Historic Shelf Play Revisited - Offshore Gabon

By

 D. M. Wallace¹, B. James¹, S. R. Jackson¹, S. G. Duc¹

(1) Perenco, London, England

 The Ompoyi discovery well, drilled in February 2002, tested at 6300 bopd, revitalising interest in the Eocene-Palaeocene Ozouri play, offshore Gabon. The well targeted a large structural closure (13 km2) and encountered a 110 m oil column and no OWC.

The salt induced structure had been originally recognised on a coarse 2D grid and unsuccessfully drilled in 1982. The original well failed due to reservoir absence, at that time assigned to stratigraphic variation. 3D seismic subsequently revealed that outside a crestal graben area, the Ozouri reservoir was present over the entire structure.

The Ozouri formation is regionally extensive, and can reach a thickness of 450 m. It is frequently oil bearing above salt induced structures, although its potential has often been overlooked by exploration programs focusing on Upper Cretaceous clastic objectives. The Ozouri Formation, broadly analogous to the Monterey Chert Formation of California, is characterised by highly variable lithologies, generally relying on natural fracturing to give good productivity. The Ompoyi discovery is located in an area where the Ozouri is a primary reservoir target due to a favourable depositional, diagenetic and structural history. Exploration continues for analogous structures in this trend.

In these days of technology driven deepwater exploration, Ompoyi is a salutary example of how basic geological work and a reappraisal of drilled structures guided by modern 3D can discover low cost reserves in the West African shelf regions.