The Lacq gas field, France's most important, was discovered in 1951 by geophysical methods. The field is just north of the major overthrust separating the southern edge of the Aquitaine basin from the Nord Pyrenees foredeep. Directly under the field is a paleo-high flanked by two strongly subsided basins: the Arzacq basin on the north and Upper Cretaceous flysch trough on the south.

The gas is trapped in a roughly elliptical anticlinal structure in which differential subsidence has played the major role.

The gas has a 15.4 percent H₂S content and occurs under strong pressure (9,700 psi at 13,200 ft) in uppermost Jurassic dolomitic strata, Purbeckian-Wealdian sandstone, and Neocomian limestone and dolomite.

Reservoirs characteristics of these rooks are poor (5 percent porosity; 0.1 md permeability at best) and production is possible only because of intense fracturing on the upper part of the structure.

Production of the field is presently 700 MMcf per day of raw gas and in-place reserves are evaluated at 8.8 trillion cu ft.

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