Origin of variations in organic matter abundance and composition in a lithologically homogeneous maar-type oil shale deposit (Gérce, Pliocene, Hungary)

Despite having an homogeneous lithology, the largest Hungarian maar-type deposit (Gérce oil shale, Pliocene) has previously been shown to exhibit substantial variations in organic matter quantity and quality with depth. This heterogeneity is also reflected, in the present study, by large variations in bitumen abundance and composition, for 23 samples from Gérce well-6 core. Based on the above bitumen data, four samples were selected that were representative of the whole set which exhibit contrasting features. Scanning and transmission electron microscopy showed the occurrence of extensively altered Botryococcus colonies in this deposit. GC/MS and GC-C-ir-MS of the saturated hydrocarbon fractions of the bitumen of these samples reveal a predominant algal contribution along with a variable bacterial input. The relative abundance of these two contributions in the four selected samples is also reflected by differences in FTIR and solid-state 13C NMR spectra of the isolated kerogens. Curie point pyrolysis/GC/MS of these kerogens revealed a relatively high terrestrial contribution in one sample and confirmed the variable input of algae and bacteria. The above differences in relative contributions account for the variations in organic matter quantity and quality observed along the core