Alkyldibenzofurans in terrestrial rocks: Influence of organic facies and maturation

Dibenzofuran (DBF), dibenzothiophene (DBT), and phenanthrene (PHE) have been determined along with their methyl derivatives (MDBFs, MDBTs, MPs) in coals, carbonaceous shales, and mudstones of Lower Cretaceous to Upper Carboniferous ages from the Elmworth gas field (Canada) and from the Sakoa (Madagascar) and Ruhr coal (Germany) basins. The samples contained mainly terrestrial organic matter with thermal maturities in the 0.7-1.9% mean vitrinite reflectance (R(r)) range. The pronounced predominance of MDBFs over MDBTs in the Sakoa samples is typical of plant debris deposited in lacustrine swamp environments. Onset of intense MDBF generation occurs at fairly low rank (0.77% R(r)) as indicated by elevated concentrations of 50-120 mu g MDBFs/g total organic carbon (TOC). Influences of lithology, organic matter type, and maturity on distributions of the target compounds were extracted from the molecular concentration data by multivariate statistical methods, such as Principal Components Analysis (PCA) and fuzzy c-means cluster analysis with nonlinear mapping (NLM). A specific source effect on 1-MDBT concentrations suggests that lichens contributed to the senescent coal-forming forest in the Sakoa coal basin. Alkyldibenzofurans hence are potential lichen biomarkers. MDBFs can be used as maturity indicators only beyond 1.0% R(r) where Ruhr coals displayed a distinct increase in the 1-/4-MDBF ratio. Copyright (C) 2000 Elsevier Science Ltd.