Charles Lamiraux Ministry of Industry Rueil-Mahnaison, France
Alain Mascle, Institute Francais du Petrole, Rueil-Mahnaison, France
Wells Drilled in the Bay of Biscay table (22447 bytes)Wells Drilled in the Channel and Sea of Iroise table (14939 bytes)Well Drilled in the Gulf of Lions table (12433 bytes) Forty nine wells (see tables) were drilled with-out success from the latter 1960s to the first half of the 1980s in French offshore areas, including the "Golfe de Gascogne" (Bay of Biscay), the Sea of Iroise (French western approaches) and the English Channel, and the Gulf of Lions (western Mediterranean).
A few wells encountered shows, with the most significant being located in the Bay of Biscay. About 10 years of reduced activity followed this first please of exploration. For a couple of years, these areas have been closely reassessed, taking into account the experience and data previously acquired. More particularly, a better understanding of tectonic processes at the origin of complex structural traps, a better taking into account of the distribution, quality, and maturation history of source rocks, together with significant improvements in seismic data acquisition-processing and basin modeling techniques, have led a few oil companies to apply for licenses in the three offshore areas (Fig. 1).(76919 bytes)
BAY OF BISCAY
Exploration in the French part of the Bay of Biscay has been carried out on the continental shelf adjacent to the Aquitaine basin and concerns the offshore part of the Parentis basin. Elf Aquitaine holds three licenses there and Esso Rep, Exxon's French subsidiary, holds one license.
Twenty-four wells have been drilled since 1966 with shows in seven. The most significant was oil tested in an early Cretaceous reservoir at Antares, but the discovery was rated noncommercial at the time. Another significant oil show was found in Barremian limestones in the Ibis-2 well.
The recent renewed interest of several oil companies in the Bay of Biscay continental shelf results largely from the close onshore Les Arbousiers (1991) and Les Pins (1993) discoveries by Esso Rep, operator, and Elf Aquitaine. The producing horizons are Lower Cretaceous "Purbeckian" fluvio-deltaic sandstones on top of structural horsts of early Cretaceous age, partly eroded by a major unconformity of Albian age.
The source rock interval is generally believed to be hosted in early Kimmeridgian marls and limestones and corresponds to marine organic matter. Other significant onshore oil producing fields along the Aquitaine coast include Parentis field-discovered as early as 1954, at that time the largest western European onshore oil field and still producing 3,800 b/d from limestones and karstified dolomites of late Jurassic-early Cretaceous age - and five smaller fields producing a combined 3,700 b/d from early Cretaceous reservoirs.
Esso Rep and Elf Aquitaine have acquired more than 1,000 sq km of 3D seismic the past 4 years, and drilling could resume as early as 1995. Present targets are still located in the vicinity of onshore producing areas.
The Parentis basin, however, extends far to the west on the outer continental shelf and upper slope.3 The structure of the basin is clearly depicted by a deep seismic profile shot in 1984 by the Ecors scientific association (Fig. 2)(23972 bytes). The Mesozoic-Tertiary sedimentary infilling is up to 6,000 m thick and lies on a continental crust severely thinned during the early Cretaceous crustal stretching event that led farther west to the oceanic opening of the Bay of Biscay in Aptian to Campanian times.
The present complexity of the basin results from two subsequent compressive events of Albian and Eocene ages at the origin of major unconformities and of large structural traps. Part of the deformation has been further enhanced by the presence of thick Triassic salt layers at depth.
IROISE SEA AND CHANNEL
The Sea of Iroise was explored from 1975-85. Thirteen wells were drilled, without any success if we except small oil and gas shows in two (Lenkett and Krogen wells).
However during the same period two fields were discovered and have subsequently been developed in neighboring areas within basins quite similar to the Sea of Iroise's basins.
On the northern coast of the British Channel, onshore Wytch Farm oil field is now producing about 75,000 b/d from two main reservoirs in early-middle Triassic braid-plain and playa margin sandstones (Sherwood formation) and Toarcian shallow marine sandstones (Bridport formation). In the Irish Celtic Sea, Ballycotton and Kinsale Head offshore gas fields are producing a combined 260 MMcfd from Albian sandstones.
A few fair to good reservoirs have been encountered in the Sea of Iroise wells in Permian, Triassic, Callovian, and Purbeckian sandstones or carbonates. The best potential source rocks belong to the Lias interval with organic matter of both marine and continental origin.
The quality, area distribution, thickness, and present state of maturation of these source rocks show however rapid variations resulting from both the initial configuration of Lias basins (rapidly subsiding grabens versus more stable platforms) and subsequent tectonic events, including a severe inversion of Mesozoic basins in Paleogene times.
Fig. 3 (41475 bytes) shows two contrasting Liassic sections. At Levneg 1, the Lias is more than 1,000 m thick, but source rocks are overmature and as a result the present residual organic content is low. At Rea Gwenn, the Lias section is only a few hundred meters thick, but the source rocks are at the threshold of the oil window. The average organic content is about 1.5% with maxima up to 4%.
Most of the wells drilled so far in the Sea of Iroise are located on the resulting large and seismicly well-imaged inversion anticlines. The Levneg 1 well was located on such an anticline. Pleistocene sediments cover Barremian marls so that about 1,400 m of Cretaceous-early Tertiary sediments have been eroded following the tectonic inversion.
A similar anticline is present near Wytch Farm, but only (me well, at Kimmeridge Bay on top of it, is producing a marginal amount of oil from fractured shares and carbonates of Dogger age. Most of the production indeed comes from close Jurassic tilted blocks pre-served from any subsequent inversion.
Inversion anticlines are related to a late Paleogene compressive event and are probably too recent in age with respect to the main period of hydrocarbon generation and expulsion, which is believed to have occured in Cretaceous times according to subsidence-maturation modeling." A better understanding of these petroleum systems is probably at the origin of recent applications for licensing in these areas from Elf Aquitaine, BHP Hamilton, and Amerada.
In the near French waters of the English Channel only one well, Nautile, was drilled in 1982 without success. However only a reduced section of Jurassic age lying at a depth of about 600 rn on lab Paleozoic red beds has been encountered at that site. On( pen-nit is still being operate( by Hunt Oil along the Anglo French median line, only 71 km south of Wytch Farm oil field, and 750 km of seismic have been shot in 1993.
GULF OF LIONS
Eleven wells were drilled from 1968-85 in the Gulf of Lions. All were dry with only minor shows encountered in one of them. However, Elf Aquitaine recently applied for an exploration permit covering most of the continental shelf.
The Gulf of Lions is a young passive margin of Neogene age, resulting from continental rifting in late Oligocene-Aquitanian times.
This margin is superimposed on a segment of the Pyrenean thrust belt that developed from late Cretaceous to late Eocene times. As a result, some of the Pyrenean thrusts have been reactivated as normal fault during the subsequent rifting, and the formation of late Oligocene-Aquitanian half-grabens is generally controlled by thin-skin extension on the upper part of the margin, while crustal thinning has been transferred farther east in the lower part of the margin.
Rifted basins are well developed offshore, but some of them are also present in the close onshore area. This is more specifically the case for the basins located to the northeast of the margin, in the Camargue area, where a small oil field, Gallician, has been discovered. About 4,840 tons of heavy, paraffinic oil was produced from 1951-59. Reservoirs are hosted in Oligocene strata with a main lacustrine source rock of similar age.
One or two additional wells on similar plays are scheduled to be drilled in 1995 in the same Camargue basin. The wells drilled in the offshore area are located on regional highs at the edge of the Oligocene basins. They generally reached a Mesozoic or Paleozoic basement directly below the post-rift Neogene cover. Deep Oligocene :Plays have thus never been tested in the Gulf of Lions ind will probably be the nam target of the next phase ),f exploration.
Recent maturation modeling 6 actually suggests that Oligocene source rocks within these basins would presently be in the oil window, or even in the gas window for the deepest horizons (Fig. 4).(18795 bytes)
CONCLUSIONS
The development of French offshore sedimentary basins can be linked to some of the main stages of rifting and basin formation in western Europe.
The first stage in late Triassic-early Jurassic times corresponds to active rifting in the North Sea and Sea of Iroise, among others basinS,7 and leads to the oceanic opening of the Central Atlantic Ocean farther south as early as Callovian times.
The second large rifting event in early Cretaceous times is well recorded in the Parentis basin and led to the opening of the Bay of Biscay in Aptian-Campanian times.
Finally, Oligocene-early Miocene rifting in southern Europe is at the origin of the western Mediterranean oceanic basin in Burdigalian times, and was followed by the large post-rift subsidence of the adjacent Gulf of Lions passive margin.
All French offshore basins have experienced complex tectonic histories and as a result their petroleum systems are quite difficult to assess. In the Bay of Biscay, superimposed rifting events in middle-late Triassic and late Jurassic-early Cretaceous times, two compressive events in Albian and Eocene times, plus halokinesis of Triassic salt in the deeper part of the offshore Parentis basin, led to complex and rapidly changing geometries of the whole basin and of individual structural or stratigraphic traps.
This in turn has affected the distribution and quality of reservoirs, the timing of the generation of oil and gas, and the geometries of migration pathways as well.
In the Sea of Iroise, the severe rifting event of Liassic age induced differential block collapses and the deposition of source rocks with different initial potential in little or rapidly subsiding areas.
The major tectonic inversion in late Paleogene times is at the origin of large structural traps, but also induced the uplifting of the deepest part of the Liassic basins. As a result, the generation-expulsion of hydrocarbons from Liassic source rocks was stopped, and little if any oil and gas were available to migrate to these large Tertiary traps. Most of the wells drilled so far were located on such structural highs, while we have learned from nearby Wytch Farm oil field that older (Jurassic) and much more subtle structural traps (preserved tilted blocks) are the right targets.
In the Gulf of Lions, Oligocene rifting developed on an overthickened crust resulting from the previous so-called Pyrenean orogen (late Cretaceous-early Oligocene) at the origin of the Pyrenees and Provence thrust belts in southern France. The first wells in this area focused on regional highs at the edge of the Oligocene rifts, which have actually never been investigated. These rifted basins are deeply buried below a thick Neogene post-rift blanket and their structures have been poorly imaged by seismic. Numerous wells in the close Camargue basin of similar age and origin have shown however that the main productive horizons, and source rocks as well, are located within the Oligocene syn-rift sequences.
These complexities can certainly explain the lack of success of the first stages of exploration in French offshore areas. We must remember that our knowledge of the geological history and petroleum plays within these basins was very speculative at that time. The poor quality of the seismic and the lack of well controls were additional difficulties, and most of the first wells were spudded on the largest structural traps as seen by the seismic, while it now appears that more subtle or deeper traps are probably better targets. For about 10 years, regional studies have been undertaken by both the oil industry 8 9 and the French scientific community 3 5 10 so that much more comprehensive data are now available to explorationists for a better assessment of these basins.
All French offshore well data will be public by yearend. New exploration and production regulations have been adopted and the procedure for obtaining exploration permits trimmed to 9 months from 2 years. Permits will be awarded without initial public inquiry, the tax system has been revised, and specific taxes and royalties have been ended for offshore fields. Applications for licensing may be filed with the French administration at any time, and neither surface rentals nor fees apply.
REFERENCES
1. Mascle, A., Bertrand, G., and Lamiraux, Ch., Exploration for and production of oil and gas in France: a review of the habitat, present activity, and expected developments, in Mascle, A. (ed.), Hydrocarbon and Petroleum Geology of France, special publication of the EAPG No. 4, Springer, Heidelberg, 1994, pp. 328.
2. Espitalie, J., and Drouet, S., Petroleum generation and accumulation in the Aquitaine basin, in Spencer, A.M. W.), Generation, accumulation and production of Europe's hydrocarbons IL special publication of the EAPG No. 2, Springer, Heidelberg, 1992, pp. 127-150.
3. Bois, Ch., and Gariel, O. Deep seismic investigation in the Parentis basin (southwestern France), in Mascle, A. (ed.), Hydrocarbon and petroleum geology of France, special publication of the EAPG No. 4, Springer, Heidelberg, 1994, ppg 173-186.
4. Deronzier, J.F., Bruneton, A., Mondon, J., and Mascle, A., Liassic source rocks from three wells from the Sea of Iroise (French western approaches) and Dorset (U.K.) outcrops: nature and modelled maturity history, in Mascle, A. (ed.), Hydrocarbon and petroleum geology of France, special publication of the EAPG No. 4, Springer, Heidelberg, 1994, pp. 157-169.
5. Seranne, M., Benedicto, A., Truffert, C, Pascal, G., and Labaume, P., Structural style and evolution of the Gulf of Lions Oligo-Miocene rifting: role of the Pyrenean orogeny. Marine and Petroleum Geology, 1995, in press.
6. Vially, R., and Trernolieres, P., (sub-Miffed) Geodynamics of the Gulf of Lions: Implication for petroleum exploration, in Structure and prospects of alpine basins and foreland, edited by P. Ziegler and F. Horvath, editions Technip Paris (in prep.).
7. Sinclair, I.K. Shannon, P.M Williams, B.P.J., Harker, S.D., an( Moore, J.G., Tectonic control o sedimentary evolution of three North Atlantic borderland Mesozoic basins, Basin Research, Vol. 6, No. 4, 1994, pp. 193-218.
8. Guennoc, P., Debeglia, N., Gorin C., Le Marrec, A., and Mauffrex A., Anatomic d'une marge passive jeune (Golfe du Lion, Suex France). Apport des donnees geo physiques, Bull. Centre Recherche Exploration-Production Elf-Aquitaine, Vol. 18, No. 1, 1994, pp. 33-57.
9. Drouet, S., Deronzier, J.P, and Mascle, A., Mer d'Iroise (French western approaches), Nonexclusive report, Institut Francais du Petrole, 1993, 60 plates.
10. Bois, Ch., Gariel, O., and Sibuet, J. Cl. (eds.), Etude de la croute terrestre par sismique profonde: met celtique, Manche et ses approches occidentales. Profits SWAT et WAM, Memoire de la Societe Geologique de France, No. 159, 1991, Vol. 1, 220 P., Vol. 2, 22 plates.
