Geology

Oil and gas fields in Upper Austria and Salzburg

The geological profile shows that the Molasse basin is an asymmetric basin dipping to the south and partly extending underneath the Alps. It consists of relatively thin Mesozoic formations overlain by several thousand of meters of Cenozoic sediments. These consist of sandstones, conglomerates and shales sourced from the uplifted Alps. Sediments from the Bohemian Massive were deposited only in the northern parts of the basin. The basin consists of  the Foreland Molasse, reaching from the crystalline rocks that outcrop as the Bohemian Massive in the north, down to the northernmost extension of the Alps) the Imbricated Molasse associated with the syntectonic ”south slope facies” , a narrow zone along the Alpine Flysch front and 3) the Subalpine Molasse overlain by the Alpine nappes.

Geological cross section through the Molasse Basin

Today’s knowledge of the geology of the Molasse Basin and its hydrocarbon potential is based on the results of approximately 850 wells, 22,000 kilometers of 2D seismic lines and almost 3100 km² of 3D seismic coverage. In Bavaria we have another 900 wells and 700 km² of 3D seismic. These are supplemented by thousands of shallow wells and maps of the near-surface stratigraphy. Although these horizons do not bear hydrocarbons, they have substantial ground water resources.

Stratigraphic table

In Upper Austria, the crude oil and associated gas were formed by thermal maturation of organic matter of the Lower Oligocene Schöneck Formation (“Fischschiefer”). The oil was not generated in the immediate vicinity of today’s reservoirs. It formed within the Subalpine Molasse that was overthrust by the Alpine nappes at a time when the Schöneck Formation entered the “oil window” (experiencing higher temperatures and pressures).  It migrated northwards to its present position.

The generation of natural gas of the Foreland Molasse is different. The quick burial of large sediment masses into a reducing deep water environment during subsidence of the basin encouraged biogenic gas generation. Bacterial activity converted the organics dispersed within the sediment into methane.

Oil reservoirs are found mainly in Upper Eocene sandstones. Cretaceous and Jurassic sandstones can also contain oil.  Natural gas, however, is mainly restricted to sandstones and conglomerates of the Oligocene (Puchkirchen Formation) and the lower Miocene (Hall Formation).