Characterization of organic matter in late Cretaceous black shales of the Eastern Alps (Kainach Gosau Group, Austria)

Bituminous marls occur at the base of the late Cretaceous Kainach Gosau Basin, Eastern Alps. They interfinger with local coaly layers, and are overlain by a turbiditic sequence. Forty outcrop samples with different lithologies were studied by Rock-Eval pyrolysis. Seven bituminous marls, one coaly sample, two siltstones from the turbiditic sequence and solid bitumen from the Paleozoic basement have been characterized in detail by organic geochemical and petrographic techniques. The organic matter from bituminous marls can be subdivided into allochthonous, terrigenous material (mainly recycled vitrinite), which is present in all samples in similar amounts, and autochthonous hydrogen-rich organic matter (alginite, liptodetrinite), which is most probably of lacustrine origin. Organic richness varies from 0.4 to 2.3% TOC. The amount of the autochthonous organic matter is controlled primarily by different oxygen concentrations. Sedimentological (lack of bioturbation) and geochemical data (e.g., pristane/phytane ratios, homohopane indices, C*30/C29Ts ratios, distribution of phenol and alkylphenols in the pyrolyzate) suggest that the water column above the sediment surface was suboxic to dysoxic, but probably not anoxic. High energy events were responsible for short periods with enriched oxygen in the water column. The organic matter in the resulting sediments is of a residual type. Drift wood was transported from the nearby shore into the lake. Paleontological evidence proves a temporary marine influence. Vitrinite reflectance and Tmax indicate maturation levels ranging from the beginning to the peak stage of oil generation. Despite the fact that some hydrocarbons have already been generated, HI values of kerogens of bituminous marls reach 605 mg HC/g TOC. Most analysed bituminous marls produce a high wax paraffinic oil during maturation and are good sources for oil. Pyrolysis-gas chromatography and biomarker data indicate that changing wax contents of oils generated from bituminous marls might be a function of different oxygen concentrations in the water column. Coaly samples have a very good gas potential.