Maturation and isotopic changes of individual hydrocarbons in lignite lithotypes through diagenesis and early catagenesis

Abstract

Maturation and isotopic changes of individual hydrocarbons in lignite lithotypes through diagenesis and early catagenesis were studied. The maturity ratios based on biomarkers and alkylaromatics which showed the best applicability were determined. Intensity of observed changes of maturity ratios decreases in the following order: mineral-rich coal > matrix coal > xylite-rich coal, indicating that mineral matter, which is dominated by clays in all lithotypes, plays an important role in the isomerisation of polycyclic biomarkers and aromatic compounds. Carbon isotopic signatures of n-C25 and n-C27 alkanes, as well as δ13C values of phenathrene and cadalene are very close in studied range of maturity, confirming that isotopic signatures are useful tracers for alteration products of biological molecules. Long-chain homologues of n-alkanes exhibit slight enrichment in 12C with increasing maturity, whereas almost no change is detected in δ13C of short-chain n-alkanes. Negligible enrichment in 13C of aromatic compounds is observed during maturation.