Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa

Home , Accra, Benin, Guinea, Guinea (region), Gulf of Guinea, Ivory Coast

5°W 0°

10° BENIN

TOGO NIGERIA

Ibadan CÔTE D'IVOIRE GHANA PORTO- NOVO

LOMÉ LAGOS ABIDJAN ACCRA GULF OF 5° GUINEA

LIBERIA

ATLANTIC OCEAN

Niger Delta

0 250 500 KILOMETERS

0°

U.S. Geological Survey Bulletin 2207-C

U.S. Department of the Interior U.S. Geological Survey Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa

By Michael E. Brownfield and Ronald R. Charpentier

U.S. Geological Survey Bulletin 2207-C

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

Posted online July 2006 Version 1.0 This publication is only available online at http://www.usgs.gov/bul/2207/C/ For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: Brownfield, M.E., and Charpentier, R.R., 2006, Geology and total petroleum systems of the Gulf of Guinea Province of west Africa: U.S Geological Survey Bulletin 2207-C, 32 p. iii

Foreword

This report was prepared as part of the World Energy Project of the U.S. Geological Sur- vey. The purpose of the World Energy Project is to assess the quantities of oil, gas, and natural gas liquids that have the potential to be added to reserves within the next 30 years. These volumes either reside in undiscovered fields whose sizes exceed the stated minimum-field- size cutoff value for the assessment unit (variable, but must be at least 1 million barrels of oil equivalent) or occur as reserve growth of fields already discovered. For this project, the world was divided into 8 regions and 937 geologic provinces, which were then ranked according to the discovered oil and gas volumes within each (Klett and others, 1997). Of these, 76 “priority” provinces (exclusive of the U.S. and chosen for their high ranking) and 26 “boutique” provinces (exclusive of the U.S. and chosen for their anticipated petroleum richness or special regional economic importance) were selected for appraisal of oil and gas resources. The petroleum geology of these priority and boutique provinces is described in this series of reports. A geologic province is a region that characteristically has dimensions of hundreds of kilometers and that encompasses a natural geologic entity (for example, sedimentary basin, thrust belt, accreted terrain) or some combination of contiguous geologic entities. Province boundaries were drawn as logically as possible along natural geologic boundaries, although in some provinces their location is based on other factors, such as a specific bathymetric depth in open oceans. The total petroleum system constitutes the basic geologic unit of the oil and gas assessment. The total petroleum system includes all genetically related petroleum that occurs in shows and accumulations (discovered and undiscovered) that (1) has been generated by a pod or by closely related pods of mature source rock, and (2) exists within a limited mappable geologic space, along with the other essential mappable geologic elements (reservoir, seal, and overburden rocks) that control the fundamental processes of generation, expulsion, migration, entrapment, and preservation of petroleum. The minimum petroleum system is that part of a total petroleum system encompassing shows and discovered accumulations along with the geologic space in which the various essential elements have been proved by these discoveries. An assessment unit is a mappable part of a total petroleum system in which discovered and undiscovered fields constitute a single, relatively homogeneous population such that the chosen methodology of resource assessment based on estimation of the number and sizes of undiscovered fields is applicable. A total petroleum system may equate to a single assessment unit, or if necessary may be subdivided into two or more assessment units such that each unit is sufficiently homogeneous in terms of geology, exploration considerations, and risk to assess individually. A graphical depiction of the elements of a total petroleum system, in the form of an events chart, shows the times of (1) deposition of essential rock units; (2) trap formation; (3) generation, migration, and accumulation of hydrocarbons; and (4) preservation of hydrocarbons. iv

A numeric code identifies each region, province, total petroleum system, and assessment unit; these codes are uniform throughout the project and throughout all publications of the project. The code used in this study is as follows:

Unit Name Code Region Sub-Saharan Africa 7

Province Gulf of Guinea 7183

Total petroleum system Cretaceous Composite 718301

Assessment unit Coastal Plain and Offshore 71830101

The codes for the regions and provinces are listed in Klett and others (1997). Oil and gas volumes quoted in this report are derived from Petroconsultants, Inc., 1996 Petroleum Exploration and Production database (Petroconsultants, 1996) and other area reports from Petroconsultants, Inc., unless otherwise noted. Figures in this report that show boundaries of the total petroleum system and assessment unit were compiled using geographic information system (GIS) software. Political boundaries and cartographic representations were taken, with permission, from Environmental Systems Research Institute’s ArcWorld 1:3 million digital coverage (1992), have no political signifi- cance, and are displayed for general reference only. Oil and gas field centerpoints, shown in these figures, are reproduced, with permission, from Petroconsultants (1996).

Contents

Foreword...... iii Abstract...... 1 Introduction...... 1 Geology of the Gulf of Guinea Province...... 1 Pre-Transform Stage...... 8 Syn-Transform Stage...... 19 Post-Transform Stage...... 19 Petroleum Occurrences in the Gulf of Guinea Province...... 22 Hydrocarbon Source Rocks...... 22 Hydrocarbon Generation and Migration...... 24 Hydrocarbon Reservoirs, Traps, and Seals...... 24 Total Petroleum Systems of the Gulf of Guinea Province...... 25 Lower Paleozoic and Lower Cretaceous Total Petroleum Systems...... 26 Cretaceous Composite Total Petroleum System (718301)...... 28 Assessment Units of the Gulf of Guinea Province...... 29 Coastal Plain and Offshore Assessment Unit (71830101)...... 29 Summary...... 29 Acknowledgments...... 31 References Cited...... 31

Figures

1. Location map of Gulf of Guinea Province, west-central Africa...... 2 2. Location map of Cretaceous Composite Total Petroleum System and Coastal Plain and Offshore Assessment Unit...... 3 3. Map of the basins, exploration wells, and major hydrocarbon discoveries in Gulf of Guinea Province...... 4 4. Generalized geologic map of west Africa...... 5 5. Paleogeographic maps of the Cretaceous separation of Africa and South America...... 6 6. Maps of four stages of the Mesozoic breakup of Africa and South America and the tectonic evolution of the Equatorial Atlantic...... 7 7. Generalized geologic map of Gulf of Guinea area, showing selected oil and gas provinces...... 9 8. Sketch map showing major fracture zones, sediment thickness, and oceanic-continental crust boundary...... 10 9. Generalized stratigraphic column within the Gulf of Guinea Province...... 11 10. Generalized composite stratigraphic column of offshore and onshore parts of the Saltpond and Tano Basins...... 12 11. Generalized stratigraphic column of Dzita-1 drill hole, Keta Basin, Ghana...... 13 12. Sketch diagram showing paleogeographic reconstruction of the Silurian Period...... 14 vi

13. Generalized stratigraphic column for central and western parts of Ivory Coast Basin...... 15 14. Schematic cross section across the Belier and Espoir fields in Ivory Coast Basin...... 16 15. Generalized stratigraphic column for offshore part of Benin Basin...... 17 16. Generalized geoseismic cross section for offshore part of Benin Basin...... 18 17. Generalized geoseismic cross section for offshore part of Keta Basin, eastern Ghana...... 21 18. Modeled burial history curves and vitrinite reflectance and temperature plots for Dzita-1 well, Keta Basin, Ghana...... 23 19. Schematic cross section showing common trap types and oil- and gas-field analogs in Gulf of Guinea Province...... 26 20. Events chart for the Lower Paleozoic Total Petroleum System in the Saltpond, Keta, and Tano Basins, Gulf of Guinea Province...... 27 21. Events chart for the Lower Cretaceous Total Petroleum System in the Ivory Coast, Tano, Keta, and Benin Basins, and the Dahomey Embayment, Gulf of Guinea Province...... 27 22. Events chart for the Cretaceous Composite Total Petroleum System (718301) in the Ivory Coast Basin, Gulf of Guinea Province...... 28

Tables

1. Summary of estimated undiscovered volumes of conventional oil, gas, and natural gas liquids for undiscovered oil and gas fields for the Coastal Plain and Offshore Assessment Unit in the Cretaceous Composite Total Petroleum System of the Gulf of Guinea Province, west Africa, showing allocations of undiscovered volumes to the offshore...... 30 2. Summary of estimated undiscovered volumes of conventional oil, gas, and natural gas liquids for undiscovered oil and gas fields for Sub-Saharan Africa, showing allocations by oil and gas province...... 30 Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa

By Michael E. Brownfield and Ronald R. Charpentier

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

5°W 0°

10°

NIGERIA BENIN

Ibadan CÔTE D'IVOIRE TOGO GHANA PORTO- NOVO

LOMÉ LAGOS ABIDJAN ACCRA GULF OF 5° GUINEA

LIBERIA

ATLANTIC OCEAN

Niger Delta

0 250 500 KILOMETERS

0°

Projection: Robinson. Central meridian: 0 EXPLANATION Gulf of Guinea Province 7183 Country boundary Gas field centerpoint Oil field centerpoint

Figure 1. Gulf of Guinea Province (7183) in west-central Africa and locations of oil and gas field centerpoints (Petroconsultants, 1996). Geology of the Gulf of Guinea Province

5°W 0°

10°

NIGERIA BENIN

Ibadan CÔTE D'IVOIRE TOGO GHANA PORTO- NOVO

LOMÉ LAGOS

ABIDJAN ACCRA GULF OF 5° GUINEA

LIBERIA

ATLANTIC OCEAN

Niger Delta

0 250 500 KILOMETERS

0°

Projection: Robinson. Central meridian: 0 EXPLANATION Coastal Plain and Offshore Assessment Unit 71830101 Cretaceous Composite Total Petroleum System 718301 Country boundary Gas field centerpoint Oil field centerpoint

Figure 2. Gulf of Guinea Province showing area of Cretaceous Composite Total Petroleum System and Coastal Plain and Offshore Assessment Unit, and oil and gas field centerpoints (Petroconsultants, 1996). Total Petroleum Systems, Gulf of Guinea Province, West Africa

10°W 5°W 0° 5°E

MALI Volta TOGO Basin Dahomey BENIN Embayment NIGERIA GHANA ISE-2 CÔTE D'IVOIRE Dzita-1 CTS-1 Sémé Aje Epiya LIBERIA Lomé Attoutu-1 W. Tano N. Tano Saltpond

S. Tano Keta-1 5° Benin Basin Foxtrot Belier Keta Basin Espoir Tano St. Paul Basin FZ Ivory Coast Basin Saltpond and Cape Central Basins Three Points

GULF OF GUINEA Romanche Fracture Zone

AFRICA

Chain Fracture Zone

0° SOUTH AMERICA

0 200 400 KILOMETERS

EXPLANATION

Fracture zone (FZ) Sedimentary basin Country boundary Oil and gas discoveries and fields Exploration well mentioned in text

Figure 3. Major features of the Gulf of Guinea Province, west Africa: Benin, Central, Ivory Coast, Keta, Saltpond, Tano, and Volta Basins, Cape Three Points, major fracture zones, and approximate locations of exploration wells and of the oil and gas discoveries and fields mentioned in the text. Mid-Atlantic Ridge and fracture zones shown in index map. Modified from Kjemperud and others (1992). Geology of the Gulf of Guinea Province

18°W 12°W 6°W 0° 6°E 12°E 18°E 24°E 16°N

Senegal Chad 7013 7066 Iullemmeden Taoudeni 7055 12°N Basin 7035 Benue West African Nigerian 7136 Shield Massive 7021 7121 ° Baffa 8 N 7105 Volta 7114

4°N West Zaire Gulf of Guinea Niger West African 7192 Precambrian Coastal 7183 Belt 7173 7211 Zaire 7225 0°

EXPLANATION 4°S

Quaternary rocks West-Central Coastal Quaternary and Tertiary rocks 7203 8°S Tertiary rocks

Cretaceous rocks ATLANTIC 12°S Triassic and Jurassic rocks OCEAN Paleozoic rocks

Precambrian rocks 16°S

Etosha Igneous rocks 7285

Subsurface salt diapir 20°S

Water WALVIS RIDGE Kalahari Oil and gas province boundary 7325 24°S Damer Belt Orange River 7311 1,000 KILOMETERS Coastal 0 500 7303

28°S

Karoo 7355 32°S

Figure 4. Generalized geologic map of west Africa (from Persits and others, 2002), showing province 36°S boundaries and selected province names and codes South African Coastal as defined by Klett and others (1997). 7363

CAMPANIAN- ALBIAN CENOMANIAN TURONIAN MAASTRICHTIAN Total Petroleum Systems, Gulf of Guinea Province, West Africa 110 M.Y. 95 M.Y. 90 M.Y. 75 M.Y.

NORTH NORTH NORTH NORTH AMERICA AMERICA AMERICA AMERICA

EUROPE EUROPE EUROPE EUROPE

AFRICA AFRICA AFRICA AFRICA

Gulf of Gulf of Gulf of Guinea Guinea Gulf of Guinea Guinea

SOUTH SOUTH SOUTH SOUTH AMERICA AMERICA AMERICA AMERICA

Walvis Walvis Ridge Walvis Walvis Ridge Ridge Ridge Tristan da Cunha

EXPLANATION Type of oceanic basins

Anoxic

Oxic

Areas of continental-margin sedimentation

Figure 5. Paleogeographic stages in the separation of Africa and South America during the Cretaceous. Modified from Tissot and others (1980). 15°W 10°W 5°W 0° 5°E 15°W 10°W 5°W 0° 5°E

Senegal Basin Senegal Basin Benue 10° Bové Basin Trough Benue Bové Basin Volta Volta Benin and 10° Trough Basin Basin Keta Basins Benin and Keta Basins Ivory Coast Ivory Coast Basin Basin 5° 5°

Para-Maranhao Basin Para-Maranhao 0 500 KILOMETERS Basin

A 0 500 KILOMETERS 15°W 10°W 5°W 0° 5°E Senegal Basin Benue C 10° Bové Basin Trough 15°W 10°W 5°W 0° 5°E Volta Benin and Basin Keta Basins Senegal Basin Ivory Coast Benue Basin Bové Basin 10° Volta Trough 5° Basin Benin and Keta Basins Ivory Coast Basin

5° Para-Maranhao Basin Geology of the Gulf of Guinea Province 0 500 KILOMETERS

B EXPLANATION Para-Maranhao Basin West African and Brazilian shields Bové and Volta Paleozoic basins 0 500 KILOMETERS Onshore Mesozoic to Cenozoic Areas of oceanic crust— coastal basins Arrows show direction Areas of thick continental crust of crustal extension D and extension Shear zones Divergent basins—Thinned Present-day 2,000-m isobath continental crust and thick clastics

Figure 6. Schematic Cretaceous stages in the Mesozoic breakup of Africa and South America and the tectonic evolution of the Equatorial Atlantic, and showing the approximate

location of the Bové, Benin, Ivory Coast, Keta, Senegal, and Volta Basins and the Benue Trough of Africa and the Para-Maranhao Basin of Brazil. A, Hauterivian, 125 Ma; B, early Albian, 110 Ma; C, late Albian, 100 Ma; D, Santonian, 85 Ma. Modified from Mascle and others (1988). Total Petroleum Systems, Gulf of Guinea Province, West Africa

Thus, the province has undergone a complex history, which we and others, 2000). Such rocks may also be present in the divide into pre-transform (late Proterozoic to Late Jurassic), Benue Trough and in the Keta and Ivory Coast Basins (figs. 3, syn-transform (Late Jurassic to Early Cretaceous), and post- 6). Transform tectonism was active between the African and transform (Late Cretaceous to Holocene) stages of basin South American plates until middle to late Albian time, when development. These three stages are referred to as the pre-rift the first oceanic crust was formed (fig. 6C) and the last con- (or intracratonic), syn-rift (or rift), and post-rift (or drift) stages nection between the two continents was breached (Blarez and by Dumestre (1985), Kjemperud and others (1992), Tucker Mascle, 1988). The end of syn-transform tectonism and sedi- (1992), and Chierici (1996). The structural basins within the mentation in the Gulf of Guinea Province coincides with this province are aligned generally east-west, with boundaries phase. A major Albian–lower Cenomanian unconformity was delimited by an east-west transform fault system (fig. 3) and a direct consequence of the final separation of the continental north-south structural arches. margins (Blarez and Mascle, 1988; Chierici, 1996; MacGregor The initial phase of the post-Hercynian opening of the and others, 2003). By Santonian time (fig. 6D), continued north Atlantic and the splitting of North America from Eurasia crustal extension resulted in the formation of major oceanic and Africa began during Late Permian–Early Triassic time crust, and the marginal basins and offshore platform of the (Lehner and De Ritter, 1977; Ziegler, 1988; Lambiase, 1989; province were subjected to an increase in clastic deposition Uchupi and others, 1976). The final breakup of Africa and and thermal subsidence, resulting in development of several South America began in the Late Jurassic in the southernmost Late Cretaceous and Tertiary unconformities. part of the south Atlantic and prograded northward during The Gulf of Guinea Province is divided structurally by Neocomian time (Binks and Fairhead, 1992; Guiraud and three major transform fault zones (figs. 3, 8): (1) the St. Paul Maurin, 1992). The area now occupied by the Gulf of Guinea fracture zone along the northwestern boundary, (2) the Roman- opened last, forming a continuous anoxic seaway in the late che fracture zone that separates the Ivory Coast and the Central Albian to Turonian (Tissot and others, 1980). A continuous and Saltpond Basins from the Keta Basin, and (3) the Chain oxic Atlantic Ocean (fig. 5) existed by the end of the San- fracture zone along the eastern boundary. Sedimentary fill tonian (Blarez and Mascle, 1988). The presence of Aptian within the Ivory Coast Basin is more than 6,000 m thick (fig. 8) evaporites and clastics in the West-Central Coastal Province to north of the Romanche fracture zone, which acted as a dam to the south (fig. 4) provides evidence that rift-basin sedimenta- the transport and accumulation of sediments to the south. tion occurred during the Early Cretaceous, associated with the The three-stage tectonic evolution in the Gulf of Guinea breakup of Africa and South America. Province allows the stratigraphic section to be divided into The early tectonic history of the Gulf of Guinea Province three main sequences (fig. 9): (1) Precambrian to Triassic is different from that of the West-Central Coastal Province intracratonic rocks and Jurassic to Lower Cretaceous conti- (fig. 3) or “Aptian salt basin” (Blarez and Mascle, 1988; nental to marginal marine rocks representing the pre-transform Mascle and others, 1988; Basile and others, 1993; MacGregor stage, (2) Lower Cretaceous to latest Albian rocks represent- and others, 2003; Brownfield and Charpentier, 2006). In the ing the syn-transform stage, and (3) Cenomanian to Holocene evolution of the “Aptian salt basin,” the rifting stage was rocks representing the post-transform stage. dominated by extensional or block faulting, forming grabens An understanding of the geology of both the African and that filled with lacustrine and fluvial sediments; this was fol- South American margins of the Atlantic, and an appreciation lowed by deposition of regional evaporates, and subsequent of their resource potential, have increased greatly in the last halokinesis (McHargue, 1990; Teisserenc and Villemin, 1990). decade. Some suggested references for general geology and The Gulf of Guinea Province shows two important geological structural evolution of the African Atlantic margin are Blarez differences compared to the passive-margin basins south of and Mascle (1988), Uchupi (1989), Doust and Omatsola the Niger Delta: (1) the influence of transform tectonics, and (1990), Teisserenc and Villemin (1990), Brown and others (2) the absence of evaporites and halokinesis. Middle Jurassic (1995), Cameron and others (1999), Mello and Katz (2000), volcanic rocks occur in the Gulf of Guinea Province, indicat- and Arthur and others (2003). ing that tectonism started no later than the Middle Jurassic (Dumestre, 1985; Kjemperud and others, 1992). Pre-Transform Stage Transform faulting was initiated between the African and South American continental plates in Early Cretaceous time The pre-transform section consists of Precambrian to (Hauterivian, fig. 6A). The thick continental crust of the Afri- Triassic rocks that crop out in the Volta and Tano Basins can and South American platform started to break up, forming of Ghana (Volta Province, figs. 3, 4). This section has been divergent basins or pull-apart grabens separated by transform penetrated by drilling in the Saltpond, Tano, and Keta Basins faults (Blarez and Mascle, 1988) in early Albian time (fig. (fig. 3) in the central and eastern parts of the Gulf of Guinea 6B). Thick continental clastics, consisting of fluvial and pos- Province (Dumestre, 1985; Kesse, 1986; Kjemperud and oth- sibly lacustrine facies, were deposited in the divergent basins. ers, 1992; Tucker, 1992). In the Volta Basin (figs. 3, 7), the In the Dahomey Embayment (figs. 3, 7), source-rock samples pre-transform section consists of intracratonic Proterozoic to have chemical and geologic characteristics similar to Lower Cambrian rocks. The upper Precambrian Buem Formation Cretaceous lacustrine source rocks in the Congo Basin (Haack overlies the Precambrian basement, and includes sandstone, 6°W 0° 6°E 12°E

Taoudeni Basin 7035 Volta Iullemmeden 7114 7055

Volta Basin Nigerian Massive 7121 8° Dahomey Embayment West African Shield Benue 7021 7136

Ivory Coast Basin

Gulf of Guinea 7183 4° West African GULF OF GUINEA Niger Delta Coastal 7192 7173

West Zaire Precambrian Belt West-Central 7211 ATLANTIC OCEAN Coastal

7203 Geology of the Gulf of Guinea Province

0°

EXPLANATION 0 500 KILOMETERS

Quaternary rocks Paleozoic and Mesozoic rocks

Pleistocene rocks Paleozoic rocks

Quaternary and Tertiary rocks Precambrian rocks

Tertiary rocks Igneous rocks

Cretaceous rocks, undivided Salt structures Figure 7. Generalized geology of the Gulf of Guinea area, showing selected petroleum provinces and the Dahomey Embayment and the Lower Cretaceous rocks Petroleum province boundary Ivory Coast and Volta Basins. Modified from Persits and others (2002). 10 Total Petroleum Systems, Gulf of Guinea Province, West Africa

15°W 10°W 5°W 0° 5°E

Porto- Novo Lagos 2 Lomé

Four North FZ Ivory Coast Abidjan Accra Basin

5° 6 2 4 6 8 5 2 4 2 3 4 3 St. Paul FZ 1 1 2 Romanche FZ 2

1 0° Chain FZ

AFRICA

0 100 500 KILOMETERS

EXPLANATION

Gulf of Guinea Province Oceanic-continental crust boundary Major fracture zones (FZ) 3 Sediment thickness, in kilometers

Figure 8. Sketch map showing major fracture zones (FZ), sediment thickness, and oceanic-continental crust boundary for the Gulf of Guinea Province. Modified from Emery and Uchupi (1984) and MacGregor and others (2003).

shale, and volcanics (Kesse, 1986). The Voltaian Series deposition possibly continued eastward to the Benue Trough, in overlies the Buem Formation, and includes sandstone, shale, Nigeria. Devonian marine sandstones and shales (fig. 11) were mudstone, conglomerate, limestone, and tillites deposited in penetrated in the Dzita-1 well (Kjemperud and others, 1992) but continental and shallow-marine environments (Kesse, 1986: drilling did not reach the basement. The Upper Devonian Tako- Kulke, 1995). radi Formation, consisting of sandstone, shale, and organic-rich In the Central, Keta, and Saltpond Basins (figs. 9, 10, 11) shale, was deposited in a shallow to restricted marine environ- of central and eastern Ghana, the pre-transform rocks range in ment. The Upper Devonian to Carboniferous Efia Nkvanta For- age from Ordovician to Triassic (Kjemperud and others, 1992; mation is composed of nonmarine and marine sandstones and Tucker, 1992). The Ordovician to Silurian lacustrine Ajua cherty shales with limestone deposited in continental to shallow Formation consists of laminated shales; it is overlain by the marine environments. The Carboniferous to Triassic Sekondien fluvial and lacustrine Elmina Formation, composed of feld- Series includes red sandstones and red-brown shales deposited spathic sandstone and minor conglomerate. Both formations are in a fluvial environment. Middle Devonian to Lower Cretaceous present only in the Central and Saltpond Basins (Kjemperud pre-transform rocks consisting of continental interbedded con- and others, 1992). The distribution of marine Silurian rocks glomerate, sandstone, and shale were observed in the onshore (Clifford, 1986), which are known to contain oil-prone black part of the Saltpond Basin (Kesse, 1986). graptolitic shales in northern Africa, is shown on the Silurian Pre-transform rocks have not been identified with cer- paleogeographic map in figure 12; as indicated, Silurian tainty in the Ivory Coast Basin (fig. 3), but there are Lower Geology of the Gulf of Guinea Province 11

EXPLANATION Ivory Ghana Tectonic Series/Stage Coast Tano Saltpond Keta Benin stage

System Sandstone Q Pleistocene Pliocene Conglomerate Miocene Shale

Oligocene Calcareous shale Tertiary Eocene Limestone Post-transform Imo Volcanic rocks Paleocene Shale Missing section, Maastrichtian Nauli Limestone major unconformity Campanian Santonian

Upper Coniacian Turonian Cenomanian Albian ? Cretaceous Aptian ? Syn-transform Barremian Hauterivian Lower Valanginian ? Berriasian Sekondi Formation ? ? ? ? Not penetrated by drilling Upper Pre-transform Block faulting

Middle ? ? Volcanic flows and intrusions,

Jurassic marks the initial breakup in the Tano Basin in the Tano

Lower ? Not penetrated by drilling

Scale change U M Triassic L ? ? U ? Lower Permian

? ?

Upper Pre-transform intracratonic rocks ? Efia Nkvanta

Lower Formation Carboniferous Not penetrated by drilling in western part of province U ? Takoradi Formation M L Devonian Elmina Formation Figure 9. Generalized stratigraphic U ? column showing age, lithology, ? L tectonic stage, and stratigraphic Silurian position of selected formations in U major basins (see fig. 3) of the Gulf M Ajua Formation of Guinea Province. Modified from

Ordovician L Kjemperud and others (1992). 1 Total Petroleum Systems, Gulf of Guinea Province, West Africa

EXPLANATION Field Age Lithology Geologic setting and tectonic stage discoveries Sandstone

Conglomerate Tertiary Shale undivided Marine facies Siltstone Oligocene unconformity Limestone

Volcanic rocks Maastrichtian

Basement Campanian Predominantly marine

sediments with minor Post-transform Unconformity continental conditions during the middle and Stratigraphic Late Cretaceous horizon of field Belier ? discovery Senonian

Cenomanian South Mid-Albian Tano unconformity faulting

North Extensional Albian Tano ?

Continental facies ? in Lower Cretaceous Aptian associated with block faulting Syn-transform Block faulting

Jurassic Volcanic rocks associated Figure 10. Generalized with the beginning of block faulting and continental composite stratigraphic column showing age, Permian separation Saltpond lithology, approximate stratigraphic horizon of field discoveries, geologic Carboniferous setting, and tectonic stage for offshore and Paleozoic sedimentation onshore parts of Saltpond in a stable intracratonic setting and Tano Basins, Ghana, Gulf of Guinea Province. See figure 3 for location Saltpond of field discoveries. The Marine brackish-water Stable Devonian middle and upper parts facies, includes source intracratonic of the Devonian section rocks of the Takoradi contain marine source Saltpond Formation rocks in the Takoradi Formation. See figure 9 for stratigraphic position of Continental (fluvial and selected formation names. lacustrine) facies Ordovician Modified from Tucker (1992). Cretaceous stage boundaries were not Precambrian West African Shield shown by Tucker (1992). Geology of the Gulf of Guinea Province 13

EXPLANATION Depth Tectonic Age Lithology in feet stage Sandstone

Pleistocene- Conglomerate and sandstone Pliocene Sandstone and shale Miocene 1,000 1 Eocene Shale

Paleocene Post-transform Siltstone

Maastrichtian 2,000 2 Limestone

Volcanic rocks

3,000 Unconformity

1 Oligocene unconformity

2 Late Cretaceous unconformity 4,000 Aptian-Albian

Index map 5,000 5°W 0°

10°

6,000 CÔTE BENIN

D'IVOIRE TOGO

GHANA NIGERIA

ABIDJAN ACCRA LAGOS LOMÉ Cretaceous 7,000 5° Dzita-1 GULF OF GUINEA Barremian ATLANTIC 8,000 OCEAN Syn-transform, block faulting

followed by extensional faulting 0°

9,000

Figure 11. Generalized stratigraphic column showing age, depth, lithology, 10,000 and tectonic stage of drill hole Dzita-1,

Hauterivian Keta Basin, eastern Ghana. Jurassic volcanics represent the onset of 11,000 block faulting in the province. Late Cretaceous unconformity at the base of Ryazanian Triassic?- Pre- the Maastrichtian represents a major Jurassic transform? period of erosion where it is estimated 12,000 that more than 1,300 m of Upper Carboniferous Cretaceous (Albian to Campanian) rocks were removed. The middle Oligocene unconformity represents a period of 13,000 Devonian Stable erosion where as much as 390 m of

intracratonic Tertiary rocks was removed. Modified TD-13,448 ft from Kjemperud and others (1992). 1 Total Petroleum Systems, Gulf of Guinea Province, West Africa and Middle Jurassic rocks (fig. 9), consisting of thick beds of Paleozoic rocks in the Gulf of Guinea Province. Similar of conglomerate and sandstone deposited in a continental Paleozoic rocks crop out at Cape Three Points (fig. 3) and are setting, which were interpreted to be older than syn- present offshore. transform-related volcanics (Kjemperud and others, 1992). Upper Jurassic pre-transform sedimentary rocks have not Chierici (1996) also reported a sequence of rocks overlying been identified with certainty in the Keta Basin (fig. 9) in east- the basement (figs. 13, 14) and below a thick section of syn- ern Ghana (Kjemperud and others, 1992), although the lower transform rocks in the basin. part of the continental Upper Jurassic to Lower Cretaceous The only Paleozoic rocks observed in the Tano Basin of Sekondi Formation (fig. 9) may correlate to the Upper Jurassic eastern Côte d’Ivoire and western Ghana are Lower Carbon- to Lower Cretaceous Ise Formation in the offshore part of the iferous siliciclastic rocks (Tucker, 1992) penetrated in the Benin Basin (fig. 15; Elvsborg and Dalode, 1985), and may CTS-1 well (fig. 3). Seismic data indicate that this sequence is represent the uppermost part of this stage. The Ise Formation underlain by strata equivalent to the Ordovician and Devo- contains conglomerates, sandstones, and shales which were nian rocks in the onshore part of the Saltpond Basin and the deposited in continental and deltaic environments. The dating offshore part of the Keta Basin. The CTS-1 well Lower Car- of the lower part of the Sekondi Formation is uncertain because boniferous and underlying rocks represent the western limit of the fluvial depositional environments where preservation of

Turkey Spain

20°S HIMAL SEA MID-EUROPEAN OCEAN

30°S

Africa 40°S

India

50°S Gondwanaland

60°S Antarctica South America EXPLANATION

70°S Silurian marine deposition Outline of Silurian paleocontinents Outline of present-day continents, dashed where uncertain

Figure 12. Paleogeographic reconstruction of the Silurian Period showing relative positions of paleocontinents and areas of deposition for marine Silurian rocks, of which oil-prone graptolitic shales constitute a major part. Outline of present-day continents shown in relative position with areas of known Silurian rocks. Modified from Clifford (1986). Geology of the Gulf of Guinea Province 15

EXPLANATION Age Lithology Tectonic stage

Miocene- Sandstone Pliocene Sandstone with shale 1

Conglomerate Paleocene- middle Eocene Shale

Siltstone

Limestone with shale partings

Campanian- Basement Maastrichtian Missing section Post-transform 2 Unconformity 1 Oligocene unconformity

Early 2 Senonian unconformity Senonian, Turonian (?) 3 Albian unconformity

Cenomanian

Syn-transform, block faulting Figure 13. Generalized stratigraphic followed by Barremian- transform column showing age, lithology, and Albian faulting tectonic stage for rocks in central and western parts of Ivory Coast Basin, Gulf of Guinea Province. Pre-transform rocks were deposited in continental environments, syn-transform rocks in continental to marginal marine environments, and post-transform rocks Jurassic to in marginal marine to open-marine Neocomian Pre-transform environments. Exploration drilling has not reached rocks older than Jurassic in the Precambrian central and western parts of the Ivory Coast Basin. Modified from Chierici (1996). 1

Belier Espoir Projected NORTH SOUTH Total Petroleum Systems, Gulf of Guinea Province, West Africa Sea level ATLANTIC OCEAN Post-transform Tertiary Tertiary

Maastrichtian

R R Post-transform Senonian Senonian unconformity 5 R R Turonian

Middle Pre-transform Albian R Albian Syn-transform R unconformity

R EXPLANATION

DEPTH, IN THOUSANDS OF FEET DEPTH, IN THOUSANDS Albian Sandstone R 10 Shale Aptian

Carbonate rocks Syn-transform Pre-transform rocks 0 1 2 3 4 5 KILOMETERS R Proven and probable reservoir rocks Vertical exaggeration 3:1

5°W 0° Index map

10° GHANA

CÔTE BENIN

D'IVOIRE NIGERIA TOGO

ACCRA ABIDJAN LAGOS LOMÉ 5° Figure 14. Schematic cross section across the Belier and Espoir fields in Ivory Coast Basin, Gulf of Line of GULF Guinea Province. The Senonian unconformity separates the continental and marginal marine syn- section OF transform rocks from the post-transform or passive margin rocks. Espoir and Belier field discovery ATLANTIC GUINEA wells are projected into the section (see fig. 3). Modified from Clifford (1986), Kulke (1995), and Chierici OCEAN (1996). Heavy black line, fault. 0° Cretaceous Tertiary Quaternary

Early Late Jurassic ? Paleogene Neogene Age Neocomian Senonian Maastrichtian Cenomanian Holocene to Valanginian Hauterivian Campanian Pleistocene Barremian Paleocene Santonian Berriasian Oligocene Coniacian Turonian Pliocene Miocene Eocene Aptian Albian or Abeokuta sandstone" sandstone" Oshoshun Formation Formation Afowo "Turonian Araromi "Albian Shale Shale Awgu Benin Ijebu Imo and Ise ? Upper Lower Lithology 1 2 3 ? transform Tectonic stage Pre- Syn-transform Post-transform Geology of the Gulf of Guinea Province (2003). (1985) andMacGregorothers Modified fromElvsborgandDalode as theAbeokutaFormationinBenin. “Turonian sandstone”isalsoknown pre-transform tectonicstage.The Upper Jurassicandpartofthe Ise Formationisconsideredtobe Province. Thelowerpartofthe part ofBeninBasin,GulfGuinea stratigraphic columnforoffshore Figure 15. 1 2 3 EXPLANATION

Sandstone withshale Shale Generalized Siltstone Oligocene unconformity Senonian unconformity Unconformity Conglomerate Sandstone Albian unconformity

1 7 1 NORTH SOUTH Sea level EXPLANATION

Benin and Ijebu Formations Total Petroleum Systems, Gulf of Guinea Province, West Africa ATLANTIC OCEAN Sandstone Lower Afowo Formation

Conglomerate 1,000 Lower Afowo Imo Shale Formation Upper Afowo Formation Sandstone and shale Imo Shale Awgu Formation Shale 2,000 Araromi Araromi Shale sandstone Sandstone with Awgu Formation siltstone and Imo Shale shale "Turonian sandstone" Basement 3,000 Middle Miocene unconformity Basal Miocene unconformity Senonian 4,000 unconformity

"Albian sandstone"

5,000

DEPTH , IN METERS ? Ise Formation

6,000 Basement

7,000 ?

Index map 5°W 0° 8,000

10° ?

CÔTE BENIN NIGERIA 9,000 D'IVOIRE GHANA TOGO

ABIDJAN ACCRA LAGOS LOMÉ 5° Figure 16. Generalized geoseismic cross section of offshore part of Benin Basin, Gulf of Guinea Province. The Albian Line of section unconformity at base of the “Turonian sandstone” is the top of the syn-transform rocks. The Senonian unconformity at GULF base of the Araromi Shale is related to tectonism in the Benue Trough; the Miocene unconformity at base of the Upper ATLANTIC OF OCEAN GUINEA Afowo Formation is related to a depositional hiatus from the Eocene to the Miocene. The Oshoshun Formation is included with the Imo Shale. Some unconformities are not shown. See figure 15 for stratigraphic positions of formations and 0° unconformities. Modified from Elvsborg and Dalode (1985). Heavy line, fault. Basement extent queried where no data. Geology of the Gulf of Guinea Province 19 fossils is low. However, the pre-transform stage is interpreted The Keta Basin of eastern Ghana (figs. 9, 11) contains to be largely a period of erosion and nondeposition in the Keta syn-transform rocks of Early Cretaceous age (Kjemperud and Basin. The eroded sediments were transported westward, and others, 1992), including the Sekondi Formation (fig. 9), in both are most likely preserved as the Jurassic pre-transform rocks in offshore and onshore areas. The Sekondi Formation consists the Tano and Ivory Coast Basins. of alternating red-brown to red sandstone, siltstone, and shale The pre-transform rocks in the Benin Basin and Dahomey deposited in a continental environment. During the earliest Embayment are represented by the lower part of the Ise Cretaceous, the basin underwent gradual subsidence, block Formation (Dumestre, 1985; Elvsborg and Dalode, 1985; faulting, and graben filling followed by extensional faulting. MacGregor and others, 2003). The Upper Jurassic to Neo- The Aptian to Albian rocks (fig. 9) are characterized by marine comian Ise Formation (fig. 15), as much as 2,000 m thick, is sandstone and shales with some organic-rich black shales, composed of sandstone, shales, and conglomerate deposited in coarse sandstone and conglomerates, and minor limestone fluvial and deltaic environments. Drilling has not reached the (Kjemperud and others, 1992). Graben filling continued until base of the formation, but seismic data indicate that it directly the middle of the Cenomanian, when uplift of the basin brought overlies basement rocks (fig. 16) in the offshore part of the about extensive erosion and the peneplanation of the Gulf of Benin Basin. Guinea Province. The Campanian is represented by marine sandstone, shale, and minor limestone and conglomerate (fig. 9). The unconformity at the base of the Maastrichtian in the Syn-Transform Stage Dzita-1 well (fig. 11) represents a major period of erosion, Data on the age of volcanic intrusives associated with during which it is estimated that more than 1,300 m of Upper initial block faulting in Liberia, southern Sierra-Leone, and Cretaceous rocks were removed (Kjemperud and others, 1992). Ghana (fig. 3) indicate that faulting started no later than In the Benin Basin (location, fig. 3), syn-transform rocks the Middle Jurassic and may be as old as Early Jurassic are represented by the Neocomian part of the Ise Formation (Dumestre, 1985; Kjemperud and others, 1992). Continental (figs. 15, 16); they are composed of sandstone, shale, and con- syn-transform rocks in the Ivory Coast Basin (figs. 3, 9, 13) glomerate deposited in fluvial, lacustrine, and deltaic environ- also show evidence that volcanic and fault activity may have ments (Dumestre, 1985; Elvsborg and Dalode, 1985; Mac- started in the Early Jurassic. Orientation of the intrusives Gregor and others, 2003). The base of the formation has not indicates that the initial fracturing and graben formation were been reached by drilling, but seismic data indicate that deposi- subparallel to the present coastline. Block faulting and graben tion was in a series of grabens and half grabens in the basin filling characterized the initial stage of tectonism, followed by (fig. 16) and that the strata may be equivalent to the Lower transform or extensional faulting in the Gulf of Guinea. Cretaceous rocks in the Keta Basin. The upper part of the Ise The oldest Mesozoic syn-transform sedimentary rocks Formation contains lacustrine algae similar to those present are extensive, continental Jurassic conglomerate and sandstone in the lacustrine Bucomazi Formation of the Lower Congo (Dumestre, 1985), with thicknesses as much as 2,000 m in the Basin (Haack and others, 2000; Brownfield and Charpentier, Ivory Coast Basin (location, fig. 3). A comparable sequence 2006) in the West-Central Coastal Province of west Africa has not been penetrated by drilling in Ghana, indicating a (fig. 4). The Ise Formation is unconformably overlain by the period of nondeposition and (or) erosion in that area (Kjempe- transgressive “Albian sandstone,” which consists of sandstone rud and others, 1992). Drilling has not encountered rocks older with some interbedded shale (figs. 15, 16) and forms the upper than Jurassic in the Ivory Coast Basin. boundary of the syn-transform stage in the Benin Basin (Elvs- During Neocomian, Aptian, and probably early and borg and Dalode, 1985). middle Albian time, more than 5,000 m (Chierici, 1996) of The end of the syn-transform stage is delineated by syn-transform sediments were deposited in continental to a major unconformity, which separates it from the marine marginal marine environments in the Ivory Coast Basin (figs. post-transform rocks of the uppermost Albian and Cenoma- 9, 13, 14). The oldest marginal marine strata are in the upper nian (Dumestre, 1985; Kjemperud and others, 1992; Chierici, Albian, and the lack of evaporites in the Lower Cretaceous 1996; MacGregor and others, 2003). This major unconformity section indicates that in the Gulf of Guinea Province the syn- is also readily recognized in the Brazilian marginal basins, transform sediments were deposited in a humid equatorial which supports the interpretation that the two continents were climate. close to one another during the Early Cretaceous and that their Lowermost Cretaceous syn-transform sediments in the geologic histories were similar during that time. Tano and Saltpond Basins (figs. 3, 10) were deposited under mostly continental conditions (Kjemperud and others, 1992; Post-Transform Stage Tucker, 1992), resulting in interbedded sandstone, shale, and limestone (figs. 9, 10). The environment became mostly The post-transform stage rocks in the Gulf of Guinea marine during the late Aptian to early Albian, resulting in a Province consist predominantly of marine Cenomanian to syn-transform sequence of alternating sandstone and shale Holocene sandstones, shales, and minor carbonate rocks (fig. with some black shale, coarse sandstone, conglomerate, and 9) deposited in alternating regressions and transgressions minor limestone. (Dumestre, 1985; Chierici, 1996; Kjemperud and others, 1992; 20 Total Petroleum Systems, Gulf of Guinea Province, West Africa

MacGregor and others, 2003) that resulted in several Late faulted terrain. The Cenomanian rocks include sandstone, shale, Cretaceous and Tertiary unconformities (figs. 9–11, 13, 15). In siltstone, mudstone, and limestone (Kjemperud and others, general, continental-margin tectonics of the province’s post- 1992; Tucker, 1992); black shale and limestone were deposited transform stage were driven by thermal subsidence (Kjempe- on the crests of the horsts, whereas turbidites accumulated in the rud and others, 1992). grabens. Turonian to Maastrichtian rocks (fig. 9) include marine A marine transgression in the Ivory Coast Basin in the sandstone, shale, limestone, and minor conglomerates (Kjempe- early Cenomanian (fig. 13) signaled the beginning of the rud and others, 1992). Major unconformities bound the Ceno- post-transform stage, resulting in the deposition of limestone manian to Maastrichtian rocks in the Tano Basin as in the Ivory on fault-block crests and of organic-rich black shale and Coast Basin. The unconformably overlying Tertiary rocks are turbidites in the grabens (Dumestre, 1985; Chierici, 1996). marine sandstone, shale, and limestone. The regional Oligocene Paleontological evidence indicates restricted water circula- unconformity is present in the Tano Basin (figs. 9, 10) separat- tion and low oxygen content. Following this transgression, a ing Miocene from the underlying Eocene rocks. middle Cenomanian regression and uplift resulted in erosion Post-transform rocks range in age from Campanian to of the upper Albian to lower Cenomanian sequence in the Holocene in the Keta Basin (figs. 9, 11, 17). Campanian strata, eastern part of the Ivory Coast Basin (figs. 13, 14). During the including marine and continental conglomerate, sandstone, regression more than 3,000 m of middle and upper Ceno- shale, and limestone (fig. 9), unconformably overlie Albian manian sediments were deposited in the central and western syn-transform rocks, and are unconformably overlain by parts of the basin, as evidenced by strata encountered in the Maastrichtian rocks. The Maastrichtian includes limestone Attoutu-1 well (fig. 3) in the northwestern part of the basin and shale with sandstone and claystone deposited in a marine (Chierici, 1996). In the Turonian, a major transgression took environment; these strata grade into continental clastics east- place that established the first communication between Atlan- ward toward the Benin Basin. Tertiary post-transform rocks tic and Tethyan waters. Paleontological analysis indicates that unconformably overlie the Maastrichtian rocks, and include restricted water circulation and a low oxygen content contin- Paleocene to Eocene marine shale interbedded with sandstone, ued through the Turonian. Mainly marine shale with some siltstone, and limestone. These rocks are separated from the sandstone characterizes the overlying Coniacian to Santonian upper Miocene marine sandstone, conglomerate, shale, and interval (figs. 13, 14). limestone (fig. 9) by a major Oligocene to Miocene unconfor- An episode of intense deformation occurred in the Benue mity. During the erosional period represented by this uncon- Trough (fig. 6) at the end of the Santonian that was reflected by formity, as much as 365 m of Tertiary rocks (Kjemperud and erosion and the development of a major unconformity (labeled others, 1992) was removed, as evidenced by the stratigraphic “Senonian unconformity” in figs. 13, 14) in the east half of the sequence penetrated in the Dzita-1 and Keta-1 wells (figs. 3, Ivory Coast Basin (Dumestre, 1985; Chierici, 1996). Campan- 11, 17). An unconformity separates the upper Miocene and ian rocks, predominantly shale with minor sandstone deposited lower Pliocene in parts of the onshore and shallow shelf (fig. during a transgression, lie unconformably on the older Cre- 9). The Pliocene to Holocene offshore units consist of uncon- taceous rocks. Paleontological evidence indicates improved solidated sands and muds. bottom-water circulation, but the waters were still anoxic. The The Cenomanian to Santonian part of the post-trans- Maastrichtian is the interval best represented throughout the form stage in the Benin Basin and Dahomey Embayment onshore part of the Ivory Coast, where it includes sandstone was influenced by transform or extensional faulting, and and shale deposited in fluvial and near-shore environments; on also was affected by deformation that took place during the the continental shelf, it includes more marine facies. Santonian in the Benue Trough to the east (fig. 6; Elvsborg The Paleocene sequence represents a major transgression and Dalode, 1985); these episodes of tectonic activity resulted in the Ivory Coast Basin, where it consists of glauconitic shale in the development of the Senonian unconformity in the with sandy shale and minor limestone. These strata are overlain Benin Basin (figs. 15, 16). The “Turonian sandstone” (fig. by lower and middle Eocene shales and marls with thin beds 15) consists of a gray to white coarse-grained, poorly sorted of limestone (Chierici, 1996), which, in turn, are overlain by a sandstone interbedded with thin shale beds overlying a shale major unconformity (figs. 13, 14) that apparently developed in and siltstone sequence deposited as a reworked fan delta in a Oligocene time. Marine Miocene rocks lie above this unconfor- marginal marine to inner shelf environment. The “Turonian mity in the offshore part of the Ivory Coast Basin, and no Oligo- sandstone” unconformably overlies the syn-transform “Albian cene rocks have been identified in offshore exploration wells. sandstone” and is the oldest post-transform unit in the Benin The Tano Basin is located in the easternmost part of the Basin. It is present over the entire Benin Basin, and, in places, Ivory Coast Basin (fig. 3), and the post-transform period in the directly overlies crystalline basement. The depocenter is Tano Basin generally has the same tectonic and stratigraphic located in the eastern part of the basin, where thicknesses are history as the rest of the Ivory Coast Basin. Post-transform as much as 1,000 m; the unit thins to the north and west. The continental margin tectonism was driven by thermal subsidence, Miocene unconformity cuts out the “Turonian sandstone” beginning in the Cenomanian (figs. 9, 10) with the first marine beyond the shelf edge, whereas the Senonian unconformity transgression into the Gulf of Guinea Province. The marine cuts into the unit in the eastern part of the shelf and along the waters most likely inundated the remaining syn-transform block shelf edge (Elvsborg and Dalode, 1985). Projected well Keta-1 NORTH SOUTH Sea 0 level Paleocene-Eocene 1 Miocene Coniacian- ATLANTIC OCEAN Campanian Maastrichtian

2 Pliocene Pre-Cretaceous- Pliocene-Holocene Aptian 3

Miocene unconformity Miocene 4

Oligocene

TWO WAY TIME, IN SECONDS TWO WAY 5 Pre-Cretaceous- Aptian 6 0 5 KILOMETERS Coniacian- Campanian

EXPLANATION Index map Geology of the Gulf of Guinea Province 5°W 0° Pliocene to Quaternary Paleocene to Fault sediments Eocene rocks Miocene unconformity 10° Pliocene rocks Maastrichtian rocks Upper Cretaceous CÔTE BENIN unconformity NIGERIA

D'IVOIRE TOGO Miocene rocks Coniacian to GHANA Campanian rocks Seismic reflector LAGOS ACCRA Oligocene rocks ABIDJAN Pre-Cretaceous Exploration well LOMÉ to Aptian rocks 5° Keta-1 Line of section

ATLANTIC GULF OCEAN OF Figure 17. Generalized geoseismic cross section of offshore part of Keta Basin, eastern Ghana. See figure 11 for stratigraphic GUINEA positions of major unconformities. Modified from Kjemperud and others (1992). 0° 1 Total Petroleum Systems, Gulf of Guinea Province, West Africa

The Coniacian Awgu Formation (fig. 15), which is pres- Devonian shales (MacGregor and others, 2003), as the alterna- ent over most of the Benin Basin, unconformably overlies the tive of an Upper Cretaceous source (see paragraph on anoxic “Turonian sandstone.” The formation, consisting of dark-gray conditions) seems unlikely. calcareous shale interbedded with calcareous siltstone and Oil seeps in outcrops of the Upper Cretaceous tar sands fine-grained sandstone, was deposited in an anoxic marine in the northern Dahomey Embayment are interpreted to be environment; below the Senonian unconformity (fig. 15), it is sourced by Neocomian lacustrine strata (fig. 15), such as were preserved in grabens. Six Maastrichtian to Holocene post-trans- drilled into in the Ise-2 well (Haack and others, 2000). These form stratigraphic units have been identified in the Benin Basin source rocks contain Type I kerogen, with total organic carbon (fig. 15): (1) the Maastrichtian to Paleocene Araromi Shale; (2) (TOC) contents as much as 4 percent and the richest source- the Paleocene to Eocene Imo Shale; (3) the Eocene Oshoshun rock intervals having hydrogen indices (HI, mg (milligrams) Formation; (4) the Miocene Afowo Formation; (5) the Pliocene hydrocarbon/g (grams) organic carbon) greater than 500. The Benin Formation; and (6) the Pliocene(?) to Holocene Ijebu geochemical characteristics are similar to those of the lacus- Formation. The units become coarser grained toward the top trine source rocks in the Neocomian Bucomazi Formation of and include argillaceous sandstone and siltstone, and shale. the Lower Congo Basin in the West-Central Coastal Province Sediment transport was from the north, and rapid sedimenta- (fig. 4). Lower Cretaceous lacustrine strata are identified as tion rates initiated growth faults (Elvsborg and Dalode, 1985) far west as the Ivory Coast Basin (fig. 13) and may include that sole out in most cases in the Araromi Shale. A depositional similar source rocks (Elvsborg and Dalode, 1985). hiatus from the late Eocene through the Oligocene resulted in Lower to middle Albian gas-prone source rocks have a major unconformity between the Eocene Imo and Oshoshun been identified in the Ivory Coast and Tano Basins (Mac- Formations and Miocene Afowo Formation (figs. 15, 16). A Gregor and others, 2003) (figs. 9, 10, 13). These source rocks second Miocene unconformity separates the lower and upper are part of a sequence consisting of as much as 5,000 m of members of the Afowo Formation. Lower Cretaceous continental to marginal marine rocks deposited in grabens in the Ivory Coast and Tano Basins (Chierici, 1996). Similar source rocks are likely present in the Keta and Petroleum Occurrences in the Gulf of Benin Basins (figs. 9, 11, 17). Anoxic oceanic conditions that characterized the middle Guinea Province Cretaceous worldwide also affected the Gulf of Guinea during the Cenomanian (fig. 5), resulting in the deposition of Oil and gas occurrences in the Gulf of Guinea Province are the sediments forming the Albian and Cenomanian black concentrated in Cretaceous reservoirs on the continental shelf shale source rocks in the Ivory Coast and Tano Basins. This and adjacent onshore extensions in two basin areas (fig. 3): (1) depositional event marks the first marine transgression in the the Ivory Coast to Tano Basins (Espoir field) of Côte d’Ivoire Gulf of Guinea and was accompanied by a decrease in rate of and Ghana, and (2) the Keta Basin (Lomé discovery) to the subsidence in the province. Anoxic conditions continued into Benin Basin (Aje field) of westernmost Nigeria. These areas the Turonian. Samples analyzed from deep sea drilling sites are associated with oil seeps and tar sand accumulations along both north and south of the Gulf of Guinea indicate that these Upper Cretaceous outcrops west of Cape Three Points (fig. 3) source rocks (fig. 13) contain more than 10 weight percent in the onshore parts of the Ivory Coast and Tano Basins, and in organic matter consisting of Type II kerogen. In contrast, the the Dahomey Embayment from Togo to western Nigeria. Most Dahomey Embayment and Benin Basin are characterized by discoveries in the province to 1995 have been located in water nonmarine to marginal marine conditions with the deposi- depths less than 500 m. Oil has also been produced from Devo- tion of coarse sandstone and interbedded shale and carbona- nian to Carboniferous Takoradi Formation sandstones sourced ceous shale (fig. 9) representing the last “land bridge” during from Devonian shales in the Saltpond field (figs. 3, 10). the opening of the Atlantic Ocean (Chierici, 1996). Middle Cretaceous source rocks in this area are expected to contain Hydrocarbon Source Rocks gas-prone Type III kerogen. The Coniacian Awgu Formation, the Maastrichtian The oldest hydrocarbon source rocks in the Gulf of Araromi Shale, and the Paleocene to Eocene Imo Shale (fig. Guinea Province are postulated to be shales in the Middle 15) contain marine source rocks in the offshore part of the to Upper Devonian Takoradi Formation (figs. 9, 10) in the Benin Basin. These source rocks contain Type II and Type Saltpond Basin and field (fig. 3) of western Ghana (Kjempe- II-III kerogen with TOC contents ranging from 2 to more than rud and others, 1992). These source rocks were deposited in 5 weight percent; they were deposited from the northwestern a brackish marine environment. Oils from the Lomé field in part of the Niger Delta westward to the Keta Basin (Haack and Keta Basin (fig. 3) are interpreted to be sourced from cor- others, 2000). The richest source rock intervals have hydrogen relative Devonian shales (MacGregor and others, 2003), and indices greater than 500. Coniacian to Paleocene rocks are seismic data indicate that similar source rocks are preserved present in the offshore part of the Keta Basin (fig. 17) and may in the Tano Basin (Tucker, 1992). Upper Albian reservoirs in contain Type II and Type II-III source rocks similar to those in Sémé and Aje fields (fig. 3) may also have been sourced from the Benin Basin. Petroleum Occurrences in the Gulf of Guinea Province

Sea level

1,500

3,000

4,500

6,000

7,500

9,000 EXPLANATION 10,500

DEPTH, IN FEET Miocene 2

12,000 Eocene 1 Maastrichtian 13,500 Aptian 1 Barremian 1 15,000 Efia Nkvanta Formation 16,500 Lower Takora Formation

350.0 300.0 250.0 200.0 150.0 100.0 50.0 Present TIME (m.a.b.p.)

0 0 Control Control point point 1,500 1,500

3,000 3,000

4,500 4,500

6,000 6,000

7,500 7,500 DEPTH, IN FEET DEPTH, IN FEET 9,000 9,000

10,500 10,500

12,000 12,000

13,500 13,500

0.0 1.0 2.0 0.0 100.0 200.0

VITRINITE REFLECTANCE (Ro) TEMPERATURE, IN DEGREES FAHRENHEIT

Figure 18. Modeled burial history curves and vitrinite reflectance and temperature plots of Dzita-1 well (figs. 3, 11) in Keta Basin, Ghana (fig. 3). Factoring in major erosion in the Cretaceous and minor erosion in the Tertiary produces a good fit with measured Ro values. Total amount of erosion was 1,800 m. Modified from Kjemperud and others (1992); (m.a.b.p.), millions of years before present. Total Petroleum Systems, Gulf of Guinea Province, West Africa

Hydrocarbon Generation and Migration and others, 2003). Hydrocarbon generation began in the Late Cretaceous and may be active to the present. The most important hydrocarbon generation within the Gulf of Guinea Province is from the upper Albian and Cenomanian source rocks, which are distributed throughout Hydrocarbon Reservoirs, Traps, and Seals the offshore part of the province; these strata are expected to The oldest proven reservoir rocks in the Gulf of Guinea increase in thickness and source rock quality into deep water. Province are Devonian to Carboniferous sandstone beds in Two main areas of hydrocarbon generation were interpreted by the Saltpond field in Ghana (fig. 3). The sands forming the MacGregor and others (2003) to exist in the province: (1) the Devonian reservoirs were deposited in shallow to restricted offshore parts of the Ivory Coast and Tano Basins and (2) the marine environments, whereas those forming the Carbonifer- offshore parts of the Keta and Benin Basins and the Dahomey ous reservoirs were deposited in a fluvial environment. Embayment (fig. 3) eastward to the Niger Delta (fig. 7) just Seismic data indicate that a thick Lower Cretaceous east of the Gulf of Guinea Province. These two probable syn-transform section in the offshore part of the Benin Basin oil kitchens are only present in the deep-water parts of the contains probable sandstone reservoir units (Elvsborg and province, where the source rocks have reached a temperature Dalode, 1985) deposited in fluvial to deltaic environments. of at least 100°C and a vitrinite reflectance (R ) of 0.6 percent o Sandstone units with favorable reservoir characteristics have (MacGregor and others, 2003), which are values equivalent to been encountered below the “Albian sandstone” (fig. 15) in having source rocks subjected to about 2,700 m of overburden. drilling in the Sémé field (fig. 3). Similar reservoir rocks may The source of hydrocarbon generation in the Saltpond Basin, exist westward across the province; for example, as much as which lies between the two areas of generation just listed, is 5,000 m of Lower Cretaceous continental to marginal marine more problematic, and may involve deeper source rocks, such sandstone and shales (fig. 9) was deposited in the Ivory Coast as the Devonian Takoradi Formation and Lower Cretaceous lacustrine rocks. Hydrocarbon generation started in the Late Basin (Chierici, 1996). Similar reservoir rocks should also be Cretaceous and continues to the present in the Ivory Coast present in the Keta and Tano Basins (fig. 9). and Tano Basins, whereas hydrocarbon generation started in Stratigraphic units that contain proven reservoirs in the the late Miocene and continues to the present in the Keta and shallow-water discoveries in the Gulf of Guinea Province are Benin Basins and Dahomey Embayment (fig. 3) eastward to mainly late syn-transform Albian sandstones and Cenomanian the Niger Delta (fig. 7). to Maastrichtian post-transform marginal marine and turbidite clastic rocks. Clastic Albian rocks are the dominant reservoir Examples of a burial-history curve and Ro and temperature plots of the Dzita-1 well in the Keta Basin are shown in type in the Espoir and Belier fields (fig. 3) in the Ivory Coast figure 18. This well was drilled in 1973 to test the hydrocar- Basin and are also known in the Tano and Keta Basins. These bon potential of Devonian and Cretaceous sandstones; figure rocks are interpreted to have been deposited in several differ- 11 shows the stratigraphic sequence recorded in the drilling, as ent depositional settings, including lacustrine, fluvial through interpreted by Kjemperud and others (1992). A model of the fluviodeltaic, and marginal marine to marine and submarine burial history that factors in both a major Cretaceous (middle fans. Marginal marine to marine upper Albian sandstone res- Albian) unconformity and a minor Tertiary (Oligocene) ervoirs in the Ivory Coast Basin (fig. 13) contain the best pet- unconformity, marking time intervals during which 1,300 m rophysical qualities, with porosities as much as 25 percent and and 360 m of material, respectively, are estimated to have been permeabilities in the hundreds of millidarcies (MacGregor and others, 2003). Many of the sandstone reservoirs are character- removed by erosion, indicates a good fit with the measured Ro values (fig. 18). This model is in agreement with the maturity ized as being laterally discontinuous and exhibiting variable mapping of the Cretaceous unconformity by MacGregor and petrophysical properties across short distances. Similar Albian others (2003), who placed the top of the oil-generation win- reservoirs have been penetrated in the Tano and Keta Basins (figs. 10, 11) below the mid-Albian unconformity (Kjempe- dow (Ro about 0.6) at about 2,700 m below the sea bed. At least two areas of hydrocarbon generation related to rud and others, 1992; MacGregor and others, 2003). Only Lower Cretaceous lacustrine source rocks are present in the the transgressive “Albian sandstone” represents the marine Gulf of Guinea Province (Haack and others, 2000; MacGregor Albian interval in the offshore part of the Benin Basin (fig. and others, 2003). These areas are associated with oil seeps 15), and the upper Albian section is missing (Tucker, 1992). and tar sand accumulations along Upper Cretaceous outcrops The potential middle Albian reservoir rocks in the Ivory Coast west of Cape Three Points in the onshore parts of the Ivory Basin were deposited in a fluvial continental environment and Coast and Tano Basins as well as in the Dahomey Embay- are characterized by poorer petrophysical qualities. ment (fig. 3); in particular, there are large in-place volumes of A steep shelf began to develop during the Cenomanian hydrocarbons in the eastern tar belt of the Dahomey Embay- along the continental margin of the Gulf of Guinea Province. ment. The total tar-belt volumes likely exceed the published MacGregor and others (2003) speculated that several south- reserves of any field in the Gulf of Guinea Province including flowing rivers supplied clastics to the continental margin prior the largest, which is Espoir field in the Ivory Coast Basin, to their capture by the ancestral Niger River. These rivers—for whose reported field size is 400 million barrels (MacGregor example, a large ancestral Tano River in western Ghana and Total Petroleum Systems of the Gulf of Guinea Province a major south-flowing river in the Benin Basin—would have channel. Seismic data indicate that undrilled channel-erosion drained extensive areas to the north during the early post- traps are commonly associated with the regional Oligocene transform period in the province and deposited large amounts unconformity from Benin westward, in the deep-water part of of clastic sediment during the Cenomanian to Maastrichtian the province. (Elvsborg and Dalode, 1985; Tucker, 1992; MacGregor and Seismic data indicate that syn-transform ponded turbidites others, 2003) now represented by the “Turonian sandstone” lying directly above the upper Albian unconformity in the (figs. 15, 16) or the equivalent Abeokuta Formation (fig. 15). western part of the Ivory Coast Basin (MacGregor and others, The downslope projections of deltas that were formed at that 2003) could be trapped against existing faults (fig. 19). Ponded time would be prospective for probable turbidite channel and turbidite traps are also observed as detached sandstone bod- ponded turbidite sandstone reservoirs (fig. 14). Because the ies in the post-transform section in the Ivory Coast, Keta, and continental shelf is steep and was subjected to several low Benin Basins, where stratigraphic trapping and updip seals are stands along the continental margin, conditions favored the the critical factors in defining potential targets. Some of the deposition of detached, deep-water sandstone units, ponded high-amplitude traps associated with the late Albian unconfor- turbidite sands, and clastic fans (fig. 14). Recent seismic data mity may be limestone units located on syn-transform highs indicate that large turbidite channels developed in the Ivory (Kjemperud and others, 1992; MacGregor and others, 2003). Coast Basin during the Maastrichtian (MacGregor and others, Untested stratigraphic traps consisting of mounded sand- 2003). The presence of large turbidite channels supports the stones and channels interbedded with the Cenomanian source interpretation that large fans or detached sandstone bodies may rocks are interpreted from seismic data in the post-transform lie in the deeper parts of the basin. In general, reservoirs in the section in the Gulf of Guinea Province (fig. 19). Thick and early post-transform section are likely to be of better quality extensive sandstone units were observed by MacGregor and than those in the syn-transform section. others (2003) above the late Albian unconformity, covering Seismic data indicate that the Tertiary section has fewer areas of more than 80 km2 in the deep-water parts of the prov- reservoirs than the Cretaceous section in the Gulf of Guinea ince, where the shelf is steeper and a greater probability exists Province. Some slope fans have been identified in the Araromi for detached sandstone units and up-dip seals. An example of Shale (fig. 15; Elvsborg and Dalode, 1985) in the overly- a stratigraphically sealed channel is Dana’s (British Indepen- ing section above the regional Maastrichtian unconformity dent) Maastrichtian West Tano oil discovery (figs. 3 and 19). (MacGregor and others, 2003). The Araromi sandstone unit Seals associated with syn-transform reservoirs are formed (fig. 16) has been interpreted as a slope fan in the Benin Basin. by both shales and faults (fig. 19), whereas the seals associated Other probable reservoir rocks could be present in the deep- with post-transform reservoirs are generally shales. water part of the continental margins in the form of detached sandstone units resulting from ponded turbidite sands (fig. 14). Proven hydrocarbon accumulations associated with fault- block traps (fig. 19) are in the upper part of the syn-transform Total Petroleum Systems of the Gulf of section throughout the Gulf of Guinea Province in shallow to Guinea Province moderate water depths (MacGregor and others, 2003). This trap type characterizes both the Espoir field (Grillot and oth- At least five total petroleum systems (TPS) exist in the ers, 1991) and the Tano field (fig. 3; MacGregor and others, Gulf of Guinea Province (7183): (1) the Lower Paleozoic TPS, 2003), and extends offshore onto the continental slope. Fault- consisting of Devonian source rocks and Devonian to Lower block traps are also along the Romanche fracture zone (fig. 3) Cretaceous reservoir rocks; (2) the Lower Cretaceous TPS, and in the western part of the Ivory Coast Basin, including the consisting of Lower Cretaceous lacustrine source rocks and 2001 Baobab deep-water discovery. Cretaceous reservoir rocks; (3) the middle Albian Terrestrial Syn-transform anticlinal traps (fig. 19), detected only TPS, consisting of gas-prone source rocks and Albian reservoir from seismic data and as yet untested, are associated with the rocks; (4) the upper Albian TPS, consisting of marine trans- terminations of regional fracture zones in two areas: (1) the gressive oil-prone source rocks and Albian reservoir rocks; and offshore parts of the Dahomey Embayment and Keta Basin, (5) the Cenomanian-Turonian TPS, consisting of open marine and (2) the western offshore part of the Ivory Coast Basin oil-prone source rocks and Albian to Upper Cretaceous reser- (MacGregor and others, 2003). Proven hydrocarbon accumula- voir rocks. The three youngest systems were combined into the tions associated with post-transitional anticlinal traps are in Cretaceous Composite TPS consisting of Albian to Turonian the Tano Basin and the eastern part of the Ivory Coast Basin marine and terrestrial source rocks and Cretaceous reservoir (fig. 3) in the Belier field. rocks. Only limited exploration and production information is Known hydrocarbon accumulations are associated with available for the Lower Paleozoic TPS and Lower Cretaceous erosional channel-fill traps (fig. 19) in the post-transform TPS. Oil production from the Lower Paleozoic TPS is limited section of the Gulf of Guinea Province in both shallow- and to the Saltpond and Lomé fields (fig. 3), whereas Lower Creta- deep-water areas (MacGregor and others, 2003). This type of ceous TPS oils have only been identified in Upper Cretaceous trap characterizes the Aje field of westernmost Nigeria (fig. 3), tar sands and oil seeps at Cape Three Points and the Dahomey where the west end of the reservoir is sealed by a shale-filled Embayment. Only the Cretaceous Composite TPS was Total Petroleum Systems, Gulf of Guinea Province, West Africa

Channel, West Tano Post-transform anticline, Belier

Stratigraphic Erosional channel, Aje

Ponded Fault block, turbidites Middle Albian unconformity Espoir Syn-transform Stratigraphic anticline updip pinchout

EXPLANATION

Conglomerate Shale Unconformity

Sandstone Siltstone Fault

Figure 19. Schematic cross section showing common trap types and oil- and gas-field analogs (see fig. 3) in Gulf of Guinea Province. Many of the traps are present only in the deep-water parts of the province and have not been tested. Barbs on fault (heavy line) show relative movement. Modified from MacGregor and others (2003). No scale.

considered for assessment, because (1) it is the most extensive, migration. These total petroleum systems were not assessed and (2) current exploration and production are mostly limited but are documented in the Gulf of Guinea Province report. to this system. One assessment unit (AU) was defined within The likely source rocks for the Lower Paleozoic TPS are the Cretaceous Composite TPS—the Coastal Plain and Off- organic-rich brackish marine shales in the Middle to Upper shore AU. Input data describing the assessment unit are given Devonian Takoradi Formation in the Saltpond field. Lomé field in the U.S. Geological Survey World Petroleum Assessment (fig. 3) oils are sourced from these shales (MacGregor and 2000—Description and results, Disk 3 (U.S. Geological Survey others, 2003), and seismic data indicate that they are preserved World Energy Assessment Team, 2000). in the Tano Basin (Tucker, 1992), although potential targets are presently untested. Reservoir rocks consist of Devonian and Carboniferous to Permian sandstone beds (fig. 10) in the Lower Paleozoic and Lower Cretaceous Total Saltpond area (fig. 3). Currently it is assumed that upper Albian reservoirs in Sémé and Aje fields are sourced from Devonian Petroleum Systems shales (MacGregor and others, 2003), because downward migration from Upper Cretaceous-sourced oils seems unlikely. Events charts (figs. 20, 21) for the Lower Paleozoic TPS Hydrocarbon generation most likely began in the Late Carbon- and Lower Cretaceous TPS graphically portray the ages of iferous and may have continued into the early Tertiary. source, seal, and reservoir rocks, as well as the timing of trap The Lower Cretaceous TPS was defined because lacus- development, and generation, migration, and preservation of trine source rocks deposited in early rift grabens have been hydrocarbons, and the critical moment. The critical moment recognized in the central and eastern parts of the province, is defined as the beginning of hydrocarbon generation and as evidenced by the presence of lacustrine oils from Upper

Total Petroleum Systems of the Gulf of Guinea Province

ACCUMULATION ACCUMULATION PETROLEUM PETROLEUM SYSTEM EVENTS

SYSTEM EVENTS

MIGRATION- MIGRATION- TIME SCALE TIME SCALE GEOLOGIC

GEOLOGIC TRAP FORMATION GENERATION- CRITICAL MOMENT SOURCE ROCK OVERBURDEN ROCK

ROCK UNIT ROCK RESERVOIR

SEAL ROCK PRESERVATION

TRAP FORMATION OVERBURDEN ROCK GENERATION- SEAL ROCK

CRITICAL MOMENT SOURCE ROCK PRESERVATION ROCK UNIT ROCK RESERVOIR 0 0 Plio Plio Neogene Mio ? 24 Neogene Mio Olig Paleogene Eoc 24 50 Olig 65 Pal L Paleogene Eoc 100 Cretaceous 50 Pal ? E 65

150 146 L L Jurassic M Cretaceous 100 X 200 E 208 L Triassic E E M 250 245 L

Permian E 290 146 L 150 L 300 Pennsylvanian M E X 323 M L Mississippian ? Jurassic 350 E 363 L E M Devonian 200 400 E 408 208 L Silurian E L 439 Triassic 450 L M Ordovician M E 245 250 L 500 E 510 Permian L E M Cambrian 550 E 570 Precambrian 600

Figure 20. Events chart for the Lower Paleozoic Total Figure 21. Events chart for the Lower Cretaceous Total Petroleum System in the Saltpond, Keta, and Tano Basins, Petroleum System in the Ivory Coast, Tano, Keta, and Benin Gulf of Guinea Province. Light-gray shading indicates rock Basins, and the Dahomey Embayment, Gulf of Guinea Province. units present (fig. 9); wavy line, unconformity. Age ranges of Light-gray shading indicates rock units present (fig. 9); wavy source, seal, reservoir, and overburden rocks and the timing of line, unconformity. Age ranges of source, seal, reservoir, and trap formation and generation, migration, and preservation of overburden rocks and the timing of trap formation and generation, hydrocarbons are shown in green and yellow. Queried where migration, and preservation of hydrocarbons are shown in green uncertain. Critical moment is defined as the beginning of and yellow. Queried where uncertain. Critical moment is defined hydrocarbon generation and migration. as the beginning of hydrocarbon generation and migration. Total Petroleum Systems, Gulf of Guinea Province, West Africa

Cretaceous tar sands and oil seeps in areas west of Cape Three Points in western Ghana, as well as in the Dahomey Embayment (fig. 3). Neocomian Ise Formation source rocks encountered in the Ise-2 well (fig. 3) have been correlated

to Upper Cretaceous oil seeps and tar sands in the northern ACCUMULATION

Dahomey Embayment (Haack and others, 2000). The Ise PETROLEUM

Formation source rocks (fig. 15) contain Type I kerogen with SYSTEM EVENTS MIGRATION-

TOC contents as much as 4 percent and hydrogen index (HI, TIME SCALE

mg hydrocarbon/g organic carbon) values greater than 500 in GEOLOGIC

TRAP FORMATION

CRITICAL MOMENT

OVERBURDEN ROCK SOURCE ROCK SEAL ROCK PRESERVATION RESERVOIR ROCK RESERVOIR ROCK UNIT GENERATION- the organically richest intervals. These Neocomian lacustrine 0 shales have similar geochemical characteristics to the Lower Plio Neogene Cretaceous lacustrine source rocks of the Bucomazi Formation Mio 24 in the Congo Delta (MacGregor and others, 2003; Brownfield Olig and Charpentier, 2006). Lower Cretaceous lacustrine rocks are identified as far west as the Ivory Coast Basin (fig. 13) and Paleogene Eoc 50 may contain source rocks (Elvsborg and Dalode, 1985). Pal 65

Cretaceous Composite Total Petroleum System L (718301) Cretaceous 100 X The Cretaceous Composite TPS was defined in the Gulf of Guinea Province (fig. 1). An events chart (fig. 22) for this total E petroleum system graphically portrays the ages of the source, seal, and reservoir rocks, as well as the timing of trap development, and generation, migration, and preservation of hydrocar- 146 150 L bons, and the critical moment. The critical moment is defined as the beginning of hydrocarbon generation and migration. M The principal source rocks for the Cretaceous Composite Jurassic TPS are Albian, Cenomanian, and Turonian marine shales with Type II and II-III oil-prone kerogen and Type III terres- E 200 trial kerogen. Lower to middle Albian Type III source rocks 208 were identified in the Ivory Coast and Tano Basins (figs. 9, 10, 13) by MacGregor and others (2003). These organic-rich Triassic L sediments were deposited in fluvial and deltaic environments; E M in the Ivory Coast Basin as much as 5,000 m of Lower Creta- 245 ceous continental to marginal marine sediment was deposited 250 L Permian in grabens (Chierici, 1996). Similar source rocks should be E present in the Keta (Kjemperud and others, 1992) and Benin Basins (figs. 9, 11, 17). Worldwide anoxic ocean conditions during the Cenomanian (fig. 5) resulted in the deposition of the Cenomanian to Turonian black shale source rocks in the Ivory Coast and Tano Basins. These source rocks (fig. 13) contain more than 10 weight percent TOC consisting of Type II kerogen. In contrast, the Dahomey Embayment and Benin Basin are characterized by nonmarine to marginal marine conditions resulting in the deposition of middle Cretaceous gas-prone source rocks containing Type III kerogen (Chierici, Figure 22. Events chart for the Cretaceous Composite Total 1996). The Coniacian Awgu Formation and the Maastrichtian Petroleum System (718301) in the Ivory Coast Basin, Gulf of Araromi Shale (fig. 15) contain marine source rocks in the Guinea Province. Light-gray shading indicates rock units offshore part of the Benin Basin. These source rocks contain present (fig. 9); wavy line, unconformity. Age ranges of source, Type II and Type II-III kerogen with TOC contents ranging seal, reservoir, and overburden rocks and the timing of trap from 2 to more than 5 weight percent; area of original deposi- formation and generation, migration, and preservation of tion was from the northwestern extension of the Niger Delta hydrocarbons are shown in green and yellow. Queried where westward to the Keta Basin (Haack and others, 2000). The uncertain. Critical moment is defined as the beginning of richest source rock intervals have HI values greater than 500. hydrocarbon generation and migration. Summary

Coniacian rocks are present in the offshore part of the Keta BCFG, respectively. Most of the undiscovered resources are Basin (fig. 17); some of these may be source rocks. expected to be in the deep-water parts of the assessment unit. Hydrocarbon generation started in the Late Cretaceous Compared to other provinces along the Atlantic coast of Africa, for the Albian to Cenomanian source rocks and continues to the Gulf of Guinea Province has only modest potential for undis- the present. For the Turonian and Coniacian source rocks, covered resources (table 2) and ranks as the third province in hydrocarbon generation possibly started in the early Ter- Sub-Saharan Africa (table 2) in terms of mean undiscovered oil. tiary and also continues to the present. Migration was either directly from adjacent source rocks or upward along faults from deeper sources. Summary Reservoir rocks are mostly Cretaceous turbidite sandstones with minor potential limestone units. The traps include The Cretaceous Composite Total Petroleum System pre-transform traps related to fault blocks, syn-transform (TPS; 718301) consists of middle Albian to Maastrichtian structural and stratigraphic traps, and post-transform strati- Type II, II-III, and III oil-prone kerogen and Type III gas- graphic traps. Seals are marine shales and shale-filled channels prone kerogen and Cretaceous reservoirs. Worldwide middle with minor fault-related seals. Cretaceous anoxic ocean conditions resulted in the formation of the Cenomanian to Turonian Type II oil-prone kerogen source rocks containing more than 10 weight percent TOC. Assessment Units of the Gulf of Guinea Cretaceous marine mudstones and shales are the primary seals. Hydrocarbon generation began in the Late Cretaceous and Province continues to the present. Two other total petroleum systems were recognized The Coastal Plain and Offshore Assessment Unit (AU; in the Gulf of Guinea Province: (1) the Lower Paleozoic 71830101) (fig. 2) includes Cretaceous reservoirs associated TPS, consisting of Devonian source rocks and Devonian with pre-transform fault blocks and syn- and post-transform to Lower Cretaceous reservoir rocks, and (2) the Lower ponded turbidites, turbidite channels, and slope fans in basins Cretaceous TPS, consisting of Lower Cretaceous lacustrine between the major fracture zones (fig. 3). This assessment source rocks and Cretaceous reservoir rocks. Although unit was defined to be co-located geographically with the these total petroleum systems are considered to have Gulf of Guinea Province boundary, with the northern bound- hydrocarbon potential, they were not assessed: current ary of the assessment unit defined as the northern limit of exploration and production data are limited to the overlying the Cretaceous rocks and the southern boundary drawn at a Cretaceous Composite TPS. water depth of 2,000 m. Although potential reservoirs in this Two important geologic differences contrast the Gulf of assessment unit could exist in the shallow-water part, most of Guinea Province with the passive-margin basins south of the the prospectivity is expected to be in the deep-water part of Niger Delta: (1) the influence of transform tectonics in the the province. Gulf of Guinea Province, and (2) the absence of evaporites and salt deformation. The province also lacks long-lived delta systems that provide depositional conditions for rapid source Coastal Plain and Offshore Assessment Unit rock burial and abundant high-quality reservoirs. (71830101) The Gulf of Guinea Province is estimated to have mean undiscovered resources of 1,004 MMBO and 10,071 BCFG Exploration in the 1970s and 1980s resulted in the dis- in undiscovered fields. Compared to other provinces in Sub- covery of several moderate-sized oil fields, including Espoir Saharan Africa, the province has a moderate potential for (1980; 400 million barrels) and the large Foxtrot gas field undiscovered resources. All of the undiscovered resources are (1982; about 1 trillion cubic feet) in the assessment unit (fig. offshore and mostly in deep water. 3). Exploration through 1995 was considered to be relatively Critical factors controlling oil and gas accumulations in immature with the discovery of only 33 fields (Petroconsul- the Gulf of Guinea Province are the presence of good reser- tants, 1996), only 4 of which are in deep-water areas. The voirs, quality and preservation of hydrocarbons, and the ability presence of one large gas field coupled with the relative lack to produce hydrocarbons at a rate that would be economic in of exploration for stratigraphic traps indicates a promising a deep-water setting. Offshore core samples and seismic data potential for future discoveries. Since the 2000 assessment indicate that erosion on the structural highs and on the province (U.S. Geological Survey World Energy Assessment Team, shelf and slope is extensive, exposing the Albian rocks on the 2000), eight discoveries have been made, with four of them in sea bed and removing potential reservoir and source rocks. deep water (IHS Energy Group, 2003). Although the deep-water parts of the Gulf of Guinea The USGS assessed mean undiscovered volumes in the Province are underexplored, they are considered to contain Coastal Plain and Offshore AU of 1,004 MMBO, 10,071 BCFG, many potential prospects. Gas resources may be substantial, and 282 MMBNGL (table 1). The estimated sizes of the larg- and may also be accessible in areas where the zone of hydro- est undiscovered oil and gas fields are 201 MMBO and 1,138 carbon generation is relatively shallow. 30 Total Petroleum Systems, Gulf of Guinea Province, West Africa

Table 1. Summary of estimated undiscovered volumes of conventional oil, gas, and natural gas liquids for undiscovered oil and gas fields for the Coastal Plain and Offshore Assessment Unit in the Cretaceous Composite Total Petroleum System of the Gulf of Guinea Province, west Africa, showing allocations of undiscovered volumes to the onshore.

[MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, million barrels of natural gas liquids. MFS, minimum field size assessed (MMBO or BCFG). Prob., probability (including both geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates not applicable]

Undiscovered Resources Field MFS Prob. Oil (MMBO) Gas (BCFG) NGL (MMBNGL) Type (0-1) F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Cretaceous Composite Total Petroleum System Coastal Plain and Offshore Assessment Unit Offshore (100% of undiscovered oil fields and 100% of undiscovered gas fields allocated to offshore) Oil Fields 2 225 901 2,117 1,004 918 3,846 9,845 4,420 29 124 339 146 1.00 Gas Fields 12 1,256 5,064 12,000 5,650 28 118 303 136

Total 1.00 225 901 2,117 1,004 2,174 8,910 21,846 10,071 57 242 642 282

Table 2. Summary of estimated undiscovered volumes of conventional oil, gas, and natural gas liquids for undiscovered oil and gas fields for Sub-Saharan Africa, showing allocations by oil and gas province.

Code Undiscovered Resources and Field Prob. Oil (MMBO) Gas (BCFG) NGL (MMBNGL) Type (0-1) F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

7013 Senegal Province Oil Fields 15 120 430 157 33 255 968 345 2 15 59 21 1.00 Gas Fields 83 414 1,276 510 3 18 58 22

Total 1.00 15 120 430 157 116 669 2,244 856 5 33 118 43

7183 Gulf of Guinea Province Oil Fields 225 901 2,117 1,004 918 3,846 9,845 4,420 29 124 339 146 1.00 Gas Fields 1,256 5,064 12,000 5,650 28 118 303 136

Total 1.00 225 901 2,117 1,004 2,174 8,910 21,846 10,071 57 242 642 282

7192 Niger Delta Province Oil Fields 17,487 39,975 65,123 40,487 28,703 70,120 133,579 74,056 903 2,292 4,608 2,459 1.00 Gas Fields 28,845 57,910 90,585 58,660 1,741 3,517 5,594 3,574

Total 1.00 17,487 39,975 65,123 40,487 57,548 128,030 224,165 132,716 2,643 5,808 10,202 6,034

7203 West-Central Coastal Province Oil Fields 9,033 27,917 56,465 29,747 17,693 56,368 123,798 61,608 827 2,739 6,483 3,080 1.00 Gas Fields 5,268 23,152 58,794 26,436 217 989 2,715 1,164

Total 1.00 9,033 27,917 56,465 29,747 22,961 79,520 182,592 88,044 1,044 3,727 9,198 4,244

7303 Orange River Coastal Province Oil Fields 23 87 312 116 46 186 701 256 3 11 43 15 1.00 Gas Fields 629 2,829 7,889 3,348 26 121 361 147

Total 1.00 23 87 312 116 675 3,015 8,590 3,603 28 132 404 163

7 Total: Sub-Saharan Africa Oil Fields 26,783 68,999 124,447 71,512 47,393 130,775 268,891 140,685 1,763 5,180 11,533 5,722 1.00 Gas Fields 36,081 89,369 170,545 94,604 2,015 4,762 9,031 5,044

Total 1.00 26,783 68,999 124,447 71,512 83,474 220,144 439,436 235,290 3,778 9,942 20,564 10,766 References Cited 1

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