Nearby finds brighten outlook for Equatorial Guinea and Namibia

Richard Bray, Steve Lawrence
Exploration Consultants Ltd.
Henley-on-Thames, U.K.

Equatorial Guinea and Namibia are situated on the northern and southern ends of the West African deepwater play fairway.

Between them lie the prolific petroleum provinces of offshore Gabon, Cabinda, Congo, and northern Angola (Fig. 1 [169,872 bytes]).

The Atlantic margins of Equatorial Guinea and Namibia share a common geological setting and tectono-stratigraphic evolution with these provinces, leading to an expectation that they may also hold the ingredients required for deepwater hydrocarbon accumulations.

Namibia and Equatorial Guinea are offering deepwater acreage in licensing rounds that close on Mar. 31 and May 10, 1999, respectively.

This article considers the geology and prospectivity of the deepwater areas of Namibia and Equatorial Guinea in the light of the recent discoveries in the intervening areas.

Equatorial Guinea's offshore territory divides into two geological provinces, the area around the island of Bioko overlying distal parts of the Tertiary Niger Delta basin, and the Rio Muni area overlying part of the West African Atlantic margin basin system (Fig. 1).

Offshore Bioko is known to have a high level of prospectivity and is already an area of active deepwater exploration. This article therefore focuses on the deepwater areas off Rio Muni.

Recent seismic data acquired for the licensing rounds have allowed a preliminary evaluation of both the Namibia and Rio Muni deepwater areas. These and well data reveal many similarities with Gabon, Congo, and Angola in terms of potential source rocks, reservoirs, and play types.

In particular, the recognition of fan or mounded features combined with the prediction of oil-mature source rocks suggests that the deepwater areas off Equatorial Guinea and Namibia will prove equally as prospective as the Atlantic margin provinces that lie between.

Basin evolution

The basins of the West African margin have formed in response to continental separation and the formation of the South Atlantic Ocean through Cretaceous and Tertiary time.

Along this margin, a thick wedge of mid-Cretaceous to Tertiary sediments built out over an early Cretaceous rifted terrain, forming a major basin system extending from the Douala and Rio Muni basins in the north to those of Namibia and South Africa in the south.

Several phases of basin development are recognized, entailing precursor rifting in the early Cretaceous followed by passive margin development during the late Cretaceous and Tertiary.

This history of development has resulted in several major tectono-stratigraphic units separated by unconformities which can be correlated along the whole margin from Namibia to Equatorial Guinea (Fig. 2 [160,738 bytes]).

An early Cretaceous rift onset unconformity separates prerift basement of Jurassic or older rocks from an early Cretaceous rift sequence.

The rift sequence is well known from Gabon, where the section comprises a nonmarine sequence of fluvial, deltaic, and lacustrine clastics. This section is regionally prospective over the relatively shallow-water areas of continental shelf and slope and holds significant prospectivity in the basins of Rio Muni and Namibia.1

However, the main prospectivity in the deepwater exploration areas is found in the postrift or drift section. The drift section is separated from the underlying rift section by the Aptian drift onset unconformity.

Drift sedimentation took place in a chain of passive margin basins that formed as continental rifting gave way to the formation of oceanic crust in the South Atlantic.

These basins are characterized by a thick wedge of drift clastic sediments which prograded seaward as the section built out over the underlying rift basins, overstepping onto oceanic crust.

Deepwater discoveries

Intense industry interest in West Africa has been recently fueled by giant oil finds in deepwater areas off Congo and northern Angola (Fig. 1).

A number of large oil fields have been discovered in the region since early 1996, and drilling success rates and the size of discoveries have been impressive.

Total discovered reserves in the region increased by approximately 3.16 billion bbl in 1997-from 14.18 billion bbl at the beginning of the year to 17.34 billion bbl at year-end, an increase of about 22%.

Approximately 2.86 billion bbl of this total is attributable to new discoveries, mostly located within the deepwater play fairway.

West Africa deepwater success first occurred in the Congo basin in 1995 with the Elf Aquitaine SA discovery of 400 million bbl Moho field in 800 m of water, tested at 5,700 b/d.

The first deepwater discovery offshore Angola was Elf's Girassol field on Block 17, in 1,350 m of water. Girassol reserves are thought to be around 1 billion bbl, and the field is expected to produce at 200,000 b/d.

Other discoveries are Dalia in 1,360 m of water, which may turn out to be bigger than Girassol; Rosa in 1,405 m of water; and more recently Lirio, tested at 11,000 b/d.

On Angola's Block 15, Exxon Corp. made three discoveries with three consecutive wells: Kissanje in 1,011 m of water tested at 10,000 b/d; Marimba in 1,289 m of water tested at 6,800 b/d; and Hungo in 1,202 m of water tested at 15,900 b/d. Potential recoverable reserves in these three fields are estimated at 1 billion bbl.

On Block 14, Chevron Corp. is planning development of Kuito in 400 m of water, with estimated reserves of 700 million bbl, which is expected to produce at around 50,000 b/d.

Other discoveries on the block are Landana in 440 m of water and Benguela in 400 m of water, which tested at 20,000 b/d.

The deepwater play

Key factors controlling the West African deepwater play can be summarized as follows:

Rich oil-prone source rocks in the pre- or post-salt section, especially in the Aptian rift-to-drift transition sequence and in the Albian to Cenomanian-Turonian early drift section.
Favorable timing of hydrocarbon generation. Maximum maturity and hydrocarbon generation are occurring at the present day due to progressive burial of early drift source rocks beneath a thick late drift sediment pile.
Vertical migration pathways provided by faults associated with salt or gravity slide structures. Overpressure caused by rapid Tertiary sedimentation has probably also influenced migration, as well as sealing.
Reservoirs and traps in the Upper Cretaceous and Tertiary late drift section, formed by stacked sand bodies deposited by a variety of gravity-driven, deepwater processes. Stratigraphically enhanced combination traps are caused by mounding, channeling, and ponding of sands around underlying structures.

The combination of circumstances responsible for the deposition of the deepwater reservoirs has existed since the mid-Cretaceous but has been particularly prevalent in the Congo and Angola basins since mid-Oligocene time.

The circumstances include the uplift and continued emergence of the West Africa margin to act as a sediment source, the northward drift of the African continent into the zone of humid tropical weathering, the lowering of sea level causing cycles of regression, and the evolution of major Niger, Ogooue, Congo, and Kwanza river systems.

The main reservoirs in the deepwater areas off Congo and Angola are stratigraphically controlled deepwater fan or turbidite deposits within the late drift section.

Their potential is demonstrated at Girassol, where the reservoir is formed by Upper Oligocene distal turbidites deposited in submarine channels. The reservoir system is composed of several individual complexes including meandering channels and sheet sands that extend over an area of 18 km by 10 km.

In the mirror-image deepwater basins of Brazil all the major fields are similar Upper Cretaceous and Tertiary turbidite and deepwater-fan deposits.

The potentially huge size of these deepwater-fan type reservoirs is further demonstrated by the stratigraphically enhanced traps in reservoir sands of the greater Marlim complex, which have a collective area exceeding 500 sq km, resulting in extremely large reservoirs both in area and in net pay thickness. These reservoirs are also characterized by excellent porosity and permeability characteristics.²

The Namibian margin

The Namibian margin is intersected in the north by the Walvis Ridge, a volcanic feature thought to have been created by plume impingement during the early Cretaceous (Fig. 3 [89,075 bytes]).

The part of the margin south of the Walvis Ridge is considered to have developed as a volcanic margin characterized by a thick wedge of seaward dipping reflectors (SDRs), suggesting that oceanic or proto-oceanic crust formed subaerially.

It is possible that rifted continental crust is absent over most of offshore Namibia and the oceanic-continental crust boundary may extend landward to Rabinowitz and La Brecque's G magnetic anomaly.

A zone of rifted continental crust may have extended some distance onshore but has since been removed by late Cretaceous and Tertiary uplift and erosion.

Only the topmost part of the SDR section is penetrated by drilling offshore Namibia, where it forms the reservoir interval at giant Kudu gas field, composed of interbedded aeolian sandstones and basalts.

The field is developed in what is thought to be the feather edge of the SDR package, and the trap has a large degree of stratigraphic control. Numerous similar trapping opportunities occur in Kudu analog situations along the broad SDR terrain.

SDR wedge-out traps are prospective for oil where downdip oil-prone Aptian source rocks are situated in oil window maturity conditions.¹

In common with the other basins on the West Africa margin, postrift sedimentation offshore Namibia is characterized by a thick wedge of drift clastic sediments which migrated seaward with time through its development.

The overall progradational nature of the wedge is due mainly to the construction of large growth-faulted delta systems, particularly during the mid to late Cretaceous.

Sediment supply was from the proto-Cunene River system to the north and the proto-Orange River in the south, with an unknown number of other drainage systems providing sediment input points.

These are now represented by ephemeral rivers due to the onset of arid climatic conditions in the late Tertiary.

Seismic stratigraphic interpretation of the Namibian postrift section over the Walvis-Luderitz-Orange basin system, to the south of the Walvis Ridge, has recognized three megasequences bounded by major unconformities.³

A claystone dominated early drift megasequence of early Aptian to mid-Turonian age overlies the drift onset unconformity (Fig. 2) and is interpreted as a complete transgressive-regressive cycle, terminated by the Turonian unconformity.

This is overlain by the Upper Cretaceous to Tertiary late drift section, divided into two megasequences by a prominent Base Tertiary unconformity.

The lower late-drift megasequence (mid-Turonian to Base Tertiary) is interpreted as a regressive-transgressive-regressive cycle, consisting of a thick claystone-dominated succession which varies in more shelfal areas to include shallow marine sands and delta plain deposits.³

In the Orange and Luderitz basins, this sequence is disrupted by zones of intense listric growth faulting, passing into a zone of toe thrusts and slump structures in the downslope basinal direction (Fig. 3).

The upper late-drift megasequence (Tertiary) is bounded by the Base Tertiary unconformity and the present-day seabed. Sediment accumulation rates during this period are thought to have decreased due to the gradual aridification of the Namibian margin during the Tertiary.

Deposition largely occurs seaward of the lower late-drift depocenters due to the progressive infilling of accommodation space in front of the prograding sediment wedge. The upper late-drift megasequence is interpreted to be wholly regressive.⁴

The recently acquired deepwater seismic data south of the Walvis Ridge show that thick development of the drift section continues down the continental rise into the deepwater basinal areas.

Deposition in the deepwater setting was dominated by gravitational mass transport processes and the formation of submarine canyons over the shelf edge and continental slope.³

These processes led to the formation of debris flow and turbidite-like deposits, submarine fans, and mound deposits. A variety of mounded features have been identified from deepwater seismic data (Fig. 4 [133,570 bytes]) and are particularly prevalent above the Turonian and Base Tertiary unconformities.

These features are similar to mounds and turbidite deposits which form deepwater reservoirs offshore Angola. Drilling on similar base of slope mounded features in the Upper Cretaceous of the shallow-water Namibian shelf has shown them to be sand-rich, and they are considered to hold good reservoir potential in deepwater Namibia areas.

The Namibe basin to the north of the Walvis Ridge has a different basin architecture and history. The northern margin of the Walvis Ridge is believed to be controlled by a transform fracture zone which probably also offset the zone of rifted continental crust oceanwards.

The Namibe basin has a rift-to-drift basin evolution more typical of the West African basins to the north. The basin is also characterized by strongly downcutting unconformities formed by submarine erosion at Base Tertiary and Oligocene levels.

The post-erosional infill sequence contains several fan or mounded features, partly controlled by erosional relief. A number of canyon systems can also be identified at the Oligocene unconformity level.

Equatorial Guinea

The Rio Muni basin lies at the northern end of the West African basin system between the Gabon basins and the Douala basin of Cameroon (Fig. 1).

As in the Namibian basins, discrete phases of basin evolution can be recognized with rifting in the Early Cretaceous followed by Late Cretaceous to Tertiary passive margin development.

Three types of crust are revealed in the region by deep seismic data: stretched (rifted) continental crust (RCC); proto-oceanic crust (POC); and true oceanic crust (OC).^{4 5}

Mapping of these crustal types has revealed a margin which steps progressively seaward toward the south along oceanic transform fracture zones (Fig. 5 [88,688 bytes]).

In the Douala basin, the rifted margin is narrow, less than 100 km wide, and is thought to represent an upper-plate margin in terms of the Wernicke simple shear model of continental separation.

In the Gabon basins to the south, the rift margin seaward of the coastline is much broader and is thought to represent a lower-plate margin. The Rio Muni rifted margin is comparable in width to that of the Douala basin, but the width of the POC is more like that in the North Gabon basin, so it is not clear whether this segment belongs to the upper or lower plates of the simple shear model.⁶

The Aptian drift onset unconformity marks the base of the drift section, overstepping westward onto newly formed oceanic crust in the deepwater areas.

A rift-to-drift transition stage is recognized between active rifting and full continental separation. At the end of active extension, lagoonal and brackish environments were created which promoted periodic deposition of evaporites in hypersaline conditions.

During the drift stage, a prism of marine sediments built out over the Rio Muni margin in response to passive-margin subsidence. A number of deepwater fan and mounded features in the Upper Cretaceous section are recognized on seismic data, similar to those recognized in the deepwater Angola and Namibia areas (Fig. 6 [72,204 bytes]).

These are considered to hold significant reservoir and trapping potential. The drift phase can be subdivided into early and late subphases by the Coniacian to Santonian tectonic episode, attributed to transform interaction during a sudden change in the pole of rotation in Atlantic opening.

This episode is manifest as a series of gravity-slide structures with listric extensional faults soleing into a basal detachment within the Aptian salt-shale sequence.

The proximal hanging wall is composed of rotated blocks usually cored by Albian carbonates, described in detail by Turner.^{7 8} The distal hanging wall is characterized by compressional stacking in the form of toe-thrusting (Fig. 6).

Gravity-slide tectonics have been related to structural relief created by the action of oceanic fracture zones causing fault reactiviation and basin inversion.⁹

Erosion of structurally uplifted parts of the gravity-slide units has resulted in a major unconformity, here designated the Senonian Unconformity.

Salt tectonics have played an important role in the development of Rio Muni and the adjacent basins of Gabon. Regionally, Aptian salt extends over RCC and POC, and in some places salt has been mobilized farther seaward as sills emanating from salt diapirs.⁴

Salt began to flow during sediment loading in the late Cretaceous forming salt anticlines, turtle backs, and piercement diapirs.^{10 11}

Prospectivity

Having reviewed basin evolution we may now consider whether Namibia and Rio Muni fulfill the criteria of the West African deepwater play.

Deepwater Namibia (Fig. 7 [41,267 bytes]):

Source rocks. Good quality oil-prone source rocks occur in the Aptian rift-to-drift transition and Albian to Cenomanian early drift section1 which can be confidently extrapolated into deepwater areas from seismic data. Basin modeling shows that large areas of the Aptian source rock and more restricted areas of the Cenomanian-Turonian source rocks are in oil maturity windows at the present-day Fig. 8 [45,938 bytes] and Fig. 9 [35,057 bytes].
Timing of hydrocarbon generation. Apatite fission track analysis (AFTA) data recognize a thermal episode during the late Tertiary, which caused maximum maturity over large parts of the region.¹² In areas where the effects of this episode are less marked, maximum hydrocarbon generation is occurring at the present day. In either case, the timing of generation is favorable.
Migration pathways. Faulting associated with skeleton Albian rifting and volcanic plateau development, differential compaction of the drift sequence, and shelf edge gravity sliding provide vertical migration access to shallow reservoirs in the overlying late drift sequence.
Reservoirs and traps. Potential reservoir sands have been proved by drilling in Upper Cretaceous mound features in relatively shallow waters, similar to features recognized in deepwater seismic. Potentially large stratigraphic traps enhanced by an underlying structural control associated with gravity slide structures, skeleton rifting, or volcanic plateau development in area of influence of proto-Orange River in the south and Cunene River to the north.

Deep water Rio Muni (Fig. 10 [78,511 bytes]):

Source rocks. Good oil-prone source rocks occur in the Aptian rift-to-drift transition and Albian to Cenomanian-Turonian early drift section. As in Namibia, these units can be extrapolated into the deepwater areas by seismic data (Fig. 11 [86,345 bytes]).
Timing of hydrocarbon generation. Basin modeling shows that maturation has occurred late in the history. Maximum maturity and hydrocarbon generation is occurring at the present-day, with large areas of Aptian and Cenomanian-Turonian source rocks in oil-mature conditions due to burial by post-Senonian unconformity sediments.
Migration pathways. Faulting associated with Coniacian-Santonian gravity sliding and post-Senonian unconformity gravity sliding provide migration access from source rocks to reservoirs in the overlying late drift sequence.
Reservoirs and traps. Sands are expected in seismically recognized fan or mound features in the Upper Cretaceous section, with combination traps formed in association with underlying gravity slide structures, situated in areas of sedimentary influence of proto-Ogooue and Sanaga rivers.

Similar setting

The Atlantic margin basins offshore Namibia and Rio Muni, Equatorial Guinea, share a broadly similar geological setting and tectono-stratigraphic evolution with the basins of Congo and Angola on the intervening part of the margin.

Good quality oil-prone source rocks in oil-mature conditions are predicted to occur in the mid-Cretaceous section over the deepwater areas of both regions.

Both regions also have potential for late drift deepwater sand reservoirs, but in contrast to Angola and Congo the main sand developments appear to be in the Upper Cretaceous rather than in the Tertiary section.

The Rio Muni basin of Equatorial Guinea displays a typical West African rift-to-drift basin evolution, characterized by salt-influenced tectonics and sedimentation via river systems draining an uplifted continental margin in humid tropical conditions.

The Upper Cretaceous section is thought to have the best hydrocarbon potential, with reservoirs positioned in migration pathways from mid-Cretaceous source rocks.

In Namibia, the Namibe basin north of the Walvis Ridge is a typical West Africa rift-to-drift basin displaying strong erosion of the shelf in the Oligo-Miocene and deepwater clastic sedimentation fed by erosion of an emergent continental land mass.

South of the Walvis Ridge, the Walvis, Luderitz, and Orange basins have evolved under the influence of volcanic margin development. In common with the Rio Muni basin, the best deepwater prospects appear in the Upper Cretaceous section, where a proto-Orange River system provided large clastic input.

Increasing aridification caused reduced clastic sedimentation through the Tertiary. In the deepwater areas, peak source rock maturity is probably occurring at the present day due to late Cretaceous and Tertiary burial, with Aptian source horizons in oil-mature conditions positioned beneath prospective Upper Cretaceous deepwater sand reservoirs.

To conclude, the new deepwater seismic data reveal similarities between Namibia and Equatorial Guinea and the intervening area of deepwater discoveries, in terms of source rocks, potential reservoir developments, and play types, enabling the trend of the West Africa deepwater fairway to be extended into these areas.

Acknowledgments

Seismic sections are presented with the kind permission of Western Geophysical. Thanks are extended to the Ministry of Mines & Energy, Equatorial Guinea, and to Namcor, Namibia, for their support in the submission of this article. All interpretations made in this article are solely the views of Exploration Consultants Ltd. AFTA is a registered trademark of Geotrack International Pty. Ltd.

References

Bray, R.J., et al., Source rock, maturity data indicate potential off Namibia, OGJ, Aug. 10, 1998, pp. 84.
Pettingill, H.S., Turbidite play's immaturity means big potential remains, OGJ, Oct. 5, 1998, pp. 106.
Bagguley, J.G., The application of seismic and sequence stratigraphy to the post-rift megasequence offshore Namibia, PhD thesis, Oxford Brookes University, December 1996.
Meyers, J.B., et al., Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa, Marine and Pet. Geol., Vol. 13, No. 7, 1996, pp. 791-835.
Rosendahl, B.R., et al., The continental margin in the Gulf of Guinea, Oil and gas habitats of the South Atlantic conference, abs., February 1997.
Brink, A.H., Petroleum geology of Gabon basin, AAPG Bull., Vol. 58, 1974, pp. 216-235.
Turner, J.P., Gravity-driven structures and rift basin evolution: Rio Muni basin, offshore Equatorial West Africa, AAPG Bull., Vol. 79, 1995, pp. 1,138-58.
Turner, J., Detachment faulting and petroleum prospectivity in the Rio Muni basin, offshore Equatorial Guinea, Oil and gas habitats of the South Atlantic conference, February 1997, in press.
Lawrence, S., et al., Tectono-stratigraphic influences on pre-salt petroleum systems of Rio Muni, Equatorial Guinea, AAPG international conference, Rio de Janeiro, November 1998.
Teisserenc, P., and Villemin, J., Sedimentary basin of Gabon-Geology and oil systems, in Divergent/passive margin basins, J.D. Edwards and P. A. Santogrossi, eds., AAPG Memoir 48, 1991, pp. 117-199.
Duval, B., et al., Raft tectonics in the Kwanza basin, Angola, Marine and Petr. Geol., Vol. 9, 1992, pp. 389-404.
Geotrack International Pty. Ltd., Thermal history, maturity development and hydrocarbon generation history of the Namibian margin assessed using AFTA and vitrinite reflectance data, nonexclusive report, 1998.

The Authors

Richard Bray is a petroleum geologist with 20 years of experience in the oil and gas exploration industry. Since graduating from the University of Hull, England, with a BSc (Hons) in geology, he has worked internationally with a number of service and consultancy organizations. In recent years he has worked closely with Geotrack International, specializing in the application of AFTA and thermal history reconstruction methods to source rock maturity and basin modeling problems. He is presently associate geoscientist with Exploration Consultants Ltd., where he is involved in the management of regional basin evaluation and hydrocarbon exploration studies. E-mail: [email protected]

Steve Lawrence is principal geological consultant with Exploration Consultants Ltd. He is widely experienced in management of exploration projects ranging from regional appraisal of exploration potential to prospect generation and evaluation. He specializes in integrated basin analysis techniques and modeling of basin development in the context of regional structure and plate tectonics and has pioneered cross-over techniques between the hydrocarbons and minerals exploration industries. Prior to joining ECL in 1978 he worked as operations and exploration geologist for Amoco U.K. and Cluff Oil Ltd. He was graduated from the University of London with a BSc (Hons) in geology in 1971.