Energy & Fuels 1997, 11, 897-901
897
Changes in the Macromolecular Structure of a Type I Kerogen during Maturation John W. Larsen* and Shang Li Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015 Received January 10, 1997X
The solvent swelling of a maturation series consisting of 5 Type I kerogens from the Uinta basin has been studied. The swelling of the kerogens in 10 solvents has been used to determine the solubility parameter (δ) of the samples. It is 9.5-10.0 (cal/cm3)1/2. The same data have been used to calculate the number average molecular weights between cross-links (reciprocal of the cross-link density) for the samples. The cross-linking increases during early maturation, and sharply above 80% maturation, but remains about constant from 20% to 80% maturation. Oil formation involves a reconstruction of the kerogen during which bond cleavage to form bitumen is accompanied by bond making to increase the remaining kerogen cross-link density. The change in cross-link density closely parallels a model for maturation in this basin published previously by Sweeney et al. (Am. Assoc. Pet. Geol. Bull. 1987, 71, 967-985).
Introduction Kerogens are all macromolecular systems and as such are amenable to study using the array of techniques developed in polymer science. The concepts and experimental techniques of polymer chemistry are now in routine use in coal chemistry and have added much to its modern development. Polymer science concepts are often encountered in discussions of Type I and II kerogens, especially in considerations of primary migration.1,2,3 The direct application of polymer chemistry techniques to the study of these two materials is rare. This is particularly unfortunate because their behaviors are much more regular than coal and the systems are much less plagued by dominant specific non-covalent interactions.4,5 In this paper, we report what we believe to be the first detailed examination of a Type I kerogen maturation series using the techniques and concepts of polymer chemistry. We have measured the cross-link density of a Uinta Basin Type I kerogen maturation series. There are several reasons for having chosen this parameter to study. Changes in the cross-link density are particularly informative. It is a sum of the bond making and bond breaking processes which occur in the macromolecular network. It is a defining characteristic of the macromolecular network. It is readily accessible using simple experiments and a thorough theoretical framework for its consideration exists. There has been no direct measurement of the cross-link density changes in a Type I kerogen maturation series, but two different maturation models that address this issue have been proposed.2 In the depolymerization model, the kerogen cracks into fragments and continuously depolymerizes. Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1978. (2) Ungerer, P. Org. Geochem. 1990, 16, 1-25. (3) Stainforth, J. G.; Reinders, J. E. A. Org. Geochem. 1990, 16, 6174. (4) Larsen, J. W.; Li, S. Energy Fuels 1994, 8, 932-936. (5) Larsen, J. W.; Li, S. Org. Geochem., in press. X
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This eventually results in the conversion of most of the kerogen to extractable compounds. In the defunctionalization model, an inert central skeleton has attached to it hydrocarbon groups of various sizes that cleave off leaving behind the central kerogen core. Apart from a correction for “dangling ends”, the cross-link density of the central core will remain unchanged. These are not the only possibilities but are the two principal speculations which have appeared in the literature. One would expect that the cross-link density will be a sensitive function of the amount of bond cleavage which has occurred in these systems and be a good descriptor of oil generation. It is independent of the quantities so far used to characterize kerogen maturation and so will provide an independent and complimentary view of the changes which are occurring. In systems which are polymerizing or depolymerizing, there are established relationships among the amount of insoluble macromolecular network (kerogen), the amount of the soluble material (bitumen), the molecular weight distribution of the soluble material, and the statistics of the bond making or bond breaking processes occurring (for example, random vs selective).6,7 If a relationship between these four quantities can be defined for kerogen maturation, it will provide a unique and deep insight into the processes occurring. Because of bitumen expulsion from our highly mature samples, we cannot fully investigate these relationships using these samples. For this study, we desired a maturation series from a formation which was well studied and understood. Five samples from the Uinta Basin were studied. They cover the region from less than 5% to 100% maturity, and there is an abundance of data on these samples and on the field from which they came.8-10 A well-studied (6) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (7) Yan, J.; Johnson, D. J. Appl. Polym. Sci. 1981, 26, 1623-1635. (8) Sweeney, J. J.; Burnham, A. K.; Braun, R. L. Am. Assoc. Pet. Geol. Bull. 1987, 71, 967-985. (9) Sweeney, J. J. Org. Geochem. 1988, 13, 199-205.
© 1997 American Chemical Society
898 Energy & Fuels, Vol. 11, No. 4, 1997
Larsen and Li
Table 1. Elemental Analyses of Uinta Basin Maturation Series of Oil Shales (Total Sample Basis)
b
sample
depth (m)
% maturation
total %C
total %H
total %N
total %S
%org C
Government 33-4a,b Brotherson 1-23B4-Aa,b Brotherson 1-23B4-Ba,b Brotherson 1-3B4b Christensen 1-33A5b
1630 2560 2780 3260 3610
