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Abstract: The Aquitaine Basin: 60 Years of Gas Exploration and Production in the Foreland of the Pyrenean Fold and Thrust Belt

Michel Le Vot, Jean-Marie Masset, Jean-Jacques Biteau

Over the last 60 years, Exploration in the Aquitaine basin has led to the discovery of about 13 TCF of gas associated with 100 MMBIs of condensate. The first gas discovery was made on the St Marcet surface anticline in 1939. However the major step was accomplished in 1951 by the discovery of the Giant Lacq field (9 TCF of gas), which was followed in 1965 by the discovery of the Meillon Field (2,5 TCF). Production started in 1944 at St Marcet, in 1957 at Lacq and in 1967 at Mellon leading to a cumulative production of 10 TCF of gas as of December 1994.

The fields are located in the immediate foreland of the Alpine Pyrenean Thrust Belt. The region shows as a result extreme structural complexity, which is also linked to the polyphased geological evolution of the area. Overprinted on the faulting pattern of the basement (variscan and Hercynian orgenies), the area is characterized by a general E-W extension during the Jurassic and the Early Cretaceous, followed by a major submeridian compression from the Late Cretaceous to the Oligo-Miocene.

In this context the traps for the fields consist in deep (3500 to 4500 m in average) faulted blocks derived from the Early to Mid Mesozoic extension, inverted at various degrees during the Late Cretaceous and Tertiary compressive events. The reservoirs are represented by Jurassic and Neocomian dolomites which show poor matrix porosities (3/5%). Excellent permeabilities are associated to intense fracturing.

In such a petroleum context, the challenge for the 90's is to evaluate the remaining potential and to optimize the development of existing fields as well as to discover new fields especially within the unexplored zones along the leading edge of the Pyrenean Fold and Thrust Belt. Recent onshore 3D seismic (over 1500 sq km2 shot from 1987 to 1993) has proven to be efficient in defining good geometry for the fields and in delineating precisely the fractured zones of the reservoirs. It has as well allowed to develop a comprehensive understanding of the area and therefore a good evaluation of the unexplored zones within this very prolific region.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France