Fluoren-9-ones and carbazoles in the Posidonia Shale, Hils Syncline, northwest Germany

Abstract

The distribution of alkylated fluoren-9-ones and carbazoles, including their benzo- and dibenzo- and/or naphtho-annellated derivatives, have been investigated in rock extracts from a maturity sequence of Posidonia Shales from the Hils Syncline of northwest Germany. The fluoren-9-one and carbazole derivatives are quantitatively the most important compound types in the low polar heterocompound fraction obtained from the Posidonia Shale bitumens by liquid chromatography. This fraction comprises 9.3–27.5% of the total bitumen depending on maturity. The concentration of the fluoren-9-ones in the bitumens systematically increases with increasing maturity. Generally, a trend of decreasing degree of alkylation is observed with increasing maturity. However, at the molecular level, less systematic maturity-dependent variations are observed for the fluoren-9-one distribution than for the carbazole distributions. A remarkable exception is the 1-ethylfluoren-9-one (1-ethylfluoren-9-one + 1,8-dimethylfluoren-9-one) ratio which linearly decreases over the entire maturity range (0.48–1.45% R r). Among the carbazoles, dibenzo-and/or naphtho-annellated derivatives become more abundant at higher maturity. An oxidation of fluorenes may be ruled out as the major process for the formation of fluoren-9-ones since Posidonia Shale was deposited under anoxic conditions and fluorenes are not present in high amounts even in samples of low maturity. The fluoren-9-ones may rather be produced by cyclisation reactions of suitable precursors in the organic matter. Particularly, 2-carboxybiphenyls could be transformed to fluoren-9-ones either catalysed by Lewis acids, e.g. clay minerals, or solely under the influence of higher temperatures. The constant ratio of fluoren-9-ones and benzofluoren-9-ones suggests that selective formation/degradation and interconversion during maturation plays a minor role. this seems to be different for the carbazoles where a significant influence of maturity on the distribution of derivatives with different numbers of aromatic rings was observed.