Organic facies distribution at the platformward margin of the Kössen Basin | Szegedi Tudományegyetem, Ásványtani, Geokémiai és Kőzettani Tanszék

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Organic facies distribution at the platformward margin of the Kössen Basin

Acta Mineralogica-Petrographica, Szeged 2002, Vol. 43, pp. 19-25

Dr. Hetényi Magdolna

Assessment of the organic facies and that of organic precursors were performed on an 83 m thick core section of the Upper Norian Kössen Formation (Hungary), exposed in Sümeg by Süt-17 borehole. Carbon-dioxide, mineral carbon and organic carbon contents, determined on a large number (70) of samples, exhibit a carbonate-rich, organic-poor sequence. Cyclic variations observed in both forms of carbon, throughout the sequence, reveal cyclic alternations of the platform-derived input and the terrestrial-derived one. Owing to the especially low organic carbon content (Corg <0.2 %), half of the investigated rocks, composed mainly of carbonate particles, can not be considered as source rocks either for oil and gas. Most of them were formed under shallow subtidal conditions in a highly oxic environment. Rock Eval pyrolyses performed on selected argillaceous samples (Corg >0.2 %) together with organic petrographical data from a small sample set, show that the immature organic matter is of predominantly terrestrial origin and composed of mainly inertinite and vitrinite deposited in oxic environment. The predominance of the highly degraded land plant remnants resulted in organic facies D and CD with very low source potential in rocks containing less than 6 % of Ccarb. Organic facies CD is also common in rocks containing more than 6 % of Ccarb. Owing to the negligible amount of oil and minor amount of gas generated by all of these samples, they are rated as non-source rocks in any commercial sense. Gas-prone C facies and B facies with marginal potential for oil generation, observed in some carbonate-rich samples, formed from a mixture of different types of precursors and accumulated also in an oxic environment. The very minor amount of algal-derived liptinite was probably preserved by inclusion in carbonate skeleton.




acta2002, carbonate platform, depositional environment, organic facies, organic precursors, petroleum source potential




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