Bernhard A. Gunzenhauser and H. Philippe Bodmer
Tiefengas Konsortium Swisspetrol/Sulzer
Kriens, Switzerland
The Tiefengas Konsortium Swisspetrol/Sulzer (TGK), a joint exploration affiliate of Swisspetrol Holding Ltd. and Sulzer Brothers Ltd., has recently concluded an extensive exploration campaign for deep natural gas in its concession area in the north alpine overthrust belt of Central Switzerland (Fig. 1).
This exploration program-which includes reflection seismic, geochemistry, gravimetry, and an analysis of seismicity together with theoretical considerations-led to the development of an unconventional gas reservoir and trapping model likely valid under the specific geological situation of the TGK area.
"Seismic valving" is a key element of this concept. TGK believes that a fractured reservoir/pressure compartment play can thus be defined in the autochthonous and sedimentary series beneath the alpine nappes.
TGK proposes a deep wildcat well in the northern part of its concession to test the gas potential of this unconventional play never before tested in Switzerland. In case of success, the play could be extended over the whole alpine overthrust belt and to other areas with similar conditions elsewhere.
EXPLORATION HISTORY
A total of 33 hydrocarbon exploration wells have been drilled since early this century, including 21 by Swisspetrol and its partners during the last 30 years.
Earlier wells drilled to test the Tertiary of the Molasse basin were usually shallow and seismically poorly defined.
A majority of all the wells had remarkable shows of oil and gas. Only the Entlebuch-1 well, drilled 1979-80 and operated by BEB Erdgas und Erdol Gmbh, Hannover, Germany, was completed as a producer of gas and condensate.
Entlebuch-1, 3 km northwest of the TGK concession area, reached the depth of 5,289 m (17,752 ft) in the Carboniferous (Figs. 1 & 2). Several fractured zones were penetrated in the Upper Jurassic to Triassic section. Gas is produced from karstified and fractured Upper Jurassic limestones and fed into the Transitgas Pipeline that crosses Switzerland from Northwest Europe to Italy.
The Swiss north alpine overthrust belt is hardly explored although large gas seepages are commonly observed at the surface and in tunnels of Central Switzerland.
CONCESSION GEOLOGY
The TGK concession area is in the northern Alps of Central Switzerland. Most of the area is covered by the allochthonous Helvetic nappes, consisting mainly of Mesozoic platform sequences.
These units now rest on an autochthonous sequence that comprises the crystalline basement and its sedimentary cover of coal-bearing Carboniferous unconformably overlain by Mesozoic sediments including dolomites of Triassic age, followed by Jurassic to Cretaceous carbonates.
Eocene residual red clay in karst pockets of Upper Jurassic limestones indicates a widespread emergence during which the Cretaceous and part of the Upper Jurassic sediments were eroded. Flysch was deposited in a foredeep north of the growing pile of the Helvetic nappes during the Eocene to lower Oligocene.
During Oligocene and Miocene a thick succession of alternatively marine and fresh water clastics (the Molasse) was deposited in the alpine foredeep. Crustal shortening during Neogene led to the thrusting of the Helvetic nappes over the flysch and over part of the Molasse and to thrusting within the subalpine Molasse north of the concession area.
SOURCE, MIGRATION
The gas production in Entlebuch-1 and the many sizable gas seepages along the alpine front indicate ample gas generation at depth.
The distribution of the gas seepages and the results of geochemical analyses suggest that the generation still takes place below the Helvetic nappes within the autochthonous sedimentary section, notably in the coal bearing Carboniferous. Reflection seismic data indicate the presence of deep Permo-carboniferous troughs in the TGK area.
Similar basins are known to exist in the northern alpine foreland, where they contain significant coal seams and bituminous shales with source potential for gas and oil, respectively.
Geochemical investigations led to the conclusion that in the concession area the bulk of gas is generated in pre-Mesozoic strata. Chemical and isotopic composition of methane in fluid inclusions trapped in vein-quartz and -calcite dated as Miocene show large resemblance between the included gases and the present seepages.
These observations are interpreted to be the result of a steady state gas generation and an episodic migration regime at least since the Miocene. The upward migration and escape of hydrocarbon gases and the distribution of the seeps and soil gases appears to coincide with the occurrence of deep reaching faults in the concession area, which is one of the main centres of earthquake activity in Switzerland.
MODEL, SEALS, TRAPS
TGK formulated a reservoir and trapping model by which the mechanical and chemical development of secondary porosity and permeability is postulated to develop through the mechanism of "seismic valving."
This applies in overpressure regimes that are loaded by the generation of gas. Once the pore pressure has reached the fracture strength of the surrounding formations and a fault with hydrostatic pressure condition opens up, the pressure differential will cause a hydraulic fracturing of the formation from near the fault into the overpressured zone.
Pressure compartments, formed as a result of active hydrocarbon generation, combined with lithologic, tectonic, and diagenetic sealing, are expected to have been episodically fractured by "seismic valving," a mechanism related to the interaction of tectonic stress and elevated pore pressure.
This mechanism facilitated the circulation of hot, aggressive fluids, leading to local leaching and enhanced secondary porosity and permeability. Of particular relevance is that both the mechanical and chemical processes leading to reservoir development are a direct consequence of the presence of hydrocarbons.
Frequent seismicity in the area is taken as a direct evidence of this mechanical process.
Velocity inversions observed on reflection seismic data and amplitude versus offset (AVO) anomalies are thought to be evidence for potentially gas filled porous trap compartments. Potential reservoirs suitable for fracturing are the brittle limestones and dolomites of the autochthonous Mesozoic section and to a lesser extent associated clastics.
POTENTIAL
A drilling feasibility study and an economic assessment indicate that expected reserves would yield substantial project returns.
Switzerland's energy requirements, its favorable hydrocarbon fiscal regime, and gas prices presently around $3 (U.S.)/Mcf for indigenous production are in favor of such a venture. Furthermore, if the gas reservoir and trapping model is proven in a drilling program, it can be used to find hitherto unexploited reservoirs under similar geological conditions elsewhere.
TGK is seeking a competent operator, and possibly additional new consortium members, to contribute financially and technically in this deep gas project and to drill the first exploratory well on a farmout basis.
TGK has identified a number of prospective areas within its concession and proposes one for the drilling of a farmout well to test potential gas reservoirs in the Upper Jurassic to Triassic carbonates of the autochthonous sequence. Secondary objectives are thin clastics within the same interval.
Additional gas bearing formations are possible in limestones of the Helvetic nappes and in sandstones of the Molasse and North-Helvetic flysch as well as in the Permocarboniferous where present. The total depth of the well is estimated to be about 7,150 m (23,450 ft).
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
