Sources of organic matter in the Eocene Maoming oil shale in SE China as shown by stepwise pyrolysis of asphaltene

摘要

Eocene Maoming oil shale was extracted and the precipitated asphaltene was subjected to stepwise pyrolysis carried out in seven temperature steps from 310 degrees C to 610 degrees C at 50 degrees C intervals. The pyrolyzate at 310 degrees C included n-alkanes and botryococcanes in a distribution similar to the extractable lipids. Long chain n-alkanols, fatty acids (FAs) and C-29 steroids were also relatively abundant. The pyrolyzate released at 360 degrees C was comparable with that at 310 degrees C, but had a relatively high content of a steradiene (C-29 Delta(3,5)-steradiene) and several C-29 unsaturated steroid ketones derived from the pyrolysis of sterols by loss of -OH and -H. FAs were not present in the pyrolyzate at 410 degrees C. Among the steroid ketones, the diones made a greater contribution, indicating that these ketones were formed as secondary products during the pyrolysis. A full suite of aliphatic hydrocarbons made its first appearance at 460 degrees C, but steroid ketones still dominated the pyrogram. Prist-1-ene, which is frequently detected in the pyrolysis of geological macromolecules, was only a minor component, as were four unknown isomers tentatively labelled as monounsaturated C-15 isoprenoid ketones. The pyrolyzates at 510 and 560 degrees C were dominated by n-alkene/n-alkane doublets which had a bimodal distribution and carbon numbers up to C-37 (at 510 degrees C). Only trace amounts of n-alkene/n-alkane doublets were produced from the 610 degrees C pyrolysis step. The results show that stepwise pyrolysis can be a fast overview tool for studying components of the asphaltene fraction and can provide useful information on the sources of organic matter (OM) in the oil shale, including polar molecules such as sterols, ketones, and fatty acids. An obvious advantage is the avoidance of complicated pre-treatments before detection using gas chromatography-mass spectrometry (GC-MS). The data confirm the importance of algal OM in the Maoming oil shale as well as the terrigenous plant contribution. (C) 2017 Published by Elsevier Ltd.