The U.S. Geological Survey (USGS) has estimated the undiscovered, technically recoverable, conventional oil and gas lying beneath a thick layer of salt along Africa's west-central coast — in the Douala-Rio Muni, Gabon, Congo, Kwanza-Benguela and Namibe basins of the West-Central Coastal Province. The salt, of Aptian age, runs through all five basins; the rocks below it are called presalt or subsalt.
The five presalt assessment units of the West-Central Coastal Province. Image from the USGS fact sheet
How the basins formed
- Rifting. As Pangaea broke up in the Late Triassic to Early Jurassic, Africa began to pull away from South America. Rifting left a long, complex belt of horsts and grabens, and the grabens filled with up to several kilometers of sediment — including organic-rich lake shales, the main source of the oil and gas.
- Sag. As rifting slowed in the Early Cretaceous, thermal subsidence made room — the sag — which filled with widespread river and lake sediments. Unusual water chemistry let microbes build mounds of chert, dolomite and limestone — now important reservoirs.
- Salt. From the late Aptian, the sea repeatedly spilled over the volcanic Walvis Ridge and across stretched continental crust; evaporation left more than a kilometer of salt. Where later halokinesis has not occurred, the salt is a key seal over the presalt oil and gas.
Lower Cretaceous rocks of the Kwanza Basin, offshore Angola: the Aptian Loeme Salt separates presalt from postsalt strata, and the Bucomazi Formation and its equivalents are the province's main source rocks. Modified from Brownfield and Charpentier (2006). Image from the USGS fact sheet
The petroleum system
USGS defined one Berriasian–Barremian Lacustrine Total Petroleum System in the presalt, with five assessment units sharing similar source rocks, reservoirs, traps, seals and timing.
| Element | What it is |
|---|---|
| Source rocks | synrift lake shales with up to 20 weight percent total organic carbon, hydrogen index up to 800 mg hydrocarbon per gram of carbon, and up to 600 m thick |
| Reservoirs | synrift conglomerates and sandstones from rivers, lake margins and deep lakes; in the sag section, widespread river and lake sandstones and microbial mounds, whose cherts and dolomites can be highly porous and permeable (like age-equivalent rocks in Brazil's Santos Basin) |
| Traps | facies changes, sandstones draped over horsts, reservoirs against tilted fault blocks, and microbial mounds |
| Seals | mudstones within the synrift section, and the evaporites above the sag reservoirs |
| Timing | oil generation may have begun in the Aptian, gas in the Oligocene |
The results
All five units were assessed with a probability of 1.0. Totals (mean, with the F95–F5 range — a 95% and a 5% chance of at least that amount):
| Resource | Mean | Range |
|---|---|---|
| Oil | 12,104 million barrels (12.1 billion) | 4,221–26,433 |
| Gas | 49,977 billion cubic feet (50 trillion) | 17,121–109,925 |
| Natural gas liquids | 2,634 million barrels (2.6 billion) | 882–5,841 |
By unit (means):
| Assessment unit | Oil (million barrels) | Gas in oil accumulations (billion ft³) | Gas in gas accumulations (billion ft³) |
|---|---|---|---|
| Kwanza-Benguela | 4,680 | 10,763 | 6,603 |
| Gabon | 3,252 | 8,780 | 5,381 |
| Congo | 2,803 | 3,364 | 5,238 |
| Namibe | 1,053 | 2,422 | 3,216 |
| Douala-Rio Muni | 316 | 411 | 3,799 |
Sources
Based on Assessment of Undiscovered Conventional Oil and Gas Resources in Presalt Reservoirs of the West-Central Coastal Province, by the USGS West-Central Coastal Province Assessment Team (Christopher J. Schenk and others), U.S. Geological Survey fact sheet; a work of the United States government in the public domain. The source's summary once calls these "continuous" resources where the rest of the assessment and its tables are for conventional resources; its "halokenesis" is corrected to halokinesis.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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