TECTONIC STYLES, REEVALUATION OF PLAYS IN SOUTHEASTERN FRANCE

Eric Deville, Alain Mascle
Institut Francais du Petrole
Rueil-Malmaison, France

Charles Lamiraux
Ministere de l'industrie-Direction des Hydrocarbures-S.C.G.H.
Rueil-Malmaison, France

Alain Le Bras
Cie. Generale de Geophysique
Massy, France

Two major onshore sedimentary basins of Mesozoic age in France, the Paris and Aquitaine basins, have been intensively explored for about 40 years and contain a significant amount of hydrocarbons which presently still accounts for 4% of oil and 10% of gas for French domestic needs.1

Nevertheless, other Mesozoic basins are also present in the southeastern part of France, in front of the Alpine and Provencal Tertiary thrust belts (Fig. 1). The level of activity here is presently very low, as the first phases of exploration in the 1950s to 1970s have been rather disappointing.

New studies have been undertaken however in this wide area in the last few years, with already some encouraging results such as the testing of oil at the Chaleyriat well south of the Jura thrust belt. Also, regional seismic profiles acquired by Cie. Generale de Geophysique (CGG) in 1980 to 1991 provide the necessary constrains to properly evaluate remaining plays, with special attention to untested deep gas prospects.

In order to attract new players in these frontier area (and in the other basins as well), the French public authorities have furthermore decided to slightly modify the policy of license attributions.2 Among other aspects the new policy will include:

A simplification of the investigation procedure related to exploration licenses by suppression of the initial public inquiry. This new awarding procedure will be delivered by the director of the Direction Generale de l'Energie et des Matieres Premieres (DGEMP) by delegation and not by the Prime Minister as before. As a consequence, the new licenses will be delivered in 1 year when 3 years were needed previously;
An improvement of the transparency (in accordance with the new European directives) during the awarding of mining rights, by indicating the criteria that will be required for the selecting of the petitioner;
A relaxing of regulations. It will be possible for instance to extend one of the validity periods of the license for a nonrenewable period of three years. It should also be mentioned that the French tax system has already been revised several times. In 1992, the departmental and communal mine taxes were halved and the finance law of 1994 has eliminated this entire tax and the royalties for the offshore fields.

S.E. FRANCE GEOLOGY

Basin development started in southeastern France as early as late Paleozoic times, when narrow continental late Westphalian-Stephanian troughs formed on top of the Variscan mountain belt, mostly along transcurrent and/or thrust faults.

During the Permian post-collisional stage of the orogeny, the general collapse of still continental basins occurred in a north-south extension regime (i.e. perpendicular to the overall east-west trend of the Variscan belt). This extension tectonics may have prevailed until middle Triassic times, although convincing data are still missing.

In late Triassic and Lias time, however, and until the Callovian the extension direction became oriented east-west to northwest-southeast, in relation with rifting events in the Tethyan area. The first accretion of oceanic crust in what will become the Ligurian oceanic basin is indeed dated from the Callovian.

The stress regime is more difficult to assess in late Jurassic and early Cretaceous times, i.e. at the time of rifting in the Bay of Biscay. It has recently been proposed that a north-south oriented extension regime prevailed in the basins of southeastern France, followed by a north-south to northeast-southwest shortening from Aptian to Cenomanian times.

Potential source rocks related to these first stages of basin development may be found in Stephanian and Autunian coal measures and oil shales (both of continental and/or lacustrine origin), as well as in early Jurassic, late Jurassic, and late early Cretaceous marine black shales. Reservoirs, with primary or secondary porosity, fractures and/or karsts, and related seals are sometimes more difficult to assess and may vary from place to place. The most commonly recognized reservoirs are early Triassic sandstones, middle Triassic, early Lias, middle Jurassic, Tithonian and Berriasian limestones and dolostones.

The present structural complexity of southeastern France results from the superimposition, in Tertiary times, of three major tectonic events:

From late Cretaceous to middle or late Eocene times prominent north-south to northeast-southwest compression events led to the formation of the Pyrenean and Provencal thrust belts in southwestern and southeastern France, respectively. Compression structures extended far to the north in what was the relative foreland and led to the development of thrusts and related folds, transcurrent faulting along previous Mesozoic normal faults, and even locally basin inversion (the Baronnies massif, for example);
From the late Eocene-early Oligocene to the early Miocene, an east-west to northwest-southeast extension was clearly recorded throughout eastern France with the development of highly subsiding fault-bounded troughs and grabens of north-south trend to the north (Rhine, Bresse, Limagne, and Valence grabens), and rather northeast-southwest to the south (Ales, Manosque, Camargue, Gulf of Lion troughs). The several thousand meters thick syntectonic infilling in the deepest parts of these basins includes rocks of potential interest for petroleum exploration such as sandstone, anhydrite and salt, lignites, and oil shales. Relative high heat flows are expected to have taken place in some of these basins at that time;
From the late early Miocene to present times, southeastern France as a whole was subjected to the late Alpine compression leading to the final emplacement of the Jura and Western Alps (Chartreuse, Vercors, Digne-Castellane) thrust belts, and moderate reverse and/or transcurrent faulting, as well as folding related with basin inversion processes in the near foreland. The maximum shortening directions show some rapid changes throughout space and time, with east-west to northwest-southeast shortening directions prevailing in the Vercors and farther north, and with northeast-southwest to north-south shortening directions to the south.

EARLY EXPLORATION

The first stages of exploration in eastern France took place in the 1950s to 1970s, and even in earlier times locally, with only a few encouraging results.

We shall mention tiny gas fields developed at the front of the Jura thrust belt. The largest of them was Valempoulieres, where less than 100 million cu m of gas were produced from late middle Triassic dolostone in a tectonic duplex of Neogene age. The source rocks are generally believed to be the underlying Stephanian coal measures.

Small quantities of oil have been produced along the western border of the Southeast basin. For instance, in Triassic strata, at Gabian, in the vicinity of a natural seep exploited as early as the 17th century; or at Saint Jean de Marvejol in early Oligocene strata, which are still mined as bituminous limestone at shallow depths.

Oil has also been produced from Oligocene reservoirs in Gallician field located in the Tertiary Camargue rift basin. Oil and gas have been tested at a few more wells, natural gas seeps are locally active, and bitumen may be found from place to place, but accumulations large enough for economic production have never been encountered.

It should also be mentioned that there are some unexpected discoveries of CO2 accumulations. Several explanations can be proposed for this lack of success. First the initial seismic data were not of good enough quality to resolve the complexity of structural traps and even to image the whole sedimentary basins. As a result, the first rigs were often unable to reach their initial targets that appeared to be at much deeper depth than originally expected. Also, the nature and the distribution of source rocks were not properly taken into account, and more specifically the relative timing of their maturation with respect to the formation of structural traps was not considered. Likewise the quality and distribution of reservoirs was (and still partly remained) poorly understood. Most of the time reservoir qualities in Mesozoic calcareous strata are greatly enhanced by fracturing or even sometimes karstification.

SEISMIC SURVEYS

Several nonexclusive regional seismic surveys were shot from 1980 to 1991 by CGG with the collaboration of Institut Francais du Petrole (IFP).

Together with the ECORS scientific deep seismic surveys shot by CGG in 1987-88 and 1990 through the Alps-Jura thrust belt and the Rhine graben respectively, these surveys yielded significant data on the geometry of the regional structures, as well as the stratigraphic traps related to the three major Tertiary tectonic events previously discussed.

The last survey shot, in September-October 1991, through the Vercors and Chartreuse massifs is more particularly attractive as it represents the first attempt ever made to explore these two alpine massifs. A few examples of structural styles in the different thrust belts and related forelands are shown in the following chapters.

JURA THRUST BELT

The frontal leading edge of the Jura thrust belt (Fig. 2) is made of narrow and elongated highly deformed zones separated by undeformed plateaus.3 4 The first ones are related to complex hanging wall deformations in Pliocene times, along ramps, with a significant amount of senestral strike-slip displacement, and originating from a basal decollement hosted in late or middle Triassic evaporates and salt layers. Some of these ramps are ancient normal faults of presumed Oligocene age.

Classic plays in the Jura thrust belt are early Triassic sandstone on top of pre-Pliocene structural highs preserved below the decollement (as the oil recently tested at the Chaleyriat well), or middle Triassic dolostone in duplex (such as at now abandoned Valempoulieres gas field).

Untested plays are present in Permian structural highs preserved below the decollement. The main source rocks are generally believed to be the coal measures and oil shales of Stephanian-Autunian age.5 Toarcian black shales could be additional mature source rocks in the inner part of the Jura where large overthrusts have duplicated the Mesozoic cover.

BRESSE BASIN

The Bresse basin is the actual foreland of the Jura thrust belt. The up to 2,000 in thick Tertiary sedimentary infilling results first from rifting-related subsidence in late Eocene-early Miocene times and secondly from a slight foredeep subsidence in Neogene times.

A typical example of tilted block related to the first event is shown (Fig. 3). The normal fault at the edge of the block deepens straight down into the basement, as do most of the faults of the Bresse basin. However, the presence of thick late Triassic, as well as late Eocene-Oligocene salt, may locally induce a vertical partitioning of the deformations with listric normal faults in the upper sedimentary cover and highly dipping faults in the strata below.6

Typical traps in the Bresse basin are related to structural closures in Mesozoic strata along the footwall of normal faults. Moreover, still untested stratigraphic traps may be found in the deepest part of half grabens in the form of lateral pinch-outs of early Tertiary synrift sandstones. The presence of Stephanian or Permian basins below the Mesozoic strata has been documented in a few wells, is suggested by gravity modeling, but is poorly imaged on the presently available seismic, although reprocessing of original data could certainly and significantly improve their quality. Potential mature source rocks include both late Paleozoic and Toarcian strata.

VERCORS-CHARTREUSE

Imbricate thrusting of Neogene age at the deformation front of these massifs is well characterized on the recently acquired seismic.

The moderately dipping ramps, above which surface anticlines have developed, branch at depth in a decollement level hosted in late Liassic or late Triassic marls or evaporates, respectively. Structural traps may possibly be found in fractured carbonates along the hanging wall of these ramps, or as preserved horsts or tilted blocks below the decollement.

In the Chartreuse massif, the stacking of tectonic sheets in the inner part of the thrust belt induced a recent (Neogene) burial of some segments of the Stephanian-Autunian and Liassic source-rocks.7 This certainly led to a late stage of hydrocarbon generation, synchronous to the development of anticlinal traps above the decollement. In the Vercors massif, sections restored prior to the Neogene thrusting show that a several thousand meters thick Mesozoic basin developed until late Cretaceous times and has subsequently been inverted (Fig. 4).

Maturation of late Paleozoic and Mesozoic source rocks likely predate the formation of present Alpine structures. That is why the primary interest in that area would be pre-Neogene structural traps preserved below the early Mesozoic decollements, such as Jurassic tilted blocks related to the initial phase of Tethyan rifting. On the other hand, Neogene traps above the decollement in fractured Jurassic-early Cretaceous carbonates could have been recently charged with oil or gas dis-migrating from earlier accumulations.

Natural gas seeps are actually found along the eastern edge of both Chartreuse and Vercors massifs, while bitumen has locally been found along Neogene fault planes.

SOUTHEAST BASIN

The Southeast basin is the thickest French sedimentary basin, where up to 10,000 m of Mesozoic and Tertiary sediments may be found locally (Fig. 5).

The intensity and cumulative effect of each of the three Tertiary tectonic events previously mentioned may show great variability in space because of the lateral changes in the sedimentary thicknesses and of the distribution of potential decollement levels (including thick salt of late Triassic age), and because of the direction and dip of previous Mesozoic normal faults with respect to the successive Tertiary paleostress fields.8 For similar reasons the distribution, quality and maturation history of potential source rocks are closely related to the local sub-basins developments and subsequent tectonic deformations.

As the basin is several hundred kilometers wide, regional seismic profiles, such as those already shot in the 1980s, are quite useful for selecting the more prospective area before any detailed exploration, on necessarily more limited area, should be undertaken. Most of the large structures have surface expressions (e.g., Fig. 5), thus allowing stratigraphic correlation to be made, at least with the younger seismic sequences, and a good estimation to be proposed of the timing of structural traps formation.

The field section at the top of the large ramp anticline seen on this section shows two intervening north-south compression events. The first one in Eocene times probably is at the origin of the relatively severe deformations of early Cretaceous strata, while the second result in the folding of the early Miocene depositional unconformity and of subjacent strata in Neogene times.

Regional studies indicate that potential plays remain untested in deep early Mesozoic carbonates.

CORBIERES

The Pyrenean and Provence thrust belts are laterally offset of about 100 km along what is called the Corbieres Transverse Zone, a complex area where the displacement of the external units of the thrust belt was to the west or to the northwest, i.e. was quite oblique to the northward vergence of thrusting that characterized both the Pyrenees and Provence segments (Fig. 6).

Such features have been described in other transverse zones and generally imply the presence of transcurrent faulting at depth and in the inner part of the belt to accommodate the relative displacement of the two adjacent segments. The Corbieres Transverse Zone is actually probably superimposed on a major Mesozoic fault zone that has been separating a relatively thick sedimentary basin to the east from a much more slowly subsiding platform to the west.

No major transcurent fault has been recognized so far on the field. Such a fault could, however, have been reactivated as a normal fault in Oligocene time, at the time of the western Mediterranean rifting. As a matter of fact, the Corbieres also represent the upper part of the Gulf of Lions passive margins, with several Oligo-Miocene troughs superimposed on the Eocene thrust belt.8

Although about 40 wells have been drilled in this area, the Corbieres can still be considered as little explored. The late Cretaceous-Paleocene foreland of the thrust belt has for instance been very little explored as only two wells have been devoted to this area. Furthermore seismic shows the presence of a thick pile of little deformed reflectors below the late Triassic decollement of the thrust belt. They are interpreted as originating from a preserved sedimentary basin of unknown age as no well has ever tested this interval.

Source rocks in the Corbieres area include late Paleozoic coals and oil shales, Toarcian and Albian-Aptian marine black shales, and several layers of coal or bituminous limestone in the Senonian-Oligocene time interval.

CONCLUSION

The basins of eastern and southeastern France can still be considered as in an immature stage of exploration.

Although a relatively large number of wells has already been drilled in some area, most were spudded on poorly defined targets, as the seismic shot before 1980 was often unable to resolve the complexity of structural traps, and even to image the full thickness of the basins.

Some other areas, such as the western Alps massifs (including the Vercors and Chartreuse thrust belts) can be considered as unexplored.

Modern regional seismic data are now available to proceed with a full evaluation of wide areas where both excellent exposures in the field and many available boreholes (all onshore French well data are freely accessible) can provide accurate constrains for their interpretation and the petroleum evaluation as well.

The detailed structural analysis, which will necessarily be undertaken to properly evaluate the prospects, will have to focus on both, the chronology of deformation with respect to the timing of oil or gas generation, and with the distribution of potential detachment levels.

As a matter of fact, the nature and distribution of structural traps along the hanging wall of major faults will be quite different if they deepen right down to the basement, or if they have developed with a flat and ramp geometry in the sedimentary cover. Both cases may be found in these basins. The presence of major decollements will furthermore have favored the preservation of older structural traps, with the possibility that they have been filled in an early stage of basin subsidence and hydrocarbon generation.

A lot of geological data are now available. Regional synthesis, including the reprocessing of seismic lines and petroleum evaluation, are for that reason now being prepared by IFP to provide a complete set of modern data on little explored area of France. They include for the moment the northern Jura and its foreland,9 the Corbieres,10 and the Vercors-Chartreuse thrust belt where non-exclusive seismic has been shot in 1991.

It should be finally mentioned that applications for licensing in France may be filed with the administration at any time, since specific licensing rounds on restricted areas do not exist. It is also possible to apply at any time for offshore seismic surveys. Such an authorization will be delivered in less than 6 months.

We thus believe that good opportunities now exist in France to explore still undisclosed deep gas prospects in some little explored areas of the French Tertiary thrust belts and also more shallow oil prospects in untested stratigraphic traps of the foreland.

REFERENCES

Mascle, A., Bertrand, G., and Lamiraux, C., Exploration for and production of oil and gas in France: a review of the habitat, present activity, and expected developments, in Mascle, A., ed., Exploration and Petroleum Geology of France, Springer-Verlag, Spec. Publ. of the Eur. Assn. of Petroleum Geoscientis, Vol. 4, 1994, pp. 1-25.
Hervet, J.C., Regulations of hydrocarbon exploration and production in France, in Mascle, A., ed., Exploration and Petroleum Geology of France, Springer-Verlag, Spec. Publ. of the Eur. Assn. of Petroleum Geoscientis, Vol. 4, pp. 45-47.
Guellec, S., Lajat, D., Mascle, A., Roure, F., and Tardy, M., Deep seismic profiling and petroleum potential in the western Alps: constraints with ECORS data, balanced cross sections and hydrocarbon modeling, in Pinet, B., and Bois, C., eds., The potential of deep seismic profiling for hydrocarbon exploration, Editions Technip, Paris, 1990, pp. 425-437.
Philippe, Y., Transfer zone in the southern Jura thrust belt (eastern France): geometry development, and comparison with analogue modelling experiments, in Mascle, A., ed., Exploration and Petroleum Geology of France, Springer-Verlag, Spec. Pub. of the Eur. Assn. of Petroleum Geoscientis, Vol. 4, 1994, pp. 317-336.
Blanc, G., Doligez, B., Lajat, D., and Mascle, A., Evaluation du potentiel petrolier des formations paleozoiques de la Bresse et de la bordure jurassienne, France, Bull. Soc. Geol. Fr., Vol. 162, No. 2, 1991, pp. 409-422.
Bergerat, F., Mugnier, J.L., Guellec, S., Truffert, C., Cazes, M., Damotte, M., and Roure, F., Extensional tectonics and subsidence of the Bresse Basin: a view from Ecors data, in Roure, F., Heitzman, P., and Polino, R., eds., Deep structure of the Alps, Mem. Soc. geol. Fr., Vol. 156, Mem. Soc. geol. Suisse 1, vol. spec. Soc. geol. It., Vol. 1, 1990, pp. 145-156.
Butler, R.W.H., The influence of pre-existing basin structure on thrust system evolution in the western Alps, in Cooper, M.A., and Williams, G.D., eds., Inversion tectonics, Geological Society, London, Spec. Pub. 44, 1989, pp, 105-122.
Roure, F., Brun, J.P., Colletta, B., and Vially, R., Multiphase extensional structures, fault reactivation, and petroleum plays in the Alpine foreland basin of south-eastern France, in Mascle, A., ed., Exploration and Petroleum Geology of France, Springer-Verlag, Spec. Pub. of Eur. Assn. of Petroleum Geoscientis, Vol. 4, 1994, pp. 237-260.
Institut Francais du Petrole, The Northern Jura and its foreland, IFP Regional report, 1994.
Institut Francais du Petrole, Corbieres, IFP Regional report, 1993.