New exploration concepts spark Swiss gas, oil prospects

Roland Schegg, Werner Leu, Emil Greber
Geoform Ltd.
Winterthur, Switzerland

A recently completed data synthesis and evaluation ¹ based on the interpretation of public well data and seismic lines and the application of modern basin-modeling tools upgrades gas and oil prospects in Switzerland.

The study gives insight into key elements of the petroleum systems of Switzerland and comments on new promising gas and oil play concepts.

Switzerland, a highly industrialized country in the heart of Europe, is 100% dependent on imported fossil and nuclear fuels. Since 1912, only 35 wells (Fig. 1 [38,977 bytes]) for oil and gas have been drilled,² and only one went into production (Entlebuch-1: 75,000,000 cu m or 2.65 bcf of gas). Over 600 seismic lines with a total of over 12,000 km were acquired since the 1960s.

The former Swisspetrol group and its partners, BEB from Germany, Elf Aquitaine from France, and Shell-Switzerland, have spent 317 million Swiss francs (about $220 million) on exploration after World War II. Due to missing economic success, foreign partners withdrew from Switzerland, and most domestic exploration companies were dismantled in 1994.

As of mid-1997 the Swiss exploration companies SEAG, TGK, Petrosvibri SA, and FREAG held four concessions.

Is this the end of the hydrocarbon exploration story in Switzerland? Probably not. During the EAGE meeting in Geneva (May 28, 1997), Patrick Lahusen, vice-chairman of SEAG, announced that a joint venture agreement with Anschutz Corp. of Denver, Colo., was expected to be signed shortly.³

Why Switzerland?

Different points that emanate from the data synthesis by Greber et al. ¹ favor a second look at Switzerland's exploration potential:

• Low exploration maturity compared with neighboring regions⁴ and new play concepts suggest a still considerable potential for undiscovered fields in Switzerland. The country has low taxes and is politically stable.
• 80% of the 19 wells drilled in the last 38 years had gas and/or oil shows (Fig. 1). Moreover, 60 locations with oil seeps/stains and more than 150 gas seepages at surface testify to ongoing or past hydrocarbon generation.
• Occurrence of mature oil and gas prone source rocks over wide areas of Switzerland, young hydrocarbon generation phases (Mid-Cretaceous and Oligocene-Miocene), and a variety of reservoir rocks (dolomites, karstified limestones, sandstones) in different trap positions (tilted fault blocks, thrusted units, anticlinal structures, stratigraphic traps) encourage further exploration.
• The presence of multiple plays of conventional (target of past exploration activities) and unconventional types make Switzerland an attractive area for further hydrocarbon exploration, especially when the proximity to existing markets and to the pipeline infrastructure is considered.

Different new and old play concepts remain to be tested, based on advances in exploration technology (drill- ing, testing, etc.). They are:

1. Permian and Carboniferous clastic plays (basin centered gas accumulations);
2. Coalbed methane plays;
3. Clastic reservoirs in triangle zones;
4. Mesozoic carbonates; and
5. Fractured and weathered basement rock plays.

Geological background

From north to south Switzerland can be divided into the following tectonic units (Fig. 1):

• Stable European foreland and its sedimentary cover: the Tabular Jura, the Variscan massif of the Black Forest and the Tertiary Rhine Graben (failed continental rift).
• Folded Jura mountain belt: Mesozoic and some Cenozoic sediments have been folded and sheared off from their substrate and thrusted to the north and west at the end of the Alpine orogeny (late Miocene-Pliocene).
• Plateau Molasse, where the Mesozoic sediments and their thick cover of Oligo- cene-Miocene Molasse have been dislocated to the north by "décollement" in the Triassic evaporites and/or basement-involved thrusts.
• The Subalpine Molasse: A stack of imbricated Molasse thrust sheets that are overridden themselves by Alpine nappes.
• Alpine nappes and massifs (e.g. Aar massif). The Helvetic Alps form the northern or external zone of the Alpine belt in Switzerland. The Southern Alps, a 50-150 km wide fold-and-thrust belt, represent the Southern orogenic wedge of the Alps.

The Tertiary North Alpine Foreland basin (NAFB) is a peripheral foredeep that formed by flexural bending of the European lithosphere in response to crustal thickening in the Alpine orogen. The so-called Molasse basin is the younger continental and shallow marine component of this foreland basin. The folded and thrusted Subalpine Molasse comprises the southernmost outcropping part of the basin, but the Tertiary clastic wedge can be traced southward far below the most internal Helvetic units.

In most places the sedimentary fill of the NAFB rests unconformably on truncated Mesozoic continental margin sediments. Underlying the megasequence boundary, defining the base of the foreland basin, Mesozoic carbonates, shales, and clastic rocks are superimposed on a Hercynian basement complex with locally deep Permo-Carboniferous troughs (Figs. 2-4) which have been drilled in northern Switzerland and in one deep well below the Subalpine Molasse.

We limit our discussion of exploration opportunities to the northern part of Switzerland (Jura Mountains and NAFB). The petroleum geology of the Helvetic Alps has been reviewed recently by Gunzenhauser and Bodmer⁵ and that of the southern Alps by Greber et al.⁶

Source rocks

Oil-prone lower Oligo- cene source rocks are present in the most internal part of the Subalpine Molasse and beneath the Alpine nappes. A well-known source rock in the Mesozoic is the Toarcian "Posidonia" shale (TOC of up to 10%), which is characterized by Type II kerogen. Its quality decreases, however, towards the Alps.

The Posidonia shale is immature in the Jura mountains and the most external part of the Molasse basin; it is in the oil window in most parts of the Plateau Molasse and in the gas generation zone towards the Alpine front.

Paleozoic sediments drilled in the Weiach well (Fig. 2 [58,847 bytes]) comprise two source rock types: Carboniferous coals (total thickness 32 m) and Lower Permian bituminous lacustrine shales. The new seismic evaluation by Greber et al.¹ indicates a widespread occurrence of Paleozoic sediments below the NAFB. Paleozoic coals are in the gas window over most parts of the NAFB.

Generation phases

Two major oil generation phases have been simulated for the Posidonia shale: Early-Middle Cretaceous and Miocene.

Hydrocarbon generation phases in Permian, Early Triassic, Late Jurassic, Cretaceous, and Miocene are suggested for the Paleozoic source rocks by basin modeling results.

Reservoirs and seals

The Tertiary Molasse sequence comprises different sandstone/shale units of varying quality. The important Mesozoic reservoir rocks (Fig. 2) are Lower Triassic sandstones, Middle Triassic dolomites, and Upper Jurassic and Lower Cretaceous limestones (karstified tops, reefs). Mesozoic reservoirs are generally better sealed than the Tertiary sandstones. The Middle Triassic dolo- mites are, for example, overlain by a thick layer of Upper Triassic evaporites.

Another promising reservoir unit consists of sandstones of the continental Permo-Carboniferous troughs below the Jura and the entire Molasse basin. In the Weiach well porosities in the sandstones may reach values of up to 10-15%. (Fig. 2). Deeply buried Permo-Carboniferous sandstones may also be a promising target as "tight" reservoirs in fractured basin-centered gas accumulations. A further large reservoir potential is related directly to the Permo-Carboniferous coal seams for coalbed methane plays.

Trap types

Based on the evaluation of 1,100 km seismic lines in the Molasse basin and the Folded Jura the following trap types have been identified (e.g. Fig. 3 [50,790 bytes]):

• Tilted fault blocks in extensional and compressional settings: They may occur in the Tertiary, the Mesozoic or Permo-Carboniferous level.
• Thrusted units in the Folded Jura, Subalpine Molasse, and Helvetic Alps. Smaller duplex structures have also been observed in intra-Mesozoic thrust systems in the external part of the Plateau Molasse.
• Various anticlinal structures: Folds, mega-anticlines related to positive flower structures, and Subalpine/ Alpine uplift structures of the late Mesozoic platform close to the Alpine front.
• Stratigraphic traps related to facies changes or unconformable contacts are observed in the Tertiary section and postulated for the Permo-Carboniferous interval, e.g. in basin-centered gas accumulations.

Play concepts

Roughly five play concepts have been located in the Jura mountains, the Molasse basin and the North Alpine border zone (Fig. 4 [26,934 bytes]), each with interesting aspects in view of the low exploration maturity of Switzerland:

1. Permian and Carboniferous clastic play (gas and oil): So far, these clastic reservoir units have only been drilled in three NAGRA (National Cooperative for the Storage of Radioactive Waste^7-8) wells (e.g. Weiach) in northernmost and in one hydrocarbon exploration well (Entlebuch-1) in Central Switzerland (Fig. 1). The new seismic evaluations indicate, however, a widespread occurrence of these Paleozoic clastics extending far beyond the proven troughs. The reservoir quality may locally be of conventional type, with "normal" porosities, but also of a fractured microporosity/low-permeability type (tight reservoir). The combination with the widespread occurrence of Carboniferous coal seams is currently one of the most promising play concepts.⁹ This play concept may have substantial potential for gas and oil below the virtually unexplored Jura fold belt. The good sealing potential of Triassic evaporites, especially in the Western Folded Jura, is an additional positive aspect for such entrapments.
2. Coalbed methane play: Untested potential is represented by coalbed methane in Carboniferous coals of the located Paleozoic troughs. Rough estimates by Kempter9 indicate that from a prospective area of roughly 100 sq km with coals of the type "Weiach," 1-3 tcf of methane could be produced. The most promising areas for this play are the northernmost Molasse basin and adjacent areas (e.g., Jura).
3. Clastic reservoirs in Subalpine triangle zone (gas): Thrusted Tertiary sandstone-rich units may form good quality and high pressured reservoirs, sealed by shale-rich neighboring formations and tight thrust planes. Due to late burial hydrocarbon generation may still be going on.
4. Mesozoic carbonates (gas and oil): Several trap types have been localized in Lower Cretaceous, upper Jurassic, and Triassic carbonates. These plays were the conventional target for oil exploration in Switzerland for over 20 years.² Improved reservoir properties are expected in reefal units and karstified tops.
5. A typical example of this play concept is the Entlebuch-1 well. The prospectivity for such plays could be greater towards the deeper and proximal parts of the NAFB, where naturally induced fractures (due to overpressures) could increase the reservoir quality (tight gas reservoir type).
6. Fractured and weathered basement rocks play (gas and oil): Some potential may be envisaged for fractured and weathered basement rocks in the vicinity of deep Permo-Carboniferous grabens, where increased stress and brittle strain may have improved reservoir quality. Overthrusting during the Saalian transpressive or the younger Alpine phases may have even juxtaposed basement rocks over sources, such as Paleozoic coals.

References

1. Greber, E., Leu, W., and Schegg, R., Hydrocarbon habitat and potential of Switzerland; Geoform internal report, May 1997.
2. Lahusen, P., and Wyss, R., Erd?l- und Erdgasexploration in der Schweiz, Ein R?ckblick; Bull. Ver. schweiz. Petroleum-Geol. u. -Ing., Vol. 62, No. 141, 1995, pp. 43-72.
3. Lahusen, P., Switzerland exploration may resume, OGJ, June 23, 1997, p. 78.
4. Brink, H.J., Burri, P., Lunde, A., and Winhard, H., Hydrocarbon habitat and potential of Swiss and German Molasse basin: A comparison, Eclogae geol. Helv., Vol. 85, No. 3, 1992, pp. 715-732.
5. Gunzenhauser, B., and Bodmer, Ph., Deep gas exploration needed in central Swiss overthrust; OGJ, June 7, 1993, pp. 53-54.
6. Greber, E., Bernoulli, D., Leu, W., Schumacher, M.E., and Wyss, R.

   , Hydrocarbon provinces in the Swiss Southern Alps-a gas geochemistry and basin modelling study; Marine Petrol. Geol., Vol. 14, No. 1, 1997, pp. 3-25.

7. Matter, A., Peters, T., Bl?si, H.-R., Meyer, J., Ischi, H., and Meyer, Ch., Sondierbohrung Weiach, Geologie (Text- und Beilagenband), Nagra NTB, Vol. 86-01, 1988, 438 p.
8. Thury, M., Gautschi, A., Mazurek, M., M?ller, W.H., Naef, H., Pearson, F.J., Vomvoris, S., and Wilson, W., Geology and hydrogeology of the crystalline basement of northern Switzerland, Synthesis of Regional Investigations 1981-1993 within the Nagra radioactive waste disposal programme, Nagra NTB, 93-01, 1994, 392 p.
9. Kempter, E.H.K., Erdgas aus Tight Reservoirs; Schweizerische technische Zeitung, Vol. 7-8, 1994, pp. 41-45.

The Authors