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New hydrocarbon plays in inverted Jurassic extensional basins of the Bohemian Alpine foreland (Eastern Alps, Austria)


Austria hosts large reserves of hydrocarbons trapped in varying structural domains and reservoirs types. This study concentrates on the regional structure and stratigraphy of the Lower Austria region (north of Vienna). A particular emphasis is given to the Autochthonous Mesozoic cover developed onto the stretched European continental margin. The studied area is located at the Alpine thrust front, where the tilted Bohemian foreland is overridden by imbricated foreland deposits (i.e., Flysch and Molasse) as well as a series of N-NW directed thrust sheets of Apulian affinity. These upper plate units rest on top of the Alpine Tethys European continental margin whose structure is characterized by a series of S-SE directed Jurassic half-grabens. These half-grabens are showing thick syn-rift siliciclastic wedges and a SE-prograding post-rift carbonate platform with facies transitions towards a marly slope and deeper water sediments. In this Mesozoic succession, source rocks locate in the Doggerian pre- to syn-rift Gresten Gp. and in the post-rift Malmian Mikulov Fm. Main reservoirs locate in the Gresten Gp. quartzites and in the cherty dolomites of the post-rift Höflein Fm. Top Seal is provided by shales of Lower Miocene Molasse sediments and the underlying Mikulov Fm marls. Careful interpretation of structure and sediment architecture from 3D depth-converted seismic data has revealed that the half-grabens underwent mild inversion upon Cenozoic shortening, but also that the lower plate underwent extension coevally with shortening in the foreland region. Evidences for mild thin- and thick-skinned tectonic inversion of the Bohemian foreland are: gentle folding of the syn-rift clastic wedges, the local low-angle reverse offset of the basement top reflectors or its sub-horizontal attitude. Syntectonic Molasse sediments onlapping onto the scarps of the external most basement extensional faults and their hanging-walls suggest that these basement faults were active upon shortening. Miocene extension also took place in the upper plate as shown by the development of the Vienna Basin. Miocene extension might have also reactivated the basement extensional faults formed in Jurassic times. Such new interpretations of the Lower Austria Bohemian basement have implications for the estimation of hydrocarbon reserves, and open potential new plays and opportunities for a mature hydrocarbon province.