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Oil and gas in the eastern part of the Drava depression originated from source rocks of very good generating capabilities - dark gray to black marlstones of Ottnangian, Karpathian, Badenian, Sarmatian and partly Pannonian age. During different tectonic regimes, algal-lipid rich kerogen formed in anoxic and suboxic marine environments. The typical terrestrial maceral vitrinite only occurs as traces.
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Hernitz et al. (1995) studied this question.