HISTOIRE GEOPHYSIQUE DU CHAMP DE LACQ*

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Abstract
Whereas the first success of petroleum exploration in France (the gas deposit in the St. Marcet anticline in the St. Gaudens region) was essentially based on geological surveys, the second (the Lacq field in the Pau region) was the fruit of reflection shooting. In fact, the Lacq anticline cannot be detected by surface geology because of the Quaternary and Tertiary cover. Besides, electric and telluric prospecting methods could not be used over this zone as an electrified railway line passes through it.As stated then, it was the reflection shooting method that revealed the Lacq structure in 1948, during a reconnaissance survey conducted by Compagnie Générate de Géophysique using a very simple technique (only one geophone per trace).By the seismic technique of 1948, quite good reflections were obtained from a level adjacent to the top Cretaceous, but only sporadic reflections were picked up from a deep level and then only on the flanks of the structure, except for the southern one where results were nil.Towards the end of 1949, the first well, La 1, using a heavy rig designed to reach deep layers, led to the unexpected discovery of the upper oil deposit at a depth of 600 m in the Upper Cretaceous (lower Senonian), while the third well, La 3, led to the discovery by blow‐out of the lower gas accumulation at a depth of 3,500 m in beds later identified as lowermost Cretaceous and, more particularly, Upper Jurassic (Purbeckian).As no great geophysical effort was called for, the upper oil deposit was rapidly developed, its depth and dimensions being modest (6 km2).As far as the deeper gas deposit is concerned, the main objective up to 1954 was to gain a picture of the central part of the anticline. Despite the use of a more detailed seismic technique, it was difficult to plot the top of the structure at the level of the uppermost Jurassic. From 1952 to 1957 the wells La 101, La 102, La 103, La 104, La 105, La 106 and La 113 were drilled, which made it possible to evaluate the gas reserves.Then began the wide‐scale and systematic exploration of the structure, by drilling on the one hand and by reflection shooting on the other. Profiles were shot perpendicular to the axis, but long enough (20 to 30 km) to cover the entire anticline (seismic surveys of 1956–57–58).The eastern pericline and the northern flank were quite easily plotted by seismic survey at the level of the top Jurassic, whereas the southern flank, which has a distinctly steeper slope than the northern one, could not be traced very far–so that the problem of the relationship between the uncomplicated structure of Lacq and the country to the south, with its complex, deep tectonics, still remains an open question.The western pericline, however, remained a subject of concern. Its relationship to the anticline of Ste. Suzanne (outcropping Jurassic) was a mystery.In 1956–57 the well SV 101 confirmed the hypothesis of an overthrust which, without affecting the anticline of Lacq, borders it to the south and west. The thrust is not very large in the south (Lagor wells) but assumes considerable proportions in the west: the Ste. Suzanne anticline actually forms part of a thrust from the south, about 5, 000 to 6, 000 m thick, as the exploration of the region by reflection shooting and drilling (SSE 101, OR 102) has shown. This exploration had been undertaken in search of a new structure under the thrust that might form an extension of the Lacq structure.In 1959 a detailed gravimetric survey (2 to 3 st./km2) was carried out in the Lacq region because the old survey map (1948–1 station/8 km2) had proved progressively inadequate for interpreting the seismic data. It was found that the structure of Lacq had only a small anomaly in comparison with its dimensions. This surprising phenomenon is still difficult to explain. Are the surfaces of equal specific gravity independent of the stratigraphic planes, or has the effect of the anticlinal roof of the Cretaceous and the Jurassic been balanced by that of a saliferous and intumescent Triassic right in the core of the structure?The small anomaly which it was possible to detect may be explained by a thick sequence of reef limestones in the centre of the anticline on the Lower Cretaceous, as indicated by drilling results. These severely fissured limestones are undoubtedly partly to blame for the bad seismic results obtained on the central part of the structure as far as the deep horizon, adjacent to the top Jurassic, is concerned. They are also responsible for the considerable anomalies in the velocity of seismic wave propagation.

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