Assessment of the Undiscovered Oil and Gas of the Senegal Province, Mauritania, Senegal, The Gambia, and Guinea-Bissau, Northwest Africa

Undiscovered, conventional oil and gas resources were assessed in the Senegal Province as part of the U.S. Geological Survey World Petroleum Assessment 2000 (U.S. Geological Survey World Energy Assessment Team, 2000). Although several total petroleum systems may exist in the province, only one composite total petroleum system, the CretaceousTertiary Composite Total Petroleum System, was defined with one assessment unit, the Coastal Plain and Offshore Assessment Unit, having sufficient data to allow quantitative assessment. The primary source rocks for the Cretaceous-Tertiary Composite Total Petroleum System are the CenomanianTuronian marine shales. The Turonian shales can be as much as 150 meters thick and contain Type II organic carbon ranging from 3 to 10 weight percent. In the Senegal Province, source rocks are mature even when situated at depths relatively shallow for continental passive margin basins. Reservoir rocks consist of Upper Cretaceous sandstones and lower Tertiary clastic and carbonate rocks. The Lower Cretaceous platform carbonate rocks (sealed by Cenomanian shales) have porosities ranging from 10 to 23 percent. Oligocene carbonate rock reservoirs exist, such as the Dome Flore field, which contains as much as 1 billion barrels of heavy oil (10° API, 1.6 percent sulfur) in place. The traps are a combination of structural closures and stratigraphic pinch-outs. Hydrocarbon production in the Senegal Province to date has been limited to several small oil and gas fields around Cape Verde (also known as the Dakar Peninsula) from Upper Cretaceous sandstone reservoirs bounded by normal faults, of which three fields (two gas and one oil) exceed the minimum size assessed in this study (1 MMBO; 6 BCFG). Discovered known oil resources in the Senegal Province are 10 MMBO, with known gas resources of 49 BCFG (Petroconsultants, 1996). This study estimates that 10 percent of the total number of potential oil and gas fields (both discovered and undiscovered) of at least the minimum size have been discovered. The estimated mean size and number of assessed, undiscovered oil fields are 13 MMBO and 13 fields, respectively, whereas the mean size and number of undiscovered gas fields are estimated to be 50 BCFG and 11 fields. The mean estimates for undiscovered conventional petroleum resources are 157 MMBO, 856 BCFG, and 43 MMBNGL (table 2). The mean sizes of the largest anticipated undiscovered oil and gas fields are 66 MMBO and 208 BCFG, respectively. The Senegal Province is underexplored considering its large size. The province has hydrocarbon potential in both the offshore and onshore, and undiscovered gas resources may be significant and accessible in areas where the zone of oil generation is relatively shallow. Introduction The Senegal Province (fig. 1), which includes the onshore and offshore (to a water depth of 2,000 m) parts of the Senegal Basin, is situated along the northwestern African coast and includes parts of Western Sahara, Mauritania, Senegal, The Gambia, Guinea-Bissau, and Guinea. The Senegal Basin is classified as an Atlantic-type passive margin or marginal sag basin of Middle Jurassic to Holocene age overlying a Paleozoic basin (Wissmann, 1982). Figure 2 is a generalized geologic map of northwest Africa showing the location of Senegal and adjoining provinces. The northern limit of the Senegal Basin is the Precambrian Reguibate Shield in Morocco, and the southern limit is the Bove Basin of Guinea (fig. 3). The eastern edge of the basin is separated from the Taoudeni Basin by Precambrian rocks of the Mauritanide Mountains that were uplifted during the Late Paleozoic Hercynian Orogeny (figs. 2 and 3). The Senegal Basin is the largest of the northwest African Atlantic margin basins (De Klasz, 1978), with a total land area of about 340,000 km2 and an offshore portion in excess of 100,000 km2. The offshore portion of the basin was limited for this study to water depths of 2,000 m or less. Three major subbasins (fig. 3) have been recognized in the Senegal Basin: (1) the Mauritania offshore subbasin, which extends north from the Senegal River to the southern part of Western Sahara; (2) the Northern subbasin, which is located north of the Gambia River to the Senegal River; and (3) the Casamance subbasin, which extends south from the Gambia River through the Casamance region into Guinea-Bissau. Senegal Basin Geology 3 There are both offshore and onshore hydrocarbon occurrences in several formations in the Senegal Basin. The best understood hydrocarbon occurrences in the Senegal Basin are in Cretaceous and Tertiary reservoirs. The lower Paleozoic rocks contain oil-prone organic matter, and recent seismic data have delineated a Permian-Triassic pre-salt clastic section that may contain hydrocarbon source rocks. The Jurassic and Lower Cretaceous rocks have been explored only nearshore where they contain Type III organic matter (terrestrial plant material) and might be potential sources of gas. At least three total petroleum systems may exist in the Senegal Province: (1) the hypothetical Lower Paleozoic Total Petroleum System, (2) the hypothetical Sub-salt total petroleum system, and (3) the Cretaceous-Tertiary Composite Total Petroleum System. Drilling and production data available for this study are mostly limited to the Cretaceous and Tertiary rocks in the basin. Therefore, only the Cretaceous-Tertiary Composite Total Petroleum System with its contained Coastal Plain and Offshore Assessment Unit was assessed in this study. Due to limited drilling and production data, total petroleum system and assessment unit boundaries can only be approximately delineated and are subject to future revisions. Senegal Basin Geology The Senegal Basin formed at the culmination of a Permian to Triassic rift system that developed over an extensive Paleozoic basin during the breakup of North America, Africa, and South America. The Senegal Basin has undergone a complex history that can be divided into pre-rift (Upper Proterozoic to Paleozoic), syn-rift (Permian to Triassic), and post-rift (Middle Jurassic to Holocene) stages of basin development. The basin can be divided into a number of subbasins aligned in a north-south direction and delimited by an east-west fault system and other structural dislocations related to syn-rift tectonics. The initial phase of the post-Hercynian opening of the North Atlantic and the splitting of North America from Eurasia and Africa began during Late Permian-Early Triassic time (Lehner and De Ruitter, 1977; Ziegler, 1988; Lambiase, 1989; Uchupi and others, 1976; Uchupi, 1989) and is represented by syn-rift rocks in the Senegal Basin. The final breakup of Africa and South America began in the Late Jurassic in the southernmost part of the South Atlantic and prograded northward during Neocomian time (Binks and Fairhead, 1992; Guiraud and 0°