Influence of migration distance, maturity and facies on the stable isotopic composition of alkanes and on carbazole distributions in oils and source rocks of the Alpine Foreland Basin of Austria

•Facies variations are highlighted by compound specific isotope analyses.•Black shales within the Schöneck Fm were deposited under brackish water conditions.•Long distance lateral migration of the oils is indicated.•Benzocarbazoles can be used to assess differences in migration distance. The Austrian part of the Alpine Foreland Basin has been studied previously by organic geochemical methods to establish oil–oil and oil–source rock correlations. The lateral distance of oil fields to the position of mature source rocks (Lower Oligocene Schöneck and Eggerding formations) beneath the Alpine nappes in the south suggests long distance migration up to 50km. Minor compositional trends in W–E direction were obtained reflecting differences in depositional environment (facies) and maturity of potential source rocks. Therefore, the oil and source rock samples are thought to provide valuable insights into carbazole distributions in relation to migration distance, maturity and facies. Compound specific carbon isotope analyses on n-alkanes and isoprenoids reflect the W–E trend, already seen within the total hydrocarbon fractions. The observed trend toward lighter δ13C values of alkanes from oil fields in a W–E direction are consistent with lower δ13C values of organic matter in unit c and similar differences obtained from sediments of the Schöneck Formation at wells Oberhofen 1 (W) and Molln 1 (E). The δ13C patterns of n-alkanes and isoprenoids are in agreement with data obtained from the potential marine source rocks (Schöneck Formation) of the oils. The hydrogen isotopic composition of short chain n-alkanes (C15–19) of most oil samples are lower than the data obtained from the extracts of the marine Permian Kupferschiefer of the Polish Zechstein Basin at similar maturation stages. Only the oils from the western oil fields yield δD values of n-alkanes comparable to those obtained from marine source rocks. The data imply brackish water conditions during deposition of ‘unit c’ of the Schöneck Formation. Differences in the position of δD of isoprenoids in relation to the respective maturation trend line are most probably related to calibration errors. Alkylated carbazoles and benzocarbazoles are only present in sufficient amounts for quantification in oils sampled from the eastern fields. Rock extracts of the Schöneck Formation from drill cores Molln 1 (5200m depth, 0.92% Rr) and from the marginally mature (ca. 0.6% Rr) samples from well Oberhofen 1 (4300m depth) obtained