Preservation of labile organic compounds in sapropelic coals from the Upper Silesian Coal Basin, Poland

Abstract

Sapropelic coals were taken for study from coal mines and coal waste dumps in Upper Silesian Coal Basin (USCB), Poland. They showed unusual molecular compositions reflecting immature characteristics. The vitrinite reflectances 0.65-1.00% indicated mature organic matter (OM) and agreed with the Rock Eval Tmax (421-444oC). The OM maturity also was supported by the geochemical ratios, i.e., alkylnaphthalenes, dimethylphenanthrenes, methylbiphenyls and dibenzofuran or phenol and its methyl derivatives in the southern part of USCB. The petrographic features showed fine-grained and laminated OM, which is typical for sapropel with a dominance of vitrinite and inertinite macerals. The presence of fusinites and natural chars reflects paleowildfires prior to coal formation and/or peat fires. It is also confirmed by the presence of anthracene, methyl-anthracenes, and oxygen-containing aromatic compounds like anthrone, xanthone, fluorenone, and benzofluoren-11-ones. Rock Eval pyrolysis data showed very similar values as the humic coals from USCB, except for the elevated values of S2 (residual petroleum potential) and Hydrogen Index (HI) reflecting the presence of hydrogen-rich liptinite macerals. The distributions of n-alkanes, methylalkanes, alkylcyclohexanes and alkylbenzenes indicate a microbiological origin associated with microorganisms, including cyanobacteria or Botryococcus. In all samples 8β(H)-homodrimane dominated and indicated an anoxic environment of the deep lakes. The preservation of tetracyclic diterpenoids, diaster-13(17)-enes, ββ-hopanes, simonellite, bisnorsimonellite, cadalene, fatty acids, aromatic aldehydes and carboxylic acids from lignin degradation in sapropelic hard coals indicates an early diagenetic generation of these compounds from sapropels. Furthermore, during coalification, some the less thermally labile compounds can be preserved with increasing coal rank, while others isomerized or aromatized to more stable counterparts.