Geology and total petroleum systems of the Gulf of Guinea province of West Africa

The Gulf of Guinea Province as defined by the U.S. Geological Survey (USGS) consists of the coastal and offshore areas of Côte d’Ivoire, Ghana, Togo, and Benin, and the western part of the coast of Nigeria, from the Liberian border east to the west edge of the Niger Delta. The province includes the Ivory Coast, Tano, Central, Saltpond, Keta, and Benin Basins and the Dahomey Embayment. The area has had relatively little hydrocarbon exploration since 1968, with only 33 small to moderate-sized oil and gas fields having been discovered prior to the USGS assessment. Most discoveries to 1995 have been located in water depths less than 500 m. Since 1995, only eight new offshore discoveries have been made, with four of the discoveries in the deep-water area of the province. Although as many as five total petroleum systems exist in the Gulf of Guinea Province, only one, the Cretaceous Composite Total Petroleum System, and its assessment unit, the Coastal Plain and Offshore Assessment Unit, had sufficient data to allow assessment. The province shows two important differences compared to the passive-margin basins south of the Niger Delta: (1) the influence of transform tectonics, and (2) the absence of evaporites and salt deformation. The province also lacks long-lived, large deltaic systems that typically result in rapid source rock burial and abundant high-quality hydrocarbon reservoirs. The USGS assessed the potential for undiscovered conventional oil and gas resources in the Gulf of Guinea Province as part of its World Petroleum Assessment 2000, estimating a mean of 1,004 million barrels of conventional undiscovered oil, 10,071 billion cubic feet of gas, and 282 million barrels of natural gas liquids. Most of the hydrocarbon potential is postulated to be in the offshore, deeper waters of the province. Gas resources may be large, as well as accessible, in areas where the zone of hydrocarbon generation is relatively shallow. Introduction The U.S. Geological Survey (USGS) assessed the potential for undiscovered oil and gas resources in the Gulf of Guinea Province (7183) as part of its World Petroleum Assessment 2000 (U.S. Geological Survey World Energy Assessment Team, 2000). The province extends from the Niger Delta west to Liberia, and includes the coastal and offshore areas of Côte d’Ivoire, Ghana, Togo, Benin, and westernmost Nigeria (fig. 1). At least five total petroleum systems have been identified in the province, but existing exploration and production data in the province are mostly limited to the Cretaceous rocks. Therefore, only the Cretaceous Composite Total Petroleum System (TPS) with its contained Coastal Plain and Offshore Assessment Unit (AU) was assessed in this study (fig. 2). Also owing to limited drilling and production data, the total petroleum system and assessment unit boundaries can only be approximately delineated and so are subject to future revisions. This report documents and supplements the oil and gas assessment reported in USGS World Petroleum Assessment 2000—Description and results (U.S. Geological Survey World Energy Assessment Team, 2000) by providing additional geologic detail concerning the total petroleum systems and assessment units and a more detailed rationale for the quantitative assessment input. Since the Gulf of Guinea assessment in 1999, three major volumes (Cameron and others, 1999; Mello and Katz, 2000; Arthur and others, 2003) on the petroleum geology of west Africa have been published, signifying increased interest and exploration activity in the region. Geology of the Gulf of Guinea Province The Gulf of Guinea Province includes the Ivory Coast, Tano, Saltpond, Central, Keta, and Benin Basins and the Dahomey Embayment in the northwestern part of the Gulf of Guinea (figs. 3, 4). These basins share common structural and stratigraphic characteristics, in that they are wrench-modified basins (Clifford, 1986) and contain rocks ranging in age from Ordovician to Holocene (Kjemperud and others, 1992); they were therefore grouped together as one province. The eastern boundary is the Niger Delta Province (Klett and others, 1997), and the western boundary is the West African Coastal Province (fig. 4). The Gulf of Guinea formed at the culmination of Late Jurassic to Early Cretaceous tectonism that was characterized by both block and transform faulting superimposed across an extensive Paleozoic basin during breakup of the African, North American, and South American paleocontinents (figs. 5, 6). Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa By Michael E. Brownfield and Ronald R. Charpentier