12 Lassalle - Aapg 1995 - Aquitaine Basin - Evolution And Hydrocarbons.pdf

The North Pyrenean Aquitaine Basin, France: Evolution and Hydrocarbons1 Robert Bourrouilh,2 Jean-Paul Richert,3 and Greg Zolnaï4

ABSTRACT The Mesozoic–Tertiary Aquitaine basin overlaps the Aquitaine block and the northern edge of the Iberian margin. Both units are situated in the southwesternmost part of the European Continent. The Aquitaine shelf is a stable platform overlying a relatively thin crustal segment; it underwent extensional block faulting and many of its folded structures are related to salt tectonism. The Iberian block is a thicker lithospheric unit that acted as a buttress. At the junction of these two structural domains the South Aquitaine or North Pyrenean area developed, where crustal stretching, rifting, wrenching, and orogenic compression were maximal from the Mesozoic to the Tertiary. The history of the basin has been a suite of rifting attempts, in the context of the progressive opening of the Bay of Biscay, which never completely succeeded. The relative plate motions and the later convergence led, nevertheless, to the formation of the Aquitaine basin and to the emergence of the Pyrenean fold belt. The Mesozoic–Tertiary sedimentary infill of the basin is mostly marine, with thick evaporites, stable platform carbonates, subsiding platform shaly-calcareous deposits, and a characteristic, diachronous turbiditic (flysch) and molasse complex.

©Copyright 1995. The American Association of Petroleum Geologists. All rights reserved. 1 Manuscript received August 4, 1993; revised manuscript received January 5, 1995; final acceptance January 25, 1995. 2 Laboratoire Cinématique Bassins et Marges, CIBAMAR, Université Bordeaux I, 33405 Talence, France. 3ELF-Aquitaine Production, 64000 Pau, France. 4Consultant geologist, formerly with ELF-Aquitaine Production, 64000 Pau, France. The authors thank the Direction Exploration of ELF-Aquitaine Production for permission to publish their work and for material support. They are indebted to their colleagues for steady support, advice, and data. Special thanks are due to Jacques Henry and to members of the ELF-Aquitaine Exploration team in Boussens, France, for continuing help over the years. We thank the critical readers of AAPG for constructive remarks. This paper was presented by the authors, as an oral presentation and poster session, at the 1988 Nice International Convention of AAPG–Society for Sedimentary Geology. It was completed, updated, and reworked in 1992–1993.

AAPG Bulletin, V. 79, No. 6 (June 1995), P. 831–853.

Events during the basin-forming, extensional, and translational periods contributed more to the generation of the hydrocarbon accumulations than did the effects of the subsequent compressive structural regime. The latter, however, may have enhanced thermal flow and, thus, maturation of hydrocarbon source rocks. INTRODUCTION The paper attempts an up-to-date regional synthesis using the results of recent exploration and facts of earlier, classical knowledge. Different types of deformation mechanisms are envisioned and followed through geological times. Both the role of geological heritage and the effects of changing stress fields are accounted for, in order to investigate their impact on hydrocarbon habitat. Regional and continental (global) considerations are blended together with local exploration data to issue a realistic interpretation useful for future research work. BASIN DEFINITION The Aquitaine basin, in the large sense, took its definitive shape during the Tertiary; elements of its general framework have been present since the end of the Variscan (Hercynian) orogeny. Its northern limit (Figure 1), the shoal of Poitiers, has been a slightly positive axis linking the Massif Central with the Armorican fold belt during the whole Mesozoic–Tertiary period. The shoal of Poitiers allowed marine connection between the future Paris and Aquitaine basins. In a somewhat similar way, its southern limit (Figures 1, 2, 3b), i.e., the northern edge of the Iberian landmass (the presentday “axial” high chain of the Pyrenees) has been an all-time relative high, but allowing marine communication. At several times, platform carbonates have formed a continuous blanket across this tectonic edge. The Pyrenean orogenic belt proper is to the north of the Paleozoic buttress—it is included within the geographic limits of the Aquitaine basin and the northern foothills of the mountain chain. To 831

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Evolution and Hydrocarbons, Aquitaine Basin, France

VA RI SC AN

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Figure 1—Location and framework. (a) Location map, (b) basin framework, (c) general north-south cross section. CAH = Celt-Aquitaine hinge line, NPF = westerly extension of North Pyrenean fault, NPT = North Pyrenean thrust, P = Paleozoic, P–TR = Permian–Triassic, J = Jurassic, Kl = Lower Cretaceous, Kuf = Upper Cretaceous flysch, Kup = Upper Cretaceous platform, Te = Tertiary, LR = La Rochelle. (Based on BRGM et al., 1972.)

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the east, the basin continues in the foreland of the Mediterranean structural domain; the ToulouseVillefranche fault, however, has acted as an effective hinge line, controlling the edge of the presentday basin. To the west, the basin was continuous until the Early Cretaceous, with what is today the eastern salient edge of the North American Continent, i.e., the Flemisch Cap and the Jeanne d’Arc Basin of the Labrador shelf. This western dissected passive margin was truncated by erosional events during the Late Cretaceous–Tertiary; it is now overlain by Tertiary to Holocene turbidites of the Bay of Biscay. Several subbasins (or “infrabasins,” hidden under the Tertiar y cover) are separated by shoals or uplifts within the basin area (Figure 2). These are, from north to south: the Charente (and the small Quercy) basin, the Gironde High, the Parentis Basin

vert. exag. 4 x

(Figure 3a) which is bounded to the southeast by the Montauban High, the Landes shoal, the Mirande basin, and the Adour basins (Figure 1), comprised of the Arzacq, Tarbes, and Comminges basins. With the exception of the east-northeast–oriented Landes shoal, most major paleostructural features exhibit northwest-southeast orientations. The southernmost part of the “autochthonous” Adour basin disappear s to the south under neath “allochthonous” segments of the North Pyrenean thrust front (Figures 1c, 2, 3b). This deep trough is the sub-Pyrenean zone. The tectonic envelope of the overthrust front is the northern limit of the North Pyrenean fold belt. To the south, it is cut off by the North Pyrenean fault, an east-southeast– trending subvertical fault complex, that runs along the Iberian margin and limits to the south the thick, sedimentary sequences of the Aquitaine domain.

Bourrouilh et al.

Figure 2—Detailed map of structural elements. AD = Audignon diapir, AU = Arbailles uplift, AxCh = axial (high) chain, BM = Basque massifs, Dax = Dax diapir, GRU = Grand Rieu uplift, Ib = Ibis structure and offshore well, LR = La Rhune massif, MH = Montauban high, NPF = North Pyrenean fault, NPT = North Pyrenean thrust, NPZ = North Pyrenean zone, S = Sarrance, SB = Salies de Béarn diapir, SP = St. Palais structure, S-PZ = subPyrenean zone, SS = Ste. Suzanne structure, TVF = Toulouse-Villefranche fault. (Adapted from Soler, 1972; Choukroune and Séguret, 1973; Zolnaï, 1975; Chiron, 1980.)

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The deep, prospective part of the Aquitaine basin, approximately one-third in surface area, lies in the southwestern portion of the above sensu lato basin. It is framed by three major structural lines, which are the Celt-Aquitaine hinge line (or flexure) to the northeast, the Toulouse-Villefranche fault to the east, and the North Pyrenean thrust front to the south (BRGM et al., 1972; Henry and Mattauer, 1972; Castéras, 1974; Winnock, 1974; DurandDelga et al., 1980). The effective western limit of the basin, the truncated edge of the Mesozoic

N.P.T

.

++ +

+ + + + + + ++ + + + + + +

N.P.F.

sequence, is offshore and remains as yet poorly defined (Boillot et al., 1984). EVOLUTION The early Precambrian to Paleozoic evolution of the basin is rather poorly known. The economic basement is composed of a full succession of Paleozoic series folded during the Variscan (Hercynian) orogeny (Bourrouilh et al., 1980)

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Evolution and Hydrocarbons, Aquitaine Basin, France

SW

POSITION OF SECTIONS

b

NE BOUGUER ANOMALY

+ 20 0 - 20

0

100 km LANDES SHOAL

PARENTIS BASIN

NORTH AQUITAINE PLATFORM Garonne R

Ocean

S.L.

....... ...... ........

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Te K.L.

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............. ....... ........

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(a) 20 ?

? MOHO (projected from west)

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50

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IBERIA

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TRANSVERSE BASEMENT WRENCH-FAULTS

TECTONIC STACK OF BASEMENT SLABS WITH MAFIC INJECTIONS

- 30 MOHO

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0

50

100 km

Figure 3—Structural cross sections. (a) Northern Aquitaine (Parentis) basin, (b) southern Aquitaine (Adour) basins. Au = Arbailles uplift, A = Audignon, D = Dax, S = Salies de Béarn, SP = St. Palais diapiric structures, NPF = North Pyrenean fault zone (westernmost limit), NPT = North Pyrenean thrust zone, P = Paleozoic, P–TR = Permian–Triassic, TRu = Upper Triassic, J = Jurassic, Kl = Lower Cretaceous, Ku = Upper Cretaceous, Te = Tertiary. (Adapted from undisclosed ELF-Aquitaine documents by J. M. Flament and BRGM et al., 1972.)

(Figures 3, 4). They are underlain by (or they include) older, Precambrian ensemble(s), which may have helped to shape the mid- to late-Paleozoic fold belts (Britanny and Pyrenees). The Variscan structural heritage of the basin is composed of fault lines and lineaments, the main directions being (1) north-northwest–south-southeast (N160°), (2) northeast-southwest (N40°), and (3) west-northwest–east-southeast (N100°) (commonly N110°, Figure 5a). Some of the basement blocks, limited by elements of the inher ited fault network, remained mobile throughout subsequent basin evolution. They became paleogeographic units with distinct subsidence histories and local deformations along their edges. Small, synsedimentary adjustment movements (on the order of 1 km or

less) in the extensional, compressional, and wrenching modes initiated block rotations, tilts, uplifts, and downwarps. Some of the north-northeast or northwest-trending old structural lines straddle the present-day (“Alpine”) Pyrenean belt (Zolnaï, 1991, Figure VI-29–31). Gentle scissors or piano-key–type differential tilting along such fault lines eventually delimited major paleogeographic domains (containing thicker vs. thinner salt deposits, or representing platform vs. basin limits), which in due course became structural compartments (e.g., Arbailles, Figures 2, 3b). The Paleozoic series, whether on the surface outcrops or in subsurface boreholes, are always moderately to strongly folded with varying degrees of metamorphism. In the Pyrenean chain, they

Bourrouilh et al.

v

Charlas Mazères

St-Marcet

250

BARREMIAN NEOCOMIAN

K.AL

Vic-Bilh PECORADE

1 2 3

JURASSIC

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CAZAUX MIMIZAN N.

. .. ...

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?

.. . . . . . . . ... . .

WEALDIAN PURBECKIAN

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150 to 1650

TRIASSIC

VOLC.

KEUPER

PERMIAN

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v

S

. ...... ... ... ...... ... ...... .

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400

300 to 3000

Castéra-Lou

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700

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0 to 2000

Montastruc

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Ledeuix

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Lagrave

v

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100

CAPROCK

Lacq

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2000

VOLC.

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? . . . . . . .. . . . . . P . Pal. . . .. . . .. . . . . . . . . .. . . . . . . .

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vv

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0-5000

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. .. . . .. . . . . . . . . . .. . . . . . . . . . . . . . . DP DANO-PALEOCENE K.S .. . .. SENONIAN ... K.CT .. .. .. . K.AL .... TURONIAN ... .... . . . .. . .. . . CENOMANIAN . .. . . .. . v v . . ... . . .

PARENTIS Mothes Lucats

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835

CAZAUX Lavergne

?? v v v

vv v

. . . . .. . ..... .. . ...........

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N.C. heavy oil

DEVONIAN

SILURIAN

C condensate

... ........ ..... .. .. ...... ..

Figure 4—Stratigraphic logs. (a) North Pyrenean trough and Arzacq-Tarbes basins, (b) Parentis Basin (adapted and modified from ESSO-Rep documents in ELF-Aquitaine and Chambre Syndicale, 1991; Mathieu, 1986; Mediavilla, 1987).

exhibit penetrative schistosity of two or three generations (Majesté-Menjoulas, 1979). To the north of the Parentis Basin, the basement of the flat undeformed North Aquitaine platform consists mainly of granites (Figures 1c, 3a). The first post-Variscan rifting attempt has taken place as soon as the Early Permian, i.e., during the late stages of the post-Variscan peneplanation. Roughly, north-south– and east-west–oriented grabens are known in the Pyrenees as well as under the basin itself; they are filled with red continental clastics and volcanic flows, with thicknesses up to 1000 m (Figures 1c, 5a, 6, 7a). One of these grabens is the transverse (northeast-southwest–oriented) Elizondo trough in the Basque country (Figure 2) (Gapillou, 1981). The Germano-type Triassic series (threefold stratigraphy) record the second rifting event (Curnelle et al., 1980, 1982; Curnelle, 1983) (Figure

5a, b). The thick upper part (Keuper) is salt bearing (Figures 4a, b; 6). The Triassic was deposited in a flat-lying basin in the center, while rift geometry prevailed in both the Parentis Basin and along the southern basin edge (near the North Pyrenean fault zone) (Figure 7b). High emerging blocks (e.g., Arbailles and Grand Rieu) (Figures 2, 3b, 6; see also hydrocarbon field sections) were formed along the southern basin edge, which alternated with deep corridors already in place during the Late Triassic. Submarine (ophitic) volcanic flows and intrusions took place at the Triassic–Jurassic transition (approximately between the Keuper salts and the Liassic anhydrites), near the Iberian-Aquitaine boundary zone. The thickest volcanic wedge was emplaced along the southernmost, rifted basin edge (several hundreds of meters), whereas the volcanic wedges taper to zero toward the present-day basin center, including the Parentis Basin (Figures 4a, b; 6).

RI AN

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Atlantic Ocean

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N100° N110°

N40°

Bibao

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Toulouse

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APPARENT TRAJECTORY OF IBERIAN PLATE (not to scale)

NORTH PYRENEAN FAULT ZONE

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Mediterranean Sea

FUTURE NORTH PYRENEAN FAULT

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TECTONO-SEDIMENTARY THRUSTS WITH CHRONOLOGY

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NORTH PYRENEAN THRUSTS

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APPARENT TRAJECTORY OF IBERIAN PLATE (not to scale)

THRUST OF BOIXOLS (Upper Maastrichtian)

APPARENT TRAJECTORY OF IBERIAN PLATE (not to scale)

PYRENEAN OROGENY

EUROPE

AQUITAINE

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Bordeaux

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San Sebastian

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Bay of Biscay

FLYSCH BASINS (u. Albian Senonian)

MEGATURBIDITES

Bay of Biscay

Atlantic Ocean

(c)

Figure 5—Paleogeodynamic maps. (a) pre-Mesozoic structural heritage, (b) the mid-Cretaceous revolution: Aptian–Albian basins, (c) Upper Cretaceous flysch furrows: Albian–Cenomanian to Senonian. Incipient compression developing from late Turonian(?) through latest Cretaceous, (the Boixols thrust is but one among several upper Cretaceous compressive structures), (d) Eocene backward migration–principal compression; salt diapirism active from Albian to present-day. Based on Bourrouilh and Doyle (1985, 1987a, b); Bourrouilh and Zolnaï (1988); and Bourrouilh et al. (1988).

pull ALBIA N -apa rt b asin s

N 160°

N 40°

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Basalt flows on the sea floor

Anoxic basins (black shales)

Fanglomerates flysch basin

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STRUCTURAL HERITAGE

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Bay of Biscay

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(a)

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836 Evolution and Hydrocarbons, Aquitaine Basin, France

Bourrouilh et al.

N.

MIGRATION OF DEPOCENTER

MIGRATION OF BAY OF BISCAY

MIGRATION OF SOUTHERN BASIN

SOUTH PYRENEAN ZONE (JACA BASIN)

MORPHOGENIC MIGRATION AXIAL ZONE ("HIGH CHAIN")

B A S I N

NORTH PYRENEAN ZONE

........ ........ ........

.

? Tertiary ?

~ ~~~ ~ ~~~ ~

~~ ~ ~~

~ ~~

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DEEP FLYSCH BASIN

........ ........ ........ ........ ........ ........ ........

CRET ACEO US

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CARBONATE PLATFORM Upper Cretac.

Lower Cretac.

... . . .

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Lias

vv

v

v

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. . . . .. .. .. . ... .. . . . . .. ... . . ... . . . . .. . . . .. . . . . . . .. .. . . . . ..

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vv

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. . .

Te ~ ~ ~

Dogger

Pillow lavas

..... . . . .. ..... ...... ...... .. .... ... ...... ...

Upper Cretac.

.... . . .... . . . ....

. . .

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SHALLOW-WATER CARBONATES

DEEP FLYSCH BASIN

S. EBRO BASIN/PLATFORM

....

FILLING

A Q U I T A I N E

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~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~

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~ ~ ~

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Albian

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Trias

Permian Paleoz.

. . . NORTH PYRENEAN THRUST FRONT

NORTH PYRENEAN FAULT

. . . .

SYNSEDIMENTARY FAULTING and DIAPIRISM

. . . .

Figure 6—North-south evolution diagram across the Pyrenees. The axis of maximal subsidence is shifted from the North Pyrenean domain (Triassic–Upper Cretaceous) to the South Pyrenean domain (Tertiary).

The lowermost Jurassic (lower Liassic) anhydrites grade upward into a dolomitic, then shallowmarine, mainly limestone platform sequence (Dogger–Malm). At the end of the Malm, the basin became partly emergent, as demonstrated by the presence of anhydrite, some coal, red shales, and important locally dominant clastics (sands) and high sand content of the carbonates, as well as the intraformational (desiccation) breccias. The first salt movements (gentle swells and salt cushions) seem to have started as early as the Portlandian (Winnock and Pontalier, 1970). Recent work showed evidence of rifting south of Bayonne during the Late Jurassic (Montagné, 1986). A new phase of basin differentiation began during the Early Cretaceous (Figure 5b, c). At the Jurassic–Cretaceous boundary, large areas of the basin became emergent and sharp erosional topographic features (cliffs, slumps, screes) coincident

with deep faults are known in the south (e.g., the Meillon ridge), as well as in parts of the Parentis Basin (Lugos; see “Hydrocarbons” section). Massive sand bodies were deposited in the Parentis Basin during the Purbeckian–Wealdian period (Figure 4b). The equivalent series of the southern Aquitaine subbasins are clastic, sandy carbonates (Lacq); sands; breccias (Meillon); and shales. During the Neocomian–Barremian period, the main subbasins (Parentis, Mirande, and ArzacqTarbes) became separate entities and the first massive salt structures were emplaced (e.g., Audignon). Salt intrusions and salt lineaments were emplaced principally along faults on the edges of the rhomb-shaped pull-apart basins that first appeared during this period. The subsiding basins became confined and were filled with black shales; some of these (e.g., Arzacq basin) record up to 1000 m of sediments.

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Evolution and Hydrocarbons, Aquitaine Basin, France

South

North ....

+ + +

...............

PERMIAN

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(a)

STRETCHING : ~ 80 km

early TERTIARY N. PYRENEAN FURROW

0

GRAND RIEU upl.

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LANDES HIGH

PARENTIS BASIN

CELT-AQUITAINE hingline

- 10

(b)

salt movements

- 20

?

?

?

- 30 MOHO

- 40 COMPRESSION ~ 30 km

ONLY EXTENSION - STRIKE SLIPPING OROGENIC FOLD BELT N.P.F.

0

PRESENT - DAY

shift in section - line

N.P.T.

Garonne R.

+

+

+

- 10

wrench - faults

(c)

?

- 20

?

MOHO

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BOUGUER GRAVITY

+ 50

Scale : Horiz. = Vert.

+ 20

0 - 50 - 100

?

- 50

- 10

0

50

100 km

- 100

Figure 7—North-south cross section of Mesozoic–Tertiary evolution (location on Figure 2). (a) Post-Paleozoic heritage. (b) Mesozoic–early Tertiary extension, affecting the whole Aquitaine basin, i.e., the Parentis and Adour/Arzacq subbasins and the North Pyrenean furrow, and initiating early salt diapirism (arrows). (c) Middle to late Tertiary compression, limited to the southernmost North Pyrenean furrow, while the other basins and shoals undergo only minor extension and wrenching, with renewed salt tectonism. NPF = North Pyrenean fault, NPT = North Pyrenean thrust.

During the late Early Cretaceous (Aptian to early Albian part of the middle Cretaceous), subsidence rates increased in all basinal areas. A set of northwest-southeast–oriented lozenge-shaped basins and uplifts (with east-west– and south-southeast–trending edges; Figure 5b) developed; their geometry indicates sinistral wrench movements occurring along a northwest-southeast axis (Figure 8). In the meantime, bioher m-capped plateaus were emplaced on highs, like the Arbailles block in the western Pyrenees, and on tilted blocks in the structural staircases of the southern basin margin (Figures 2, 3b, 4, 7b) (Canérot and Delavaux, 1986). Along the southern, mobile edge of the basin, the differential tilting of some of the blocks produced dips as high as 30°.

During the late Albian, yet another important structural reorganization took place in connection with the ongoing opening of the North Atlantic Ocean. This revolution or third rifting episode can be related to further sinistral extensional wrenching. The event can be recognized all over the Aquitaine basin (and beyond its limits) and has fundamentally reshaped the paleogeographic pattern (Figure 5c). Subsidence rates strongly increased. Synsedimentary tectonism along the inherited fault pattern was renewed and enhanced halokinesis. High subsidence rates in the basin centers and penetrative diapirism along the basin edges were coeval and structurally linked events (Figures 7, 8, 9). Westward-opening rifted furrows were thus reactivated or newly formed: the Parentis Basin to the

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Oil or gas producing structures

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Diapirs

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Figure 8—Structural map of the southernmost Aquitaine basin (North Pyrenean domain). Inset: the opening of the rhomb basins is due to a combination of inherited faults and northwest-southeast–oriented regional senestral shear couple. The synchronous salt diapirism follows the border faults of the Albian rhomb basins. The Pyrenean thrust front, composed by en echelon segments, partly overlaps the rhomb basins and the foreland platform.

north, and the North Pyrenean trough to the south, each containing several thousands of meters of rhythmic sediments (Feuillée et al., 1973; Souquet et al., 1977, 1985). The already existing ArzacqTarbes pull-apart basins in the southern domain received over 3000 m of shales during this time. This evolution can be considered as a result of the limited easterly shift of the Iberian block, eventually generating an east-west–oriented sinistral megashear zone. These opening pull-apart basins merge into a North Pyrenean flysch furrow during the Cenomanian (Curnelle et al., 1980). Several inherited structural lines became of major importance during this period. The west-east to east-southeast–oriented North Pyrenean (Figure 10) fault was initially a braided fault system, probably

much wider than at present (Figure 7b, c). It stretches over 300 km (200 mi) between the easternmost Pyrenees and the Basque country, where it splits into several east-west–oriented branches, which separate the different elements of the Basque Massifs. Its trace is lost beyond the Basque transversal zone (Schoeffler, 1982), another compound fault group oriented northeast-southwest, which connects the western Pyrenees proper and the Cantabro-Asturian chain, the latter being the offset westerly continuation of the Variscan fold belt. This ancestral transverse feature, active since at least the Permian, actually straddles across the South Aquitaine and Nor th Iberian domains. Presently it can be followed to the north (Dax) and the south (Pamplona) beyond the limits of the alpine fold belt proper (Rat, 1983) (Figures 2, 11, 12).

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Evolution and Hydrocarbons, Aquitaine Basin, France

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Figure 9—“Albian revolution” compares (a) north-south field section near Sarrance (North Pyrenean furrow, see Figures 2, 7), and (b) north-south subsurface section of the Vic Bilh field (north Tarbes basin). The tectonic events during the late Early Cretaceous are very similar in these two remote areas (fold belt vs. foreland basin). The Albian fine clastics directly overlap the Triassic evaporites and shales, and demonstrate intense salt diapirism. The Tarbes basin has not been affected by any of the later, compressive events. (J. Henry and J.-P. Richert, unpublished ELF-Aquitaine documents.)

Both these major fracture zones, the North Pyrenean fault and the Basque transversal, reacted repeatedly during the Mesozoic and Tertiary tectonic evolution. In the Parentis Basin the coarse clastics, which were derived from the northeast (the Massif Central; B. Martin, 1975, personal communication), were spread out in deep channels and deep-sea fans on the basin slope and floor, near its northern edge, within a mainly shaly-marly rhythmic (flyschtype) sedimentation. In the North Pyrenean trough, the clastic elements were of southern and eastern origin (basement uplifts) or they were derived from the north (from the carbonate platforms; Figure 5c). The southern edge of the flysch furrow had been leaning

against the Iberian margin, along what was to become the North Pyrenean fault (Figures 6, 10, 11). This fault, nevertheless, also became involved in the shearing process. Faulted tectonic blocks (of 10-km magnitude) calved from the main Iberian landmass, were uplifted and tilted, and in some instances rotated. Large areas of the present-day high chain became emergent [e.g., to the south of the Mendibelza massif in the Basque country (Boirie and Souquet, 1982), or in the central Pyrenees], offering source areas for coarse clastics that flowed into the deep-sea arm (“aulacogen”; Souquet and Debroas, 1981) to the north (Figures 4a, 6). Above the basal breccias (wildflysches), the first thick sandy-shaly turbidite (flysch) sequence was deposited (Albian–Cenomanian). In the meantime, huge tholeiitic basalt masses (pillows and lava flows as well as sheeted dikes and sills) were emplaced within the flysch furrow, between Bilbao and the area south of Pau. In the Parentis Basin (Figure 4b), this type of rhythmic sedimentation was limited to the Albian, while in the North Pyrenean trough it lasted through the rest of the Cretaceous and throughout the Paleocene. The Nor th Pyrenean f lysch sequences are extremely thick, up to 5000 m (Figure 4a). They are diachronous, since the receiving tectonic basins shifted laterally (east to west and south to north). Facies changes to finer, shaly, and/or platform-type calcareous sediments were frequent in the slower subsiding areas. With time, the clastics became finer grained and more calcareous. The initial narrow and long upper Albian furrow has progressively evolved into an open embayment, which together with the Parentis Basin projects the outline of the present-day Bay of Biscay (Figure 1b). During the Turonian, general (eustatic) withdrawal of the sea resulted in more carbonate platform-type sedimentation, even inside the flysch troughs, where finely brecciated, layered limestones are typical. Inside the North Pyrenean mobile domain and the Parentis Basin, a series of east-west– and northeastsouthwest–oriented diapiric salt lineaments were formed during the Albian–Senonian (Figure 6). These aligned or offset salt walls of the North Pyrenean domain, and the salt cushions of the southern Arzacq and Parentis basins were, in some places, capped by clastic-carbonate plateaus, e.g., on the St. Marcet structure, at Salies-de-Béarn, or farther to the west in the Bidache area (Figure 2). Minor bioherms developed above salt cushions in the more quiet waters of the Parentis Basin at Mimizan (Figure 3a) or on the Lacq structure in the southern Arzacq basin (see “Hydrocarbons” section). Olistostromes composed of Triassic material (salt, shales) were also emplaced, mainly into the mobile domains, during the timespan of the

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Figure 10—Tectonic evolution diagram. (a) Paleozoic heritage and later wrenching, (b) Albian distension (transtension), (c) Tertiary compression and main effects, (d) late Tertiary extension and renewed relief-forming.

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rhythmic (flysch) sedimentation of the late Albian to early Tertiary (Stevaux and Zolnaï, 1975). The diapirism and the emplacement of olistostromes enhanced, through the flow of the evaporites, the subsidence initially due to crustal stretching. Within the North Pyrenean furrow, the salt structures acted as barriers and influenced the paleogeography of the turbidites. The entire fast subsiding, rifted, turbidite-filled, mobile belt became progressively the North Pyrenean zone, where the major part of the orogenic deformation was going to take place (Zolnaï, 1971). Salt lineaments of varied orientations as well as circular diapirs were also formed farther to the north and northwest of the North Pyrenean furrow, on the stable platform and in the Parentis Basin (Figures 6, 7b). The salt lineaments are located mostly along major faults and fault intersections, such as the edges of the Aptian–Albian rhomb grabens (e.g., the huge Audignon anticline to the north of the Arzacq basin, diapirs near the city of Dax, most oil-producing structures of the Parentis Basin, and the Ibis structure 60 km offshore, etc.) (Figures 2, 3, 7, 9). The second major f lysch furrow, of midSenonian age, may have been related to yet another extensional period, here considered as the fourth (major) rifting attempt. The flysch sedimentation in the deep North Pyrenean furrow by now became fine grained, dominated by marls, and ended with a thick mudstone ensemble during the Maastrichtian. This extensional period was probably the last and was certainly limited to the westernmost part of the North Pyrenean zone, because the eastern part of the area had been under compression since the Turonian. The Iberian compression progressed diachronously from east to west, crushing the North Pyrenean belt during the Late Cretaceous (see following paragraphs).

On the former Iberian carbonate platform (Bourrouilh and Alhamawi, 1993), another upper Senonian flysch domain was emplaced along the high chain of the present-day central and western Pyrenees. Its connection with or independence from the North Pyrenean equivalent flysch trough is under study. During the entire Late Cretaceous, two extensive carbonate platforms developed, covering to the north the major part of the Aquitaine basin and to the south the persistently high Iberian block (Figures 6, 7b). The massive carbonate series overlie the older sediments and/or the basement mostly with angular unconformity, except in the middle part of the Adour and Parentis depocenters, where thicknesses reach 1000 and 2000 m, respectively. These two carbonate platforms thus “framed” the f lysch belt. No transitional facies are known between the platforms and the North Pyrenean trough sediments, indicating the sharp sea-f loor topography resulting from the abrupt rifting events. During the Tertiary, predominantly marine sedimentation continued, at first with calcareous sediments. Rhythmic sedimentation continued in the southwestern reaches of the Aquitaine basin during the Paleocene (the third thick, f lysch sequence). These turbidites were deposited to the west and the north of the older flysch complexes; turbiditic sedimentation continued until the Holocene in the present-day offshore area, i.e., in the deeper parts of the Bay of Biscay. There is no structural (rifting) event related with these periods of shaly-calcareous “passive” basin fillings (Figure 5d). On the northwestern and the southern portions of the present-day Pyrenees, the thick sandycalcareous flysch sequence was deposited during the entire latest Cretaceous–Paleocene–Eocene period (Figure 6).

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Evolution and Hydrocarbons, Aquitaine Basin, France

BOUGUER ANOMALY AND STUCTURAL MAP SOUTHERN AQUITAINE BASIN AND PYRENEES

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Figure 11—Elements of regional geophysics. (a) Gravity (Bouguer anomaly) and structural map: the east-west row of positive gravity anomalies corresponds to arches of the North Pyrenean thrust envelope, suggesting that the gravity anomalies are due to the compressive events of the Pyrenean orogeny. The “heavy” belt corresponds to the North Pyrenean fold belt (stacking of heavy Paleozoic structural slabs with possible mafic masses at depth; see Figure 4b). The “light” anomalies to the north indicate the thick sedimentary column of the sub-Pyrenean trough (North Pyrenean basins), overthrusted by equally light structural imbricates (Triassic to Tertiary). The steep gradient to the south shows the edge of the thick Iberian block where Variscan granitic intrusions abound (see BRGM-ITGE, in press). (b) ECORS (Étude de l’écorce terrestre et océanique par réflexion et réfraction sismiques) profile—offshore Bay of Biscay (adapted from Pinet et al., 1987). (c) ECORS profile—Central Pyrenees (adapted from Roure et al., 1989). NPF = North Pyrenean fault, NPT = North Pyrenean thrust, NPyZ = North Pyrenean zone, SPyZ = sub-Pyrenean zone.

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In the southern Aquitaine area, along what was evolving into an orogenic foreland, sedimentation became progressively more clastic with large masses of coarse continental fanglomerates (“poudingues”) deposited in successive sequences in a row of

50 km

Eocene–Oligocene sedimentary fans. The greater part of the Aquitaine basin to the north of the orogenic foreland remained marine until the general filling of the basin caused the sea to retreat to the west. The present-day Bay of Arcachon can be considered

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Figure 12—Tectonic arches and Trans-Pyrenean faults. The inherited structural patterns of the Iberian block and that of the area to the north of the Pyrenees are very similar; some of these features actually straddle the Pyrenees. The Basque, Lannemezan, and Corbières arches (to the north) and the thrust front of the Graus-Tremp basin (to the south) form an interlocking “double indenter” system, elements of which can be traced back to the Permian/earliest Mesozoic (using Soler and Puigdefabregas, 1970; Séguret, 1972; Guimera and Alvaro, 1990; and Roure et al., 1988, modified). Abbreviations same as Figure 2, except: B = Boltana shear zone, BA = Basque arch, BT = Basque transversal, CA = Corbières arch, CG = Cerdagne graben, GTB = Graus-Tremp basin, LA = Lannemezan arch, NPyZ = North Pyrenean zone, SD = Sierra de la Demanda uplift, SF = Segre fault, SPT = South Pyrenean thrust, SPyZ = South Pyrenean zone.

as the last marine inlet on the present-day continent; it coincides with part of the deep Parentis Basin (Figure 1b). The total thickness of the Tertiary reaches 3000 m in the Tarbes basin and averages 2500 m along the sub-Pyrenean trough (to the north of the overthrust front); it is up to 2000 m in the Parentis Basin and between 500 and 1000 m on the shoals and basinmargin areas (BRGM et al., 1972). Orogenic compression started as early as the mid-Late Cretaceous (Turonian–Senonian) in the eastern part of the Pyrenees (Henry and Mattauer, 1974; Souquet et al., 1977; Souquet and Deramond, 1989) and propagated progressively to the west in the North Pyrenean basin. Compression of both the north and south margins was marked by the development of giant, linear, gravity slope sequences or

“evolutionary mass-flow megaturbidites” (Bourrouilh et al., 1983), which reached 90 km in length and a thickness of about 60 m. Similarly, in the southern Pyrenean basin, but during the Eocene, compression was accompanied by the emergence of a syntectonic cycle of giant mass-flow megaturbidites extending over 140 km, and more than 200 m thick (Soler and Puigdefabregas, 1970; Johns et al., 1981). The main north-south compressional event of the southern Aquitaine margin was during the middle Eocene (Figure 5d), as dated by the first massive conglomeratic sequences. The major part of the shortening took place along the Aquitaine-Iberian limit, i.e., in the North Pyrenean belt where the crust had earlier been the most extensively stretched (Figure 7b, c). The main Alpine metamorphism occurred

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Evolution and Hydrocarbons, Aquitaine Basin, France

within the North Pyrenean fault zone, which represents the most strongly compressed, schistosed, crushed part of the belt (Choukroune, 1976), along the northern edge of the Iberian block. Former normal faults and hinge lines were inverted into overthrusts, the northern limit of the compressed zone corresponding to the earlier boundary between the flysch furrow and the carbonate platform. This longlived tectonic edge had thus become the North Pyrenean thrust front (Figures 5d, 6, 7c). The North and South Pyrenean thrust fronts are actually tectonic envelopes, and not continuous, unique, sole faults. They are composed of en echelon segments that offset each other, in places with important offsets (Figures 2, 8, 11, 12). Several structural arches exist: the Basque arch along the northwestern Pyrenees, the Lannemezan arch in the central northern Pyrenees, the Graus-Tremp arch in the southern Pyrenees, etc. Two muchexplored surface anticlines are centered in the northern arches: the Ste. Suzanne structure to the west and the “Petites Pyrénées” in the central zone. The Corbières arch continues the Pyrenees proper to the northeast; it is related to regional sinistral wrenching (Figures 1b, 5d, 11, 12). The geometry of the thrust sheets and the amplitude of the overthrusting seem to be directly related to the amount of salt in place; in areas of little to no salt, parallel sets of recumbent folds and/or reverse faults can be found (Figure 7c), while overthrust sheets of some magnitude (10+ km) are present in areas of thick evaporites, e.g., in the Ste. Suzanne–Salies de Béarn and the Lannemezan areas (Figures 2, 3b, 8, 12, 13, 14c), or in the GrausTremp area in the southern Pyrenees (Figures 11, 12). Many of the earlier salt lineaments and diapirs have in the process been transformed into lubricating layers of overthrust sheets within the mobile belt (see “Hydrocarbons” section). In the North Pyrenean fold belt, several overthrust faults may be present in the same northsouth traverse, in en echelon arrangement, but none of the deep boreholes has so far provided evidence of large-scale structural stacking, i.e., the superposition of several (more than one) full overthrust units or rafts (Figures 8, 13, 14a–d). Along the southern edge of the compressed belt (i.e., along the North Pyrenean fault) the Paleozoic basement and the Mesozoic–Tertiary cover are, in places, deformed together; local structural stacks thus form tightly faulted, imbricate complexes, with northerly or southerly vergences (Figures 3b, 7c, 14c, d). To the north of this narrow zone, the Keuper salt complex acts as effective separation between the Paleozoic and Mesozoic–Tertiary “stockworks” (Henry, 1966). During the orogeny, several compressional episodes followed each other, the younger,

structurally higher faults (sole faults of allochthonous units or rafts) deforming the earlier, structurally lower faults (e.g., in the Ste. Suzanne structure) (Zolnaï, 1971; Henr y et al., 1989). The deformation style was mainly influenced by the facts that (1) the already fractured basement complex was involved in the compression, setting the sites for the major dislocations, and (2) major segments of the sedimentary column (e.g., Keuper salt, Lower Cretaceous marls, Albian to Tertiary f lysch and molasse series) were still extremely incompetent during the period of compression. Large, open synclines and anticlines were formed first; the synclines later became overthrusted, overturned, and/or thinned, “ironed out” by the overriding tectonic units. Specific structures were thus generated, which would seem uncommon in areas of greater crustal rigidity, and provided with more competent sedimentary sequences (there are very few true “duplexes” in the Northern Pyrenees). The complexity of the final structural picture makes paleogeographic and palinspastic reconstructions extremely difficult and hazardous. To the north, beyond the North Pyrenean thrust front, i.e., in the Adour basin and the Landes platform and Parentis Basin areas, there is no evidence of any major, regional compression involving thrust or crustal shortening. All folds here can be accounted for by salt tectonism triggered by small adjustments of the basement in the normal faulting or wrenching mode (Figures 1c, 2, 3a, 7c). Most of the latter structures were created along rejuvenated basement faults, principally of the east-west (N110°) set in the West Pyrenean foreland, or along northeast- and east-southeast–trending faults in the Parentis Basin. Late in the Pyrenean orogeny, probably during the Miocene, the two transverse basement directions were reactivated as well, again in the wrenching mode: N40° (mostly right lateral) and N160° (right and left lateral) (Figures 2, 3b, 10). These strike-slip movements, evidenced by field observations (Richert 1967, 1968; Henry et al., 1968), by high-resolution seismic studies, and by regionalscale mapping (Zolnaï, 1975, 1991), were also of small, kilometer-order amplitudes. They nevertheless generated wrench corridors, intricate fold zones, important outcrop shifts, vaulted structural “slabs” (fault-bounded narrow domes), f lower structures, local overturned limbs, etc., many of which are salt cored. The same late orogenic strikeslip movements contributed to shape the imbricated tectonic arches (Séguret, 1972), which are characteristic for both sides of the Pyrenees (Figure 12). The very latest Pliocene to Holocene episodes of the Pyrenean orogeny seem to have been an important relief inversion, which uplifted the former southern (Iberian) margin to form the present-day

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Figure 13—Hydrocarbon field location map.

high “axial” mountain chain (+2000 to +3500 m), where Paleozoic outcrops dominate (Figures 2, 7c). The same process (relief inversion) dropped the central zone of the severely eroded, Mesozoic– Tertiary North Pyrenean fold belt from 1000 m up to below sea level. [In the true axis of this young folded belt, i.e., in the presently low-lying North Pyrenean foothills, the St. Palais area (Figure 3b), over 5000 m of Cretaceous–Tertiary sediments, must have been eroded.] This inversion movement, possibly a late- or postorogenic isostatic adjustment, has also deformed the former overthrust sheets, steepening those of southern vergence to near vertical, thus inducing the cross-sectional asymmetry of the North Pyrenean structural “fan” (Figures 7c, 11c). The major part of this inversion took place again near the North Pyrenean fault zone, which is a still-active structural line, as expressed by abrupt topography and earthquakes (Figure 3b).

This sudden uplifting of the northern rim of the Iberian margin may have enhanced (or triggered with the aid of the Keuper decollement horizon) gravitational tectonic phenomena on the gently south-dipping southern side of the Pyrenees, toward the Ebro basin. In the meantime, within the North Pyrenean f lysch sequences, the steep northerly tilting caused layer-on-layer slippage, amplifying or even newly creating “structural” features above buried obstacles such as paleotopographic reliefs (e.g., the Pau anticline, coincident with the Meillon subcrop relief; see Henry, 1969; Haller and Hamon, 1993) (Figure 7c). It should finally be emphasized that the alpine orogenic complex of the Pyrenees is devoid of granitic intrusions and synorogenic volcanism; there are no ophiolitic masses and the known metamorphism is restricted to the trace of the North Pyrenean fault complex (Choukroune, 1976). This indicates that the major orogenic event in the area

846

Evolution and Hydrocarbons, Aquitaine Basin, France

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Figure 14—Hydrocarbon field sections. (a–f) are located in the South Aquitaine or North Pyrenean domain, (g–i) in the North Aquitaine, Parentis Basin. Compare with stratigraphic logs in Figure 3a, b, and regional cross sections in Figure 4a, b. (a) Lacq: the “synchronous high” hides a salt cushion, formed at the intersection of inherited northsouth and east-west basement faults, from the latest Jurassic on (modified from A. Nicolaï et al., unpublished work; see also Winnock and Pontalier, 1970). Stratigraphical abbreviations as in Figures 1c and 4. (b) Meillon: successive erosional and faulting phases on the northern slope of the old Grand Rieu basement high resulted in a structuralstratigraphic trap. The Pau surface anticline may be due to late or postorogenic “neo-tectonism,” atop the deep buttress of the Jurassic structural/erosional relief (cliff), itself superposed to basement wrench zone (modified from de Chevilly et al., 1969; Henry, 1969; Haller and Hamon, 1993; and A. Nicolaï et al., unpublished work). Pal = Paleozoic, Perm = Permian, lJ = Lower Jurassic, m-u J = Middle to Upper Jurassic, lK = Lower Cretaceous, uK = Upper Cretaceous, lTe = lower Tertiary, uTe = upper Tertiary. (c) Lannemezan-Bonrepos-Montastruc: structural traps within the belt of the North Pyrenean thrust complex, where salt diapirs were emplaced during the earlier extensional (transtensional) period, reworked during the Pyrenean compressions (from Morange et al., 1992). Stratigraphical abbreviations as in Figures 1c and 4, and DP = Dano-Paleocene. SL = sea level. (d) St. Marcet: the surface anticline in the “little Pyrenees” Range has been detected by field-geological survey. The slightly recumbent deep diapir has been active since the Early Cretaceous (from ELF-Aquitaine et al., 1991). Stratigraphical abbreviations as in Figure 14b, and DP = Dano–Paleocene, E = Eocene, E–O = upper Eocene–Oligocene. (e) Vic Bilh and Casteralou fields, situated on the flanks of an Albian salt lineament, along the northern edge of the Tarbes infrabasin. The Jurassic erosional traps are sealed by the Lower Cretaceous marls (from Mauriaud, 1987; compare with Figure 9b). (f) Lagrave: an old “transverse” hinge line has become, during the (late-) Pyrenean orogenic phases, a senestral wrench zone, as indicated by the set of nonconjugate normal faults. Light oil has thus migrated from deep, mature sources into the shallow habitat (from ELF-Aquitaine et al., 1991). Stratigraphical abbreviations as in Figure 14d. (g) Parentis Basin, main types of traps: the Parentis field has been generated by deep, nonpiercing diapirism, the Mimizan bioherm is salt-related as well, the Cazaux stratigraphic trap has become a flat anticline during later salt movements. Stratigraphical abbreviations as in Figures 1c, 4, or 14c (from ELF-Aquitaine et al., 1991; see also Mediavilla, 1987).

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Figure 14—Continued. (h) Cazaux: the upper set of oil pools are trapped in separate lenses of a north-south–oriented channel or deep-sea fan near the northern limit of the Parentis Basin. Synsedimentary Albian faulting was essential in the emplacement of sand bodies; it also permitted Jurassic-generated oil to be emplaced in the upper pools (from Bessaguet and Martin, 1977). (i) Lugos: the erosional relief that contributed to the mixed structural-stratigraphic trap is somewhat similar to that of the Meillon trend in the North Pyrenean area (Figure 14b). Figures 12g–i are based on exploration documents of ESSO-REP, in ELF-Aquitaine et al. (1991).

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Evolution and Hydrocarbons, Aquitaine Basin, France

has been the Variscan one (tectonics with major crustal shortening, penetrative metamorphism, and granitization). Parts of this structural heritage or regional pattern were rejuvenated during the Mesozoic–Tertiary extensions, and the whole fabric was eventually overprinted and refolded by the Late Cretaceous to Tertiary, Pyrenean-Alpine orogenic convergence (Figure 7c). RECENT INTERPRETATIONS (1) The inherited basement structure constantly influenced the sedimentation and paleogeography of the whole area. The northeast-, north-northwest–, and east-west–trending structural lines are well known in both neighboring cratons; their emplacement on both sides of the Tertiary fold belt of the Pyrenees shows a continuity, which has been somewhat blurred by the Pyrenean-Alpine events but which can still be recognized (Figure 12). The North Pyrenean fault, the Basque transverse system, and the Cerdagne fault lineament (Figure 10d) cutting across the eastern Pyrenees are but the most prominent elements of this old fault web. Much of the recurring movements along these faults took place in the wrenching mode. The northwest-oriented faults acted during the early movements (Cretaceous); the east-west fault group was active since the Permian, under extension, wrenching, and compression, while northeast-trending faults underwent important wrenching rejuvenation during the late orogenic events (Figures 3b, 7c, 10). The similarity of the Permian to Lower Cretaceous sedimentary sequences, present in southern France and northern Spain, argues in favor of the Iberian landmass being relatively autochthonous in relation to the rest of Europe. (2) The deep structure of the North Pyrenean fault shows a northerly dip, the deep “spur” of the thick Iberian plate slipping underneath the very much thinned South Aquitaine plate during the process of crustal convergence (Figures 3b, 7c, 11c). The gravity image is very relevant all along the Pyrenees. From north to south, the foreland basin (i.e., the sub-Pyrenean zone) exhibits negative anomalies; a lineament of positive anomalies corresponds to the belt of maximal compression (the North Pyrenean belt), whereas the “axial” high chain (where crustal thickness is maximal and where granitic stocks abound) corresponds to another domain of negative anomalies. There is a direct correlation between the emplacement of the positive gravity anomalies in the orogenic belt and the shape and amplitude of the overthrust sheets. The structural arches (concave to the south, see Figure 11a) envelop the positive anomalies, suggesting a common origin for the two. The observed

rows of east-west–trending geophysical anomalies can therefore be best accounted for by the stacking and imbricating produced by the Pyrenean orogenic shortening (Zolnaï, 1971). (3) The appearance and evolution of the set of rhomb basins in the North Pyrenean belt and Parentis area, and the inner organization of these pull-apart structures, can be best explained by regional shearing. The wrench movements are, however, inferred from the basin geometries rather than from direct observation. The rhomb basins of the Aptian–Albian period indicate a northwest-southeast–oriented left-lateral shear component caused by the opening of the North Atlantic oceanic arm in connection with the slow easterly drift and counterclockwise rotation of the Iberian landmass (Bourrouilh et al., 1988; Zolnaï, 1991, Figure VII-8). These first pull-apart basins (Arzacq, Tarbes) are limited by south-southeast– and east-west–oriented faults. The north-northwest–oriented faults are abrupt (indicating the direction of the shearing), whereas the eastwest–trending ones form progressive stairs that contribute to the regional dip. South of the individual Aptian-Albian pull-apart basins (Figure 5c), the single late Albian-Cenomanian east-west–oriented furrow took shape. This furrow is composed of a row of smaller rhomb basins, place of the first flysch sedimentary cycle (Souquet et al., 1981, 1985). This structural event was initiated by the ongoing left-lateral shearing, which was reoriented from northwest-southeast to east-west. The second major f lysch trough (mid-Senonian) was coeval with the partial oceanization of the floor of the Bay of Biscay, which itself was the result of the slight counterclockwise rotation of Iberia (Figure 5b, c); new basaltic masses were emplaced, which engulfed and spread over parts of an already thinned continental crust. The east-west magnetic “anomaly 34” (beyond the western limits of our maps) is dated at 85 Ma, i.e., mid-Senonian. This oceanic event thus corresponds in time to the end (and not to the beginning) of the Cretaceous rifting period, which was triggered by the sinistral east-west shearing. The lateral displacements necessary to generate all the rhomb basins and rifts need not have been great (Zolnaï, 1991)—a few kilometers suffice to generate the known basins. All that direct field observation could provide so far are shifts on the order of 10 km in the central Pyrenees (Debroas, 1987), along the southern edge of the Albian– Cenomanian flysch furrow. Ongoing field work may supply further evidence for the assessment of the offset(s) produced along the North Pyrenean fault. (4) The amplitude of the north-south, extensional, and compressive movements may have been

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relatively modest too. The total north-south stretching of the Aquitaine basin is estimated to have been about 80 km, of which the later orogenic compressions were going to recover approximately 30+ km (Figure 6). The moderate counterclockwise rotation of the Iberian plate, which accompanied the sinistral wrenching events (Albian to mid-Senonian), must not have exceeded a few degrees at this stage (Figure 5) (see Zolnaï, 1991, Figure V-4). All of these relatively moderate movements have nevertheless substantially thinned and weakened the basin floor. The opening of the Parentis and Adour basins, as well as the partial oceanization of the Bay of Biscay (Aptian–Senonian), were consequences of this moderate extension and of the rotation of the Iberian landmass. The Senonian to Eocene orogenic events all along the Pyrenees (and probably in the Provence area as well) took place along the weakened southern wedge. It is significant that the orogenic compressions directly affected only the narrow North Pyrenean zone. The larger North Aquitaine basin (including the Parentis subbasin), which has been equally stretched during the rifting events of the Mesozoic, remained undisturbed during the compressive events, except for some minor adjustment movements (shearing), which triggered important salt tectonism but no regional shortening. (5) There is some controversial evidence in favor of a moderate dextral wrench component, in conjunction with the major compressive event during the Pyrenean orogeny, resulting in transpression (Figures 5d, 6). This might have pushed the Iberian landmass back nearer to its original position, prior to the Albian sinistral transtensional wrenching. This late, dextral movement could have taken place mainly along or near the North Pyrenean fault, too. The compression (shortening) is maximal along this line, expressed as schistosity and local metamorphism, which are limited to this narrow belt. Small masses of mantle rocks, i.e., lherzolites (peridotite-type rocks) are also known along the North Pyrenean fault corridor, usually at crossings with transverse, north-south–oriented fault lines. Their extrusion may have been facilitated by recurring shear movements able to create local extensional zones (e.g., in “releasing bends”). The straightness and verticality of the North Pyrenean fault give further evidence in favor of the at least partially strikeslip character of the late Alpine movements. (Wrench faults and corridors are, in fact, mostly vertical to subvertical, whereas thrust faults issued directly from crustal shortening are, as a general rule, low angle or flat, and present arcuate traces on maps.) Mapping evidence for right-lateral wrenching exists in the Basque area (La Rhune massif, Figure 2) and in the Jaca basin (Spain) from the Oligocene (Guimera and Alvaro, 1990; Thomas and Delfaud, 1990).

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(6) Above-average, differential basin subsidence rates, coeval with major uplifts due to diapirism along the edges of the same basins, are most typical of the area (Figures 6; 7b, c; 8). Such balanced (“paired”) movements occurred during the Aptian to early Albian (in the Arzacq basin), as well as during the Albian (in the Parentis Basin and along the sub-Pyrenean zone). It seems to be beyond doubt that the ef fects of the or iginal crustal stretching were enhanced in the whole area by the migration of the Keuper salts, flowing from the basin floors to their edges. The shattering of the fault zones, due to the wrench component, may have triggered these diapiric movements. They resulted in intricate basin geometries next to the diapiric complexes (see “Hydrocarbons” section). The remobilization of the thick (>1000 m) late Triassic–earliest Jurassic evaporitic masses enhanced the effects of the tectonic subsidence in the North Pyrenean mobile belt too, where salt lineaments and circular mushroom-shaped diapirs came into existence within the furrow, all through the rifting events. In the western Pyrenees, the first major (late Albian to Cenomanian) turbidite sequence was deposited to the south of a row of mid-basin salt lineaments and to the north of the faulted southern margin, i.e., the North Pyrenean fault (Figure 6). Most of the second major turbidite sequence (mid-Senonian) was, to the contrary, deposited to the north of these salt structures and to the south of another major hinge line, which was going to become the North Pyrenean thrust front and which corresponded roughly to the southern rim of the earlier, Lower Cretaceous rhomb basins (Arzacq, Tarbes). (7) Interpretations using stacked overthrust sheets and supposing large-scale crustal shortening (several hundreds of kilometers for the northern side of the Pyrenees alone) have sometimes been proposed. These interpretations are based on longrange lateral projection of the various recognized structures and on the superposition of the latter. Such approaches usually do not stand up to threedimensional analysis and/or paleogeographic restorations. The solution to the structural complexity of the folded area should be sought to better understand the effects of the transverse fault systems, which delineate the tectonic compartments. More detailed analysis of the synsedimentary movements of each of these pre-existing basement blocks allows a better understanding of the local structural picture, which changes from one compartment to the other. In fact, each tectonic compartment has its own specific “signature” or its own answer to the tectonic forces; anomalies, therefore, cannot be carried over from one compartment to another.

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HYDROCARBONS Although surface seepages have been known in the southern part of the basin since the early 19th century, serious exploration started only in the late 1930s. It resulted in the St. Marcet gas discovery, made in 1939 on Bastille Day (July 14) (Perrodon, 1980, 1985). The upper Lacq “shallow” (650 m, approximately 2000 ft) heavy-oil field was discovered in 1949 based on gravimetry and some seismic work. This was followed in 1951 by the deep Lacq sour-gas discovery, which tapped a true giant at 3550 m (11,000 ft) in the basal Cretaceous. The next important discoveries were made at Parentis, in 1953 in the Barremian, and at Cazaux, in the Albian sandstones in 1959. After a long series of dry holes, the Meillon sour-gas field was tapped in 1964, at a depth of 5000 m. The Pecorade–Vic Bilh oil fields were drilled in 1974–1979, while the interesting Lagrave oil field was completed in 1984. A dozen offshore wells, drilled in the western reaches of the Parentis Basin in the late 1960s and early 1970s, did not yield commercial results. The latest oil discovery, in 1992, was made onshore, in the northernmost part of the Parentis Basin near Arcachon with the Les Arbousiers 1 well. The Aquitaine basin is one of the most explored areas in western Europe; before the North Sea Basin discoveries, it was the most productive too, with the giant Lacq and Meillon gas discoveries (1951 and 1964, respectively) near the northern rim of the Pyrenees and the oil-producing Parentis Basin (1954–1959) to the north, near Bordeaux. Mature source rocks are in the Jurassic shaly carbonates and possibly the Lower Cretaceous shales (see Figure 4a, b). Most reservoir rocks are situated near the Jurassic–Early Cretaceous boundar y (sands, sandy carbonates, and dolomites), within the Jurassic (dolomites), or in the Lower to middle Cretaceous (Albian; clastics). Natural fracturing is an important production factor in the Jurassic reser voirs of the deep gas fields of the subPyrenean area (Adour basins: Lacq, Meillon, etc.). The Upper Cretaceous carbonates are poor reservoirs, where not fractured. All discoveries with the exception of St. Marcet were made using seismic data, the evolution of which has enabled the exploration to progress over the decades. Gravimetric and magnetic methods have also been used systematically. Another controlling factor of exploration was the development of deep drilling, especially in the flysch-covered domain, where turbine drilling techniques were introduced and developed to obtain adequate penetration rates. Most purely structural habitats (e.g., St. Marcet gas field, shallow Lacq oil pool, and deep Lacq gas field, Parentis Basin oil fields) (Figures 13, 14a–i)

are related to some sort of salt movement during sedimentation. The first wave of exploration started on surface structures in the “Petites Pyrénées” area (St. Marcet) and at Ste. Suzanne. A series of diapiric structures was then drilled with no commercial success, until the improved seismic and drilling techniques allowed deeper exploration (deep Lacq pool) (Figure 14a). The discovery in the Parentis Basin was also made possible with better resolution seismic, which was able to detect the “infrabasin” hidden under the relatively quiet Tertiary cover (Figure 14g–i). Several of the more recent discoveries are subunconformity traps, which, like the gas fields of the Meillon trend (Figure 14b), remained unexplored because of the difficulty of penetrating a very thick and abrasive flysch sequence. The fields along the salt ridges (Casteralou, etc.) are structural-stratigraphic, related to early salt movements (Albian–Aptian) (Figure 14c–e). Their discovery needed better regional understanding and new exploration concepts. The role of synsedimentary movements needed to be recognized; synsedimentary movements produced sets of erosional traps on both sides of the eroded diapiric trends. Almost all productive areas are situated within the deeper basinal domains (Arzacq-Tarbes and Parentis basins), where the Pyrenean orogeny (compressive folding) had little to no direct effect and where maturation was sufficient but not excessive. Most of the purely compressive, alpine folds have proved disappointing (the St. Marcet and Lacq fields correspond to habitats formed already during the Cretaceous halokinetic events). Another important impact of the synsedimentary movements (mostly extensional, with a wrench component) has been in the shaping of confined, “starved” lozenge-shaped basins (Arzacq-Tarbes) or rifts (Parentis), ideal for both hydrocarbon generation and preservation. The same early (preorogenic) movements have triggered the salt movements. Commercially interesting but regionally limited discoveries have been made in clastic or clastic-carbonate deep-sea fans and “slumped” masses in the Parentis Basin at Cazaux (Figure 14h), and within the North Pyrenean flysch trough at Mazères (to the south of the Meillon ridge). The emplacement of these reservoir bodies also needed the presence of abrupt synsedimentary reliefs, structural and/or erosional. Although source maturation may have been late (Tertiary subsidence and burial), early trap formation appears to have been a controlling factor in all major discoveries: Early Jurassic to Early Cretaceous abortive salt doming (Lacq, Parentis) and Early to Late Cretaceous structural-erosional traps (Meillon, Vic Bilh, Pecorade, Lugos) (Figure 14b, e, i). One of

Bourrouilh et al.

the more recent discoveries, at Lagrave in the North Pyrenean domain, seems to be a fractured anticlinal trap related to a north-south fault system with a sinistral wrench component (Figure 14f). The reservoirs here are Upper Cretaceous carbonates, but the maturity of the oil points toward a deeper origin. Another Upper Cretaceous structural trap is the upper Lacq pool where platform carbonates contain some heavy oil. The reservoir in the St. Marcet gas field is brecciated Upper Cretaceous carbonates, which cap a salt diapir that grew in the Senonian flysch furrow. The Ledeuix gas field is a fractured reservoir in Cretaceous volcanic rocks. The small Lannemezan gas pool is the only purely subthrust discovery so far, although the Lacq field is partly overthrusted along its southern margin. All of the deep gas is sour, the H2S content being in the 10% range. Various amounts of condensate occur in most of the gas fields. The discoveries amount to a total of approximately 50 × 106 MT (300 million bbl) of recoverable oil and 210 × 109 m3 of recoverable gas (6000 tcf). Drilling density is still rather low; about 700 wells have been drilled so far on the 30,000 km2 (12,000 mi2), triangular-shaped southern productive area, for an average of 1 well/4 km2 (1 well/1.5 mi2). Many of these wells are old and shallow and were based on now obsolete exploration concepts. CONCLUSIONS So far, hydrocarbon exploration has mainly concentrated on known Mesozoic (mainly uppermost Jurassic to Lower Cretaceous) target horizons. Other intervals within the Mesozoic sequence may be considered as little known and underexplored, especially in the light of more recent discoveries and concepts. The Tertiary and the Paleozoic series (Mullan, 1984), although potentially hydrocarbon bearing, are virtually unexplored, and there are still prospects in the Lower Cretaceous (Albian–Aptian) series as well. For decades, the principal aim of exploration has been the search of “alpine,” compressive-structural traps, especially those of the N110° set located in the overthrust belt and/or those directly related to salt diapirs in the basin itself. Reinterpretation in the light of the extensional-wrenching concept and targeting “synchronous,” combination structural-stratigraphic habitats may lead to new discoveries. The extremely complex lithological and structural evolution of the Aquitaine basin has led to a great variety of lithological and structural forms and hydrocarbon habitats. Each structure represents a specific history. Even within the same structural trend or target, neighboring wells may exhibit

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different settings, and almost every development well has to be regarded as if it were a wildcat. This certainly turns exploration into an extremely complex, multidisciplinary process, but it maintains hope for new discoveries for those with thorough knowledge, up-to-date technology, and imagination. REFERENCES CITED Bessaguet, J., and B. Martin, 1977, Evolution de l’interprétation du champ albien de Cazaux au cours de son exploitation: Pétrole et Techniques, Février 1977, no. 241, p. 21–35. Boillot, G., L. Montadert, M. Lemoine, and B. Biju-Duval, 1984, Les marges continentales actuelles et fossiles autour de la France: Paris, Masson. Boirie, J. M., and P. Souquet, 1982, Le poudingue de Mendibelza: dépôt de cone sous-marin du rift albien des Pyrénées: Bulletin des Centres de Recherches ELF-Aquitaine, v. 6, no. 2, p. 405–436. Bourrouilh, R., and M. Alhamawi, 1993, Données nouvelles sur l’évolution de la marge Ibérique dans les unités tectoniques des Eaux Chaudes, Vallées d’Aspe et d’Ossau, Pyrénées Atlantiques, France: Comptes Rendus de l’Académie des Sciences, Paris, t. 317, sér. II, p. 979–985. Bourrouilh, R., and L. J. Doyle, 1985, Petroleum geology, tectonics and sediments of the French Pyrenees and associated Aquitaine basin: AAPG Field Seminar (June/July), 103 p. Bourrouilh, R., and L. J. Doyle, 1987a, Sedimentology of carbonate gravity deposits of the interplate basin: Conference Field Studies (September 2), Carbonate Gravity Sedimentation SEPM Research Conference, Aïnhoa, France, p. 1–23. Bourrouilh, R., and L. J. Doyle, 1987b, Sedimentology of migrating carbonate gravity deposits: Basque Coast: Conference Field Studies (September 4), Carbonate Gravity Sedimentation SEPM Research Conference, Aïnhoa, France, p. 24–39. Bourrouilh, R., and G. Zolnaï, 1988, Iberia versus Europe—effects of continental break-up and round-up on hydrocarbon habitat (abs.): AAPG Bulletin, v. 72, p. 990. Bourrouilh, R., T. Coccozza, M. Demange, M. Durand-Delga, S. Gueirard, G. Guitard, M. Julivert, F. J. Martinez, D. Massa, R. Mirouse, and J. B. Orsini, 1980, Essai sur l’évolution paléogéographique, structurale et métamorphique du Paléozoique du sud de la France et de l’ouest de la Méditerranée: Mémoire du BRGM, no. 108, Colloque C6, 26ème Congrès Géologique International, Paris, p. 159–188. Bourrouilh, R., F. Coumes, and B. Offroy, 1983, Mécanismes séquentiels et évènements exceptionnels du flysch Sénonien Nord-Pyrénéen. Corrélations par les dépôts gravitaires profonds: Bulletin de la Société Géologique de France, sér. 7, v. XXVI, p. 1223–1234. Bourrouilh, R., J.-P. Richert, and G. Zolnaï, 1988, North Pyrenean intraplate basin: evolution and hydrocarbons (abs.): AAPG Bulletin, v. 72, p. 990. BRGM (French Geological Survey), ELF-Re., ESSO Rep., SNPA, 1972, Géologie du bassin d’Aquitaine: Atlas, 27 plates, scale 1:1,000,000. BRGM-ITGE, in press, Synthèse géologiques des Pyrénées, J. C. Chiron and A. Barnolas, coordinators: 2 vol., abt. 1000 p., 1000 figures, 64 plates. Canérot, J., and F. Delavaux, 1986, Tectonique et sédimentation sur la marge nord Ibérique des chaînons béarnais, Pyrénées basco-béarnaises: Comptes Rendus de l’Académie des Sciences, v. 302, sér. II, no. 15, p. 951–956. Castéras, M., 1974, Les Pyrénées, in J. Debelmas, ed., Géologie de la France: Paris, Doin, v. II, p. 296–345. Chiron, J. C., coordinator, 1980, Carte tectonique de la France, scale 1:1,000,000: Orléans, BRGM. Choukroune, P., 1976, Structure et évolution tectonique de la

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ABOUT THE AUTHORS Robert Bourrouilh Robert Bourrouilh received his Thèse d’Etat Sciences from the University of Paris on sedimentology and structure of Baleraric Islands and Western Mediterranee. He held academic positions at the universities of Paris, Pau, and now Bordeaux. He was a visiting professor at University of Southern California and University of South Florida. He organized AAPG field seminars on sedimentology and petroleum geology of Aquitaine Basin and Pyrenees with the support of ELFAquitaine, ESSO, and TOTAL. He is associate editor of the Journal of Sedimentary Research. J.-P. Richert J.-P. Richert is presently an international structural expert with ELFAquitaine. He joined this group in 1966 after gaining a doctorate in 1964 at the University of Strasbourg (under Professor Michel Ruhland). His whole career has been devoted to the application of tectonic analysis methods to oil and gas exploration, especially concerning seismic data interpretation in compressive and extensive tectonic regimes.

Greg Zolnaï Greg Zolnaï holds diplomas of geology from the universities of Budapest, Hungary (1953) and Paris, Sorbonne, France (1958). He spent his career in exploration of coal, water, and hydrocarbons, mostly with the Aquitaine and ELF groups, 1957–1988. He has worked in Hungary, France (Paris and Aquitaine basins, Pyrenees), North Africa, Australia–South East Asia, Surinam, and North America (U.S. and Canada). Presently he does research work and carries out teaching assignments on regional and structural geology (wrench tectonics).