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Organic geochemical investigations of brown coal lithotypes. A contribution to facies analysis of seam banding in the Helmstedt deposit

1986; Elsevier BV; Volume: 10; Issue: 1-3 Linguagem: Inglês

10.1016/0146-6380(86)90058-6

1873-5290

Elfriede Winkler,

Hydrocarbon exploration and reservoir analysis

Samples of light, medium light and dark lithotypes from the Eocene Helmstedt brown coal deposit were examined by organic geochemical methods (Soxhlet-extraction, liquid and gas chromatography, GC/MS). The study is part of a facies analysis of banded brown coals. There are significant differences between the lithotypes concerning the extractable organic matter yields, the composition of the extracts and the composition of the alkane fractions. On average the light brown coals produce three times as much extractable organic matter as the dark ones. Asphaltenes and resins are the main components of the extracts of the light lithotypes. Hydrocarbons and N, S, O compounds of lower molecular weight are less abundant than in the dark lithotypes. The alkane fraction of light lithotypes contains more n-alkanes of high molecular weight and more saturated triterpenoids (hopanes excluding homohopane) but less unsaturated polycyclic compounds (rearranged sterenes, triterpenes) and less homohopane. The generally low concentrations of cyclic diterpanes (norabietane, norpimarane) do not reveal any relationship to the three lithotypes. Based on these results there is no support for the conventional assumption that the light and dark brown coal lithotypes originated from different plant communities of the paleomoor. The results rather suggest that the banding within the seam of the Helmstedt brown coal deposit results from different biogeochemical degradation and transformation processes. These processes are considered to be dependent on the environment of the paleomoor.