Anal. Chem. 2007, 79, 5642-5650
Pyrolysis Comprehensive Two-Dimensional Gas Chromatography Study of Petroleum Source Rock Frank Cheng-Yu Wang* and Clifford C. Walters
Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801
Detailed compositional analyses of sedimentary organic matter can provide information on its biotic input, environment of deposition, and level of thermal maturation. Pyrolysis-gas chromatography (py-GC), often coupled with a mass spectrometer (py-GC/MS), is one technique used to provide this information. New developments in comprehensive two-dimensional gas chromatography (GC×GC or 2D-GC), coupled with pyrolysis (py-GC×GC), offer the prospect of providing more complete and quantitative compositional information of complex organic solids, such as kerogen and coals. This study will describe applications of pyrolysis-GC×GC to the characterization of petroleum source rocks using flame ionization detector (FID) and sulfur chemiluminescence detector (SCD). In the hydrocarbon analysis by FID, paraffins, naphthenes, and aromatics form distinct two-dimensional separated groups. In the analysis with SCD, sulfur-containing compounds can be distinguished as different classes, such as mercaptans, sulfides, thiophenes, benzothiophenes, and dibenzothiophenes. Single components or summed bands of homologous components can be analyzed qualitatively and quantitatively. With these detailed molecular fingerprints, the relations between kerogen composition and its biotic input, environment of deposition, and thermal maturation may be better understood. Petroleum is generated from organic-rich sedimentary rocks. This organic matter is a mixture of preserved biopolymers and altered biochemicals cross-linked into a complex organic solid termed kerogen.1,2 Knowledge of the amount of kerogen, its generative potential, and the quality of petroleum it can produce when heated (either naturally in geologic basins or artificially in the laboratory) is an essential element in developing and risking exploration and exploitation initiatives. A detailed compositional analysis of a source rock can provide information on the biotic input, the environment of deposition, and its level of thermal maturation. * To whom correspondence should be addressed. E-mail: Frank.C.Wang@ exxonmobil.com. (1) Tissot, B.; Welte, D. H. Petroleum Formation and Occurrence; SpringerVerlag: New York, 1984. (2) Peters, K. E.; Walters, C. C.; Moldowan, J. M. The Biomarker Guide, Biomarkers and Isotopes in Petroleum Exploration and Earth History, 2nd ed.; Cambridge University Press: New York, 2005; Vols. 1 and 2.
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Detailed compositional analysis of kerogens and coals includes both solid-state techniques (such as NMR,3-6 FT-IR,7,8 XPS,9,10 and X-ray analysis near-edge spectroscopy10-13) and degradation techniques (such as thermal pyrolysis14-17 or selective chemical bond cleavage reactions18-20) that break the kerogen into smaller fragments. Pyrolysis of source rock is run under several different conditions such as open-system high-temperature pyrolysis (∼300650 °C at rapid heating rates, e.g., 25 °C/min)21,22 and in closed systems pyrolysis (sealed tube heating at ∼275-350 °C for 2472 h), either in the absence23 or presence24 of additional water. The advantages of open-system pyrolysis techniques are easy sample preparation, controlled thermal treatment, and rapid quantitative analysis of major components by GC-FID, or qualitative analysis of minor components by GC/MS. (3) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Kwiatek, P. J.; Solum, M. S.; Hu, J. Z.; Pugmire, R. J. Energy Fuels 2002, 16, 1507-1515. (4) Suggate, R. P.; Dickinson, W. W. Int. J. Coal Geol. 2004, 57, 1-22. (5) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992, 6, 414-431. (6) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (7) Iglesias, M.; del Ro, J. C.; Laggoun-Defarge, F.; Cuesta, M. J.; Sua´rez-Ruiz, I. J. Anal. Appl. Pyrolysis 2002, 62, 1-34. (8) Lis, G. P.; Mastalerz, M.; Schimmelmann, A.; Lewan, M. D.; Stankiewicz, B. A. Org. Geochem. 2005, 36, 1533-1552. (9) Kelemen, S. R.; Rose, K. D.; Kwiatek, P. J. Appl. Surf. Sci. 1993, 64, 167174. (10) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Kwiatek, P. J.; Sansone, M.; Walters, C. C.; Cohen, A. D. Energy Fuels 2006, 20, 635-652. (11) Vairavamurthy, A.; Wang, S. Environ. Sci. Technol. 2002, 36, 3050-3056. (12) Wiltfong, R.; Mitra-Kirtley, S.; Mullins, O. C.; Andrews, B.; Fujisawa, G.; Larsen, J. W. Energy Fuels 2005, 19, 1971-1976. (13) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93-97. (14) Eglinton, T. I.; Larter, S. R.; Boon, J. J. J. Anal. Appl. Pyrolysis 1991, 20, 25-45. (15) Philp, R. P. In Natural and Laboratory-Simulated Thermal Geochemical Processes; Ikan, R., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003; pp 297-323. (16) Larter, S. R.; Horsfield, B. In Organic Geochemistry Principles and Applications; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993; pp 271-288. (17) Horsfield, B. In Advances in Petroleum Geochemistry Vol. 1; Brooks, J., Welte, D. H., Eds.; Academic Press: London, 1984; pp 247-292. (18) Michaelis, W.; Albrecht, P. Naturwissenschaften 1979, 66, 420-421. (19) Hatcher, P. G.; Nanny, M. A.; Minard, R. D.; Dible, S. D.; Carson, D. M. Org. Geochem. 1995, 23, 881-888. (20) Ho¨ld, I. M.; Schouten, S.; Van der Gaast, S. J.; Sinninghe Damste´, J. S. Chem. Geol. 2001, 172, 201-212. (21) Tegelaar, E. W.; Noble, R. A. Org. Geochem. 1994, 22, 543-574. (22) Behar, F.; Lewan, M. D.; Lorant, F.; Vandenbroucke, M. Org. Geochem. 2003, 34, 575-600. (23) Horsfield, B.; Dueppenbecker, S. J. J. Anal. Appl. Pyrolysis 1991, 20, 107123. (24) Lewan, M. D. Philos. Trans. R. Soc. London, Ser. A 1985, 315, 123-134. 10.1021/ac070166j CCC: $37.00
© 2007 American Chemical Society Published on Web 06/23/2007
In all analyses, complex peak coelutions limit the reliability of quantitative data. Although the qualitative detection and identification of pyrolysate compounds is routinely performed by coupled pyrolysis-gas chromatograph/mass spectrometer (py-GC/MS) systems, accurate quantitative analysis is difficult. Typically, these analyses are semiquantitative, at best, and are based on relative peak intensities of either individual ions or ion sums. Rarely are measurements taken that account for variations in detector response. New developments in comprehensive two-dimensional gas chromatography (GC×GC) offer the prospect of analyzing complex mixtures without requiring mass spectrometry.25-29 GC×GC may be considered a “continuous” heart-cutting form of a conventional single heart-cutting multidimensional GC that has been established for many years. GC×GC employs a single GC containing two separation columns of different selectivity connected in series. The first column is typically of conventional length, whereas the second column is short. A modulation unit, placed between the two columns, is key to accomplish this twodimensional separation by performing solute focusing from the first column and reinjection into the short, high-speed second column.28,30,31 The refocusing process during the modulation operation enhances sensitivity.25,29,31 The greater separation and enhanced sensitivity of the GC×GC technique forms a unique capability for the analysis of complex mixtures. When coupled with a universal FID or element-selective detectors, quantitative analysis of both major and minor components is possible as many coelution problems are eliminated. Unprecedented identification also is possible when GC×GC is coupled to a mass spectrometer.31,32 New visualization and data processing techniques have been developed to display and analyze the two-dimensional retention pattern, and the number of peaks that can be resolved and quantified in the GC×GC chromatogram have been dramatically increased.27,28,33,34 These advances enable GC×GC to become an excellent technique to analyze complex mixtures, such as natural crude oils and refined petroleum products. In this study, we investigate the coupling of pyrolysis-GC×GC (py-GC×GC) with FID and element-selective detectors to characterize kerogen found in immature petroleum source rocks. The advantages of this method are numerous as it affords the prospect of routine quantitative analysis of the pyrolysates through enhanced chromatographic separation as opposed to py-GC/MS where reproducible, semiquantitative analysis is difficult. Coupled (25) Adahchour, M.; Beens, J.; Vreuls, R. J. J.; Brinkman, U. A. T. TrAC, Trends Anal. Chem. 2006, 25, 438-454. (26) Bertsch, W. J. High Resolut. Chromatogr. 1999, 22, 647-665. (27) Dallu ¨ ge, J.; Beens, J.; Brinkman, U. A. T. J. Chromatogr., A 2003, 1000, 69-108. (28) Go´recki, T.; Panic, O.; Oldridge, N. J. Liq. Chromatogr. Relat. Technol. 2006, 29, 1077-1104. (29) Ong, R. C. Y.; Marriott, P. J. J. Chromatogr. Sci. 2002, 40, 276-291. (30) Pursch, M.; Sun, K.; Winniford, B.; Cortes, H.; Weber, A.; McCabe, T.; Luong, J. Anal. Bioanal. Chem. 2002, 373, 356-367. (31) Adahchour, M.; Beens, J.; Vreuls, R. J. J.; Brinkman, U. A. T. TrAC, Trends Anal. Chem. 2006, 25, 540-553. (32) van Deursen, M.; Beens, J.; Reijenga, J.; Lipman, P.; Cramers, C.; Blomberg, J. J. High Resolut. Chromatogr. 2000, 23, 507-510. (33) Hollingsworth, B. V.; Reichenbach, S. E.; Tao, Q.; Visvanathan, A. J. Chromatogr., A 2006, 1105, 51-58. (34) Reichenbach, S. E.; Ni, M.; Kottapalli, V.; Visvanathan, A. Chemom. Intell. Lab. Syst. 2004, 71, 107-120.
py-GC×GC systems are by design cheaper and more robust than py-GC/MS systems and offer the promise of automated data processing. EXPERIMENTAL SECTION Petroleum Source Rocks. The petroleum source rocks presented in this study are from the Green River (Uinta basin), Paradox (Paradox basin), and Monterey (Santa Maria basin) formations. They are representative of source rocks containing algal-rich (type I), a low-sulfur marine (type II), and a high-sulfur marine (type IIS) kerogen, respectively. Samples of Green River and Paradox shales were taken from interior, nonoxidized portions of rock outcrop; the Monterey formation samples are washed well cuttings. No further cleaning or processing was performed beyond course grinding immediately prior to analysis. Pyrolysis-Gas Chromatography/Mass Spectrometry. Pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) was performed using a CDS (Oxford, PA) Pyroprobe 1000 coupled to an Agilent 6890 gas chromatograph-7673 mass spectrometry detector (MSD) (Agilent Technology, Wilmington, DE). About 5-40 mg of ground whole rock was placed in the pyroprobe. A start signal begins data acquisition and heats the pyroprobe to 300 °C. Pyrolysis was initiated at 4 min, then ramping at 5 °C/ms to 610 °C, and held for 1 min with the pyrolysates trapped by liquid nitrogen at the head of the GC column. Cold trapping stops at 5.5 min. During this time, an initial oven temperature of 30 °C is held for 10 min and then ramped at 4 °C per min to 310 °C. Separation was performed using a J&W Scientific DB-5MS fusedsilica column (30 m × 0.25 mm i.d. × 0.25 µm film). Helium was used as a carrier gas at constant flow of 1.1 mL/min. The MS was operated using electron impact ionization (70 eV), and full scan data were acquired scanning from m/z 45 to m/z 450 at 1 scan/s. Pyrolysis-Comprehensive Two-Dimensional Gas Chromatography (py-GC×GC). A micro-oven type of pyrolyzer (Frontier Lab. model 2020iD, Quantum Analytics, Foster City, CA) was used in this study. A small piece of rock (approximately 2 mg) sample was deposited into a sample cup, which was then mounted on pyrolyzer connected to the injection port of a GC×GC. Pyrolysis occurred by dropping the sample cup through the micro-oven held at a calibrated temperature of 650 °C. The GC×GC system consists of an Agilent 6890 gas chromatograph (Agilent Technology, Wilmington, DE) configured with split/splitless inlet, columns, and detectors. Separation was achieved using a first-dimensional BPX-5 column (30 m, 0.25 mm i.d., 1.0 µm film) and a second-dimensional BPX-50 capillary column (3 m, 0.25 mm i.d., 0.25 µm film), both purchased from SGE Incorporated (Austin, TX). A dual jet thermal modulation assembly (Zoex Corp. Lincoln, NE), which uses liquid nitrogen for trapping, is placed between the first and the second dimension columns. The detection system contained both flame ionization detector (FID) (Agilent Technologies Inc.) and a sulfur chemiluminescence detector (SCD) (model 355, GE Analytical Inc., Boulder, CO). The FID and SCD setup and the analysis conditions were based on recommendations from the manufacturer specifications. The pyrolysis products (pyrolysates) were split 50:1 by the 300 °C injection. The oven temperature was ramped from 60 °C at 3 °C/min to 390 °C. Helium was used as a carrier gas at Analytical Chemistry, Vol. 79, No. 15, August 1, 2007
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Figure 1. (A) Conventional py-GC/MS TIC of Green River shale. The numbers in the figures are numbers of carbons in the molecules. (B) Py-GC×GC-FID chromatogram of Green River shale.
constant flow of 6.2 mL/min. The modulation period was 10 s. Chemstation (Agilent Technologies Inc.) was used for instrument control and data acquisition with the sampling rate for the detector set at 100 Hz. The total experiment and the data acquisition time were 110 min. After data acquisition, data were further processed for qualitative and quantitative analysis. The qualitative analysis was to convert data to a two-dimensional image that was processed by the program Transform (Research Systems Inc., Boulder, CO). The two-dimensional image was further treated by PhotoShop (Adobe System Inc., San Jose, CA) to generate publication-ready images. A proprietary 5644
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program developed in-house was used for the quantitative analysis. RESULTS AND DISCUSSION The utility of py-GC×GC is illustrated by the examination of five source rocks: a low-sulfur type I algal-kerogen from a lacustrine environment (Green River shale) and a series of type II marine kerogens with variable sulfur content (Oxford < lowsulfur Monterey < Paradox < high-sulfur Monterey). All samples are immature with respect to oil generation (
