Geochemical Significance of n-Alkane Compositional-Trait Variations in Coals
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- 作者：Charles R. Nelson ; Wenbao Li ; Iulia M. Lazar ; Kristine H. Larson ; Abdul Malik ; and Milton L. Lee
- 刊名：Energy & Fuels
- 出版年：1998
- 出版时间：March 1998
- 年：1998
- 卷：12
- 期：2
- 页码：277 - 283
- 全文大小：195K
- 年卷期：v.12,no.2(March 1998)
- ISSN：1520-5029
文摘
The compositional traits of C9-34n-alkanes were measured in supercritical CO2extracts from14 U.S. coals of varied geologic ages and wide thermal maturity range(lignite through low-volatile bituminous). The analysis data exhibit no unique componentdepletion pattern signaturesdiagnostic of known types of postgeneration physical, chemical, ormicrobiological degradationprocesses that commonly affect crude oil in sedimentary rocks. TheC9-34 n-alkanes inPaleoceneand Upper Cretaceous age coals exhibit bimodal carbon-numberdistribution profiles that stronglyresemble those of the biogenic n-alkanoic acids present inbrown coals. The compositional traitsimilarities between these n-alkanes andn-alkanoic acids and the covariance of the bulkcoalorganic matter atomic oxygen-to-carbon (O/C) ratios andcarbon-preference index (CPI) valuesoffer tangible evidence for the existence of a genetic linkage betweenthese two series ofcompounds. Our analysis results indicate that theC2-5 alkanes andC6+ hydrocarbons in coalsattain their maximum abundances over the thermal maturity interval from0.50 to 0.72% R0,which, in turn, strongly suggests that these two groups of compoundsare formed concurrentlyby similar overall reaction processes during coal maturation. Thecompositional traits of theC4-5 alkanes andC9-34 n-alkanes incoals appear to uniquely mimic those of the alkaneproductsformed by mineral-catalyzed defunctionalization and cracking ofn-alkanoic acids, which suggeststhat mineral catalysis rather than temperature-controlled thermolysismay be a critical variablecontrolling the formation and compositional traits of natural gas andC9+ n-alkanes during coalmaturation. These mechanistic insights should be useful to thoseseeking to formulate improvedgeochemical models for predicting hydrocarbon evolution during coalmaturation.