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The Cretaceous and Tertiary Senegal-Mauritanian basin was almost completely unknown below the surface, before its reconnaissance was started in 1952 by the French "Bureau de Recherches de Petrole."
The great depth of the basin was at first indicated by airborne magnetometer and refraction seismograph tests and checked by a deep stratigraphic well (3,403 m., 11,165 ft.). Active exploration was carried on since 1955, three companies being active now in the French part of the basin.
Reflection and refraction seismograph and geological core holes were effective tools for exploration, gravity having been useful only locally. A marine section of about 5,000 meters (16,400 ft.) is known in deep wells, from Lower Cretaceous (Aptian) up to Miocene, mainly shale and sands in various proportions, with some limestones, overlapping the basement toward the East. The basin is open toward the Atlantic Ocean. The regional westward dip is very low and structure is mainly controlled by deep north-south flexures and by basement uplifts. A first gas well producing 3½ m.c.f./day was drilled near Dakar in 1959.
In Ivory Coast, a narrow coastal basin, almost as deep as broad on land, was discovered by gravity and checked by refraction-seismograph since 1952. Active exploration was started in 1957. Marine, lagoonal, and terrestrial reflection-seismograph and geological coreholes were done at first in the French part of the basin, before spudding deep exploratory tests.
The northern border of the basin is a fault system, and structures are related to plunging noses with southward dip. A marine section of about 2,700 m. (7,856 ft.) was drilled, with mainly shale, sands and conglomerates ranging from Aptian or Albian up to Miocene. Below Aptian or Albian, possibly non-marine dark shales and sandstones, or continental red-beds, were encountered. Good shows (tar sands and heavy oil seepages) occur in the eastern part of the basin, in Ivory Coast and in Ghana.
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