GEOLOGY OF OFFSHORE BIOKO
Offshore Bioko contains an active petroleum system comprising reservoir sands that lie structurally above voluminous marine shales containing oil-prone source rocks. The most important of these are the marine shales of the basal Pliocene Qua-Iboe and the Miocene Akata (Isongo) Formations which contain mixed Type II and Type III .
A thick sequence of Tertiary to Quaternary sands and shales was deposited as a wedge of clastic sediments in the distal Niger Delta basin setting. Shelf edge sedimentation has led to deposition of delta slope and basinal clastics by a variety of gravity driven processes including gravity sliding, debris flow and turbidity currents. Progradation of the delta has been accompanied by shale diapirism and growth faulting with associated rollover structures, and a zone of compressional toe-thrusting has developed at the foot of the delta slope.
The reservoirs at the Zafiro field complex occur in debris flow and slump sandstones of the Qua-Iboe Formation, comprising large, channelised sandbodies deposited in deep water settings. The Alba field produces from a deep water sand in the Isongo Formation.
Simple four-way dip structural closures and toe-thrust structures are common within the Isongo Formation whereas the Qua Iboe Formation contains a number of structurally enhanced stratigraphic traps. Good seals for all traps are provided by the interbedded Qua-Iboe and Isongo shales.
Regional seismic lines show that Mid to Late Cretaceous strata extend far into the ultra-deep water and certainly beneath the present-day volcanic intrusives such as Bioko Island. This is evidenced by the oil seeps on the volcanic trend in Sao Tome and the occurrence of Turonian ammonites on Bioko. The presence of volcanics in this setting therefore need not preclude hydrocarbon occurrences in the vicinity of Bioko and the other islands in the chain, including Annobon.
Exploration is at an immature stage in offshore Bioko and holds considerable potential for future discoveries. The deep water areas to the south and west of Bioko hold potential, particularly within large fan-like mound features recognised on seismic data in front of the toe-thrust limit, which are as yet untested by drilling.