removal is indicated by decreases in % Fe, % Spy,and % ash and corresponding increases in percentages of other elements. From 89 to 100%of the pyrite was removed by the chromous chloride treatments. The lack of significant alteration of the organic matter in the kerogen is indicated by comparing the atomic ratios in Table I11 to those in Table I and by spectroscopic examination (FTIR and NMR). These results are discussed below. The pyrite levels before and after treatment and the efficiency of pyrite removal are summarized in Table IV. Pyrite removal is nearly complete in two of four cases, was 93% in one, and was 89% in the worst case. A number of reasons are possible for variations in efficiency, and further work, no doubt, will improve the technique. The lower efficiency (93%)for the Eagle Ford sample may be attributed to a variation in grinding procedure between the two treatments. However, the reason for the still lower recovery from the South Elwood sample (89% after four treatments) is unknown. Possibly, the difficultyis related to mechanical properties of the organic matter making grinding less efficient and/or morphology of the pyrite (euhedral cubes with low surface area). This kerogen has the highest So& ratio (highest sulfur content) which may possibly make the polymer more elastic (rubberlike) and difficult to break away from occluded pyrite. The necessity of fine grinding and multiple treatments is expected because of the intimate association of pyrite with the organic matter. Some pyrite exists as exposed grains, but much exists as particles either coated by or occluded in the organic matter. Pyrite grains of different morphology (i.e., framboidal, anhedral, or euhedral) must be exposed to the reducing reagent in order to react. Of course, this is also the case with other chemical treatments (e.g., HN03 and/or LiAlH4, and the necessity of fine grinding or multiple treatments is well-known. Lack of wettabilityof kerogen by the aqueous medium is a potential problem, but the initial addition of ethanol appears to handle this problem. Qualitative X-ray diffraction analysis (XRD) confirms that pyrite is the major crystalline mineral in the initial kerogen preparations and that it is decreased below detection limitsafter CrC12 treatment except for the 6653-C sample. XRD spectra for the Kimmeridge sample 6656 before and after treatment are shown in Figure 1,a and b, respectively. Pyrite peaks are absent after treatment, and remaining peaks are attributed to titanium oxides (rutile and anatase) known to be minor components of sediments that are resistant to dissolution in HCl/HF.4 Pyrite present in initial samples was reduced to below XRD detection limits as expected from the chemical data
Energy & Fuels, Vol. 7, No. 3, 1993 409
Pyrite Removal from Kerogen
I
I
I
t
5.00
1
11.00
17.00
23.00
29.00
35.00
41.00
47.00
53.00
59.00
65.00
47.00
53.00
59.00
65.00
DifTraction angle 2 0
b
5.00
I
11.00
17.00
23.00
29.00
35.00
41.00
Diffraction angle 2 8
Figure 1. (a, top) XRD diffraction patterns of the Kimmeridge kerogen (6656) before chromous chloride treatment: (1)titanium oxide/rutile syn: TiOz; (2) titanium oxide/anatase syn:TiOz, (3) iron sulfide/marcasite syn:FeSz; (4) iron aulfide/pyrik syn:FeSz. (b, bottom) XRD diffraction patterns of the Kimmeridge kerogen (6656) after chromous chloride treatment: (1) titanium oxide/rutile syn: TiOz, (2) titanium oxide/anatase syn: TiOz.
in three of the four treated samples. XRD data are not
available from the fourth sample (South Elwood 66534) because of limited sample availability. However, at the 3% pyrite level indicated by chemical data it should be detectable by XRD. It may be noted that the large “amorphous hump” at 10-30° 28 in Figure 1 is characteristic of kerogen. Data in Table I11 show no significant changes in H/C, OK, N/C, and So& atomic ratios and, therefore, indicate no change in gross elemental composition of the organic
matter in kerogen. The variations that are shown are small and probably within the reproducibility for data of this type which have several sources of experimentalerrors. In general, the differences between two kerogen preparations from the same rock (the -A and -Bsamples) are as large as differences before and after the CrClz treatment (samples -Band 42). More data are required to detect any significant changes or trends, but none are expected. FT-IR spectra for the South Elwood kerogen before and after treatment (Figure 2) are qualitatively identical,
Acholla and Orr
410 Energy &Fuels, Vol. 7, No. 3, 1993
1
I
31
4000
3620
3240
2860
2480
2100
1720
1340
%O
580
WAVENUMBER (rm.)
Figure 2. Comparison of FT-IR spectra of the South Elwood kerogen (6653): 6653-B,before chromous chloride treatment; 66534,after four chromous chloride treatments. Table V. Infrared Spectral Factors before and after Treatment A C sample case aliphatic carboxycarbonyl 0.83 0.60 before 6656-B 0.83 0.63 6656-C after 0.47 0.67 6648-B before 0.69 0.49 6648-C after 0.81 0.43 6653-B before 0.79 0.43 6653-C after 0.68 0.52 1675-1-B before 0.53 0.70 1675-1-C after Table VI. (3-13 NMR Estimate of Aromatic and Aliphatic Carbon ( % ) before and after Chromous Chloride Treatment aromatic aliphatic sample case carbon carbon 41 59 before 6656-B 42 58 after 6656-C 39 61 before 6648-B 40 60 6648-C after 33 67 before 6653-B 69 31 after 6653-C 67 33 1675-1-B before 65 35 1675-1-C after
indicating minimal changes in CH, C=C, and C - 0 bond types. A more quantitative evaluation is provided by an “aliphatic factor” (A) and a “carboxycarbonyl factor” (C) defined by eqs 1 and 2, where I is the intensity of the
c = (&=o)/(Ic+) + I c e )
(2)
peaks at 2921 (CHz), 2848 (CH3), 1710-1704 (C=O), and 1610 (C-C) cm-l. These infrared ratio factors (Table V) confirm no detectable compositional changes in the kerogen structure (within experimental error of about 5-10% 1. Another comparison by 13CNMR (Table VI) shows that aromatic and aliphatic carbon distributions are identical within the accuracy of the method before and after treatment. Possible reactions of chromous chloride with organic sulfur in the kerogen and possible reduction of other sulfur compounds are a concern that should be mentioned. Other~’2-l~ have examined these questions and conclude that organic sulfur does not evolve HzS by the prescribed treatment. Results from our experiments confirm this conclusion. Most inorganic sulfates, elemental sulfur, and acid-soluble sulfides are removed in the initial kerogen
preparation and therefore are of little concern. However, sulfates and elemental sulfur may form from oxidation of pyrite during storage of kerogen preparations and may require removal for some purposes. Barium sulfate can be a special problem. It is generally a minor component in sedimentary rocks (usually negligible in terms of sulfur contribution) but can be aserious drillingmud contaminate in core and cutting samples. It does not react with chromous chloride and, if present, remains in the treated kerogen residue. A barium analysis is recommended if it is suspected. The method described in this paper can be used for separation of pyritic- and organic-sulfurfractions for sulfur isotopic measurements if suitable care is taken. The HaS evolved when most of the pyrite is reduced should give valid isotopic ratios for pyrite sulfur. However, valid isotopic ratios for the organic sulfur are more difficult to obtain, especially when So, is in low abundance. Careful analytical control should establish that pyrite sulfur remaining is a small fraction relative to the organic sulfur in the residual kerogen. Two of the residues in Table I1 (66534 and 1675-14) retain enough pyrite S to affect the 6% values of the organic sulfur if the two forms differ significantly in isotopic ratios. This observation causes some concern about the accuracy of organic 634Svalues derived from CrC12treatment of whole rocks and sediments because the relative amounts of Spyand Sorg in the residue are not easily established if iron containing minerals are still present.’5J6J8 However, based on our experience with petroleum source rocks, 634Svalues of Spyand Sotgseldom differ by more than 15460 and -3 7% contamination seldom leads to errors greater than the precision of 634Smeasurements (h0.3-0.5960). Ideally, the Spy/Sorg ratio should be less than 0.03 in samples used for organic 634S measurement.
Conclusions The chromous chloride procedure offers an improved method for the preparation of pyrite-free kerogen with minimal or no alteration of the chemical structure in the kerogen. The availability of pyrite-free kerogen can decrease interferences in many chemical and spectroscopic methods of analysis and promises to provide improved determinations of kerogen structural units. For example, the method can provide kerogen preparations suitable for XANES evaluation of organic sulfur bonding in kerogen which is of interest regarding the effect of sulfur on oil generation kinetics and changes in sulfur bonding with maturation.9110,20,21 This procedure also provides a means of preparing unaltered kerogen concentrates for the study of catalytic or mineral matrix effectsin laboratory studies of conversion of kerogen to oil and gas using synthetic kerogen-mineral mixtures in hydrous or anhydrous pyrolysis experiments. Acknowledgment. We thank J. T. Edwards (MEPTEC) for developing special sample preparation procedures for XRD analysis, S. E. Schramm for assisting with the NMR analysis, and T. 0. Mitchell for providing some of the samples and reviewing the manuscript. We also thank Mobil Research and Development Corp. and MEPTEC for permission to publish these results. ~
(20) Waldo, G. S.; Carbon, R. M.K.;
Moldowan,J. M.; Patere, K. E.; Penner-Hahn, J. E. Geochim.Cosmochim. Acta 1991,55,801-814. (21) Waldo, G. S.; Mullins, 0. C.; Penner-Hahn, J. E.; Cramer, S. P.
Fuel 1992, 71, 53-59.
