A study in early diagenesis: Biomarker composition of a suite of immature coals and coaly shales

Abstract

Abstract A suite of samples from the Funan Mine, N.E. China, has allowed a comparison to be made between hydrocarbon-rich coals and coaly shales. These are immature coals and the known geological history suggests that they have experienced only a mild geothermal history. Analyses of the di- and sesquiterpenoid components show that compounds based on the kaurane skeleton are absent unlike Australian brown coals; and the diterpenes isopimarane, pimarane and 16α (H)-phyllocladane relate to a different community of higher plants such as Pinaceae and Taxodiaceae from those recognized in Australian brown coals. A consideration of geochimical parameters and biomarker composition leads to the conclusion that the geolipid concentrations in the sample suite are environmentally determined. It is concluded, therefore, that 22R-17β (H),21β (H)-bishomohopanoic acid and the 22R-17α (H),21β (H)-homohopane, both predominant components in their respective classes, are derived from different sources. Labdanic acids have been identified in these coals. They are likely to be derived from resin constituents of the coal and not from microbial biomass. They could potentially be developed as a maturation parameter. Cis and trans isomers of a triaromatic hydrocarbon based on the oleanane skeleton which has been previously identified in Victorian brown coal are present in the Funan coals. The more oxidizing the environment the less advanced is the isomerization of cis to trans and again the environment of deposition is the controlling factor. This study emphasizes the importance of understanding pathways of early diagenesis for the interpretation of data derived from more mature samples.