Assessment of undiscovered oil and gas resources of four west Africa geologic provinces

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 four geologic provinces located along the northwest and west-central African coast and extending offshore to a water depth of 4,000 meters (m) (fig. 1). From north to south, the provinces are: (1) the Senegal, containing the passive-marginal Senegal Basin of Middle Jurassic to Holocene age; (2) the Gulf of Guinea, characterized by transform tectonics; (3) the Niger Delta, containing more than 9,100 m of sediments; and (4) the West-Central Coastal, containing the Aptian salt basin and dominated by both rift and sag tectonics including the Congo Basin. These provinces were assessed previously as part of the USGS World Assessment 2000 (U.S. Geological Survey World Energy Assessment Team, 2000), resulting in estimated mean volumes of 71.5 billion barrels of oil (BBO), 235.2 trillion cubic feet of gas (TCFG), and 10.8 billion barrels of natural gas liquids (BBNGL).

More than 275 new fields have been discovered in the four western Africa provinces since the 2000 assessment. The provinces were reassessed because of increased activity and because of new oil and gas discoveries within the provinces. The assessment was based on geology 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, seven conventional total petroleum systems and nine assessment units (AU) in the four provinces were defined, and the undiscovered, technically recoverable oil and gas resources were assessed (table 1).

Several geologic studies have reported on the potential for undiscovered hydrocarbon resources and descriptions of the total petroleum systems, assessment units, hydrocarbonsource rocks, reservoir rocks, and potential traps for hydrocarbon accumulation for the four provinces (Tuttle, Brownfield, and Charpentier, 1999; Tuttle, Charpentier, and Brownfield, 1999; Brownfield and Charpentier, 2003; Brownfield and Charpentier, 2006a, b).

Figure 1. Locations of the four assessed geologic provinces located along the northwest and west-central coast of Africa.

Resource Summary

The results of the USGS assessment of undiscovered, technically recoverable conventional oil and gas resources in the west African provinces are given in table 1. The mean volumes are estimated at (1) 2,350 million barrels of oil (MMBO), 18,706 billion cubic feet of gas (BCFG), and 567 million barrels of natural gas liquids (MMBNGL) for the Coastal Plain and Offshore AU in the Senegal Province; (2) 4,071 MMBO, 34,451 BCFG, and 1,145 MMBNGL for the Coastal Plain and Offshore AU in the Gulf of Guinea Province; and (3) 15,534 MMBO, 58,221 BCFG, and 6,326 MMBNGL for the Agbada Reservoirs and Akata Reservoirs Aus in the Niger Delta Province. The West-Central Coastal Province was divided into the Gabon Subsalt, Gabon Suprasalt, Central Congo Delta and Carbonate Platform, Central Congo Turbidites, and Kwanza-Namibe AUs. The estimated mean volumes for these five AUs are 49,736 MMBO, 75, 8790 BCFG, and 2,877 MMNGL. This assessment for the west Africa provinces indicates that most of the oil and gas potential remains in the offshore waters.

References For Additional Information

Table 1. West Africa Provinces assessment results for undiscovered, technically recoverable oil, gas, and natural gas liquids. [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

Assessment results are available at the USGS Central Energy Resources Team website: http://energy.cr.usgs.gov/oilgas/noga/ or contact Michael E. Brownfield, the assessing geologist (mbrownfield@usgs.gov).

West Africa Provinces Assessment Team

Michael E. Brownfield, Ronald R. Charpentier, Troy A. Cook, Timothy R. Klett, Janet K. Pitman, Richard M. Pollastro, Christopher J. Schenk, and Marilyn E. Tennyson.

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