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The Jura – Molasse System: Hydrocarbon System and Exploration

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The Jura – Molasse System: Hydrocarbon System and Exploration The Jura – Molasse system: Hydrocarbon system and exploration by Simon Freitag (329303) Abstract The Molasse Basin stretches from Switzerland to Germany and ends in the east of Austria. Its main lithostratigraphic units, which were formed from Middle Oligocene to Upper Miocene, are underlain by a Mesozoic basement. The first major phase for hydrocarbon (HC) generation began at the end of the Jurassic. Another one was initiated in Oligocene-Miocene times. The main driving mechanism for migration and accumulation of the hydrocarbons is the excess pressure, resulting from the Alpine orogeny in the south of the basin. Fold- and fault-structures of the Unfolded and especially Folded Molasse formed good traps for an accumulation of hydrocarbons mainly in Germany. However, vertical fault zones may lead to a pressure drain-off and thus to a prevention of migrations and aggregation of hydrocarbons. In combination with the missing Lower Marine Molasse, these aspects may be reasons for the scarce occurrence of hydrocarbons in the Swiss part of the Molasse Basin, in contrast to the German and Austrian parts. There, all of the HC generating source rocks and reservoir rocks are existent. Furthermore, a general southwest dipping direction leads to the migration of oil and gas from Swiss parts to German areas where they accumulate. That is why the history of oil and gas exploration in Germany and Austria was successful in comparison to Switzerland, although the search started in all three countries at about the same time. At the moment, there are still two oil fields being exploited in Germany while in Austria, this number is much bigger for both oil and gas fields. In Switzerland, there has only been drilled one moderately successful well in its history. But since 2005, the search activity has picked up again. In Germany and Austria, the focus will mainly be on storage of gas in exploited HC fields. Content Introduction ............................................................................................................................................ 1 Geological Setting................................................................................................................................. 1 Evolution of the Swiss Molasse Basin (SMB) .................................................................................. 2 Hydrocarbon Systems .......................................................................................................................... 4 Permo-Carboniferous – Triassic-Jurassic-Neogene ................................................................... 4 Posidonia Shale – Cretaceous-Neogene ...................................................................................... 5 Migration and accumulation of hydrocarbons .................................................................................. 7 Exploration history ................................................................................................................................ 9 Outlook for future explorations ......................................................................................................... 11 Conclusion ........................................................................................................................................... 11 References .......................................................................................................................................... 13 Introduction The search for hydrocarbons in the Molasse Basin (MB), which is located to the north of the Alps, reaches back to the early 20th century. However, some areas of the MB are, compared to others, enriched with oil and gas including regions in Austria and South Germany. Switzerland by contrast, was moderately successful in the search for hydrocarbons only once in its exploration history. This may be ascribed to the Alpine orogeny, which by tectonic weight-loading led to a slight dipping of the rock layers to the south. The result of this process was the migration of the hydrocarbons to the north by northeast. Even so, there is still some potential for yet undiscovered hydrocarbon reservoirs in these three countries left. This paper deals with the four main hydrocarbon systems in the Jura and especially the Molasse Basin in the Alpine foreland. Furthermore, a short overview on the history of the hydrocarbon exploration of Switzerland, South Germany and Austria will be given as well as an outlook for future exploration projects in these countries. Geological Setting The Molasse Basin is located to the north of the Alps (Fig. 1). Its lateral expansion reaches about 700 km and stretches from France across Switzerland and Germany to Austria. In the Bavarian foreland, it reaches its maximum width of 130 km thus decreasing to both sides to 20 km in the most western part at Lake Geneva and to 10 km to the eastern part of the MB (Ford, 2004; Ibele, 2011). There, it forms a connection to the Vienna Basin and the Pannonian Basin further east. The Molasse 1 sediments, which are deposited on top of a Mesozoic basement, reach a thickness of up to 6 km (Ford, 2004). The western part of the Molasse Basin is situated in between the Jura Mountains in the north and the Prèalps or Swiss Alps to the south of Switzerland. In the southwest, it is confined by Lake Geneva, whereas by Lake Constance and the Rhine in the northeast. The middle part of the MB is located in South Germany. It is confined to the area between the Danube in the North and the Northern Calcareous Alps in the south. In the westerly part of the Molasse Basin, mainly in East Bavaria and Upper Austria, it is bordered by the Bohemian Massif to the north. Further to the west, the Molasse Basin merges to the Vienna Basin. In the south, it ends at the Alpine system. For a more detailed insight into the tectonics and evolution of the Molasse Basin, have a look at the paper “The Molasse Basin: tectonics, sea level and basin dynamics” of Eva Görke. The Jura Mountains are located in the north-western part of Switzerland at the border to France. They have a length of about 370 km and a width of some 75 km. Along the western and northern border of this fold-and-thrust belt, the Bresse Depression and the Upper Rhine Graben are situated (Becker, 2000). This mountain range was formed by the so-called “Fernschub-mechanism” during the Alpine orogeny and its formation began in the Cenozoic Era (Sommaruga, 1997). A more detailed review on the evolution and geometry of the Jura Mountains and the Fernschub mechanics is given by Tabea Kleineberg (“Evolution and geometry of the Jura Mountains: Fernschub mechanics”). Figure 1: Molasse Basin location and generalised distribution map of the reservoirs in Germany and Austria (Véron, 2005). Evolution of the Molasse Basin The Molasse Basin is a result of the Alpine orogeny. The subduction of the European plate by the African plate started in the late Mesozoic. The collision of the two continents and several microcontinents in-between occurred during the Cenozoic (Lower Tertiary, 66 – 23 Ma) and is still continuing (Ziegler, 1999). In this process, the weight of the orogenic wedge increased dramatically thus forcing the lithosphere 2 to bend downward, which resulted in the formation of a deep marine foredeep. This basin then was filled up with the so-called “Molasse” sediments. The basement of this foreland basin is dipping slightly to southeast (1° to 3°) (Sommaruga, 1997). The stratigraphy of the MB can be subdivided into the basement rocks, which consist of Palaeozoic and older rocks with Mesozoic rocks above, and the Molasse sediments (Fig. 2). The crystalline, metamorphic basement was formed by the collision of the two continents Gondwana and Laurasia, also called the Variscan orogeny. During the latest stage of this orogeny, crustal thinning along normal faults and strike-slip faults formed several basins (Bachmann, 1987). The Mesozoic sediments are provided by Triassic to Cretaceous limestones and sandstones. These were deposited in an epicontinental sea that dried out occasionally. The Triassic can be subdivided into the “Buntsandstein”, consisting of conglomerates and sandstones, the “Muschelkalk”, characterized by marine sandstones, marls and dolomites as well as evaporates, and the “Keuper”, which also contains evaporates and dolomites, but additionally lignites. During the Jurassic, predominantly limestones and marls were deposited (Liassic, Dogger and Malm) (Bachmann, 1987). Some of these layers form very important source rocks. Cretaceous sediments were deposited in shallow waters, forming calcareous marls or limestones (Bachmann, 1987), which are good reservoir rocks today (Véron, 2005). The Tertiary, which represents the late stage of Alpine orogeny and the evolution of the Northern Alpine Foreland Basin (NAFB), can be subdivided into four lithostratigraphic groups. These are: Lower Marine Molasse (LMM): 35 to 30 Ma Lower Freshwater Molasse (LFM): 30 to 20 Ma Upper Marine Molasse (UMM): 20 to 16,5 Ma Upper Freshwater Molasse (UFM): 16,5 to 5 Ma Some of these sedimentary rocks provide fairly good gas generators, reservoir rocks as well as seal layers (Véron, 2005). During the last stage of the alpine orogeny, the Jura Mountains in the northwest of the Swiss Molasse Basin were created after the Serravalian (Burkhard, M. & Sommaruga, A., 1998). Because sediments of Miocene age are involved in this folding, the formation had to happen during the Miocene and Pliocene. As already mentioned, the Triassic evaporite pillows
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