U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3034

March 2011

World Petroleum Resources Project

Assessment of Undiscovered Oil and Gas Resources

of the West African Coastal Province, West Africa

Printed on recycled paper

Introduction

The main objective of the U.S. Geological Survey’s 

(USGS) World Petroleum Resources Project is to assess the 

potential for undiscovered, technically recoverable oil and 

natural gas resources of the world, exclusive of the United 

States. As part of this program, the USGS recently completed 

an assessment of the West African Coastal Province (fig. 1), 

an area of about 202,715 square kilometers (km2) that covers 

parts of Guinea, Liberia, and Sierra Leone. This assessment was 

based on data from oil and gas exploration wells and published 

geologic reports. At the time of the assessment, the province 

contained no discovered fields and only 10 exploration wells 

had been drilled—and it is considered to be underexplored for 

its size.

The West African Coastal Province developed in two 

phases: (1) the syn-rift phase was initiated during the Early 

Cretaceous and resulted in the formation of deep grabens and 

The West African Coastal Province along the west African coastline recently was assessed for undiscovered, technically 

recoverable oil, natural gas, and natural gas liquids resources as part of the U.S. Geological Survey’s (USGS) World Oil and 

Gas Assessment. Using a geology-based assessment methodology, the USGS estimated mean volumes of 3.2 billion barrels of 

oil, 23.63 trillion cubic feet of natural gas, and 721 million barrels of natural gas liquids.

GUINEA

MALI

COTE D'IVOIRE

LIBERIA

SIERRA LEONE

GUINEABISSAU

BURKINA

FASO

16°W 14°W 12°W 10°W 8°W 6°W

12°N

10°N

8°N

6°N

4°N

0 50 100 200 MILES

0 50 100 200 KILOMETERS

ATLANTIC OCEAN

EXPLANATION

West African Coastal 

Province

Mesozoic-Cenozoic 

Reservoirs AU

INDIAN

OCEAN

ATLANTIC OCEAN

WEST AFRICAN

COASTAL PROVINCE

AFRICA

Figure 1. Locations 

of the West African 

Coastal Province 

and MesozoicCenozoic Reservoirs 

Assessment Unit 

(AU), west Africa.
half-grabens; and (2) the passive margin-transform phase began 

in the late Albian and continues to the present. The total thickness of the Mesozoic to Cenozoic section is about 5,000 meters 

(m) on the outermost part of the continental shelf and thickens 

to as much as 10,000 m in the basin depocenters.

The West African Coastal Province was assessed for the 

first time because of increased exploratory activity and interest 

in its future potential. The assessment was geology-based and 

used the total petroleum system (TPS) concept. The geologic 

elements of a TPS include hydrocarbon source rocks (source 

rock maturation and hydrocarbon generation and migration), 

reservoir rocks (quality and distribution), and traps for hydrocarbon accumulation. Using these geologic criteria, the USGS 

defined the Cretaceous Composite TPS with one assessment 

unit (AU), the Mesozoic-Cenozoic Reservoirs AU (fig. 1), 

encompassing about 188,550 km2, that includes the offshore 

parts of the province to a water depth of 4,000 m. The TPS was 

defined to include Cretaceous marine source rocks, including 

the Cenomanian-Turonian source containing Type II kerogen 

ranging from 3 to 10 weight percent total organic carbon. Possible lacustrine source rocks may be present in grabens that developed in the Lower Cretaceous. The AU contains Cretaceous and 

Paleogene clastic reservoirs and traps that mostly are associated 

growth-fault related structures, rotated fault blocks within the 

continental shelf and below the mid-Cretaceous unconformity, deep water fans, turbidite channels and sandstones, slope 

truncations along the present-day shelf and paleoshelf edge, 

and Cretaceous and Paleogene stratigraphic pinch-outs along 

the eastern basin margin. The primary seals are Cretaceous and 

Paleogene marine mudstones and shales.

Exploration wells on the continental shelf and upper slope, 

in water depths ranging from 100 to 470 m (IHS Energy, 2009), 

have demonstrated the existence of an active petroleum system 

containing Cretaceous marine source rocks that have produced 

hydrocarbons most likely since the Late Cretaceous and that 

the hydrocarbons have migrated into Cretaceous and Paleogene 

reservoirs.

Resource Summary

Using a geology-based assessment, the USGS estimated 

mean volumes of undiscovered, technically recoverable conventional oil and gas resources for the Mesozoic-Cenozoic 

Reservoirs AU in the West African Coastal Province (table 1). 

The mean volumes are estimated at 3,200 million barrels of oil 

(MMBO), 23,629 billion cubic feet of gas (BCFG), and 721 

million barrels of natural gas liquids. The estimated mean size 

of the largest oil field that is expected to be discovered is 783 

MMBO, and the estimated mean size of the expected largest gas 

field is 4,695 BCFG. For this assessment, a minimum undiscovered field size of 5 million barrels of oil equivalent (MMBOE) 

was used. No attempt was made to estimate economically 

recoverable reserves.

For Additional Information

Assessment results are available at the USGS Central 

Energy Resources Science Center website: http://energy.cr.usgs.

gov/oilgas/noga/ or contact Michael E. Brownfield, the assessing geologist (mbrownfield@usgs.gov).

West African Coastal Province Assessment Team

Michael E. Brownfield, Ronald R. Charpentier, Christopher 

J. Schenk, Timothy R. Klett, Troy A. Cook, and Richard M. 

Pollastro.

Reference Cited

IHS Energy, 2009, International petroleum exploration and production database [current through December 2009]: available 

from IHS Energy, 15 Inverness Way East, Englewood, Colo. 

80112 USA.

Province, total petroleum

systems (TPS) and

assessment units (AU) 

Field 

type

Largest 

expected 

mean

field size

Total undiscovered resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

West African Coastal Province–Cretaceous Composite TPS

Mesozoic-Cenozoic

Reservoirs AU

Oil 783

Gas 4,695

Total coventional

resources

[Largest expected mean field size in million barrels of oil and billion cubic feet of gas; MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, 

million barrels of natural gas liquids. Results shown are fully risked estimates. For gas accumulations, all liquids are included as natural gas liquids (NGL). 

Undiscovered gas resources are the sum of nonassociated and associated gas. F95 represents a 95-percent chance of at least the amount tabulated; other 

fractiles are defined similarly. AU, assessment unit; AU probability is the chance of at least one accumulation of minimum size within the AU. 

NGL, natural gas liquids. TPS, total petroleum system. Gray shading indicates not applicable]

Table 1. West African Coastal Province assessment results for undiscovered, technically recoverable oil, gas, and natural gas liquids.

801 2,713 7,305 3,200

801 3,713 7,305 3,200

1,047 3,662 10,847 4,492 27 98 297 121

4,862 16,202 43,437 19,137 149 506 1,374 600

5,909 19,864 54,284 23,629 176 604 1,671 721