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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.
1
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
2
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
3
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)
4
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
5
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)
6
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).
7
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
8
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).
9
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
> 100 mD
10 – 100
1 – 10
0,1 – 1
0,01 – 0,1
0,001 – 0,1
distance
depth
(km)
NW SE permeability
0
2
4
6
LMM
LFM
CB
PC
TM – L
TU
JL
JM
JU
CL
OMM
10 20 30 40
10
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)
11
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)
12
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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References
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