Energy & Fuels 1994,8, 1494-1497
1494
Potential for Amorphous Kerogen Formation via Adsorption of Organic Material at Mineral Surfaces A. N. Bishop*??and R. P. Philp School of Geology and Geophysics, 810 Sarkeys Energy Center, University of Oklahoma, 100 E Boyd St., Norman, Oklahoma 73019 Received May 5, 1994@
The traditional model for the formation of kerogen in sediments is that biopolymers undergo biochemical degradation, followed by polymerization and condensation, resulting in an insoluble product. An alternative method of kerogen formation has been postulated, based on observations that certain biopolymers may be highly resistant to microbial degradation, indicating that these components can be selectively preserved. Clay minerals have strong adsorption and catalytic properties, with a high effective surface area, and are commonly abundant components of organicrich sediments. Hence, clays are likely to adsorb organic matter in sediments. Following mineral dissolution of a sediment, clay-adsorbed organic material may give rise to an insoluble polymer, which would possess many of the characteristics of amorphous kerogen. Although the quantitative importance of clay adsorption as a vector of carbon preservation is currently unclear, in certain environments this process could account for the formation of amorphous kerogen.
Introduction Kerogen, defined as the fraction of sedimentary organic matter which is insoluble in common organic solvents,l accounts for over 90% of the total organic carbon content of sedimentary rocks.2 It is conventionally subdivided into two main categories (see Tyson3for a recent review): structured, organic particles typically of recognizable biological affinity, e.g., spores, algae, and wood (vitrinite); and structureless, more commonly referred to as amorphous kerogen, which lacks morphological integrity. The latter, obtained following dissolution of the mineral matrix, may in some instances represent more than 90% of the kerogen content of organic rich sediments (e.g., Senftle et al.,4 Powell et a1.5 1. The traditional model for the formation of amorphous kerogen is that biopolymers undergo biochemical degradation, followed by polymerization and condensation.6 The products thus formed, chiefly humic and fulvic acids, become increasingly insoluble with time, as a result of further random polymerization and condensation steps (Figure 1). Philp and Calvin7 suggested that certain biopolymers, present in the cell walls of algae and bacteria, may be resistant to biochemical degrada+ Present address: Fossil Fuels and Environmental Geochemistry (Postgraduate Institute), NRG, Drummond Building, The University, Newcastle upon Tyne, NE1 7RU, U.K. Email: andy.bishop@newcastle. ac.uk. Abstract published in Advance ACS Abstracts, September 1,1994. (1)Durand, B. In Kerogen: Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 13-35. (2) Hunt, J. M. Petroleum Geochemistry and Geology; W. H. Freeman & Co.: San Francisco, 1979; pp 44-67. (3) Tyson, R. V. Sedimentary organic matter: Organic facies and palynofacies; Chapman Hall: London, in press. (4) Senftle, J. T.; Yordy, K. L.; Barron, L. S.; Crelling, J. C. Tech. Pap. Eastern Oil Shale Symp., Lexington, KY 1987, 155-167. (5) Powell, A. J.;Dodge, J. D.; Lewis, J. Proc. ODP Sci. Rep. 1990, 112B. 297-321. (GiTissot, B. P.; Welte, D. H. Petroleum Formatior, and Occurrence; Springer: Berlin, 1984; p 70. (7) Philp, R. P.; Calvin, M. Nature 1976,262, 134-136.
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Figure 1. Polymerizatiodcondensation kerogen formation pathway (aRer Tissot and Welte6).
tion, resulting in their selective preservation (Figure 2). Such resistant biopolymers, now termed algaenans and bacterans, are considered by some to be an important source of amorphous k e r ~ g e n . ~ , ~ (8)Tegelaar, E. W.; de Leeuw, J. W.; Derenne, S.; Largeau, C. Geochim. Cosmochim. Acta 1989,53,3103-3106. (9) de Leeuw, J. W.; Largeau, C. In Organic Geochemistry; Engel, M. A., Macko, S. A,, Eds.; Plenum Press: New York, 1993; pp 23-72.
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Energy & Fuels, Vol. 8, No. 6, 1994 1495 paper is to discuss the potential significance of mineral adsorption as a mechanism of kerogen formation, and in particular, the amorphous variety.
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Clay Minerals and Organic Carbon The substantial volume of literature published on Biosynthesis organidinorganic interactions in soils indicates that clay minerals are likely to be the dominant mineral species responsible for organic adsorption in siliciclastic source LMW biorocks (see Theng,16J7Burchill et a1.,18Clapp et al.,19and Biopolymers molecules Hayes and Minge1grin2Ofor reviews). The clay-humus I interaction plays a major role in mineral cycling, profile I \ I Condensation I development, and aggregate stabilization, processes of Selective Selective polymerization lnco~oratlon fundamental importance for the fertility of soils. In preservation I J preservation sediments, a strong association exists between the 7 Natural ) , I distribution of clay-rich and organic-rich deposits (see vu1ca;nization 4 review by Calvert and Pedersen21 1. Premuzic et noted a statistically significant relationship between Resistant J R&,,Pt percent clay and organic carbon in data from seafloor biopolymers sediments, taken from water depths of up to 2000 m. However, other factors, such as the hydrodynamic behavior of clay and organic particles, could also play a Maturation I role in this process. Maturation Clay mineral adsorption of organic carbon will take 4 place either in the water column or in the sediment following deposition. Hedges23assessed the uptake of Aliphatic and aromatic glucose, valine, and stearic acid by clay minerals, under hydrocarbons, plus NSOcom ounds a range of different solute concentrations. The results indicated that, although clay minerals have the potenFigure 2. Selective preservation kerogen formation pathway tial to preferentially remove dissolved organics from sea (after Tegelaar et a1.9 water, the quantities involved were unlikely to account RomankevichlO suggested that organic components for the high carbon contents of organic-rich sediments. adsorbed by clay minerals could be protected during However in a related study, Hedges24suggested that diagenesis from microbial degradation. Bishop et al.ll partitioning of melanoidins between aqueous solutions used osmium tetroxide staining t o demonstrate the and clay minerals could result in increases of concentraassociation of organic matter with clays in an SEM tion in the particulate phase of 100-1000 times. study of sediments from the Peru Margin. More reMacromolecular organic matter accounts for a sigcently, a number of studies have illustrated the impornificant proportion of the dissolved organic content tance of the organo-mineral association in the preser(DOC)of sea water.25 For molecules with similar shape, vation of organic carbon. Keil and Hedged2 and Keil polarity, and charge characteristics, larger molecules et al.13 have noted a relationship between the amount will be more strongly adsorbed than smaller equivaof organic material preserved in coastal marine sedilentx2" Hence, the interaction of macromolecular orments and total mineral surface area. This association ganic components and clays in the water column, or in may be related t o mineral adsorption, although it has sediment pore waters, may provide an opportunity for been suggested that organic carbon may be trapped by uptake of organics by mineral surfaces. micropores present on mineral surfaces.14J5 Within A key process in the clay adsorption hypothesis, as such pores the processes of humification and polymermentioned above, is that the organic materials are ization can take place, with the organic material steri(16) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Adam cally protected from microbial degradation by the pore Hiker: London. 1974. size. Hence, mineral adsorptiodentrapment of sedifi7) Theng, B. K. G. Formation and Properties of Clay-Polymer mentary organic materials may contribute to an overall Complexes; Elsevier: Amsterdam, 1979. (18) Burchill, S.; Hayes, M. H. B.; Greenland, D. J. In The Chemistry background of organic deposition, and in some environofSoil Processes: Greenland, D. J.. Haves, M. H. B.,. Eds.;. John Wilev ments this process could represent the primary carbon &d Sons Ltd.: Chichester, U.K., '198i; pp 221-401. (19) Clapp, C. E.; Harrison, R.; Hayes, M. H. B. In Interactions at preservation pathway. the Soil Colloid-Soil Solution Interface; Bolt, G. H., de Boodt, M. F. The concept of mineral adsorption as a major process D., Hayes, M. H. B., McBride, M. B., Eds.; Khwer Academic Publishin the biogeochemistry of organic carbon clearly has ers: Dordrecht, 1991; pp 409-468. (20) Hayes, M. H. B.; Mingelgrin, U. In Interactions at the Soil important consequences €or the diagenesis and maturaColloid-Soil Solution Interface; Bolt, G. H., de Boodt, M. F. D., Hayes, tion of sedimentary organic matter. The aim of this M. H. B., McBride, M. B.,Eds.; Kluwer Academic Publishers: DorMineralization
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(10)Romankevich, E. A. Geochemistry of Organic Matter in the Ocean; Springer-Verlag: Berlin, 1984. (11)Bishop, A. N.; Kearsley, A. T.; Patience, R. L. Org. Geochem. 1992,4, 431-446. (12) Keil, R. G.; Hedges, J .I. Chem. Geol. 1993, 107, 385-388. (13) Keil, R. G,Tsamakis, E.; Fuh C. B.; Giddings J. C.; Hedges J. I. Geochim. Cosmochim. Acta 1994, 58, 879-893. (14) Mayer, L. M. Geochim. Cosmochim. Acta 1994,58,1271-1284. (15)Mayer, L. M. Chem. Geol. 1994, 114, 347-363.
drecht, 1991; pp 323-407. (21) Calvert, S.E.; Pedersen, T. F. In Organic Matter: Productivity, Accumulation and Preservation in Recent and Ancient Sediments; Whelan, J., Farrington, J. W., Eds.: Columbia University Press: New York, 1992; pp 231-263. (22) Premuzic, E. T.; Benokovitz, C. M.; Gafiey, J. S.; Walsh, J. J. Org. Geochem. 1982,4, 63-77. (23) Hedges, J. I. Geochim. Cosmochim. Acta 1977,41,1119-1123. (24) Hedges, J. I. Geochim. Cosmochim. Acta 1978,42, 69-76. (25) Farrington, J. Mar. Chem. 1992,39, 59.
1496 Energy & Fuels, Vol. 8, No. 6, 1994
protected from microbial degradation during diagenesis, as a result of their being adsorbed. The protection of proteins and enzymes from microbial degradation, via clay adsorption, is well documented (see Stotzky,26and Burnsz7for reviews). Lynch and Cotnoil.28observed that carbohydrates and proteins were protected from degradation by montmorillonite, but this effect was much reduced with kaolinite and illite. Stotzky26noted that, in general, complexation of organics by clays inhibits degradation by microorganisms. As yet it is unclear whether or not this represents a direct protective effect of the clays toward the organics, or an unknown indirect process. Potential protective mechanisms probably include the masking of microbial attack sites by the adsorption mechanism, including steric effects if the components are adsorbed between the clay layers preventing microbial entry.27 A number of sedimentary organic geochemical processes have been attributed to the effects of clay minerals. Tissot and Welte6 suggested that the discrepancy in the polar contents of crude oils and source rock extracts was a product of clay mineral adsorption. The cracking of sedimentary organic matter to form petroleum was related by Hunt2 in part at least to the catalytic effect of clays. Similarly, the rearrangement of sterenes to form diasterenes, which ultimately undergo a hydrogenation step resulting in the formation of the diasteranes, has also been attributed to the catalytic effects of clay minerals,29such that the relative concentration of diasteranes in oils is now routinely used to assess the clay content of the original source rocks. Hence, the question arises as to how, when, and where sedimentary organic matter comes into contact with clays for such processes to take place. Adsorbed Organic Matter as a Source of Amorphous Kerogen. It is widely acknowledged that some organic material in siliciclastic sediments is adsorbed by mineral surfaces, but the extent of adsorption, and how this relates to the products identified during conventional geochemical characterization, are unknown. When a sediment is demineralized, the physical structure of any mineral adsorbed organic matter will be largely lost, giving rise to an amorphous organic residue. If this product is completely soluble in organic solvents, the sorbed organic matter will give rise to bitumen 2 (the extract of a kerogen which was isolated from preextracted rock). Conversely, if it is an insoluble polymer, the residue would possess the characteristics of amorphous kerogen. Irreversibility of polymer adsorption on clay minerals has long been recognized (e.g., Silberberg3O), and the catalytic effects of clays are well documented. It is likely that reactions mediated by the clays will affect the structure of any adsorbed components, with the polymerization and condensation of such material leading to the formation of an insoluble geopolymer. Figure 3 summarizes such a mechanism for amorphous kerogen formation via mineral adsorption. (26) Stotzky, G. In Interactions of Soil Minerals with Natural Organics and Microbes; Huang, P. M., Scnitzer, M., Eds.: Soil Science Society of America: Madison, WI, 1986; pp 305-428. (27) Burns, R. G.In Soil Colloids and their Associations in Aggregates; de Boodt, M. F., Hayes, M. H. B., Herbillon, A,, Eds.; Plenum Press: New York, 1990; pp 337-364. (28) Lynch, D.L.; Cotnoir, L. J . Soil Sci. SOC.Proc. 1956,367-370. (29) Sieskind, 0.; Joly, G.; Albrecht, P. Geochim. Cosmochim. Acta 1979,43,1675-1679. (30) Silberberg, A. J.Phys. Chem. 1963,66,1884-1907.
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Figure 3. Proposed clay mineral adsorption kerogen formation pathway. Greenland31noted that when clays are dried, a much greater area of any adsorbed organic components will come into contact with the mineral matrix, increasing the number of bond sites. The probability of simultaneously desorbing the molecule from each point of attachment is low; thus the polymer following drying is irreversibly adsorbed. The significance of this process for geochemical characterizationis that sediment samples are normally dried prior to extraction and/or demineralization; consequently, material may become insoluble as a result of the preparation technique. Unpublished data from experiments conducted at the University of Oklahoma suggest that this mechanism is unlikely t o significantly affect the extract yield in clay-rich sediments. In these experiments, an extract of a coal was stirred in dichloromethane with montmorillonite, until the solvent had completely evaporated. The clayextract mixture was then Soxhlet extracted in dichloromethane/methanol (93/7, v/v) for 72 h. The yield of extract obtained showed that the concentration retained by the clay was insignificant. However, these results may not necessarily apply to all extracts, particularly those containing high concentrations of dissolved macromolecular components, e.g., asphaltenes, which may behave differently. A potential link between the presence of mineral adsorbed organic material and amorphous kerogen was reported by Bishop et al.ll In that study, samples of an upwelling sediment from the Peruvian continental margin (ODP Leg 112) were found to have a negligible particulate organic matter content, even when viewed under the SEM, despite having TOC contents of up to 10%. The lack of identifiable organic constituents was (31)Greenland, D.J. Soil Sci. 1971,111, 34-41.
Potential for Amorphous Kerogen Formation
also supported by palynological assessments of the same samples, which showed that on average over 95% of the organic matter was present as amorphous k e r ~ g e n . ~ To account for this shortfall between the abundance of visible organic components and TOC, osmium tetroxide (Os04) staining was used to determine the distribution of the “missing” organic material. Osmium tetroxide reacts with unsaturated double bonds and phenols, but not with any of the minerals commonly found to occur in sediments.ll The distribution of organic material can be indirectly assessed by X-ray mapping the sample for osmium, the results of which showed that the organics were relatively more concentrated in the clay laminae than the diatom layers. This was confirmed by fine scale geochemical studies.32 Osmium maps taken at higher magnifications indicate that the organic matter was associated with pyrite and clay minerals, with high concentrations localized in detrital iron-rich clays. Quantitative Significance of This Mechanism. The results of the Peru Upwelling study suggest that this process could represent an important source of amorphous kerogen in such sediments; however, no precise figures for the actual concentrations produced are currently available. Numerous studies have been conducted in the field of soil science on the quantitative uptake of synthetic polymers by various clay minerals (see Theng17and Clapp et al.19for reviews). Adsorption levels of poly(vinylalcoho1)sas high as 0.8 g per gram of clay in aqueous solutions have been reported for Nam~ntmorillonite.~~ For poly(ethyleneglycol)s,concentrations of polymer in clay of up to 20% have been noted.34 These figures, corresponding t o TOCs between approximately 15 and 35%, could clearly account for the organic content of virtually all source rock sequences, assuming that such sediments were composed wholly of smectite. However, sediment mineralogies are considerably more heterogeneous, as is organic composition; thus, such a comparison is unrealistic. But it is interesting to note that TOC values as high as 7% have been reported in clay fractions isolated from recent marine sediments.13 A strong association is documented for the occurrence of clay-rich and organic-rich sediments. As the smec(32)Aplin, A. C.; Bishop, A. N.; Clayton, C. J.; Kearsley, A. T.; Mossmann, J.-R.; Patience, R. L.; Rees, A. W. G.; Rowland, S. J. In Evolution of Upwelling Systems; Summerhayes, C. P., Prell, W. P., Emeis, K. C., Ed.; Geological Society: London, 1992; pp 131-149. (33) Theng, B. K. G. Clays Clay Miner. 1982, 30, 1-10. (34) Parfitt, R. L.; Greenland, D. J . Clay Miner. 1970, 8, 305-315.
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tites have the highest adsorbance capacities of the clay mineral group, it would be expected that smectite distribution in marine sediments would match that of the sediments with the highest TOCs. However, Premuzic et a1.22noted that there was not a statistical correlation between TOC and clay composition, indicating that additionalkompeting processes exert important controls on carbon concentration. Such processes could include preservation, hydrodynamic sorting, and/or mineral surface entrapment.
Conclusions Documented organo-clay interactions demonstrate that organic carbon is preserved to some extent via clay mineral adsorption. Clays are capable of catalyzing reactions such as polymerization and condensation, which would assist in the formation of an organic polymer. Dissolution of the minerals themselves would leave a structureless geopolymer which would possess many of the observed characteristics of amorphous kerogen. The clays may also catalyze hydrocarbon cracking, which is likely to take place at elevated temperatures as part of the petroleum generation process. The quantitative significance of this process is as yet unclear and is unlikely to account solely for the amorphous kerogen content of most source rocks. Rather, this mechanism is likely to work in tandem with the more traditional models of kerogen formation such as polymerizatiodcondensation and selective preservation. Amorphous kerogen represents one of the largest pools of organic carbon in the lithosphere. Consequently, a steady background of mineral adsorption to the total global flux of organic matter deposition could have important implications for the general biogeochemistry of carbon. Although organo-mineral interactions have been widely studied in soil science and catalytic chemistry, they have been largely overlooked in organic geochemistry. This paper demonstrates the need for such work in the study of sediments to evaluate the effects of a highly reactive mineral group on the preservation and diagenesis of sedimentary organic materials. Acknowledgment. We thank M. J. Collins (University of Newcastle) and M. H. Engel (University of Oklahoma) for valuable discussions, and R. V. Tyson (University of Newcastle) for helpful comments on the manuscript. The constructive remarks of two anonymous reviewers are also appreciated.
