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Kerogen Chemistry 3. Shale Elemental Analysis and Clay Dehydroxylation John W. Larsen,*,# Koh Kidena, Ryuichi Ashida,# and Harsh Parikh Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, and The Energy Institute, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802-2303 Received January 5, 2005
For some shales, the isolated organics have a lower H/C ratio than the H/C ratio of the whole rock, both being measured by combustion analysis. The primary cause of this is water produced by clay dehydroxylation during the combustion analysis of the rock that contributes to the rock’s measured “hydrogen content”. Organics are also lost to the aqueous phases during dissolution of the minerals in aqueous HCl and HF.
Introduction When working with a maturation series of Bakken kerogens, significant differences were observed in the H/C ratios of the whole rock and the H/C ratios of the organics isolated by dissolving the inorganic minerals, both ratios measured by combustion analysis. Accurate elemental analyses are necessary to follow and to understand the chemical changes occurring in this petroleum source rock as it matures. We and others have treated the organics present in a source rock as a sol-gel system.1-4 Kerogens are insoluble cross-linked macromolecules (gel) and the smaller soluble organic molecules (bitumens) are the sol. To fully characterize this sol-gel system, accurate elemental analyses of the whole organic system are required. It was necessary to discover the origin of the discrepant elemental analyses. Treating the kerogen-bitumen system as a sol-gel system has been done recently by us1,2 and a few others,3 and effectively by workers at ExxonMobil,4 but is not common. It provides a useful framework for considering kerogen reactivity. The polymer chemistry of sol-gel systems is well developed.5,6 Polymerization of monomers to give oligomers and finally an insoluble cross-linked polymer gel is well understood. The sol reacts, its molecular weight distribution changes, and eventually insoluble gel is formed and the sol concentration decreases. The reverse process, the fragmenting of a gel to give first small molecules and then larger ones * Author to whom correspondence should be addressed. # Current address: The Energy Institute, 209 Academic Projects Building, The Pennsylvania State University, University Park, PA 16802-2303. (1) Larsen, J. W.; Li, S. Energy Fuels 1997, 11, 897-901. (2) Larsen, J. W.; Parikh, H.; Michels, R. Org. Geochem. 2002, 33, 1143-1152 and references therein. (3) Ritter, U. Org. Geochem. 2003, 34, 319-326. (4) Kelemen, S. R.; Freund, H.; Siskin, M.; Curry, D. J.; Xiao, Y.; Olmstead, W. N.; Gorbaty, M. L. Bence, A. E. U.S. Patent Appl. 2004/ 0019437, 2004. (5) Painter, P. C.; Coleman, M. M. Fundamentals of Polymer Science, 2nd ed.; Technomic Publishing Co.: Lancaster, PA, 1997. (6) Sperling, L. H. Introduction to Physical Polymer Science, 3rd ed.; Wiley-Interscience: New York, 2001.
has also been well characterized. Changes in the crosslink density of the gel and changes in the amount of and molecular weight distribution of the sol all vary as a function of time following a kinetic law that can be determined. This is exactly the kinetic framework needed to understand kerogen decomposition. Even used qualitatively, the sol-gel model has value. For example, the sometimes proposed pathway for oil formation, kerogen f bitumen f oil, is inconsistent with the expected behavior of cross-linked macromolecular kerogen. Low-molecular-weight molecules are expected to be the earliest products of kerogen fragmentation, not the last. On average, large-molecularweight regions will be bonded to the macromolecular matrix at many points. A pendent alkyl chain has one bond tying it to the insoluble macromolecular matrix. If bond breaking is random, the alkyl chain will be liberated much before the large bitumen molecule. Only one bond needs be broken to free the alkane from the network, while many must be broken to free the large bitumen molecule. Furthermore, because the alkane has a solubility parameter very different from the kerogen’s, there will be a large thermodynamic driving force for its expulsion from the kerogen network. The hydrogen content of kerogen is a controlling factor in the amount of petroleum that can be formed.7 It is often used in a Van Krevelen diagram to follow kerogen maturation.8 Most geochemists use rock-eval pyrolysis to evaluate the H/C ratios of the kerogen in source rocks because the technique is inexpensive and can be applied to the whole rock.9 Because we desired accurate atom balances, we measured H/C ratios directly by using combustion analysis. Baskin has compared the results of direct and rock-eval analyses and presents several examples of poor correlations between hydrogen indices (HI) obtained from rock-eval data and measured kero(7) Hunt, J. M. Petroleum Geochemistry and Geology, 2nd ed.; W. H. Freeman: San Francisco, 1996. (8) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: New York, 1984. (9) Baskin, D. K. AAPG Bull. 1997, 81, 1437-1450.
10.1021/ef050004v CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
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gen H/C ratios.9 Whelan and Thompson-Rizer have also reviewed the literature and concluded that the correlation between kerogen elemental analyses and rock-eval HI data is good provided the kerogens are isolated and extracted after isolation.10 Because we wish to follow all of the organics, extraction before analysis is not an option. Early work on kerogen H/C analysis has been reviewed.11 Our problem is different than obtaining the H/C analysis for a kerogen. Kerogen can be isolated by dissolving the inorganic phase, and the bitumen can be removed from it by extraction leaving the insoluble organic solid for combustion analysis. To use the solgel model, we need to start from the elemental analysis of the sol-gel system, both the insoluble kerogen and the soluble bitumen. A first step in understanding the chemistry of petroleum formation from kerogen is to establish as best we can the atom balance for the process. The whole shale was therefore subjected to combustion analysis. The shale H/C ratios thus obtained are significantly higher that the H/C of the organics that are isolated from the shale by dissolving most of the rock in first aqueous HCl and then aqueous HF. There are three possible explanations for the decrease in H/C ratios: (1) Hydrogen is lost during the kerogen isolation process. (2) Not all of the carbon is burned during the elemental analysis. (3) There is a source of hydrogen in the whole rock in addition to the organics. Each of these deserves comment. Hydrogen loss during demineralization can happen in two ways. Organic material having a high H/C ratio can escape as the minerals are dissolved. Oils may be freed or gases may be liberated as the rock is dissolved. If they have higher H/C ratios than the remaining kerogen, the isolated kerogen will have a lower H/C than the whole rock. Or, if the demineralization results in oxidation of the organics, the isolated kerogen will have a lower H/C ratio than the whole rock. All of the carbon may not be burned during the analysis. Coal elemental analyses get special treatment because it is sometimes difficult to achieve complete carbon combustion. The same may be encountered with shales. Carbon that is inert and difficult to combust or carbon that is shielded from the oxygen by the inorganic matrix may not burn. Finally, anything inorganic that produces hydrogen during the combustion analysis will increase the measured H/C by adding inorganic H to the organic H. Because hydrogen is measured as water in combustion analyses, any minerals that liberate water during combustion analysis will increase the measured H/C analysis of the whole rock resulting in the isolated kerogen having a lower H/C ratio than the whole rock, not because the kerogen H/C is low, but because the whole rock H/C is erroneously high. Organic geochemists usually have not needed a full atom balance. Their usual procedure is to analyze kerogens after demineralizing a rock sample that had been extracted with methylene chloride. Such data are not useful for our purpose. But there are early data that are useful to us. Smith studied 10 independent oil shale
samples from separate cores in the Mahogany zone of the Green River formation, most from the Piceance Creek basin. Elemental analyses were done on the raw shale and on the “organic concentrate” prepared by dissolving minerals first in aqueous HCl followed by aqueous HF.12 The H/C mole ratio was 1.73 ( 0.03 for the raw shales and dropped to 1.53 ( 0.01 for the organic concentrate. The roughly 10% drop in H/C was essentially the same for all 10 samples and is large enough to be significant. Oxidation of the organics during HCl-HF treatment as had been proposed by Dancy and Giedroyc13 was rejected as an explanation for the H/C decrease. The acids used to demineralize the shales were extracted with, among other solvents, chloroform and ether. The organics isolated by extraction from the aqueous acids were less than 0.15% of the organic concentrate, an amount insufficient to be responsible for the observed H/C decrease. The origin of the decrease in H/C ratio resulting from demineralization was not identified. Dancy and Giedroyc reported decreases in H/C after demineralization of Kimmeridge, St. Hilaire, and Ermelo shales.13 The decreases were between 5% and 13%. The decreases were ascribed to oxidation during demineralization. They were not able to follow changes in O/C to test whether oxidation was occurring. A more varied data set is found in the review of procedures for kerogen isolation by Durand and Nicaise.14 For a peat, a lignite, and a torbanite, they report H/C and O/C for original samples after HCl treatment and after HF treatment. Also measured were H/C and O/C for HCl- and HF-treated samples of kukersite and Green River shale. Mineral dissolution decreases H/C for all of these samples. Importantly, the O/C ratios are constant and do not increase on acid treatment. This rules out oxidation during demineralization by these acids. In aqueous systems, oxidation would lead to an increase in the O/C ratio. The literature data clearly establish the existence of decreases in the H/C ratio as a result of shale demineralization. Oxidation during demineralization was ruled out as the origin of the drop in H/C ratios during demineralization. Loss of organics to the acids occurs but is not sufficient to explain the entire decrease. Both Smith12 and Dancy and Giedroyc13 expressed concern with clay dehydration, but neither addressed the issue directly experimentally.
(10) Whelan. J. K.; Thompson-Rizer, C. L. In Organic Geochemistry, Principles and Applications; Engel, M. H., Macko, S. A. Eds.; Plenum: New York, 1993. (11) Durand, B.; Monin, J. C. In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; Chapter 4.
(12) Smith, J. W. Reports of Investigations 5725; U.S. Bureau of Mines: Washington, DC, 1961; pp 1-16. (13) Dancy, T. E.; Giedroyc, V. J. J. Inst. Pet. 1950, 36, 593-603. (14) Durand, B.; Nicaise, G. In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; Chapter 2.
Experimental Section Drying. Samples were dried in a vacuum oven at 100 or 110 °C to constant weight. This gave Bakken shale samples with H/C of 1.43 or 1.38. The samples were allowed to cool in the oven under vacuum, and dry N2 was introduced to break the vacuum. To test whether slow diffusion of water from the shale might result in incomplete drying, a sample of Bakken shale was heated for 2 weeks under vacuum at 100 °C in an Abderhalden drying pistol containing powdered P2O5 drying agent. Demineralization. The powdered Bakken shale samples were demineralized using standard treatment with HCl fol-
Shale Elemental Analysis and Clay Dehydroxylation
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Table 1. Combustion Analysis of Bakken Shales and the Organics Produced by Demineralization. shale shale NDGS 105 NDGS 607 NDGS 527 NDGS 1858
HCl/HF
28.1 15.8 15.8
1.44 19.8 1.17 8.92 0.90 10.9 21.1
1.05 0.85 0.68 0.73
shale
CrCl2
Tmax (°C) % organics H/C % ash H/C % ash H/C 419 354 79 143
Table 2. Molar H/C Change during the Demineralization of NDGS105
1.54
1.09
4.24
0.75
lowed by HF.14 This resulted in samples containing between 6.7% and 21.1% ash. Ash contents were determined by elemental analysis (Galbraith Labs, Knoxville, TN) or oxidation in a TA Instruments thermogravimetric analyzer (TGA) Model 2950. In the TGA runs, the samples were heated in a stream of air from 25 to 1000 °C at a rate of 50 °C/min. Both of the techniques gave the same ash contents. Acid demineralization with the aid of ultrasound and acid treatment for as long as 240 h each with HCl and HF gave little further reduction in ash. Treatment with CrCl2 successfully removed most of the remaining minerals and was used with the high ash samples.15 The duration of acid treatment had no effect on either the elemental analyses or the solvent-swelling ratio of the isolated kerogen. Kimmeridge clay was satisfactorily demineralized by 24 h treatments with aqueous HCl followed by aqueous HF following the standard procedure.14
Results and Discussion Discrepant Analyses of Bakken Samples. Three samples of Bakken shales of different maturities and the organics remaining after demineralization were subjected to combustion analyses carried out by Galbraith Laboratories. The demineralization was carried out by treating the shales with aq HCl followed by treatment with aq HF.14 In addition, two samples were further demineralized by using CrCl2 solutions to remove pyrite using a standard procedure.15 The geological provenance of the samples and a great deal of their geochemistry has been described.16-18 The sample numbers are those assigned by the North Dakota Geological Survey. The results are contained in Table 1. The demineralized organic samples have significantly lower H/C values than do the shale samples. The changes are so large that these samples provided good candidates for further study. The first issue to be addressed is the timing of the decrease. Is it larger after aq HCl or aq HF treatment? To determine the timing of the H/C decrease, the organics were analyzed at each step of the demineralization process. Two replicate samples were studied independently, and the results are shown in Table 2. There is a slightly larger drop in H/C after HCl treatment, but significant decreases occur after both acid treatments. HCl removes less mineral matter, but has the largest effect on the H/C ratio. The removal of pyrite with chromous chloride does not reduce the H/C ratio. (15) Acholla, F. V.; Orr, W. L. Energy Fuels 1993, 7, 406-410. (16) Meissner, F. F. Petroleum Geology of the Bakken Formation Williston Basin, North Dakota and Montana. In Petroleum Geochemistry and Basin Evaluation; Demaison, G., Murris, R. J., Eds.; AAPG: Tulsa, OK, 1984; pp 159-179. (17) Price, L. C.; Ging, T.; Daws, T.; Pawlewicz, M.; Anders, D. Organic metamorphism in the Mississippian-Devonian Bakken Shale North Dakota portion of the Williston Basin. In Hydrocarbon source rocks of the greater Rocky Mountain Region; Woodward, J., Meissner, F. F., Clayton, J. L., Eds.; Rocky Mtn. Assn. Geol.: Denver, CO, 1984; pp 83-133. (18) Price, L. C.; Daws, T.; Pawlewicz, M. J. Pet. Geol. 1986, 9, 125162.
after aq HCl
after aq HF
H/C
% ash
H/C
% ash
H/C
% ash
1.43 1.43
74 74
1.25 1.26
66 66
1.13 1.13
21 21
Drying. A high H/C ratio for shale might result from incomplete drying. All samples were dried to constant weight under vacuum at 100 or 110 °C. There was no effect of drying time on the H/C ratio. To investigate whether slow diffusion of water from the shale was occurring, a sample of Bakken NDGS 105 shale was heated at 100 °C for 2 weeks under vacuum and in the presence of P2O5 drying agent. This sample had an H/C ratio of 1.38, the same as when dried to constant weight in a vacuum oven. The measured shale H/C is not erroneously high because of incomplete drying. Loss of Hydrogen or Hydrogen-Rich Components. We first considered again the possibility that the demineralization might oxidize any organics. While all acids have the potential to oxidize, aqueous HCl and HF are not generally regarded as oxidizing. Oxidation by air is possible. To address this issue, we demineralized NDGS105 using HCl and HF for 48 h each at 5565 °C and then again by treating it with the acids for 168 h each. The H/C value before and after 48 h treatment were 1.43 and 1.13, respectively. After 168 h of acid treatment, two samples of this shale had H/C values of 1.05 and 1.09, respectively. If oxidation was responsible for the drop in H/C, a 3.5-fold increase in reaction time should have resulted in more than an additional drop of 0.05 in H/C. These values do not support oxidation as the explanation for the observed decrease and confirm earlier work that demonstrated the absence of oxidation during demineralization.14 This argument fails if the shale contains easily oxidizable organics that are quickly lost. It is unlikely that easily oxidizable compounds would exist in the amounts necessary to give the large decreases in H/C that are observed. These results, together with the absence of any increase in O/C ratio reported by Durand and Nicaise,14 lead us to conclude that oxidation is not responsible for the drop in H/C. A more likely explanation is the loss of high H/C organics during the demineralization. Any high H/C organics that escape as gases or are freed from the shale and are discarded with the water layer will result in a lower H/C for the remaining organics. To evaluate this possibility, we carried out the demineralization as usual except the aqueous acids had a layer of methylene chloride to dissolve any organics freed by the demineralization process. The organics were then isolated by evaporating the methylene chloride at room temperature in a stream of dry N2. Volatile organics are lost during this process.19 The data are given in Table 3. The loss of ca. 3% (H/C ) 1.60) of the organics does not explain the observed decrease in H/C from 1.43 to 1.13. That organics are liberated during the demineralization confirms Price’s contention that organics were extruded from the Bakken kerogen and are present in “cracks and parting laminae in the rocks” and are “ready for expulsion from the rocks”.20 Such material would float free as the rocks are dissolved. (19) Ahmed, M.; George, S. C. Org. Geochem. 2004, 35, 137-155.
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Table 3. Demineralization of NDGS 105 with Methylene Chloride Extractant shale (g) mass
H/C
15.014 1.43 15.218 1.43
organics in CH2Cl2 (mg) mass/HCl mass/HF 64.54 68.46
35.03 37.28
total
solids (g) H/C
mass
H/C
99.57 1.60 105.74 1.60
3.70 4.60
1.13 1.13
Table 4. Analysis of Bakken NDGS105 Shale and Its Combustion Residue shale
residue
%C
%H
H/C
%C
%H
16.26
1.87
1.38
