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

Despite having an homogeneous lithology, the largest Hungarian maar-type de posit (Gerce oil shale, Pliocene) has previously been shown to exhibit subs tantial 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 Gerce well-6 core . Based on the above bitumen data, four samples were selected that were rep resentative of the whole set which exhibit contrasting features. Scanning a nd transmission electron microscopy showed the occurrence of extensively al tered Botryococcus colonies in this deposit. GC/MS and GC-C-ir-MS of the sa turated hydrocarbon fractions of the bitumen of these samples reveal a pred ominant algal contribution along with a variable bacterial input. The relat ive abundance of these two contributions in the four selected samples is al so reflected by differences in FTIR and solid-state C-13 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 con tributions account for the variations in organic matter quantity and qualit y observed along the core. (C) 2000 Elsevier Science Ltd. All rights reserv ed.