Exploration Adventures in France’s Aquitaine Basin

There are two main oil and gas producing sedimentary areas in France: the Paris Basin and the Aquitaine Basin. The presence of oil seeps and bitumen deposits has been known since ancient times in the western part of the Aquitaine basin, a 35,000-square-kilometer triangular polygon bordered to the north by the city of Bordeaux, to the east by Toulouse and to the south by the Pyrenees Mountain chain separating France from Spain.

These hydrocarbon shows were observed close to surface anticlinal structures such as the Sainte-Suzanne dome, southwest of Pau or in the caprocks of salt diapirs at Salies de Bearn and Dax to the north.

At the end of the 19th century, with the petroleum exploration and exploitation results in the United States, Poland and Romania’s Carpathian Mountains in mind, local elders formed companies and started core drilling campaigns. Between 1896 and 1926, almost 30 wells were completed, some of them reaching more than 750 meters total depth. However, liquid and gaseous hydrocarbons were discovered only in small quantities.

After 1926, petroleum research started with the creation of companies with public or private funding, including Pétrole National and Syndicat de Recherches et Mines d’Hydrocarbure. No real petroleum successes were announced at this stage, though ten wells targeted surface anticlines.

The Great Depression stopped all efforts for a while. Modern emerging geophysical techniques and a French national priority with associated funding facilitated a new exploration effort just before World War II. The seal concept was seen as mandatory to guide new exploration programs.

Discoveries in a Difficult But Hopeful Era

Structural and stratigraphic investigations in Pyrenees Mountain outcrops (figures 2 and 3) were proposed by Léon Bertrand and completed along with electrical geophysical surveys. Two drilling rigs were purchased in the United States and a first drilling location was chosen by the CRPM (which became Régie Autonome des Pétroles) on the Saint-Marcet anticline, 70 kilometers southwest of Toulouse.

Spudded in January 1939, the well found hydrocarbons on Bastille Day (July 14, 1939), establishing a gas condensate field in Jurassic dolomites. A drill stem test demonstrated economic flow rates, and then drilling efforts were concentrated on this area known as the Petites Pyrenees.

Seven wells were drilled, one of them revealing the structural complexity of the elongated and narrow trend by missing the target. RAP decided to develop this structurally complex, salt-cored anticlinal field after applying for a very large concession encompassing the field limits.

The German occupation after 1943 forced RAP to accelerate the field delineation, development and production.

Saint-Marcet, was a multi-reservoir field and produced for 50 years, yielding 275 billion cubic feet. After WWII the exploration effort accelerated in the whole southeast Aquitaine Basin or Comminges area.

Unfortunately, the very lucky Bastille Day strike of Saint-Marcet-1 was followed by wildcat failures over the course of 20 years in this area south of Toulouse and east of Tarbes. Lack of real structures and flushing by fresh water were two of the causes of these very disappointing results.

At that time there were no experienced employees in the region to drill and produce oil fields, so these essential personnel came from Alsace where they had been working at Pechelbronn, a small and shallow old oil field. These newcomers also provided local training.

The small city of Boussens became the operational base of RAP (the mother company of ERAP then Elf) with a library, training center and core storage. A new state-owned company, Société Nationale des Pétroles d’Aquitaine was created in 1942 to explore the perimeter outside the concession granted to RAP, that is, the rest of the Pyrenean foreland. Geophysical surveys were carried out in the area, using magnetotelluric, electrical and gravimetric techniques after World War II.

A series of anomalous lineaments were observed below the Molasse layers. Those trends correspond to the Vic Bilh and Pecorade field structural trends, which were discovered 30 years later.

Image Caption

Figure 3 : Aerial view of the area of the Petites Pyrenees southwest from Toulouse. Saint Marcet is located in the foreland.

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There are two main oil and gas producing sedimentary areas in France: the Paris Basin and the Aquitaine Basin. The presence of oil seeps and bitumen deposits has been known since ancient times in the western part of the Aquitaine basin, a 35,000-square-kilometer triangular polygon bordered to the north by the city of Bordeaux, to the east by Toulouse and to the south by the Pyrenees Mountain chain separating France from Spain.

These hydrocarbon shows were observed close to surface anticlinal structures such as the Sainte-Suzanne dome, southwest of Pau or in the caprocks of salt diapirs at Salies de Bearn and Dax to the north.

At the end of the 19th century, with the petroleum exploration and exploitation results in the United States, Poland and Romania’s Carpathian Mountains in mind, local elders formed companies and started core drilling campaigns. Between 1896 and 1926, almost 30 wells were completed, some of them reaching more than 750 meters total depth. However, liquid and gaseous hydrocarbons were discovered only in small quantities.

After 1926, petroleum research started with the creation of companies with public or private funding, including Pétrole National and Syndicat de Recherches et Mines d’Hydrocarbure. No real petroleum successes were announced at this stage, though ten wells targeted surface anticlines.

The Great Depression stopped all efforts for a while. Modern emerging geophysical techniques and a French national priority with associated funding facilitated a new exploration effort just before World War II. The seal concept was seen as mandatory to guide new exploration programs.

Discoveries in a Difficult But Hopeful Era

Structural and stratigraphic investigations in Pyrenees Mountain outcrops (figures 2 and 3) were proposed by Léon Bertrand and completed along with electrical geophysical surveys. Two drilling rigs were purchased in the United States and a first drilling location was chosen by the CRPM (which became Régie Autonome des Pétroles) on the Saint-Marcet anticline, 70 kilometers southwest of Toulouse.

Spudded in January 1939, the well found hydrocarbons on Bastille Day (July 14, 1939), establishing a gas condensate field in Jurassic dolomites. A drill stem test demonstrated economic flow rates, and then drilling efforts were concentrated on this area known as the Petites Pyrenees.

Seven wells were drilled, one of them revealing the structural complexity of the elongated and narrow trend by missing the target. RAP decided to develop this structurally complex, salt-cored anticlinal field after applying for a very large concession encompassing the field limits.

The German occupation after 1943 forced RAP to accelerate the field delineation, development and production.

Saint-Marcet, was a multi-reservoir field and produced for 50 years, yielding 275 billion cubic feet. After WWII the exploration effort accelerated in the whole southeast Aquitaine Basin or Comminges area.

Unfortunately, the very lucky Bastille Day strike of Saint-Marcet-1 was followed by wildcat failures over the course of 20 years in this area south of Toulouse and east of Tarbes. Lack of real structures and flushing by fresh water were two of the causes of these very disappointing results.

At that time there were no experienced employees in the region to drill and produce oil fields, so these essential personnel came from Alsace where they had been working at Pechelbronn, a small and shallow old oil field. These newcomers also provided local training.

The small city of Boussens became the operational base of RAP (the mother company of ERAP then Elf) with a library, training center and core storage. A new state-owned company, Société Nationale des Pétroles d’Aquitaine was created in 1942 to explore the perimeter outside the concession granted to RAP, that is, the rest of the Pyrenean foreland. Geophysical surveys were carried out in the area, using magnetotelluric, electrical and gravimetric techniques after World War II.

A series of anomalous lineaments were observed below the Molasse layers. Those trends correspond to the Vic Bilh and Pecorade field structural trends, which were discovered 30 years later.

Gravimetric prospecting indicated other promising anomalies in the Pau area, and focused seismic reflection surveys confirmed the presence of a positive anomaly below the Molasse blanket. Two exploration wells were completed in this area, Pau-H1 and Lacq-1, the latter proving the existence of a heavy-oil-bearing shallow (less than 1,000 meters) field in the Upper Cretaceous carbonates. Oil flow was 150 barrels per day and the field’s recoverable reserves were 30 million barrels. Upper Lacq field was the second Aquitaine exploration success in 1949, 10 years after Saint-Marcet.

Deep was Beautiful

Before World War II, Standard Oil of New Jersey (Esso-Standard) carried out geological investigations of the Pyrenean Foreland Basins. The recommendations of Esso experts and of a contracted French geologist were to explore two different areas with analogues in the Middle East. These were the western Pyrenean foothills (where RAP and SNPA were still active), and the Gironde area south of Bordeaux, where deeply buried structures could contain large hydrocarbon fields thought to be analogous to the Arabian Shield or the Zagros forelands.

An application for a permit was of course only possible after the War. The final application embraced the studies of Bordeaux University geologists and a vast area of 17,630 square kilometers was granted to Esso REP without competition. Esso REP, the French subsidiary of Standard Oil, established an office and geoscience teams were hired (mainly former Bordeaux University students) and managed by the American Ray P. Walters, as exploration leader, and Hans Hlauschek as chief geologist, to begin geological and geophysical studies.

The discovery of the Upper Lacq oil field gave a second impetus to the Esso studies, and gravimetry and magnetotelluric surveys were acquired. They were followed in 1952 by a seismic reflection campaign, with CGG as the contractor. After an initial phase of testing, the parameters were adjusted to the local conditions that included a surface cover composed of aeolian sands, as the huge Landes Forest had been intensively planted on this sandy plain during the 19th century.

This led to the decision to drill the Mano-1 north of the main zone of interest. Mano-1 revealed a surprising stratigraphic column (top Cretaceous at 220 meters, which was much higher than expected) and minor oil shows at the base of the Lias. Having obtained this excellent first deep stratigraphic calibration, a structure was recognized in the vicinity of Biscarosse Lake. The rig was rapidly moved from the Mano-1 location to the Parentis site, and the Parentis-1 spudded in October 1953. The Tertiary series was found to be thicker than at Mano-1, and the Cretaceous also was much deeper than predicted. Top/Barremian was at 2,000 meters’ depth.

After some hesitation, it was decided to mobilize a more powerful drilling rig to reach the Jurassic, which had oil shows at Mano-1. Drilling resumed in March of ’54 and an oil-bearing layer was discovered in the Lower Cretaceous. A drill stem test produced oil of excellent quality (32 degrees API, low gas/oil ratio), at a flow rate estimated at 2,100 barrels per day. The appraisal activity confirmed the large size of the field (a faulted anticline with 210 million barrels of proved plus probable reserves), but also the high complexity of the Barremian-Aptian fractured carbonate reservoirs.

Esso REP then significantly increased its exploration efforts, unfortunately with a lot of failures but small discoveries in the same Parentis Aptian-Barremian carbonate play.

The Deep Lacq Gas Field

Encouraged by its first success in the Lacq area during the same period of time (early ‘50s), SNPA decided to deepen the Lacq-3 appraisal well spudded in 1951. Below the Upper Cretaceous, Albian to Aptian excellent reefal reservoirs were drilled but were totally wet. Below the Aptian marls, (one of the main sealing intervals of this part of the South Aquitaine area), the well penetrated a calcareous formation in the Aptian-Barremian section with gas shows (same age as Parentis) and was cored.

During the coring operations, a violent gas kick occurred. A high H2S content (more than 10 percent) and CO2 explained the very rapid corrosion of the drill pipes and the resulting kick. The gas flow could be controlled, flared and then stopped thanks to the pioneer oil well firefighter Myron Kinley, who was called from the United States to help SNPA engineers (see figures 4 and 5).

This huge discovery had high initial pressure (9800 psi), high temperature (140 degrees C), and sour gas in a fractured and poor quality carbonate reservoir (porosity of 2-4 percent). The top/Barremian was at 3,150 meters depth.

Most experts considered these characteristics highly challenging and thought the chances to execute viable and safe exploitation of the Deep Lacq gas field to be very poor or nil.

However, considering the expected size of the field, its location in the south of France, and the urgency of economic recovery and growth after World War II, SNPA was allowed to tackle the challenge to develop these strategic resources with the help of its main funder, the French government.

Two appraisal wells confirmed the size of the discovery. Deep Lacq field (9 trillion cubic feet) was put on stream in 1957 after the design and construction of a large, dedicated desulphurization plant. One of the main challenges of Lacq development was to separate the sulphur from the commercial gas, utilizing the amine method.

At the end of 1957, after these 20 years of exploration, two significant fields were proved and developed in the Aquitaine area. A large number of wells, whether wet or dry, provided subsurface control throughout the basin. There was no confidentiality for well data, so all information was made available for all companies.

It soon appeared that the foreland was separated into two sedimentary sub-units – the North Aquitaine or Parentis and the South Aquitaine with the Adour-Arzacq (Lacq Field) to the west and the Comminges to the east (Saint-Marcet). The Landes Saddle (with surface salt diapirs) corresponds to the high in-between the two sub-basins (figure 6).

Some of the main plays and petroleum systems were identified at that stage of exploration. The Pyrenean compression, the Lower Cretaceous extensional-phase tectonics, and the presence of a Keuper Triassic evaporitic section were the key factors for structure and the associated fracturing. Carbonate reservoirs in the Cretaceous and marly sealing sections were identified as main play components, and source generative intervals were demonstrated in the Liassic and Kimmeridgian.

The relinquished areas of the initial huge SNPA permit and RAP concessions became attractive targets for newcomers, mainly French companies that merged later to form the Elf Aquitaine Group. Open acreage became very sparse and permits were granted with a high level of drilling commitment.

The Difficult Search for Pearls After the Big Diamonds Were Found

Most drilling operations results were negative, except in the Parentis Basin where Esso-REP drilled intensively in its own backyard and discovered the Cazaux field in 1959. Three years later, two new clastic plays, Albian turbidites and the so-called Purbeckian lithofacies that was known in Lacq yielded oil reserves of about 85 million barrels.

In the South Aquitaine Basin, huge improvements in seismic led SNPA to identify a possible truncation below the Upper Cretaceous erosional surface, down to the Tertiary Meillon anticline (a pop-up structural trend), just south of Pau. This prospect was drilled by two wells: Lons-1 (which was dry) and Meillon-1. Both were completed in the Jurassic.

Only Meillon proved the presence of Tithonian and Kimmeridgian gas-bearing dolomitic reservoirs. The first appraisal well proved that the field was indeed a truncated fault block at the northern edge of a thick Albian to Aptian trough lying directly on Triassic. The following appraisal wells demonstrated that the Meillon-Saint-Faust field is a significant gas discovery formed in an elongated and compartmentalized fault block, with two Jurassic sour gas-bearing dolomitic reservoirs (2 trillion cubic feet of recoverable reserves).

Other much smaller discoveries (Rousse, Ucha) were found south of Pau and Meillon by SNPA in 1967 and 1970 by taking advantage of better seismic quality and easy connections to the existing Lacq plant. SNPA, after 20 years of producing this black gold treasure, decided to invest in pharmaceuticals and created a new affiliate called SANOFI which has since been spun off and become a world leader in that business.

Offshore Disappointments

As elsewhere around the globe, activities moved offshore.

Esso applied for a license, called Landes Atlantiques along trend with the prolific Barremian to Aptian carbonate axis (Parentis and marginal on-trend fields). The first well, Antares-1, was drilled in 1966 and found a small oil-bearing reservoir in the Barremian carbonates. The Parentis offshore basin was explored by 20 exploration wells, all dry except Antares and an oil show at Ibis-2 well. Deeper carbonate deposition and instability related to the opening of the Parentis Basin (Bay of Biscay) explain the lack of commercial hydrocarbons despite the presence of the mature Kimmeridgian source rocks known elsewhere in the Aquitaine.

Exploration efforts were significantly increased after the first petroleum crisis in 1974. In ’75, SNPA discovered Pecorade oil and gas field in the northern Arzacq Basin in Barremian and Upper Jurassic dolomites.

The next year, SNPA and Elf merged. Elf was a commercial brand of the former RAP (Elf-ERAP) which was created at the end of the ‘60s and very active in marketing and services, including Formula 1 competitions. Still state-owned, the new company was named “Elf Aquitaine.”

This merger led to a rationalization of operations, portfolios and sharing play concepts and experiences, particularly with a large spectrum of operations in Africa, the North Sea and the Americas, for which the new group was better equipped to invest.

Elf Aquitaine and Esso REP found another field in 1979, Vic Bilh, in the same area as Pecorade. In light of high costs and basin complexities, Elf Aquitaine and Esso REP decided to become joint-venture partners on new permits delimited from the relinquished areas of former licenses.

The last discovery in the Arzacq Basin was Lagrave in 1984, and it was a lucky one. An Upper Jurassic prospect, it contained light oil in the Upper Cretaceous shelf carbonate reservoirs that were approximately equivalent in age to the Upper Lacq producing layer.

Great Expectations for 3-D Seismic

Both Esso REP and Elf Aquitaine recorded modern 3-D surveys on their respective production and exploration blocks. Vic-Bilh was the first acquired in 1982, just after the first appraisal wells. Most of the fields were covered with 3-D surveys, as were exploration areas in the fold and thrust belt southwest from Pau, such as Lacq West and Gurs, totaling 2,000 square kilometers.

The declines of Deep Lacq and Parentis fields were the best incentives for searching additional nearby production. They used 3-D seismic interpretation of the producing fields to better delineate the traps (Lacq is in fact a reverse-faulted anticline) and the complex associated fracturing networks. However, it was not the expected game-changer, and operators were unable to significantly increase resources. Since existing mature fields could not deliver additional volumes, exploration remained mandatory to achieve production replacement.

Elf Aquitaine’s operational subsidiary remained in Boussens. It became a fantastic school of structural and petroleum geology – senior specialists coached generations of junior geologists to tackle global exploration challenges. Despite many attempts, most efforts remained unsuccessful.

A Deep Well Failure

In the 1980s, a complex prospect was identified below thrusted sheets southwest of Pau as a possible seismic analog to the Lacq or Meillon anticlinal faulted blocks (with 1 trillion cubic feet expected recoverable reserves). Considering the large stakes and the risk of production ending in the upcoming 10 years, the decision was made to drill this very risky prospect with the expectation of a new little Lacq.

Bellevue-1D, the deepest well in Europe with a 6,710-meter total depth, was drilled on the basis of 2-D seismic interpretation and field work. Unfortunately, the well failed and was plugged in 1993, after drilling its last 3,500 meters in a vertical Pyrenean fold flank. In the same poorly explored fold and thrust belt, the seismic imagery improved, thanks to new 3-D seismic campaigns, but this didn’t sufficiently de-risk the expected prospects near Pau.

The Antares Failure and a Short Comeback

Elf Aquitaine decided to revisit the offshore area, applying for permits with the expectation of improvement in exploration success rates due to modern 3-D seismic acquisition. The main challenge was to work in coordination with the French Army, because of their missile launch training center activities near Mimizan. After obtaining an agreement to drill during the Army center’s short vacation time, they drilled a new well. Antares-3 confirmed the limited size of the accumulation after a production test, acidizing and stimulation. This was the end of Gulf of Biscay petroleum province expectations.

In the Parentis onshore basin, hopes were rejuvenated in the vicinity of Arcachon where Esso (partnering with Elf Aquitaine) found Les Arbousiers oil field in 1991. They chose the prospect location according to a concept of wells having drilled fault blocks and missed Purbeckian targets in the 1950s and ‘60s. Les Pins, Les Mimosas and Courbey fields followed in 1994 and 1996 (totaling 30 million barrels, including Les Arbousiers) in the same fault block trends. After a successful 3-D survey covering the whole Arcachon Basin with very touchy environmental constraints, there were no further discoveries.

Petroleum System

Despite scattered field occurrences, this is a very efficient petroleum province with a fair to poor generative system, but excellent vertical migration of hydrocarbons through fault systems and superimposed source and reservoir seal pairs. The key aspects of this petroleum province are illustrated in figures 6, 7 and 8, and most of details can be found in Biteau et al., Petroleum Geoscience, 2006, and in an AAPG Bulletin paper to come.

The generative systems are found mainly in the Upper Jurassic (Kimmeridgian, as in the North Sea) and marginally in the Lias and the Upper Barremian. Local presence of sour gas is the result of a process of thermochemical sulphate reduction using methane and buried anhydrite layers. Pyrenean tectonics resulting from Iberian and European plate collision and Aptian-Albian former extensional phases associated with the opening of the Bay of Biscay have formed the geometries of the main traps. The role of the Keuper Triassic ductile evaporite is a main driver of the thin skin tectonic style.

Today

Esso REP, the second major company operating in the French E&P landscape, sold all their assets to Vermilion, a Canadian junior, which is now operating all these mature fields (at a current rate of 4,500 barrels per day). Elf Aquitaine became a private company in the 1990s and merged with TotalFina in 2000 to form the fifth largest publicly-traded oil company in the world. Producing assets were sold to Geopetrol, a French company, in 2014.

Lacq gas production stopped, and the wells were plugged and abandoned in 2013. The vast industrial complex that came to life in 1957 is now active for purposes other than gas exploitation. Lacq was an exceptional gas field without water and a textbook pressure-versus-volume decline.

This was the end of French oil operations in its birthplace, representing a great adventure and sparking the economic growth of the Bearn (Pau) area. TotalEnergies still maintains its world E&P Technical Center in Pau where it employs around 5,000 geoscientists and engineers.

Between 1939 and 2021, more than 650 exploration wells were drilled in the Aquitaine Basin, and 2,000 square kilometers of modern 3-D modern seismic was acquired. Four big discoveries and around 30 modest fields were found, mostly in the 1950s and ‘60s, totaling 2.5 billion barrels of recoverable reserves. Exploration in this petroleum province is probably now complete after 60 years of very active operations.

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