PETROLEUM SYSTEMS IN THE AUSTRIAN SECTOR OF THE NORTH ALPINE FORELAND BASIN: AN OVERVIEW

D. Gross^1*, R.F. Sachsenhofer¹, A. Bechtel¹, R. Gratzer¹, M.-L. Grundtner¹, H.-G. Linzer², D. Misch¹, L. Pytlak¹ and L. Scheucher²

¹ Montanuniversitaet, Chair in Petroleum Geology, Department of Applied Geosciences and Geophysics, Peter-Tunner-Str. 5, A-8700 Leoben, Austria.

² Rohöl-Aufsuchungs AG, Schwarzenbergplatz 16, A-1015 Vienna, Austria.

^*corresponding author: doris.gross@unileoben.ac.at

Key words: North Alpine Foreland Basin, Austria, Central Paratethys, Oligocene, Miocene, source rocks, petroleum system, microbial gas, thermogenic hydrocarbons.

Two separate petroleum systems have been identified in the Austrian sector of the North Alpine Foreland Basin: a lower Oligocene - Cenomanian/Eocene oil and thermogenic gas system; and an Oligocene-Miocene microbial gas system. Recent studies by both academic and industry-based research groups have resulted in an improved understanding of these petroleum systems, which are reviewed in this paper.

Lower Oligocene organic-rich intervals (up to 12 %TOC; HI: 400-600 mgHC/gTOC), capable of generating slightly more than 1 t of hydrocarbons/m², are the source rocks for the thermogenic petroleum system in the Austrian sector of the North Alpine Foreland Basin. The present-day distribution of this source rock is controlled by submarine mass movements which removed a large part of the organic-rich interval from its depositional location during the late early Oligocene. The transported material was redeposited in locations to the south which are at the present day buried beneath Alpine thrust sheets. In addition, source rock units were incorporated into Molasse imbricates during Alpine deformation. Hydrocarbon generation began during the Miocene, and the oil kitchen was located to the south of the Alpine thrust front. Hence, lateral migration over distances of up to 50 km were required to charge the mainly Eocene and Cenomanian non- and shallow-marine sandstone reservoir units. Hydrocarbons are in general trapped in structures related to east-west trending normal faults, and differences in source rock facies resulted in the development of separate western and eastern oil families. Surprisingly, with the exception of some fields in the eastern part of the study area, associated gas contains varying (and sometimes very high) percentages of primary and secondary microbial methane. The composition of oil in some fields is influenced by both biodegradation and water washing. Post-Miocene uplift in the Austrian sector of the basin had further effects on biodegradation and the consequent formation of secondary microbial gas, and also resulted in re-migration.

The upper Oligocene to lower Miocene succession (Puchkirchen Group, Hall Formation) provides both source and reservoir rocks for the microbial petroleum system in the Austrian sector of the North Alpine Foreland Basin. TOC contents (<1.0 %) and HI values (<140mgHC/gTOC) of pelitic source rocks are typically low. Microbial gas was generated shortly after deposition during early diagenesis and was subsequently fixed in gas hydrates. Basin subsidence and high sedimentation rates resulted in decomposition of the hydrates below their stability zone, and reservoirs were filled during the early Miocene. Subsequent mixing of microbial gas with thermogenic gas and condensates is widespread. However, biodegradation has prevented precise determination of the fraction of thermogenic hydrocarbons present in gas samples. Reservoir sandstones were deposited within a deep-marine channel belt along the axis of the North Alpine Foreland Basin, and reservoir quality depends on the precise position within this belt. In the study area, gas is trapped in compaction anticlines or at channel margin pinch-outs and additional traps are formed by imbrication structures.

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