During the formation of the Atlantic Ocean, which was preceded in Aptian time by the deposition of a thick layer of salt, the Gabon basin filled with two kinds of sedimentary deposits; mainly sandy continental and littoral deposits in the east and marine deposits in the west. The latter are generally shaly, but a few thick sandstones are present far to the west intercalated in marine shales. The Grondin field was discovered in one of these sandstones called the Batanga, which is of Maastrichtian age.

The sandstones are generally clean and have good porosity. They contain some shales and can reach a gross thickness of more than 200 m. The trap at Grondin is an anticlinal salt structure without noticeable piercing. A median fault divides the structure in two parts, with the upper area's productive formation thinner due to an internal unconformity. The oil field is a unique pool, with a unique oil-water contact, and a small gas cap. The source rocks are marine shales of post-salt age, particularly the Turonian.

The Grondin field, situated 40 km offshore, was discovered in 1971 by Elf Gabon and was developed rapidly. Initial reserves are estimated at 30 million tons (approximately 200 million bbl).