Triassic Diapiric Salt Structures, Southwestern Aquitaine Basin, France

Abstract

The Aquitaine basin of southwestern France extends 190 miles along the north side of the Pyrenees and 230 miles along the Atlantic Ocean. It spreads over 35,000 square miles and is filled with nearly 30,000 feet of sediments above the basement of pre-Hercynian rocks. Above the continental Carboniferous and Permian, the Triassic consists of a lagoonal evaporite sequence of dolomite, gypsiferous clay, salt, and anhydrite. The maximum undisturbed thickness of the is 7,000 feet. Inasmuch as the Trias is overlain by 8,500-23,000 feet of limestone and marl beds, it is unstable and where affected by the tectonic phases of the Pyrenees it has been thrust or intruded upward to the surface in the form of salt domes, piercement folds, and salt dikes along fault planes. These diapiric salt structures are grouped together under the generalized name of accidents diapiriques by the French geologists. The diapiric salt structures of the southwestern part of the Aquitaine basin may be classified as follows. 1. Isolated salt domes: roughly oval with the long axis parallel with the Pyrenean structures. Have a cap of clay and gypsum. Rocks on the flanks are tilted up to 90°, and in places are overhung by the Trias; locally, there are blocks of the surrounding beds (Jurassic) imbedded in the salt mass. 2. Piercement folds with Triassic salt plug: the northern flank is downfaulted and broken. The Trias is injected along a fault up to the surface. 3. Anticlines with deep diapiric plug: discovered by drilling. At St. Marcet, the diapir bears a cap of Cretaceous sedimentary conglomerate, which is a reservoir of an important gas field. 4. Large Triassic massives: formed at the junction of various fractures. 5. Diapiric salt dikes along faults: like dikes, overturning the flanks at the surface. 6. Erratic outcrops: probably in connection with invisible faults. All these various diapiric features are connected with deep fractures of the Hercynian basement which are reflected in the cover. The difference of density and plasticity of salt might explain the uplift of the isolated domes as in the Gulf Coast of Louisiana and Texas. The Pyrenean orogenic phases played a leading part in the formation of the anticlinal structures. In most diapiric structures, both orogenesis and differences in density have combined to influence their growth. The study of unconformities and sedimentary conglomerates permits one to conclude that the uplift of the Trias has been discontinuous and connected with the various Pyrenean phases. The upthrust of the diapirs to the surface was generally at the end of Oligocene (Aquitainian). The Trias upthrust has provided migration channels for hydrocarbons, and numerous asphalt seeps have long been known on the flanks of the diapirs or in the Miocene sediments overlapping them. Some deep wells have been drilled on the flanks of the deep or outcropping domes or to the cap rock in order to explore the sedimentary conglomerate formations. Almost all these wells encountered impregnations of asphalt and of very heavy oil, but no commercial production. However, at St. Marcet, an important gas field was discovered in 1939 which yielded 23.7 million cubic feet of wet gas per day in 1950, and at Lacq an oil dome without any connection with Triassic salt structures was discovered in 1949, which yielded 5,400 BPD, in May, 1951.