Geochemical study of biomarkers in Eocene coal from the Bourgas Basin

An initial study of biomarker compositions of three coal samples from the Bourgas Basin is presented. Biomarkers were analyzed by gas chromatography–mass spectrometry (GC-MS) to assess organic matter source in the studied samples. The alkane distributions are dominated by long–chain n-alkanes (n-C27 to n-C31), usually found in higher terrestrial plants rich in cuticular waxes. The maximum relative intensity is determined at n-C31. Carbon preference index (CPI) values higher than 8.0 reflect a marked predominance of odd over even-numbered alkanes. The non-aromatic polycyclic hydrocarbons are dominated by hopanes, indicating microbial (bacterial) activity in the ancient swamp environment. The differences in the precursor vegetation are related to varying contributions of gymnosperms versus angiosperms during peat accumulation in the Bourgas Basin. The depositional environment was likely dynamic, with alternating oxidizing and reducing conditions.