Alistair Chaney (Tullow Oil, South Africa) - PETROLEUM SYSTEMS DEVELOPED ALONG THE NW AFRICA OFFSHORE MARGIN: CHALLENGES FACING EXPLORATION & PRODUCTION COMPANIES
PETROLEUM SYSTEMS DEVELOPED ALONG THE NW AFRICA OFFSHORE MARGIN: CHALLENGES FACING EXPLORATION & PRODUCTION COMPANIES
Chaney, Alistair John1
1Tullow Oil 3rd Floor, Building 11, Chiswick Park, 566 Chiswick High Rd,, London, NA, W4 5YS, United Kingdom
The offshore NW African Atlantic margin extends from northern Morocco to the Guinea Fracture Zone in the south and comprises up to 6km of Mesozoic and Cenozoic silicilastics and carbonates overlying Triassic salt.
The primary source rocks are Albian, Cenomanian and Turonian mudrocks, with lesser contributions from Palaeocene intervals that are thought to have reached marginal maturity in some areas, and restricted Liassic shelfal carbonates. Deep water gravity deposits provide the main reservoirs and include confined canyon systems and unconfined turbidite lobes. Less well explored plays include carbonate reefs and deltaic topsets further up-dip. The most prominent trapping mechanism along the length of the NW African margin is related to halokinesis. Allochthonous salt diapirs and canopies (largely absent from offshore Western Sahara, Senegal, Gambia and Guinea Bissau) have generated proven and unproven traps including sub-salt, salt stock, and supra salt closures. Other trapping mechanisms include tilted fault blocks (some are counter regional faults), stratigraphic pinchout (Banda gas discovery), extensional roll-over folds, and toe trust compressional folding.
The under explored nature of the NW African Atlantic margin provides E&P companies with numerous challenges. The lack of detailed well control makes prediction of sandstone problematic, particularly as many of the canyons appear to be by-pass systems. Where geobodies have been mapped, lack of seismic imaging makes it difficult to differentiate with confidence between prospective sheet turbidite sandstones and reservoir-poor debrites, particularly in the deeper Cretaceous section. Other pitfalls have recently come to light after drilling. These include reef-like morphologies along the carbonate platform break of slope that are possibly contourites of siliceous ooze, and bright seismic amplitude anomalies that are not hydrocarbon filled sands, but low velocity shales.
These challenges are generally prospect specific, but there are further difficulties which relate to the cost of exploration in immature basins. Increasing water depth places restrictions on rig availability, and requires increasingly larger prospects to be considered commercially viable. As the restrictions imposed by the water depth drive companies to explore deeper in the basin it becomes more likely that prospects will be overpressured as they are in other deep passive margins (e.g. Nigeria, Borneo). This will only increase well costs, placing greater pressures on commerciality.
