Influence of Alpine Tectonics on Source Rock Distribution, Hydrocarbon Generation and Migration in the Austrian Part of the Molasse Basin

Sachsenhofer, Reinhard F.¹; Linzer, Hans-Gert ²; Bechtel, Achim ¹; Dunkl, Istvan ³; Gratzer, Reinhard ¹; Gusterhuber, Juergen ¹; Hinsch, Ralph ²; Sperl, Hanns ²
(1) Montanuniversitaet Leoben, Leoben, Austria. (2) Rohoel-Aufsuchungs AG, Vienna, Austria. (3) University of Goettingen, Goettingen, Germany.

The E-W trending Molasse Basin evolved from Late Eocene to Miocene times north of the Alpine thrust front. Shortly after the deposition of shallow marine sequences, Lower Oligocene source rocks up to 70 meters thick were deposited during a rapid deepening of the basin.

In the present paper tectonic data, source rock data and data from accumulated oils are used to constrain the timing of hydrocarbon generation and migration and to understand the distribution of oil deposits.

The facies of the Oligocene source rock is laterally uniform. However, the uniform pattern was disturbed by submarine mass movements, triggered by the load of the advancing nappe system. The mass flows moved significant amounts of source rocks from the northern paleo-slope into the deep basin. Consequently two different source rock facies can be observed in the oil kitchen beneath the Alps: an original one and a facies influenced by mass flows.

The extension of both facies can be assessed using a few deep wells drilled through the Alpine nappes and information on the areas where source rocks have been removed by mass flows. Additional information is derived from Molasse Imbricates which include Oligocene source rocks of both facies deposited beneath the Alps and transported northward during thrusting. Incorporation of Oligocene source rocks into Molasse Imbricates also implies that the potential source rocks have been removed from the oil kitchen.

In general there is a good regional fit between the distribution of the source rock facies in Molasse sediments beneath the Alps and the updip occurrence of two different oil families. Moreover, oil is largely missing north of areas suffering from tectonic erosion. However, characteristics of some oils aligned along the northern front of the Alps suggest that they have not been generated from authochthonous sediments, but from the deepest parts of the Molasse Imbricates, which entered the oil window only during the last stage of overthrusting.

The integration of all data suggests that most oils were generated during Early Miocene time as a result of deep burial of the source rocks beneath the Alpine nappes. Additional oil was generated during Early/Middle Miocene time when parts of the Molasse Imbricates became mature. A young (Pontian?) uplift phase probably resulted in the termination of hydrocarbon generation. The amount of uplift varied significantly in a W-E direction. We speculate that this fact caused re-migration of hydrocarbons.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.