SHALLOW HYDROCARBON INDICATIONS ALONG THE ALPINE THRUST BELT AND ADJACENT FORELAND BASIN: DISTRIBUTION AND IMPLICATIONS FOR PETROLEUM EXPLORATION

D. Misch^a*, W. Leu^b, R. F. Sachsenhofer^a, R. Gratzer^a, B. Rupprecht^a and A. Bechtel^a

^a Department of Applied Geosciences and Geophysics - Chair of Petroleum Geology, Montanuniversitaet Leoben, Peter-Tunner-Straße 5, A-8700 Leoben, Austria

^b Geoform Geological Consulting and Studies Ltd., 1844 Villeneuve, Switzerland.

^* Corresponding author, David.Misch@unileoben.ac.at

Key words: shallow gas, microbial gas, seepage.thermogenic hydrocarbons, North Alpine Foreland Basin, Molasse Basin, Austria, Switzerland, Vienna Basin, surface seeps, shows.

Shallow oil and gas shows are common in the Alpine thrust front (including the Flysch Zone) and the North Alpine Foreland Basin in Switzerland, southern Germany and Austria, but have not hitherto been evaluated systematically. In the vertically-drained Vienna Basin and the easternmost part of the Flysch Zone, shallow oil and gas shows and seeps often coincide with deeper-lying hydrocarbon accumulations, and gas shows occur along major faults – for example within the urbanised area of the city of Vienna. The number of gas shows decreases in the Vienna Basin away from (to the south of) the subcrop of the main thermogenic source rock (the Upper Jurassic Mikulov Formation); however shallow accumulations of microbial gas occur in that area. To the west, along the northern margin of the laterally-drained North Alpine Foreland Basin, oil shows have been recorded in both Austria and Switzerland; microbial gas shows are common in addition to thermogenic hydrocarbons. Typically the shows form regional clusters along river valleys and occur above shallow gas accumulations.

A Lower Oligocene organic-rich interval represents the main source of oil / condensate and thermogenic gas in the Upper Austrian part of the North Alpine Foreland Basin, whereas the composition of oil shows within the Calcareous Alps to the south indicates the presence of mature Mesozoic source rocks within the Alpine nappes. This implies the presence of an additional, as-yet untested petroleum system. Thermogenic gas, occurring in Permo-Triassic evaporitic rocks in the Calcareous Alps, as well as microbial gas in younger sediments, has frequently been encountered during salt mining and tunnelling activities.

A surprising discrepancy has been found in different parts of the study area between the number of hydrocarbon shows and the number of economic fields. Whereas the number of fields and shows are approximately in proportion in the Vienna Basin and the Austrian sector of the North Alpine Foreland Basin, shows appear to be “under-represented” in Germany. By contrast in Switzerland, despite a high number of shows especially in the North Alpine Foreland Basin and the Jura fold-and-thrust belt, no economic production has been established to date. Future exploration will show whether this is due to poor reservoir/trap quality, or if undiscovered resources are in fact present. The presence of oil shows generated from Mesozoic and Oligocene source rocks in the SW German and Swiss parts of the North Alpine Foreland Basin suggests the occurrence of multiple petroleum systems; these systems should be delineated in future studies.

Few surface seeps have been recorded in less populated parts of the study area such as the high Alps, possibly due to sampling bias. However, this bias does not explain the low frequency of recorded hydrocarbon shows in the German part of the North Alpine Foreland Basin. This may be because the geological setting there is in general less favourable for the migration of thermogenic gas into shallow reservoirs and its preservation in shallow traps.

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