Polycyclic Hydrocarbons in Oil Shale--Origin and Geochemical Fate: ABSTRACT

Abstract

A series of pentacyclic triterpenes of the hopane type is present as the main components of the polycyclic hydrocarbon fraction of the Messel oil shale, a terrestrial sediment of Eocene age, from near Darmstadt, Germany. The series ranges from C27 to C32, and the members differ only by the length of the side chain on the 5-membered ring. The C27, C31, C32 compounds, which have never been detected in nature, probably arise from geomicrobiological degradation and alkylation reactions of a C30 precursor taking place in the first stages of sedimentation. Pentacyclic triterpenes usually have been considered as paleoecologic makers indicating a higher plant origin of part of the organic matter of sediments and pe roleum. Recent studies, however, on the chemistry of bacteria and blue-green algae have shown that these sources do contain pentacyclic triterpenes, commonly in significant amounts, and that these triterpenes are of the hopane type. These results, combined with analysis of the olefinic and oxygenated polycyclic fractions of the shale, seem to indicate a prokaryotic origin of the triterpenes isolated from the Messel oil shale. Moreover, analysis of the polycyclic hydrocarbons isolated from several recent and ancient sediments, as well as from crude oils, shows that triterpenes of the hopane type are widely present in these sources, and suggests that a significant proportion of the triterpenes found in sediments and oil in general could be of prokaryotic origin. End_of_Article - Last_Page 766------------