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Lacq, near the southern edge of the Aquitaine basin, is France's most important gas field. Hydrocarbons are trapped beneath two unconformities--shallow Lacq (pre-Tertiary) produces oil at 2,100-2,300 ft (640-700 m), and deep Lacq (pre-Aptian) produces gas at 11,650-13,300 ft (3,530-4,060 m). The field is 10 mi (16 km) long, 6 mi (10 km) wide, and covers 25,000 acres (10,000 ha.); gas production of 700 million ft3/day comes from 31 wells.
Trapping is primarily anticlinal, although porosity and permeability have considerable influence; the highest values are on the crest of the anticline. The anticline was formed progressively in the course of deposition of the Cretaceous and Tertiary, as were many other structures in the Aquitaine. The present structure at Lacq shows 4,500 ft (1,400 m) of closure at the top of the Neocomian; the south flank is considerably steeper than the north.
The gas reservoir is divided into an upper calcareous part (650-1,000 ft or 200-300 m thick) comprising Neocomian and Valanginian (Early Cretaceous), and a lower dolomitic part (latest Jurassic; 500-650 ft or 150-200 m thick). Porosity range for the upper part is 0.1 to 6 percent, with permeability less than 0.1 md; porosity in the lower part is 5-6 percent and permeability is 0.1 to a few millidarcys. Fracturing is very important to both parts of the reservoir. Initial reservoir pressure was 9,670 psi at 13,120 ft (4,000 m) and temperature was 260°F (130°C). Abnormally high pressure indicates a closed reservoir of constant volume, without a water drive. Cumulative production to the end of 1967 was 1.76 trillion ft3, with a pressure drop of 3,370 psi. Gas reserves a e estimated at 8.8 trillion ft3.
Lacq gas is sour; volumetric percentage of H2S is 1.25 lb of sulfur per 100 ft3 gas. It constitutes important sulfur reserves and production, and France now ranks third among western sulfur producers.
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