A gas column in the Leopard-1 well was discovered by the company with about 200m of net gas pay in a pre-salt reservoir.
Located around 145km off the Gabonese coast, west of Gamba, Leopard-1 well has been drilled in water 2,110m deep to a total vertical depth of 5,063m.
Shell plans to undertake an appraisal program, along with its partners, in order to further identify the resource volumes.
Shell Upstream international director Andy Brown said: "Shell has been exploring in Gabon for over 50 years. This latest deep water discovery is a testament to the innovation of our explorers in pursuing new plays, and application of our global sub-surface expertise.
"We are proud to be sharing this success with CNOOC Limited, our partner in the licence."
Shell operates the license with 75% stake and CNOOC holds the remaining interest.
The frontier discovery follows the success in recent deep water exploration in the heartlands for Shell Exploration in the Gulf of Mexico and Malaysia.
