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For comparing the oil- and gas-generation potential of humic coals, two immature subbituminous coals [Australia Gippsland, an oil-prone Eocene coal (Ro = 0.37%, HI = 237.5), and a gas-prone Rocky Mountain Upper Cretaceous coal (Ro = 0.45%, HI = 110.5)] as well as an exinite-poor, vitrinite-dominant, Gulf Coast Wilcox lignite (Ro = 0.32%, HI = 183.7) were selected for dry pyrolysis at 300 degrees C from 2 to 1000 hr. The results indicate that Australia Gippsland Eocene coal (GEC) generated four times as much liquid hydrocarbon, i.e., C[12+]-saturated and aromatic hydrocarbons, as the Rocky Mountain coal (RMC), whereas both coals have the same potential for generation of gaseous hydrocarbons. Wilcox lignite (WL) generated as much liquid hydrocarbon as GEC. The two coals (GE and RMC) and lignite (WL) exhibit similar H/C ratios, have high pristane/phytane ratios of 5.0, 4.5, and 5.1, respectively, and (sigma)C values of -25.7, -24.9, and -25.5 parts per mil in RMC, GEC, and WL, respectively.
The study suggests that the relative abundances of various macerals (exinite, vitrinite, and inertinite) are not critical to the hydrocarbon-generating potential. Rather, the amount of dispersed algae and bacterial material (unidentified) in the coal probably is more significant. Depositional environment of the source organic material also may be important because it could control preservation of organic constituents. Because of the small stable isotope variations in C, H, N in the original coal kerogen as well as residual kerogen after pyrolysis of the three coals, it seems unlikely that isotopic composition can characterize coal depositional environments.
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