Undiscovered Oil and Gas Above the Salt Off West-Central Africa, USGS Assessment

The U.S. Geological Survey assessed undiscovered, technically recoverable conventional oil and gas in five basins of Africa's West-Central Coastal Province — the Douala-Rio Muni, Gabon, Congo, Kwanza-Benguela and Namibe basins. The study focused on postsalt reservoirs: rocks younger than a widespread layer of late Aptian salt.

The results

F95 means a 95 percent chance of at least that amount; F5 a 5 percent chance. The results are fully risked.

The 10 conventional postsalt assessment units in the West-Central Coastal Province. USGS.

How the margin formed

• Rifting. The province's history began as Africa separated from South America in the Late Jurassic to Early Cretaceous, leaving a broad, complex belt of horsts and grabens along its length. As rifting waned, the subsiding "sag" filled with widespread river and lake sediments.
• Salt. From the late Aptian, seawater repeatedly spilled over the Walvis Ridge onto stretched continental crust and evaporated, depositing more than 1 kilometer of salt. Seafloor spreading began in the latest Aptian, and the margin became passive.
• Carbonates and shales. Albian seas built broad carbonate platforms, with mudstones and marls offshore, including Cenomanian–Turonian organic-rich shales. From the Santonian to the Eocene, thin advancing clastic sequences alternated with carbonate platforms.
• The big sand wedge. Uplift and westward tilting of the continental margin in the Oligocene pushed a major clastic sequence offshore. Its stacked slope-channel and basin-floor fans hold most of the major reservoirs and are the main oil and gas reservoirs of the Congo, Gabon and Kwanza-Benguela basins.

Source rocks, reservoirs and traps

All 10 assessment units belong to a single Mesozoic–Cenozoic Composite Total Petroleum System, whose elements are broadly similar but differ in detail from basin to basin.

Reservoirs include Albian to Coniacian platform-margin carbonates (reefs, buildups, dolomites and karst zones); minor slope-channel and basin-floor sands that bypassed the platforms at low sea level; Santonian to Eocene shelf, slope-channel and basin-floor sandstones sealed in shale; and the Oligocene and younger fans.

Salt movement shapes the traps. Carbonate traps are mostly stratigraphic, though sliding carbonate blocks also formed structural traps. Salt diapirs, pillars and overhangs steered where sands were deposited, and gravity sliding on the salt created extensional traps upslope and folds, faults, anticlines and thrusts downslope. Because salt and fluids may still be moving, traps may have been altered and oil and gas remigrated or lost. Salt deformed especially during the Albian to Cenomanian and the Oligocene to Holocene.

Timing. Thermal maturity varies with burial, heat flow and source quality. The synrift lake shales generally reached oil maturity in the Aptian, and gas generation may have begun in the Oligocene.

The assessment units

Separate units were defined for Cretaceous and Cenozoic reservoirs in each basin.

Oil accumulations also hold gas and natural gas liquids, and gas accumulations hold liquids; the full breakdown is in the fact sheet.

Sources

Rewritten from U.S. Geological Survey Fact Sheet 2024–3011, "Assessment of undiscovered conventional oil and gas resources in postsalt reservoirs of the West-Central Coastal Province," by the USGS West-Central Coastal Province Assessment Team (public domain), in hubnx's own words.