Diterpanes, triterpanes, steranes and aromatic hydrocarbons in natural bitumens and pyrolysates from different mimic coals

Abstract

There is a significant difference in the distribution of terpanes in natural bitumen extracted from four coals identified as Rocky Mountain coal (RMC), Australian Gippsland Latrobe Eocene coal (GEC), Australian Gippsland Latrobe Cretaceous coal (GCC), and Texas Wilcox lignite (WL). Whereas pentacyclic triterpanes are dominant in GEC, GCC and WL, diterpanes strongly predominate in the bitumen of RMC. This indicates that resin is a more important constituent of RMC than in the other coals and releases the diterpenoids at an early stage of diagenesis. Furthermore, the composition of diterpanes is also different among these coals. For example, tricyclic diterpanes are the only diterpanes present in RMC, whereas tetracyclic and tricyclic diterpanes are both present in GEC and GCC and tetracyclic diterpanes are most abundant in GEC. However, diterpanes are nearly absent in WL. Surprisingly, the diterpenoid content is negligible in the pyrolysates of all coals. The sterane and triterpane distributions in the natural bitumen of coals are very different from those in pyrolysates. C 31 αβ 22R hopane predominates in the m/z 191 mass fragmentograms of the natural bitumens, whereas a homologous series of hopanes (C 27–C 35 ; except C 28) is present in the coal pyrolysates. C 29 steranes dominate in all of the coal bitumens as well as pyrolysates. C 27 steranes are absent in the natural bitumen, but are generated in pyrolysates during lengthy heating of coal kerogen. This indicates that formation of secondary steranes occurs by cracking the side chain. C 29 monoaromatic steroid hydrocarbons are abundant in the natural bitumen of the four coals. The m/z 253 distribution is similar for GEC, RMC and WL, but it differs from the more mature GGC, which has a lower content of 20R of both 5 α and 5β C 29 MA-steroids. Triaromatic steroid hydrocarbons are only present in GCC, whereas significant amounts of tetra- and triaromatic triterpenoid hydrocarbons are present in the other three coals (GEC, RMC and WL). The ratio of benzo(e)pyrene/perylene may be potentially useful as a maturity parameter because the amount of benzo(e)pyrene increases with thermal stress, whereas perylene decreases with heating time. Benzohopanes are widely present in four humic coals but become unstable after 10 h heating at 300°C.