The U.S. Geological Survey (USGS) assessed undiscovered, technically recoverable, conventional oil and gas resources in the Douala-Rio Muni, Gabon, Congo, Kwanza-Benguela, and Namibe Basins of the West-Central Coastal Province of Africa (fig. 1). The assessment focused on the potential for undiscovered conventional oil and gas resources in reservoirs stratigraphically below a regionally extensive layer of Aptian salt (presalt or subsalt reservoirs) that is present in all five basins (fig. 2).

The tectonic evolution of the West-Central Coastal Province began with the initial separation of Africa from South America as Pangaea began to fragment in the Late Triassic to Early Jurassic (Torsvik and others, 2009; Guiraud and others, 2010; De Matos and others, 2021; De Matos, 2021). Rifting formed a broad complex region of horsts and grabens that occur throughout the length of the province (Jackson and Hudec, 2009; Baudino and others, 2018). As much as several kilometers of synrift clastic sediments were deposited in the grabens, including organic-rich lacustrine shales that represent major petroleum source rocks. As rifting waned in the Early Cretaceous, thermal subsidence formed accommodation space (or sag) that filled with regionally extensive fluvial and lacustrine sediments. Complex water chemistries led to microbial fixation of chert, dolomite, and limestone in mounds that are important oil and gas reservoirs. A series of transgressions beginning in the late Aptian caused marine waters to intermittently flow over the volcanic Walvis Ridge and across extended continental crust; associated evaporation resulted in deposition of more than one kilometer of late Aptian salt (Karner and Gambôa, 2007; Greenhalgh and others, 2012; Cowie and others, 2016; Pichel and others, 2023). Where subsequent salt halokenesis has not occurred, salt forms an important regional seal for oil and gas contained in some presalt reservoirs.

The USGS defined a Berriasian–Barremian Lacustrine Total Petroleum System (TPS) within the presalt stratigraphic section and five conventional assessment units (AUs) within this TPS (fig. 1). The five conventional AUs in general share similar petroleum source rocks, reservoirs rocks, traps, seals, and timing of oil and gas generation (Beglinger and others, 2012). The main petroleum source rocks are Berriasian–Barremian synrift organic-rich lacustrine shales with total organic carbon (TOC) content as much as 20 weight percent, hydrogen index values as much as 800 milligrams hydrocarbon per gram of TOC and thickness as much as 600 meters (Brownfield and Charpentier, 2006; Greenhalgh and others, 2012). Synrift reservoirs in this TPS include alluvial-fluvial conglomerates and sandstones, and marginal lacustrine to deep-water lacustrine sandstones. Reservoirs in the overlying sag section are extensive fluvial and lacustrine sandstones and an unusual type of reservoir related to microbial deposition from chemically complex waters (Saller and others, 2016; Cui and others, 2023; Moragas and others, 2023). Detailed descriptions of these microbial reservoirs have also been published from occurrences in the age-equivalent rocks in the Santos Basin of Brazil (Cazier and others, 2014; Farias and others, 2019; Gomes and others, 2020). These reservoirs typically form mounds or build ups composed of microbially deposited chert, dolomite, and limestone, in which the cherts and dolomites may exhibit excellent porosity and permeability. Traps in the TPS are formed by facies changes within the synrift section, by draping of sandstones over horsts, by reservoirs adjacent to tilted fault blocks, and by stratigraphic traps formed by microbial mounds within the sag section. Seals are provided by intraformational mudstones within the synrift section and by evaporites overlying the sag reservoirs. Oil generation may have begun from synrift lacustrine shales in the Aptian, and gas generation may have begun in the Oligocene (Brownfield and Charpentier, 2006). Assessment input data are summarized in table 1 and in Schenk (2024).

The USGS quantitatively assessed oil and gas resources in five conventional AUs in basins of the West-Central Coastal Province of Africa (table 2). For undiscovered, technically recoverable continuous oil and gas resources, the mean totals are 12,104 million barrels of oil (MMBO), or 12.1 billion barrels of oil, with an F95 to F5 fractile range from 4,221 to 26,433 MMBO; 49,977 billion cubic feet of gas (BCFG), or 50 trillion cubic feet of gas, with an F95 to F5 fractile range from 17,121 to 109,925 BCFG; and 2,634 million barrels of natural gas liquids (MMBNGL), or 2.6 billion barrels, with an F95 to F5 fractile range from 882 to 5,841 MMBNGL.

Assessment results are also available at the U.S. Geological Survey Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Christopher J. Schenk, Tracey J. Mercier, Cheryl A. Woodall, Phuong A. Le, Andrea D. Cicero, Ronald M. Drake II, Geoffrey S. Ellis, Thomas M. Finn, Michael H. Gardner, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, Heidi M. Leathers-Miller, Kristen R. Marra, Kira K. Timm, and Scott S. Young