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Energy & Fuels 1997, 11, 144-149
Analysis of Polycyclic Aromatic Hydrocarbons in Kerogens Using Two-Step Laser Mass Spectrometry Qiao Zhan,† Renato Zenobi,*,† Peter R. Buseck,‡ and Stan Teerman§ Department of Chemistry, ETH Zu¨ rich, Universita¨ tstrasse 16, CH-8092 Zu¨ rich, Switzerland, Departments of Geology and Chemistry/Biochemistry, Arizona State University, Tempe, Arizona 85287-1404, and Western Australian Petroleum Pty. Ltd., Perth, Western Australia 6001, Australia Received August 1, 1996. Revised Manuscript Received October 7, 1996X
Kerogen-bearing rocks from the Green River, Phosphoria, and Mowry formations were analyzed for polycyclic aromatic hydrocarbons (PAHs) using two-step laser mass spectrometry (L2MS). This method allows sensitive analyses to be carried out within minutes, and samples can be introduced into the instrument directly without pretreatment or processing. L2MS spectra were recorded from intact rocks and rock powder as well as from demineralized samples using a wellestablished acid digestion procedure to isolate the kerogen. The main findings were as follows: (i) Spectra from demineralized samples and from pulverized rocks look almost identical. This similarity shows that grinding is sufficient as a sample preparation method for L2MS. Conventional acid digestion does not alter the PAH compositions of these samples. (ii) For immature and marginally mature samples, demineralization and grinding induce a change in the mass spectra compared to data taken from fracture surfaces of intact rock. This preliminary study shows the potential usefulness of L2MS for direct chemical analysis of complex materials such as kerogen; it may be useful to distinguish among kerogens of different maturities.
Kerogen is naturally occurring, insoluble, organic matter that is of considerable interest because it can generate petroleum and gas. It is the oldest identifiable carbonaceous matter on Earth. Kerogen is generally of high molecular weight; it is widespread in sedimentary rocks and forms the bulk of the organic material in those rocks. It also occurs in certain meteorites and low-grade metamorphic rocks. Most kerogen formed from the residues of organisms and land-plant material that degraded into complex carbon-rich structures that include heteroatoms, mainly H, O, N, and S.1 Kerogens can be envisaged as physical mixtures chiefly composed of selectively preserved and sometimes partially altered, resistant biomacromolecules.2 With time and gradual heating, the carbonaceous compounds gradually lose small molecules such as CO2, H2O, and alkanes, and the remaining mass becomes progressively enriched in carbon. Ultimately, planar aromatic structures are formed that contain an increasing fraction of graphitic carbon and large polycyclic aromatic hydrocarbons (PAHs).1 The origin of PAHs in young kerogen (e.g., the marine sediments of Buzzards Bay, Massachusetts3 ) may be attributed to aromatization of steroids and triterpenoids, to various oxidation and reduction processes, and to polymeric precursors syn-
thesized by organisms such as fungi.4 The PAHs in kerogens in older rocks are more complex, comprising a larger range of alkyl derivatives than those in modern sediments.4 Knowing the PAH content in the sample helps in understanding geochemical cycles and diagenesis of geochemical samples, which are related to the kerogen’s oil-/gas- bearing potential.5-7 A classification into types I, II, III, and IV4,8 using so-called van Krevelen diagrams based on elemental analyses for atomic H, C, and O and the H/C and O/C ratios can be made for kerogens. According to this classification, young type I kerogens exhibit a H/C ratio >1.5. Type I kerogens are excellent potential oil sources. The H/C ratio of young type II kerogens is around 1.25, and young type III kerogens have a H/C ratio below 1. Type II kerogens are good potential oil sources, while type III kerogens are more likely to generate gas. Type IV kerogens contain little hydrogen and are essentially inert as a source of fuel. The older and more mature the kerogen sample, the lower the H/C and O/C ratios become. The type and maturity of a kerogen sample is also reflected in its molecular composition. For instance, increasingly large PAHs contain less and less hydrogen, with graphite being on the extreme end (H/C ratio ) 0). In the present work, we studied PAHs directly on the surfaces of freshly cleaved rocks as well as in rock
* Author to whom correspondence should be addressed (telephone +41-1-632 4376; fax +41-1-632 1292; e-mail
[email protected]). † ETH Zu ¨ rich. ‡ Arizona State University. § Western Australia Petroleum Pty. Ltd. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Durand, B. Kerogen, Insoluble Organic Matter from Sedimentary Rocks; Editions Technip: Paris, 1980; pp 13, 126, 143. (2) Tegelaar, E. W.; de Leeuw, J. W.; Derenne, S.; Largeau, C. Geochim. Cosmochim. Acta 1989, 53, 3103-3106. (3) Blumer, M.; Youngblood, W. W. Science 1975, 188, 53-55.
(4) Tisso, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1978; p 148. (5) Eglinton, T. I.; Douglas, A. G.; Rowland, S. J. Adv. Org. Geochem. 1987, 13, 655-663. (6) Chaffee, A. L.; Fookes, C. J. R. Org. Geochem. 1988, 12, 261271. (7) Shadle, L. J.; Seshadri, K. S.; Webb, D. L. Fuel Process. Technol. 1994, 37, 101-120. (8) Whelan, J. K.; Thompson-Rizer, C. L. Organic Geochemistry: Principles and Application; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993; pp 289-346.
Introduction
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PAHs in Kerogens
powders and kerogen isolates, collected in different locations and representing different depositional settings. We compared relative PAH signals from different thermal maturities in rocks of different ages. Analytical Procedures A variety of physical and chemical methods have been applied to study kerogens. Thermogravimetric and differential thermal analyses are often used to investigate their thermal degradation to compare samples from different origins.9 Petrographic identification of the individual macerals that make up a kerogen assemblage can be useful; however, only generalized chemical information can be inferred from such an analysis.1 Electron microscopy has also been used to study kerogen microstructures.10 Elemental analyses can be performed with several standard analytical methods. However, sample preparation is often difficult and time-consuming because of the need to first isolate the kerogen, generally done using acid dissolution of the host rock. Sample preparation steps such as grinding and drying may be necessary because of heterogeneity and hygroscopic characteristics. Information about the molecular composition of kerogens can be obtained with nuclear magnetic resonance (NMR) spectroscopy11,12 and with infrared spectroscopy.13 Rock-Eval pyrolysis, an open-flow pyrolysis technique that can be used to infer kerogen types, has become a standard for the evaluation of source rocks. Pyrolysis followed by chromatography14 and flash pyrolysis followed by gas chromatography/mass spectrometry with compound-specific isotope ratio monitoring are also powerful discriminatory tools for studying kerogens.15 Mass spectrometry has been applied to the study of the molecular composition of kerogens, for example, in the work of Zakett et al.16 More recently, matrix-assisted, laser desorption ionization (MALDI) mass spectroscopy was used by Li et al. to study high molecular weight constituents of kerogens.17 Peaks extending from 1000 u up to around 10 000 u were baseline-separated; heavier ions up to m/z ) 50 000 and even above were also observed. Although it is interesting that MALDI mass spectra of kerogens show these signals at very high mass, no molecular interpretation was given. Studies of PAHs in kerogen have been carried out using NMR,6 hydrous pyrolysis followed by chromatography,5,8 and negative chemical ionization-charge inversion MS-MS.16 Because of the low vapor pressures of higher molecular weight PAHs and their small concentration in natural samples, special extraction and concentration processes were often necessary. Reference compounds were employed as internal standards for identification and quantitation of kerogen constituents, for example, by comparing chromatographic retention times and relative peak areas. For such experiments,6-8 between tens of minutes and several hours per run is normally necessary. Analyses done using two-step laser mass spectrometry (L2MS),18,19 the method used in the present work, take only (9) Kalkreuth, W.; Macauley, G. Bull. Can. Pet. Geol. 1984, 32, 3851. (10) Buseck, P. R.; Huang, B. J.; Miner, B. Org. Geochem. 1988, 12, 221-234. (11) Barwise, A. J. G.; Mann, A. L.; Eglinton, G.; Gowar, A. P.; Wardroper, A. M. K.; Gutteridge, C. S. Org. Geochem. 1984, 6, 343349. (12) Premovic, P. I.; Jovanovic, Lj. S.; Michel, D. Appl. Spectrosc. 1992, 46, 1750-1752. (13) Ganz, H.; Kalkreuth, W. Fuel 1987, 66, 708-711. (14) Horsfield, B. Rev. Palaeobot. Paleontol. 1990, 65, 357-365. (15) Goni, M. A.; Eglinton, T. I. J. High Resolut. Chromatogr. 1994, 17, 476-488. (16) Zakett, D.; Ciupek, J. D.; Cooks, R. G. Anal. Chem. 1981, 53, 723-726. (17) Li, C. Z.; Herod, A. A.; John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Humphery, P.; Champman, J. R.; Rahman, M.; Kinghorn, R. R. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1994, 8, 823-828. (18) Zenobi, R.; Philippoz, J.-M.; Buseck, P. R.; Zare, R. N. Science 1989, 246, 1026-1029.
Energy & Fuels, Vol. 11, No. 1, 1997 145 several minutes, and nonvolatile compounds are easily accessible up to several thousand mass units. In L2MS, two consecutive laser pulses perform desorption and ionization of the analyte molecules. First, a pulsed infrared laser heats the sample surface. The minerals in sedimentary rocks, or other materials that strongly absorb the infrared laser, can be regarded as such a surface. In a rapid, laser-induced thermal desorption process, intact molecular adsorbates desorb from the surface. After a suitable time delay, typically 20 µs, the plume of desorbing molecules is intercepted with a pulse from a tunable ultraviolet (UV) laser. This leads to soft ionization of the desorbed species in a resonance-enhanced multiphoton ionization (REMPI) process.20 We use REMPI in its two-photon variety: a first photon induces a transition to an excited state only for molecules that resonantly absorb the laser wavelength. Then, a second photon ionizes the excited molecule. If the UV laser wavelength coincides with an absorption band of the analyte, this process is resonance enhanced and will be many orders of magnitude more efficient than nonresonant multiphoton ionization, giving rise to a strong optical selectivity. Finally, a time-of-flight (TOF) mass spectrometer is used for mass separation of the ions. The highly efficient ion production by REMPI and the high throughput of the TOF mass spectrometer result in the overall high sensitivity of L2MS. For example, a surface concentration of 5 × 10-6 monolayers of aniline deposited on silica was still detectable with L2MS in a test experiment,21 corresponding to an absolute sensitivity of 300 amol. PAHs are a class of compounds that are efficiently ionized by REMPI and that show virtually no fragmentation,20 yielding L2MS spectra that are dominated by parent ion peaks. Aliphatic compounds that are also present in kerogen are not ionized by REMPI because they lack a strong UV chromophore. Typically there is a one-to-one correspondence between the number of PAHs in a sample and the number of peaks in the L2MS spectrum, with the exception of isomers that are not separated by L2MS. Due to the optically selective detection, we can also avoid all chemical extraction methods that could contaminate or otherwise alter the sample. Therefore, it is possible to directly analyze minute quantities of selected PAHs in complex matrices such as sedimentary rocks without the need for prior sample demineralization, extraction, purification, separation, or preconcentration steps. Parts-per-million to parts-per-billion detection limits are reached for PAHs. However, the samples investigated here have higher PAH concentrations.
Experimental Section Samples. The samples used in this work (Table 1) are from four locations in the western United States. Initial geochemical characterization of the kerogens, obtained by standard techniques4,22 at Western Australian Petroleum, included the total organic carbon content (TOC) for whole rocks, the H/C ratio for kerogens isolated by acid digestion, and the RockEval maturation parameter. These data are also summarized in Table 1. The Permian Phosphoria sample is very thermally mature yet is organic rich. The original TOC was probably in the low 20% range, similar to the Little Sheep Creek Montana sample. The remaining organic matter consists of postmature (cooked) amorphous material that was once oil-prone and solid bitumen. The solid bitumen represents nonexpelled material. The vitrinite reflectance (R0) of this sample is approximately 2.25%. At this level of maturity, the Tmax value does not (19) Voumard, P.; Zhan, Q.; Zenobi, R. Rev. Sci. Instrum. 1993, 64, 2215-2220. (20) Lubman, D. M. Laser and Mass Spectrometry; Oxford University Press: New York, 1990; pp 353, 510. (21) Voumard, P.; Zhan, Q.; Zenobi, R. Chem. Phys. Lett. 1995, 239, 89-94. (22) Peter, K. E.; Moldowan, J. M. The Biomarker Guide, Interpreting Molecular Fossils in Petroleum and Ancient Sediments; Prentice Hall: Englewood Cliffs, NJ, 1993.
146 Energy & Fuels, Vol. 11, No. 1, 1997
Zhan et al. Table 1. Sample Descriptiona-c
CRC 47940-15 (Figure 1) well/location
Montpelier Hot Spring Mine, Idaho formation Phosphoria Meade Peak member geological age mid-Permian (250 million years) lithology phosphatic mustone maceral 75% postmature, composn amorphous oil-prone; 20% solid bitumen; ∼5% vitrinite and inertinite TOCd (wt %) 9.7 H/Cd 0.41 Tmaxd (°C) 566 kerogen type II
CRC 50786-121 (Figure 2)
CRC 47924-19 (Figure 3)
Colorado corehole 1, Colorado Green River unknown member mid-Eocene (50 million years) oil shale 95% amorphous oil-prone; 5% vitrinite + inertinite + structured liptinite
Little Sheep Creek 1, Montana Phosphoria Retort member mid-Permian (250 million years) phosphatic shale >95% amorphous oil-prone; tr of inertinite and vitrinite; tr of structured liptinite
7.9 1.56 441 I
19.9 1.31 429 II
CRC 49330-34 (Figure 4) Wyoming Mowry shale Cretaceous (100 million years) shale 55% amorphous oil-prone; 5% structured liptinite; 30% vitrinite; 10% inertinite 2.06 1.01 430 II/III
a Amorphous organic matter as used here consists of unstructured material as identified microscopically by analysis of the isolated kerogen. Depending on the original biogenic precursors and preservation, thermally immature amorphous organic matter can be oilprone, gas-prone, or inert. We used both geochemical (Hl, H/C) and petrographic properties (fluorescence and texture) to distinguish different types of amorphous material. b Organic components such as pollen, spore, cuticle, algae (structured liptinite), and coalified woody tissues (vitrinite) have distinct morphological properties that can be identified microscopically. These represent structured organic matter components with known properties. c We determined the amorphous content plus vitrinite, liptinite, inertinite, and solid bitumen by petrographic analysis (on isolated kerogen) using transmitted and reflected light. d TOC, total organic carbon; H/C, atomic hydrogen to carbon ratio; Tmax, Rock-Eval maturation parameter.
provide an acurate characterization of thermal maturity because of the samll and very wide S2 peak. However, both the Tmax and R0 generally suggest an overmature rock. From the atomic hydrogen-to-carbon ratio (H/C), one can infer the kerogen type by referring to the van Krevelen diagram1. The Permian Phosphoria sample consists of a homogeneous, marine, type II kerogen that contains homogeneous, oil-prone, marine, amorphous organic matter. The Cretaceous Mowry Shale represents a near-shore marine shale that contains a type II/III kerogen made up of a mixture of oil-prone, amorphous organic matter and gas-prone terrestrial components. The Eocene Green River sample contains a homogeneous, lacustrine, type I kerogen. The Phosphoria Meade Peak member is postmature, while the Mowry shale and Green River samples are immature and the Phosphoria Retort member is a marginally mature sample. Sample Preparation. We divided each sample into three parts: intact rock, rock powder (20-100 µm diameter), and an acid digest. The kerogen was isolated by a typical acid digestion procedure23 that included crushing the rock sample into fine particles (
