Diagenetic trends of lignin phenols in Mahakam Delta coals: correlation between laboratory thermal maturation and natural samples

Abstract

Changes in the general structure of the lignin-derived polymers in a series of increasingly mature, huminite-rich coals from the Mahakam Delta have been investigated by various analytical techniques. The coal series reveals a general reduction in the proportion of oxygen-substituted aromatic carbons as determined by CPMAS 13C NMR, the relative yield of phenol during flash pyrolysis/GC and by the chemical characterisation of lignin phenols, benzoic and hydroxybenzoic acids from CuO oxidation. Several U.S. coals were also analyzed for their lignin phenols composition for comparison. The trends in the yields of lignin phenols from the Mahakam Delta coals were compared with those determined previously from artificially matured wood specimens. It appears that laboratory simulation can predict to some extent the chemical compositional changes taking place in lignin polymers during natural coalification. Thermal (geochemical) degradation plays an important role in the diagenetic alteration of lignin polymer over geological time. The precursors of lignin phenols can survive into the subbituminous rank but are rare or absent above this rank. This is apparently a general characteristic of the coalification process irrespective of the depositional setting or lignin-type. The data from coals in conjunction with our earlier studies on the kinetics of wood pyrolysis imply that lignin phenols might be used as a qualitative palaeovegetation indicator, and possibly a palaeoclimatic marker at least up to the subbituminous rank.