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Energy & Fuels 2005, 19, 240-250
Assessment of Thermal Evolution of Kerogen Geopolymers with Their Structural Parameters Measured by Solid-State 13C NMR Spectroscopy Zhibin Wei,*,† Xiuxiang Gao,‡,§ Dajiang Zhang,‡,§ and Jiang Da‡ Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305-2115, Laboratory Center, Research Institute of Petroleum Exploration and Development, PetroChina, P.O. Box 910, Beijing, 100083, People’s Republic of China, and Key Laboratory of Petroleum Geochemistry of CNPC, P.O. Box 910, Beijing, 100083, People’s Republic of China Received June 17, 2004. Revised Manuscript Received November 1, 2004
The sidebands in the solid-state 13C NMR spectra of 13 polycyclic aromatic hydrocarbon compounds associated with kerogen structure were suppressed with cross polarization (CP), magicangle spinning (MAS), and total sideband suppression (TOSS). The chemical shift values of these model compounds were obtained under various chemical circumstances, which were subsequently used to determine the chemical shifts of aliphatic and aromatic carbons in kerogen structure via CP/MAS/TOSS 13C NMR measurements. Dipolar dephasing (DD) was used to obtain the spectra of nonprotonated carbon, discriminating protonated and nonprotonated carbons in the aromatic cluster. Consequently, the structural parameters for different carbons were characterized. Highresolution solid-state 13C NMR measurements were conducted on a suite of kerogen macromolecules isolated from source rocks originated from the Kuqa depression, Awati River, and Kapusaliang River of the Tarim Basin in northwestern China to develop an NMR-associated method for monitoring the thermal alteration of kerogens. These rocks covered a broad maturity range, as indicated by the Ro values of 0.52%-1.81%. Our results suggest that the aromaticity (fa) and aromatic cluster size (χb) are effective parameters for assessment of the thermal evolution of kerogens that resulted from the geological heating process undergone in the ancient sediments.
1. Introduction Over the last several decades, the rapid development of NMR techniques has led to extensive applications of high-resolution solid-state NMR spectroscopy in the study of fossil fuels, soils, and humic substances.1-4 Bartuska et al.5 first reported high-resolution solid-state 13C NMR spectra of coals. Solid-state NMR spectroscopy was then applied to characterize the organic matter in oil shales, humic substances, kerogens, and their precursor biopolymers. Recently, great achievements have been made in the investigation of hydrocarbon-generating potential, structural evolution, organic carbon composition, and oil- and gas-generating mechanisms of coals, oil shales, and kerogens.6-9 It has become one of * Corresponding author. Telephone:+1-650-723-9057. Fax: +1-650723-8489. E-mail address:
[email protected]. † Stanford University. ‡ PetroChina. § Key Laboratory of Petroleum Geochemistry of CNPC. (1) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 1, 173-181. (2) Mann, A. L.; Patience, R. L.; Poplett, I. J. F. Geochim. Cosmochim. Acta 1991, 55, 2259-2268. (3) Bharati, S.; Patience, R. L.; Larter, S. R.; Standen, G.; Poplett, J. F. Org. Geochem. 1995, 23 (11-12), 1043-1058. (4) Dria, K. J.; Sachleben, J. R.; Hatcher, P. G. J. Environ. Qual. 2002, 31, 393-401. (5) Bartuska, V. J.; Maciel, G. E.; Schaefer, J; Stejskal, E. O. Fuel 1977, 56, 354-357. (6) Boucher, R. J.; Standen, G.; Patience, R. L.; Eglinton, G. Org. Geochem. 1990, 16, 951-958.
the most powerful research tools to elucidate the chemical structures of complex macromolecules such as kerogen. As the precursor of petroleum hydrocarbons, kerogen has long drawn more attention from organic geochemists and is regarded as an important analytical object in petroleum exploration. The kerogen macromolecule naturally occurs in ancient sediments, because of the polymerization of organic matter from biomass such as plants, algae, and bacteria.10 The organic components are kerogen-dominated in source rocks and other sediments, thus leading to the geochemical and economic significance of kerogen. Most importantly, the structure of kerogen directly influences the conversion behavior of source rocks. This has led to many attempts to unravel its chemical structure using numerous analytical methods, including pyrolysis, chromatography, spectroscopy, and elemental analysis. However, none of these techniques has yielded any detailed information about kerogen structure.11 Fortunately, the advent of solid-state NMR techniques makes possible the recogni(7) Wilson, M. A.; Fargue, E. L.; Gizachew, D. Solid State 13C NMR for Characterizing Source Rocks. APEA J. 1994, 210-215. (8) Wang, Z.; Cheng, K.; Zhao, C.; Lu, Q. Chin. Sci. Bull. 1997, 42 (6), 478-481. (In Chin.) (9) Ge´linas, Y.; Baldock, J. A.; Hedges, J. I. Org. Geochem. 2001, 32, 677-693. (10) Tissot, B. P.; Welte, D. H. Kerogen: Composition and Classification. In Petroleum Formation and Occurrence, 2nd ed.; Tissot, B. P., Welte, D. H., Eds.; Springer-Verlag: Berlin, 1984; pp 131-159.
10.1021/ef0498566 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/29/2004
Thermal Evolution of Kerogen Geopolymers
tion of the chemical structure of kerogen. This may provide geologists with a novel insight into the events in geological history, particularly, the thermal evolution of organic matter and source rocks in ancient sediments. During the NMR measurements, important structural information about the carbon skeletons and functional groups of kerogen can be determined directly from the resultant NMR spectra without any destructive effects on the original samples. With the development and improvement of solid-state 13C NMR techniques including cross polarization (CP), magic-angle spinning (MAS), total sideband suppression (TOSS), and dipolar dephasing (DD), further advances have been made, in terms of the structure, properties, evolution, and petroleum generation of kerogens by many researchers,8,12-14 providing further understanding of kerogen structure and the role of kerogen in oil and gas generation. Furthermore, various parameters regarding the chemical structures of kerogen can be obtained using solidstate 13C NMR. This allows exploration of the thermal evolution of kerogens, and assessment of the thermal maturity of source rocks, because chemical compositions and structural features of kerogen change systematically throughout its evolution after geological heating in the subsurface. Of particular interest is the aromatic cluster size (χb) of kerogen, which was adopted as a maturity parameter of source rocks by Wang and Cheng.15 The parameter χb was determined to have a strong linear relationship with vitrinite reflectance at different evolution stages of carbonate kerogen, although some uncertainties still remain regarding whether lithology changes affect its value and whether this parameter is a reliable maturity marker. While solid-state 13C NMR spectroscopy provides a potential route to quantify chemical information in samples, some inherent constraints exist.16 The interpretation of a recorded 13C NMR spectrum essentially relies on chemical shift (δ) values, because the spectrum must be subdivided into regions, each of which is assumed to be associated with a specific type of carbon structure group.16 Many researchers have studied these values of various carbon functional groups in fossil fuels using 13C NMR techniques and have assigned the corresponding chemical shifts for different carbon types.12,17,18 However, the given δ values for diverse carbons in kerogen, to some degree, are still in dispute, because kerogen is a mixture of various maceral components, resulting in overlapped peaks with a wide range of chemical values of aliphatic and aromatic carbons. Because the structural parameters of kerogen are derived from the proportions of each structural (11) Miknis, F. P. Methods of Oils Shale Analysis. In Composition, Geochemistry and Conversion of Oil Shales, Snape, C., Ed.; NATO ASI Series, Series C: Mathematical and Physical Sciences, Vol. 455; Kluwer: Dordrecht, The Netherlands, 1995; pp 191-209. (12) Wilson, M. A.; Batts, B. D.; Hatcher, P. G. Energy Fuels 1988, 2, 668-672. (13) Axelson, D. E. Fuel 1987, 66, 195-199. (14) Petsch, S. T.; Smernik, R. J.; Eglinton, T. I.; Oades, J. M. Geochim. Cosmochim. Acta 1992, 65 (12), 1867-1882. (15) Wang, Z.; Cheng, K. Study on the Thermal Evolution Degree of Source Rocks Developed in Early Paleozoic or Older Eras. In Organic Geochemistry: Development and Applications to Energy, Climate, Environment and Human History; Grimalt, J. O., Dorronsoro, C., Eds.; The Basque Country: Spain, 1995; pp 478-480. (16) Cookson, D. J.; Smith, B. E. Energy Fuels 1987, 1, 111-120. (17) Trewhella, M. J.; Poplett, J. F.; Grint, A. Fuel 1986, 65, 541546. (18) Lille, U.; Heinmaa, I.; Pehk, T. Fuel 2003, 82, 799-804.
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carbon type, the accuracy of the assignment of chemical shifts undoubtedly has a considerable effect on the utility of these parameters. All of these will have an enormous impact on the assessment of the thermal maturity of source rocks and may distort petroleum resource evaluation. This paper intends (i) to use high-resolution solidstate 13C NMR to determine the accurate chemical shift values of different structural carbons in kerogen, using several pure organic compounds with a variety of chemical structures; (ii) to examine the differences, if any, in aromatic cluster size between kerogens isolated from shales and carbonates; and (iii) to assess the thermal evolution of kerogen dispersed in ancient sediments, using calculated structural parameters from 13C NMR spectra of kerogen. 2. Experimental Section 2.1. Geochemical Analyses of Samples. The 18 shale samples in this study were obtained from core drills and outcrop in the Kuqa Depression, the Awati River, and the Kapusliang River of the Tarim Basin in northwestern China. Geochemical analyses include Rock-Eval pyrolysis, elemental analysis, and vitrinite reflectance measurements. The sample set covers a broad range of maturity levels, as indicated by vitrinite reflectance data (Ro ) 0.52%-1.81%). The sampling depths for the cores are given in Table 1. Rock-Eval pyrolysis was conducted on ca. 30 mg of finely ground whole rock. Table 1 shows the obtained results of the Rock-Eval pyrolysis. Total organic carbon (TOC) values range from 0.46 wt % to 6.7 wt %. The measured Tmax values are in the range of 438-572 °C. Together with the production index (PI), this data may be used to roughly reflect the thermal maturity of source rocks.19 Table 1 shows that most samples have low hydrogen index (HI) values. The elemental and Rock-Eval data suggest that Type II and Type III kerogens for the studied samples consist mainly of remains of algal and higher plants. 2.2. Preparation of Kerogen Concentrates. To obtain kerogen concentrations of 300-500 mg for CP/MAS/TOSS 13C NMR measurements, typically 30-100 g of each shale sample was used, depending on its TOC. The 18 kerogen isolates selected for the NMR analyses were prepared as follows. The core and surface shales were first dried and ground to
