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Geochemical Significance of n-Alkane Compositional-Trait Variations in Coals Charles R. Nelson*,† Basic Research Group, Gas Research Institute, Chicago, Illinois 60631
Wenbao Li, Iulia M. Lazar, Kristine H. Larson, Abdul Malik, and Milton L. Lee*,‡ Department of Chemistry, Brigham Young University, Provo, Utah 84602 Received July 7, 1997
The compositional traits of C9-34 n-alkanes were measured in supercritical CO2 extracts from 14 U.S. coals of varied geologic ages and wide thermal maturity range (lignite through lowvolatile bituminous). The analysis data exhibit no unique component depletion pattern signatures diagnostic of known types of postgeneration physical, chemical, or microbiological degradation processes that commonly affect crude oil in sedimentary rocks. The C9-34 n-alkanes in Paleocene and Upper Cretaceous age coals exhibit bimodal carbon-number distribution profiles that strongly resemble those of the biogenic n-alkanoic acids present in brown coals. The compositional trait similarities between these n-alkanes and n-alkanoic acids and the covariance of the bulk coal organic matter atomic oxygen-to-carbon (O/C) ratios and carbon-preference index (CPI) values offer tangible evidence for the existence of a genetic linkage between these two series of compounds. Our analysis results indicate that the C2-5 alkanes and C6+ hydrocarbons in coals attain their maximum abundances over the thermal maturity interval from 0.50 to 0.72% R0, which, in turn, strongly suggests that these two groups of compounds are formed concurrently by similar overall reaction processes during coal maturation. The compositional traits of the C4-5 alkanes and C9-34 n-alkanes in coals appear to uniquely mimic those of the alkane products formed by mineral-catalyzed defunctionalization and cracking of n-alkanoic acids, which suggests that mineral catalysis rather than temperature-controlled thermolysis may be a critical variable controlling the formation and compositional traits of natural gas and C9+ n-alkanes during coal maturation. These mechanistic insights should be useful to those seeking to formulate improved geochemical models for predicting hydrocarbon evolution during coal maturation.
Introduction Coals are organic matter-rich sedimentary rocks whose geological evolution generates a complex hydrocarbon mixture composed of alkane gases and crude oil type constituents.1-8 Geochemical models of n-alkane evolution during coal maturation implicitly assume a preserved genetic linkage between their observed compositional traits and those of their presumed biogenic * To whom correspondence should be addressed. † E-mail:
[email protected]. ‡ E-mail:
[email protected]. (1) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1967, 31, 2389-2397. (2) Brooks, J. D.; Gould, K.; Smith, J. W. Nature 1969, 222, 257259. (3) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969, 33, 1183-1194. (4) Bartle, K. D.; Jones, D. W.; Pakdel, H.; Snape, C. E.; C¸ alimli, A.; Olcay, A.; Tugrul, T. Nature 1979, 277, 284-287. (5) Hunt, J. M. Petroleum Geochemistry and Geology; Freeman: San Francisco, 1979. (6) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1984. (7) Rice, D. D.; Clayton, J. L.; Pawlewicz, M. J. Int. J. Coal Geol. 1989, 13, 597-626. (8) Clayton, J. L. In Hydrocarbons from Coal; Law, B. E., Rice, D. D., Eds.; American Association of Petroleum Geologists: Tulsa, 1993; pp 185-201.
precursors.2-6 This assumption may not be universally applicable, since n-alkane compositional traits are vulnerable to alteration by postgeneration physical, chemical, and microbiological degradation processes.5,6 The possible occurrence of such compositional trait alteration was recently inferred from a depleted C15+ n-alkane anomaly in solvent extracts obtained from San Juan Basin Fruitland Formation coal.9-11 One way to substantiate this interpretation is to demonstrate concurrent depletion of light oil (C6-14) n-alkanes.5,6 However, during the solvent extraction of hydrocarbons from coals and other sedimentary rocks, the detection and quantification of C6-14 hydrocarbons generally suffer because of their preferential susceptibility to volatility fractionation or loss.6,12 In this study, coal samples from various locations and different geologic ages were extracted using neat supercritical CO2. This technique allows the recovery of (9) Clayton, J. L.; Rice, D. D.; Michael, G. E. Org. Geochem. 1991, 17, 735-742. (10) Michael, G. E.; Anders, D. E.; Law, B. E. Org. Geochem. 1993, 20, 475-498. (11) Scott, A. R.; Kaiser, W. R.; Ayers, W. B., Jr. Am. Assoc. Pet. Geol. Bull. 1994, 78, 1186-1209. (12) Hunt, J. M. Science 1984, 226, 1265-1270.
S0887-0624(97)00112-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/24/1998
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Nelson et al.
Table 1. Coal Sample Location and Analytical Data
sample
location (county, state)
age, rankg
Vitrinite reflectance (% R0)
Beulah-Zapa Wyodak-Andersona Illinois No. 6a Blind Canyona Pittsburgh No. 8a Intermediate Fruitlandb Lewiston-Stocktona Basal Fruitlandb Basal Fruitlandc Upper Freeporta Pottsvilled Pocahontas No. 3a Pottsvillee Pottsvillef
Mercer, ND Campbell, WY St. Clair, IL Emery, UT Greene, PA La Plata, CO Logan, WV La Plata, CO La Plata, CO Indiana, PA Jefferson, AL Buchanan, VA Jefferson, AL Jefferson, AL
Paleocene, lig Paleocene, sub Pennsylvanian, hvb Cretaceous, hvb Pennsylvanian, hvb Cretaceous, hvb Pennsylvanian, hvb Cretaceous, hvb Cretaceous, hvb Pennsylvanian, mvb Pennsylvanian, mvb Pennsylvanian, lvb Pennsylvanian, mvb Pennsylvanian, mvb
0.28 0.31 0.46 0.50 0.72 0.72 0.77 0.79 0.86 0.99 1.28 1.42 1.43 1.46
a Premium coal sample from Argonne National Laboratory. b Valencia Canyon Southern Ute No. 32-1 well, San Juan Basin (32 T33N R11W). c Southern Ute Tribal H-1 well, San Juan Basin (18 T32N R10W). d Pratt seam, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). e Mary Lee seam, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). f Black Creek seam, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). g lig ) lignite; sub ) subbituminous; hvb ) high volatile bituminous; mvb ) medium volatile bituminous; lvb ) low volatile bituminous.
Figure 2. Supercritical CO2 extract yields as a function of the coal thermal maturity (vitrinite reflectance, % R0). Figure 1. Dynamic supercritical CO2-extraction apparatus.
C6-14 hydrocarbons without subsequent loss (evaporation) during the removal of the extraction agent. Experimental Section The coals evaluated in this study came from sediments ranging in geological age from Paleocene to Pennsylvanian (see Table 1) and from subsurface deposits unaffected by natural weathering processes13 and ranged in thermal maturity from lignite through low-volatile bituminous rank (74.1% to 91.8% carbon, dry, mineral matter free basis). Proximate, ultimate, carbon isotopic, vitrinite reflectance, and maceral analyses were performed according to ASTM or other standard procedures by commercial testing or university laboratories. Coal samples were dynamically extracted with neat supercritical CO2 (2 h, 120 °C, and 200 atm) using the apparatus shown in Figure 1 and described in detail elsewhere.14 Approximately 1 g of powdered coal (ground to pass a 200-mesh screen) was placed in an extraction cell (3.5-mL volume) and sandwiched between silanized, washed glass beads of 250-µm outer diameter to minimize dead volume. A Lee Scientific Series 600 high-pressure syringe pump was used to introduce SFC-grade CO2 into the extraction cell. The extraction was essentially complete after 1 h, as evidenced by a negligible change in yield with longer extraction time. The extracted analytes were collected in 0.5 mL of cold (-5 °C) carbon disulfide. Soxhlet solvent extraction and chemical class fractionation procedures were used to isolate the C15+ hydrocarbon compo(13) Nelson, C. R. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Elsevier: New York, 1989; pp 1-32. (14) Li, W.; Lazar, I. M.; Wan, Y. J.; Butala, S. J.; Shen, Y.; Malik, A.; Lee, M. L. Energy Fuels 1997, 11, 945-950.
nents.15,16 The supercritical fluid and solvent-extracted analytes were analyzed by gas chromatography/mass spectrometry using a Hewlett-Packard 5890 gas chromatograph and a Hewlett-Packard 5970 mass-selective detector. The extract compound quantitation, identification, and carbon-preference index (CPI) analysis were performed using the procedures described in detail previously.15-17
Results and Discussion Bulk Extracts. Gross compositions for the supercritical CO2-extracted C6+ hydrocarbons from 14 U.S. coal samples are shown in Figure 2. The coal-sample origins are described in Table 1. The C6+ hydrocarbon content ranges between 0.02 and 0.27 wt % of the bulk coal organic matter. The predominant compounds include a homologous series of C9-C24-34 n-alkanes, C6+ branched and alkylcyclic alkanes, alkylbenzenes, and alkylnaphthalenes. The aliphatic-to-aromatic ratios of the extracts typically ranged from 3 to 8. Numerous factors can potentially affect the content and compositional traits of petroleum-like hydrocarbons in coals. It is generally acknowledged that the hydrogen content of the bulk coal organic matter and the abundances of different macerals (vitrinite, liptinite, and inertinite), in particular the abundance of the hydrogen(15) Chang, H.-C. K.; Nishioka, M.; Bartle, K. D.; Wise, S. A.; Bayona, J. M.; Markides, K. E.; Lee, M. L. Fuel 1988, 67, 45-57. (16) Carlson, R. E.; Critchfield, S.; Vorkink, W. P.; Dong, J.-Z.; Pugmire, R. J.; Lee, M. L.; Zhang, Y.; Shabtai, J.; Bartle, K. D. Fuel 1992, 71, 19-29. (17) Garcia-Gonzales, M.; Surdam, R. C.; Lee, M. L. Am. Assoc. Pet. Geol. Bull. 1997, 81, 62-81.
Significance of Variation in Coals
rich maceral liptinite, exert strong controls on the ability of coals to generate oil and gas during thermal maturation.18 However, both pyrolysis and field studies indicate that no simple, universally applicable correlation exists between the atomic hydrogen-to-carbon (H/C) ratio or the maceral composition of coals and the amount and compositional traits of oil products except when the atomic H/C ratio is greater than about 0.9 or the liptinite content is greater than 20-25%.18-25 The suite of coals studied here have atomic H/C ratios of 1.0) of the C4-5 alkane branched-to-normal-chain ratios in some of these reservoir gas samples could be the result, at least in part, of clay catalyst selectivity effects. Methane δ13C Values. The stable carbon isotope values of the methane in natural gas vary greatly depending, in part, upon whether the methane was formed by biogenic or thermogenic reactions.5,7 Methane δ13C values lighter (i.e., more negative) than -55% relative to the carbon isotopic standard Peedee Belemnite (PDB) are commonly regarded as being derived from biogenic reactions, whereas more positive values are regarded as being derived from thermogenic reactions.5,7 The stable carbon isotope values for the methane in natural gas produced from Fruitland Formation coal seam reservoirs (δ13C1 ) -46.6 to -40.5% vs PDB)7,11 vary somewhat irregularly as a function of coal maturation, which has been interpreted as being due, at least in part, to bacterial modification of the methane isotopic signature through the formation and (51) Saxby, J. D.; Bennett, A. J. R.; Corcoran, J. F.; Lambert, D. E.; Riley, K. W. Org. Geochem. 1986, 9, 69-81. (52) Tang, Y.; Jenden, P. D.; Nigrini, A.; Teerman, S. C. Energy Fuels 1996, 10, 659-671. (53) Mango, F. D. Org. Geochem. 1996, 24, 977-984.
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tion and mixing to explain the stable carbon isotope properties of the methane in natural gas produced from Fruitland Formation coal seam reservoirs. Conclusions
Figure 9. Variation of thermogenic methane stable carbon isotope composition as a function of progressively increasing peat thermal maturity (data from ref 50).
mixing of isotopically light biogenic methane with isotopically heavier thermogenic methane.11 This interpretation is based, in part, on the assumption that the observed δ13C1-value irregularity is a compositionaltrait anomaly and that thermogenic δ13C1 values should covary in a unidirectional way with the thermal maturity of the coal.11 Peat is the organic matter precursor of humic coals.6 The thermogenic methane formed by closed-system pyrolysis of dry peat exhibits a bimodal stable carbon isotope evolution profile (see Figure 9).50 Under lowthermal-stress conditions (5000 h at 100 °C) the thermogenic methane is isotopically light (δ13C1 ) -68% vs PDB) but becomes progressively heavier (more positive δ13C1 values) with increasing thermal maturation (lower atomic H/C ratios in Figure 9). The isotopic value of the initially formed thermogenic methane (δ13C1 ) -68% vs PDB) is significant from a geologic interpretation perspective, since it falls within the range of values commonly regarded as signifying a biogenic origin. At higher levels of thermal maturation there is a reversal of the δ13C1-evolution trend due to dilution by isotopically lighter thermogenic methane. A similar bimodal δ13C1-evolution profile is observed during coal pyrolysis.54 The δ13C1 values for two natural gas samples from Fruitland Formation coal seam reservoirs are plotted next to the bimodal δ13C1-evolution profile for peat shown in Figure 9. This bimodal δ13C1-evolution profile is significant from a geologic interpretation perspective, since it clearly indicates that thermogenic δ13C1 values do not covary in a unidirectional way with progressive coal maturation. Thus, it is not necessary to invoke the occurrence of isotopically light biogenic methane forma(54) Friedrich, H.-U.; Juntgen, H. Adv. Org. Geochem. 1971, 639646.
The compositional-trait data for the C9-34 n-alkanes in the suite of 14 U.S. coals studied here exhibit no unique component depletion pattern signatures diagnostic of known types of postgeneration physical, chemical, or microbiological degradation processes that commonly affect crude oil in sedimentary rocks. Although our results question both the occurrence and importance of postgeneration compositional-trait alteration of nalkanes in coals, they do not necessarily imply that these processes can never occur. However, before the compositional-trait alteration process occurrence is invoked, it is important to first determine whether there is indeed a compositional-trait anomaly to explain. The reliable detection of such compositional-trait alteration process occurrence requires a very careful analysis, which can be greatly facilitated by supercritical CO2 extraction and detailed analysis of the C6+ hydrocarbons. Hydrocarbon evolution during organic-matter maturation in coals and other sedimentary rocks is clearly a complex process. Attempts to formulate geochemical models for the hydrocarbon-evolution process are complicated by the problem of an excessive number of variables. The C2-5 alkanes and C6+ hydrocarbons attain their maximum abundances over the thermal maturity interval from 0.50 to 0.72% R0, which, in turn, strongly suggests that these two groups of compounds are formed concurrently by similar overall reaction processes during coal maturation. The compositional traits of C4-5 alkanes and C9-34 n-alkanes in coals appear to uniquely mimic those of the alkane products formed by mineral-catalyzed defunctionalization and cracking of n-alkanoic acids. Thus, mineral catalysis merits consideration as a master variable capable of integrating the many diverse factors that can affect the formation and compositional traits of natural gas and C9+ n-alkanes during coal maturation. These mechanistic insights should be useful to those seeking to formulate improved geochemical models for predicting hydrocarbon evolution during coal maturation. Acknowledgment. This research was funded by the Gas Research Institute, Contract Nos. 5088-260-1640 and 5091-260-2239. EF970112K
