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 acted as sliding surface for the overlain sediments, which were thrusted towards the northwest and thus forming the folded Jura Mountains, mainly exposing Jurassic sediments. This process is called “Fernschub”-mechanics and involved also small parts of the north-western Swiss Molasse Basin.1

1 (http://de.wikipedia.org/wiki/Jura_(Gebirge), 2013) 3

Figure 2: Geological profile of the Swiss Molasse Basin (Schegg, 1999, modified by author).

Hydrocarbon Systems

The term “hydrocarbon system” represents a concept, which comprises all processes from the generation of hydrocarbons to their entrapment and accumulation in reservoir rocks. These processes are also dependent on the age and distribution of source and reservoir rocks, as well as the timing of hydrocarbon migration and trapping (Ziegler, 1999). In the geological history of the Molasse Basin, two main hydrocarbon generation phases can be assigned. The first one took place in the Permo-Carboniferous sediments, which are situated in Variscan orogeny related graben-structures, during Late Jurassic times (Véron, 2005). The second one happened in the internal parts of the MB close to the Alpine front during Oligocene-Miocene times. Two other hydrocarbon systems are of secondary importance (Véron, 2005). Figure 3 shows a summary of the main hydrocarbon systems in the Swiss Molasse Basin, but can basically be accounted for the total Molasse Basin.

Permo-Carboniferous – Triassic-Jurassic-Neogene

Besides the already mentioned “Autunia” shale (total thickness 32 m), which was deposited during the Lower Permian (“Rotliegend”), Upper Carboniferous coal seams and bituminous shales may also be accounted to potential source rocks. These bituminous sedimentary rocks are confined to narrow rift graben, formed during the Variscan orogeny. The Maturity of this oil and gas zone increases from northwest to southeast towards the Alpine front. This is due to increasing experienced pressure and temperature resulting from the overlain thrust belt (Schegg, 1999). As the hydrocarbons rose due to overpressure zones in the frontal part of the orogen, they got trapped in antithetic, tilted fault blocks of the Unfolded Molasse area and thrust sheets or subthrust fault blocks in the Folded Molasse region, mainly in the German Western Molasse area, which were formed during Oligocene to Mid Miocene times (Véron, 2005). The part of the Molasse Basin that got incorporated in alpine deformation is addressed to as “Folded Molasse” area, whereas the more distal part of the MB is called “Folded Molasse” area, because it was passively involved in the deformational tectonics ( (Ibele, 2011). Its formation happened in the time from Late Eocene to Late Miocene (Véron, 2005). The just mentioned overpressure zones and the slight dipping to the southeast of the Mesozoic basement additionally forced the hydrocarbons of the Permo-Carboniferous to migrate upwards and lateral to the

4 north-eastern, where they accumulate in Jurassic, Triassic and Neogene reservoirs in the German Molasse Basin (Schegg, 1999). Further mechanism for migration will be discussed in the following passage. Potential reservoir rocks in the Molasse Basin are mainly Triassic sediments, lacustrine (“Stuben”), littoral – deltaic (“Rhaetian”) and nearshore shallow marine chert sandstones of the “Keuper” as well as dolomites (“Trigonodus”) of the “Muschelkalk” in particular (Schegg, 1999; Véron, 2005; Chevalier, G., Diamond, L.W. & Leu, W., 2010). The marine-brackish “Chattian Baustein Beds” of Neogene (LMM) age form a further reservoir rock in the western part of the MB (Véron, 2005). Potential reservoir rocks above the mentioned ones are Lower/Middle sandstones and Upper Jurassic carbonates (Schegg, 1999; Chevalier, G., Diamond, L.W. & Leu, W., 2010; Véron, 2005). All of these reservoir rocks are mainly sealed by Intraformational shales. In addition, evaporites partly seal Triassic reservoirs and Lower Tertiary and Upper Jurassic reservoirs are sealed by shales and marls of the Early Tertiary (Véron, 2005).

The Posidonia Shale – Cretaceous-Neogene

In the Lower Jurassic (Toarcian), the Posidonia Shale was deposited in the “Jura” sea. This formation is built up of fine-grained sediments with several bituminous limestones embedded in-between. It’s well known for the exceptional good preservation of complete skeletons of fossil marine fish and reptiles.2 The name results from the frequently occurring bivalve species “Posidonia bronni” of the family “Posidoniidae”. These sediments were deposited on the sea floor in the ancient Tethys Ocean under anoxic conditions.3 The “Opalinus” shale also may provide a source rock of Middle Jurassic age although with a lower potential than the Posidonia Shale (Schegg, 1999). Similar to the Permo-Carboniferous source rocks, the oil and gas maturity of the Posidonia Shale is relatively low in the distal parts of the SMB (northwest) and increases towards the Alpine front (southeast). The reason therefore is that the sediments reach higher depth and temperatures there thus conditions, which promote hydrocarbon generation (Véron, 2005). Lower densities than surrounding rocks and particularly excessive pressure force these HC to rise. They eventually get trapped by antithetic, tilted fault blocks in the Unfolded Molasse area, formed during Oligocene to Mid Miocene times, and by subthrust fault blocks and especially thrust sheets in the Folded Molasse region of mainly Lower Austria (“Waschberg Zone”), created during Late Eocene to Late Miocene times (Véron, 2005). As reservoirs rocks, Middle and Upper Jurassic carbonates and partly sandstones play the most important role in this hydrocarbon system (Schegg, 1999). Concerning the carbonates, these were deposited in a reefal environment during the Upper Jurassic and are providing a fracture reservoir. The sandstones are represented by Dogger-beta sandstones (Véron, 2005).

2 (http://en.wikipedia.org/wiki/Posidonia_Shale, 2013) 3 (http://de.wikipedia.org/wiki/Posidonienschiefer_(Jura), 2013) 5

The Lattorfian Fish Shale – Cretaceous-Neogene

The Oligocene (“Sannoisian” stage) marine fish shale “Lattorfian Fish Shale” is accounted to the primary source rocks for oil of the Molasse Basin (Véron, 2005). Again, traps of the Unfolded and Folded Molasse in the “Waschberg Zone” of Lower Austria are responsible for the accumulation of oil. Petroleum arrives from deeper parts of the basin after migrating upwards along pathways with updip direction. The reasons for its migration are again the overpressure in zones close to the alpine front as well as buoyancy (Véron, 2005). The reservoir rocks range from the Upper Austria’s Dogger sandstones and carbonates and the Cenomanian sandstone to Upper Eocene sandstones and Oligocene carbonates and sandstones (Véron, 2005). Sandstones of the Cretaceous (“Cenomanian”) provide one of the main reservoir rocks for hydrocarbon deposits of the Austrian Molasse. These are for example the Voitsdorf and Schwanenstadt fields (Véron, 2005). Most of these reservoirs are sealed by intraformational shales. However, the sandstones of the Dogger reservoirs are additionally caulked by Upper Jurassic carbonates. Lower Tertiary shales may have the same effect on the Campanian reservoir (Cretaceous) (Véron, 2005). All in all, the hydrocarbon deposits which are sourced by the Lattorfian Fish Shale are restricted to the areas of the Austrian and German Eastern Molasse as well as to parts of the German Western Molasse (Véron, 2005).

The Oligocene – Oligocene-Lower Miocene

The source rock that generates the hydrocarbons, mainly gas, of Oligocene age is the sandstone of the Rupelian (LMM) (Leu, W. & Oester, H., 2012). The sandstones of Rupelian, Chattian, Aquitanian and Burdigalian age (LFM and UMM) provide reservoir rocks (Véron, 2005). Sandstones of the Lower Marine Molasse were deposited in marine-brackish (“Baustein”) and deltaic channels (“Chattian and Aquitan”) environments as well as turbidits (from west to east). Latter sediment series is called the “Puchkirchen Formation” and this hydrocarbon field was the first commercial oil field discovered in the Austrian Molasse Basin (Véron, 2005). The sandstones of the Upper Marine Molasse comprise the basal, transgressive marine Gendorf Sandstone of the Burdigalian in the Bavarian part of the MB. The Hall Formation is situated in the area of Upper Austria (Véron, 2005). Although there are hardly any reservoirs existent in the Freshwater series (Lower and Upper), some of the transition zones like the brackish freshwater environment provide a number of hydrocarbon fields. One of these zones is called the “Oncophora Beds” and they occur in Lower and Upper Austria. This formation stores several fields, e.g. the Wildendurnbach, Roseldorf and Alt Prerau fields. They show very high porosity and permeability values: 13 to 33.8% and up to 3000 mD (average is 30 mD) (Véron, 2005).

Trap formation took place from Oligocene to Middle Miocene and is characterized by antithetic fault blocks and stratigraphic traps (sealed off bodies of channel sands). In addition, intraformational Oligocene and Miocene shales seal the reservoir rocks (Véron, 2005). The hydrocarbons, which are produced in or close to the reservoir rocks, the gas migrates due to the same reasons as it is the case for the other petroleum systems, with respective retardation. The process of gas generation is still ongoing (Véron, 2005).

All of the Tertiary and Mesozoic reservoirs in the Molasse Basin are ultimately sealed by the Oligocene fish shale formation (Véron, 2005). Furthermore, because of the deep wells that were drilled over the last hundred years, it is well known nowadays that the quality of the reservoir rocks in the Swiss Molasse Basin is fairly bad due to low porosity and permeability. Reasons therefore are given by de Haller et al. (2011). These are mainly the overcompaction of the sediments related to the late uplift and erosion as well as strong and complex diagenetic alterations and cemen- tations (De Haller, 2011).

Migration and accumulation of hydrocarbons

For a comprehensive understanding of a hydrocarbon system in the Molasse Basin, it’s important to know, where all the potential source and reservoir rocks are located, as well as how these are interconnected and why the hydrocarbons migrate in the first place. These questions should be answered in the following. A connection of the mother rock with the reservoir rock is either given and controlled by the basin-fill geometry or by fault systems (Schegg, 1999). Latter may also work as pressure releasing structures leading to a prevention of oil and gas migration, but this aspect will be discussed later in this passage. The basin-fill geometry of the Swiss Molasse Basin is characterized by a succession of alternating highly permeable dolomites, limestones, sandstones and low permeability shales, marls and evaporites (Fig. 4) (Schegg, 1999). The main driving mechanism constitutes the excess pressure evolution. Very high pressure gets built up in the frontal part of the alpine orogeny, namely the Subalpine Molasse. The degree of pressure depends on the overlying weight, which is made up of the sedimentary and tectonic load (Schegg, 1999). This overpressure zones are probably the reason for driving deep fluids updip into the foreland (Schegg, 1999). Nevertheless, the excessive pressure is not the only factor the behaviour of hydrocarbons in this dynamic system depends on. It also hangs on the depth and type of source rocks, the chronology of generation and squeezing out of HC in relation to the structural development of the Swiss Molasse Basin and the

Figure 3: Generalized petroleum system chart for the Swiss Molasse Basin with key source rocks, generation/expulsion history, reservoir units, seals and tectonic history. This classic diagram does only present the conventional hydrocarbon systems and not unconventional resource plays (modified from Greber et al. 2004) (Leu, W. & Oester, H., 2012). hydrogeological conditions. Latter factor comprises lateral and vertical lithological changes such as permeability and porosity of the Molasse sediments as well as faults (Schegg, 1999). Referring to just mentioned faults as influencing factor, their development and properties are highly complex. Dependencies are on lithology, deformation mechanisms and stress conditions at different depths to name some important of them. As already mentioned, faults can act as pressure releasing and migration pathway. When this happens for example at the front of the Subalpine Molasse, a scenario by Schegg et al. (1999) indicates that the oil of the Posidonia Shale wouldn’t migrate upwards into the reservoir rocks thus an absence of HC in the Middle and Upper Jurassic carbonates. However, faults may be sealed again either by cataclastic rocks or by processes which close these pathways, such as pressure solution, compaction or cementation (Schegg, 1999).

NW permeability SE OMM

CL 0 LFM JU

JM LMM

JL 2 TU depth TM – L (km) PC

CB 4

> 100 mD

10 – 100 0,01 – 0,1 6 1 – 10 0,001 – 0,1 distance 0,1 – 1 10 20 30 40

Figure 4: Modelled present-day permeability (mD) (Schegg, 1999; modified by author).

Exploration history

Switzerland

The exploration history of hydrocarbons in Switzerland is several hundred years old. Since 1714, large asphalt deposits have been mined in the Val de Travers, a valley in the Swiss Jura Mountains. There, Cretaceous limestones got impregnated by hydrocarbons already before the folding of the Jura Mountains. This process included thrust-related uplift, deformation and at the same time erosion of the MB which stopped the HC generation. Until today, it is not known which source rocks were responsible for their production (Schegg, 1999). The history of subsurface hydrocarbon exploration in Switzerland started hundred years ago and since then, only 37 deep wells have been drilled. At the beginning, it was mainly looked for conventional reservoirs in the Tertiary to Palaeozoic section of the Swiss Molasse Basin. But with time, the focus shifted to oil/gas in the Mesozoic and to gas in reservoir rocks of Permo-Carboniferous age (Leu, W. & Oester, H., 2012). In the first decades until 1950, only small ventures of independent petroleum companies drilled for oil. The decisions, where to locate the wells, were made based on areas, where oil seeped out of the soil, surface geology and sometimes on hints of geomantic specialists. Although seismic data of this area already existed since 1954, it was used economically as hydrocarbon search tool only after 1960 (Leu, W. & Oester, H., 2012). In the time after 1956, over 27 deep wells were drilled by the Swisspetrol Group. This group is a federation of over ten local Swiss companies and two major foreign petroleum companies; these were the BEB-Shell-Exxon and ELF Aquitaine (Lahusen, P.H. & Wyss, R., 1995). Only one of these 27 deep wells was

9 moderately successful (Entlebuch-1), which commercially produced gas in the time between 1985 and 1996 (Gisler, 2011). When the Swisspetrol Group was dissolved in 1993, only the SEAB (Schweizerische Erdöl AG) in eastern Switzerland and the Petrosvibri SA in Vevey remained and kept on searching for hydrocarbon. During 1995 to 2005, only one well was drilled called the Weiach-2 (Leu, W. & Oester, H., 2012).

Austria

The hydrocarbon exploration and production in Austria began about 100 years ago. Just like it was the case in Switzerland, the focus was on simple HC targets at that time. Except of two moderately successful wells, Neusiedl 1 and Zistersdorf- Gaiselberg, no major HC discoveries were made until 1950. In this year, the giant Matzen field in the Vienna Basin was detected due to the usage of single-fold and six-fold 2 seismic (Hamilton, 2000). In this area, hydrocarbons are stored in over 20 different reservoir rocks.4 In Austria, the Vienna Basin is by far the biggest hydrocarbon producer. Until 1999, 108,5 MM tonnes/oil and 72,1 Bm³ gas in total have been produced in Austria, most of it in the Vienna Basin (Hamilton, 2000). Since the beginning of the exploration history in Austria, 4400 wells have been drilled, 900 thereof in the Austrian Molasse Basin (Hamilton, 2000).

Germany

The first proven oil finding was made at the Tegernsee in the 15th century. In 1904, a first well was drilled which encountered oil in a depth of 500 m. After that, ten further exploration wells were conducted. However, these wells are already watered-down again since 1912. During the time of the World War II, no search for hydrocarbons or exploration was run. Right after this period, the search for oil picked up again and HC were exploited since 1954. Until today, around 60 wells have been drilled which have pumped up about 6,9 Million tonnes/oil and 18,2 Mrd. m³/gas from this time to 2000. Most of them are located in an area southeast of Munich close to the border of Austria.5 Nevertheless, only a very small part of the hydrocarbons exploited in Germany are situated in the German Molasse Basin. Most of the hydrocarbons - namely 94% - in Germany are exploited in Lower Saxony and Schleswig-Holstein. However, three oil fields are still located in the German North Alpine Foreland Basin. These are Aitingen (Bavaria), Hebertshausen (Baden-Wuerttemberg) and Schwabmünchen (Bavaria).

4 (http://www.wabweb.net/history/oel/noe.htm, 2011) 5 (http://de.wikipedia.org/wiki/Erdölförderung_in_Deutschland#Alpenvorland) 10

The First and biggest one was found in 1976 and is still producing. The oil field Heberthausen is exploited since 1982 and the last one since 1968.6

Outlook for future explorations

Since 2005, the rate of hydrocarbon exploration wells in Switzerland increases again. Today, more than two third of the SMB and the Jura Mountains are reserved by in total eighteen exploration permits (Leu, W. & Oester, H., 2012). Seven joint venture exploration companies, that own these permits, concentrate on four different types of hydrocarbon exploration techniques. Two companies focus on the extraction of conventional shallow oil and gas from the Mesozoic-Palaeozoic sections of the Jura Fold belt and the northern rim of the Molasse Basin. Another one tries to get shallow heavy oil out of Tertiary sediments in the Geneva area. Three further companies drill for deep conventional and unconventional tight gas in the southern part of the MB. One of them was successful lately, because the Noville-1 well (3500 m TVD) encountered a tight gas accumulation in Palaeozoic clastic rocks. The last exploration company normally wants to explore unconventional shale gas in the Jurassic section of the SMB. However, this process can’t keep going at the moment due to an exploration ban in Canton Fribourg and a hydraulic fracturing moratorium in the Canton Vaud (Leu, W. & Oester, H., 2012). In the near future, two further exploration wells are planned. Although the exploration of shale gas is still banned at the moment, Oester & Leu (2012) assume that this form of hydrocarbon could play a major role in the transition to a renewable energy supply of Switzerland. A first estimation of the potential recoverable shale gas in the Aalenia-Toarcian results in ~50-100 Mrd. m3. The annual gas consumption of Switzerland accounts for 3,5 Mrd. m3. Further studies have to be conducted to show that an usage of this unconventional resource is profitable and that it has no negative impact on the environment, (Leu, W. & Oester, H., 2012). In Austria, the focus is more on the storage of gas in already exploited HC fields in the near future. Nevertheless, there are still some smaller hydrocarbon fields available in areas, where exploration is already processed.7

Conclusion

The migration of hydrocarbons in the Molasse Basin is mainly controlled by the basin-fill geometry, which comprises a succession of highly permeable sedimentary layers and low-permeable shales and marls, as well as fault zones. Due to a tectonic weight-loading created by the alpine orogeny located to the south of the MB, its rock layers also dip this direction. This results in the upwards-migration of generated hydrocarbons to the north to north-east. The main driving mechanism for the movement of hydrocarbons is the excessive pressure evolution in the alpine

6 (http://de.wikipedia.org/wiki/Erdölförderung_in_Deutschland#Alpenvorland) 7 (http://www.wabweb.net/history/oel/noe.htm, 2011) 11 forefront, which is also created by the tectonic weight-loading of the Alps. However, major fault zones may act as migration pathways for hydrocarbons. Besides the dipping of the layers to the south, this might be another reason for missing hydrocarbons in Switzerland. That is the case when in the past generated oil or gas may have left the petroleum system through these fault zones before it could have been trapped in overlying reservoirs. However, fault-sealing processes like cataclasic rocks or pressure solution could have stopped the escape of the hydrocarbons. Taking the stated aspects into account, it may be explained, why there are hardly any hydrocarbons found in Switzerland, but in Germany there are. Most important hydrocarbon systems in the Molasse Basin are the Posidonia Shale – Cretaceous- Neogene system with the Posidonia Shale as main source rock, and the Lattorfian Fish Shale – Cretaceous-Neogene system with the Lattorfian Fish Shale as oil generating rock. Even so, there are several smaller ones in the MB, too. Although, the Jura-Mountains form good trapping structures, they contain hardly any hydrocarbons. This may be accounted to thrust-related uplifting, deformation and at the same time erosion of the MB, which stopped the generation and accumulation of hydrocarbons in this area. Most of the HC reservoirs can be found in the Austrian part of the Molasse Basin, with the Matzen field representing the biggest one. There, the oil is stored in more than 20 different reservoir rocks. Despite most of the HC fields already being discovered, there are still some yet undiscovered economical fields thought to be existent in all of these three countries. However, the focus will be more on the storage of gas in already exploited HC fields and possibly on the exploration of shale gas than of petroleum in the near future.

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