The origin of hydrocarbon source rocks in the Austrian Mollasse Basin (Eocene-Oligocene transition)

Organic-rich rocks (Schöneck Formation) from five wells representing a section across the northern paleo-slope of the Austrian Molasse Basin were studied using a multi-method approach including petrography, X-ray diffraction, organic and inorganic geochemistry, and C- and N-isotopes of the organic material. Organic matter accumulation commenced during the latest Eocene and was dependent on variations of currents, salinity, organic productivity, and nutrient supply. Initial marl deposition (and phosphorite-formation) under dysoxic conditions was followed by organic-rich mud deposition associated with benthic and photic zone anoxia. This paleoceanographic situation resembles the first isolation of the western Central Paratethys. Deposition of organic-rich rocks terminated during lower Oligocene time when changing current patterns and decreasing salinity caused a breakdown of water column stratification. Even though the Schöneck Formation is an excellent source rock, major differences exist in the petroleum potential of the Schöneck Formation deposited in upper and lower basin slope settings. The Schöneck Formation from the upper slope is characterised by TOC contents up to 12% and HI values between 500 and 600 mgHC/gTOC. In the lower slope settings, TOC values are generally below 2%.