Petroleum Exploration Contribution to the Structural History of "Golfe du Lion"

Robert Curnelle

Oil exploration of the "Golfe du Lion," which began in 1968, has not led to a single discovery. It has, however, enabled us to gather a wealth of both geological and geophysical data. Over 18,000 km of seismic lines have been recorded; the most recent campaign was undertaken in 1984. Gravimetric and magnetic surveys were also carried out. A portion of these surveys were operated by the Institut Francais du Petrole. Eleven wells were drilled, 9 in water depths not exceeding 150 m, correspond to normal conditions, and 2 in deep water (1,714 and 1,246 m of water). Of the 11 wells, 9 encountered pre-Tertiary substrata made up of Paleozoic or granitic rocks associated with the Pyrenean axial zone. This substratum constitutes the Pyrenean domain's eastern termination. Two wells those nearest the coast, encountered a Mesozoic series with an imbricate structure containing, locally, Paleozoic schists.

Petroleum exploration has thus strongly contributed to the knowledge of the post-Hercynian structural history of the "Golfe du Lion." It shows three stages: (1) Pyrenean orogenesis which started in the Late Cretaceous and culminated during the Eocene (40-75 Ma), (2) Oligocene rifting associated with the oceanic accretion of the Provcenal basin, and (3) post-Messinian deformations due to salt tectonics. Pyrenean deformation of the area seems to have been controlled by its Hercynian inheritance: the major transcurrent faults of the Cevenole belt, Nimes, and the Durance. They favored displacement of the deformation in a northeasterly direction along the fault corridors, inside which the overthrust units were put into place. These east-west-oriented structures show northward vergence. The Pyrenean axial zone in this downthrown part of the belt consists of progressively more northerly units offset by transverse faults. The Oligocene rifting is represented by a series of horsts and deeply subsiding grabens linked to preexisting major faults. The horsts are deeply eroded and the Mesozoic carbonates are karstified. The extension of the Messinian evaporitic deposits known throughout the Mediterranean basin is located north of the shallower deep-water well. The gravitational deformation of the salt is expressed by a large number of listric faults which originate in the salt bodies. The sedimentary sequence ends with a thick discordant, erosional, undeformed Pliocene-Quaternary series.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.