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

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 50 km. 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 (5200 m depth, 0.92% Rr) and from the marginally mature (ca. 0.6% Rr) samples from well Oberhofen 1 (4300 m depth) obtained sufficient amounts of carbazoles. The significantly different concentrations of benzocarbazoles in the extracts (around 10 μg/g at Oberhofen 1; > 50 μg/g at Molln 1) are related to maturity differences, whereas the predominance of 1-methylcarbazole in the Oberhofen samples is inferred to reflect differences in source rock facies. In the oils from the eastern deposits, neither the concentration ratios of methylcarbazoles, nor the benzocarbazole ratio show indications for the influence of facies differences. Maturity of oil expulsion is estimated on the basis of MPI 1 (Rc = 0.76-0.88%), confirmed by sterane isomerization parameters. The results argue for a minor influence of maturity differences on the benzocarbazole ratios of the oil samples. The obtained ratios and the reservoir source fractionation parameters, calculated from the data of Molln 1 source rocks and the respective oils, show a systematic decrease with the estimated migration distance. The results argue for the applicability of benzocarbazoles for the assessment of relative differences in migration distances in oil plays characterized by minor differences in source rock facies and maturity during oil expulsion.