The Gulf of Guinea is characterised by a number of Mesozoic sedimentary wedges, where Cretaceous to Tertiary sequences extend through the shelf to the onshore: the Côte d’Ivoire and Tano Basins, the Dahomey Embayment (Ghana, Togo, Benin and Nigeria) and the Niger Delta. These onshore and shelfal Mesozoic wedges are additionally underpinned in part by Devonian and Jurassic failed-rift basin sediments associated with earlier intracontinental extensional tectonic phases active while Africa still formed a part of Gondwana (Kaki et al).
Palaeozoic and Mesozoic Plays
The Dahomey Embayment, the focus of this article, is the least explored of these sedimentary wedges. In the onshore setting, just 11 exploration wells have been drilled to date, all targeting Mesozoic reservoirs: three in Ghana and eight in Nigeria. Five of these encountered hydrocarbons, whilst an onshore aquifer well drilled at Sazue in south-west Benin in 1932 reported gas that was analysed as C1, C2 and C3, strongly indicating a thermogenic source.
In the shelfal setting, the 24 wells drilled have witnessed considerable exploration success within both Mesozoic and Palaeozoic intervals. The Lomé field, offshore Togo, has some 80 MMbo in place within Lower Cretaceous and Devonian clastics, charged from Devonian brackish marine source rocks, whilst the Seme and Dahomey Fields in Benin contain recoverable reserves of about 100 MMbo within Turonian rock charge (Brownfield and Charpentier, 2006).
Across the Atlantic Ocean, onshore exploration success has been prolific in the conjugate Potiguar Basin of Brazil. With approximately 1,200 wells drilled and over 100 oil fields discovered, this success suggests that the remaining potential within the onshore Dahomey Embayment may be considerable.
Recent geological and exploration work, conducted to assess the exploitable petroleum resources of the onshore and shelfal acreage of the Dahomey Embayment, has sought to apply knowledge and insight from exploration efforts across the region. Vintage seismic, exploration well data and modern airborne gravity gradiometer and magnetic survey results have been integrated to build a valid geological interpretation; when combined with new regional play concepts this has resulted in the reassessment of the stratigraphy, petroleum systems and hydrocarbon prospectivity of the Palaeozoic and Mesozoic basins.
Proven Palaeozoic Failed Rift Plays
The Devonian to earliest Carboniferous sedimentary basins that underpin the Dahomey Embayment receive fleeting mention in technical publications (Kaki et al), despite being the target of exploration drilling in the shelfal waters of Togo in the 1970s and ’80s. Where they have been penetrated, such as in the Lome-2 and Haho-1 wells, marginal marine sandstones with good potential qualities have been encountered, bearing liquid hydrocarbons expelled from a similar age source rock.
Over the Benin shelf and onshore areas, large half-graben structures, with an approximately east-west orientation, can be interpreted in seismic data and extrapolated in airborne gravity gradiometry data. The gravity data also permits the half-graben sedimentary fills to be calibrated to the structures and first-pass depth to basement calculations to be made. Geological modelling suggests that the Devonian-Carboniferous sedimentary basins are extensive throughout the Dahomey Embayment and are probably unconformably overlain by a thin Middle Jurassic sedimentary cover. The boundary between the earliest Carboniferous and Middle Jurassic successions is subtle within seismic data and thus structural interpretations tied to the available airborne gravity data have been made to help map this boundary.
The extent to which these sedimentary basins seen deep within the Dahomey Embayment are connected to the better understood Devonian-Carboniferous basins of the margin is unclear. Encouragement can be taken, however, from the number of gas discoveries made within the Parnaiba Basin, onshore Brazil and the commercial quantities of oil discovered within the Saltpond Basin on the present-day shelf offshore Ghana, both in reservoirs of comparable age.
Enigmatic Middle Jurassic Plays
The controls of the presence of a Middle Jurassic depositional succession, interpreted throughout the shelf and onshore and constrained by penetration by the Lome-1 well on the offshore shelf of Togo, are poorly understood. This enigmatic succession represents a brief episode of pre break-up sedimentation and possibly failed rifting before the onset of the break-up of Gondwana. The Middle Jurassic succession is interpreted to contain deposits of continental clastic sands, possibly equivalent to the Sekondi Sandstone Formation seen on the Ghana Shelf.
In onshore and shelfal seismic, the Middle Jurassic sands are interpreted as being present within a succession of horst blocks with an east-north-east to west-south-west orientation. These provide numerous exploration drilling targets, with multiple blocks exhibiting areal extents of over 140 km2. Due to the enigmatic nature of this succession, the depositional setting is unknown and thus the likely reservoir qualities need further de-risking. Initial basin modelling studies suggest that even with a small amount of uplift, preserved reservoir qualities of these sands should be good.
The boundary between the pre break-up Middle Jurassic and Mid-Cretaceous to Tertiary successions is seen in seismic data as a strong reflector, typical of a boundary representing a period of non-deposition and possibly undergoing uplift and erosion.
Devonian Source Rock Modelling
Basin modelling studies have been conducted on the Devonian section with pseudowell locations in both offshore-shelf and onshore settings. These studies have sought to model the potential maturity of the mid-Devonian Takoradi Formation equivalent source rock, although an earlier Lower Devonian Elmina Formation equivalent and a later Lower Carboniferous Efia Formation equivalent source rock may also be present.
The extent of burial and subsequent uplift and erosion that took place after the deposition of the Devonian-Carboniferous succession and prior to the deposition of the Upper Jurassic-Lower Cretaceous succession is poorly understood. Consequently, basin model parameters have been considered that take account of this variation. Using regional heat flows from the results of wells drilled on the shelf in Ghana, particularly the Dzita-1 well (Brownfield and Charpentier, 2006), initial results are encouraging, showing a likelihood of the mid-Devonian source rock interval reaching expulsion maturity during the early Tertiary. The timing of this expulsion would be suitable to charge both Devonian marginal marine reservoirs within half-graben structures as well as Middle Jurassic-age continental to marginal marine reservoirs within horst and graben structures.
Source rock geothermal modelling, conducted on pseudowell #1, shows the Devonian Takoradi equivalent source rocks reaching maturity for oil in the Late Cretaceous.
Prolific Mesozoic Post Break-Up
The Mesozoic sedimentary wedge that extends from the offshore to throughout the shelfal and onshore setting of the Dahomey Embayment was deposited subsequent to the break-up of the Gondwana supercontinent as the South Atlantic Ocean formed.Composite section showing offshore and onshore seismic data. Note the large Jurassic horst structure in the onshore, overlying the Devonian rifted section. The horst block exhibits an areal extent of 52km2 with 4-way closure.
The onset of break-up in the Dahomey Embayment is understood to have occurred in the latest Jurassic to earliest Cretaceous, resulting in the clastic continental sediments of the Ise Formation, deposited within a rift-valley habitat (Kaki et al). The Ise Formation sands are understood to only be present outboard of the present-day shelf edge as well as to the east of the Benin shelf, where they form the lower reservoir of the Seme North and Seme South oilfields. This is corroborated by seismic data, suggesting an up-dip limit of the break-up unconformity and associated sedimentation.
In the onshore setting, gravity gradiometer data allows the interpretation of two large post break-up transform fault complexes that likely influence the deposition and distribution of Mesozoic sediments. Present-day Lake Aheme lies within the western set of the transform fault complexes, and depth-to-basement calculations made on the gravity gradiometer data, calibrated to seismic, suggest that there may be up to 2,700m of sediment fill. The timing of the onset of these transform faults requires further investigation but they have probably influenced the distribution of depositional facies throughout the Mesozoic.
The earliest Mesozoic post break-up deposits in the shelfal and onshore settings are Albian continental to marginal marine sandstones (Kaki et al) that unconformably overstep the underlying enigmatic Middle Jurassic horst-block and Devonian failed-rift half-graben successions. These Albian sands, which are seen in seismic data to progressively onlap the underlying Middle Jurassic horst blocks, represent the basal unit of a thick succession of continental to fluvial-deltaic to shallow marine sands and form the main reservoir of the Seme North oilfield as well as the Aje gas-condensate field across the border in Nigeria.
Unconformably overlying the Albian sands are the regionally transgressive Cenomanian-Turonian marine shales that form both a regional seal to the Albian reservoirs and are a prolific source rock in the deeper waters throughout the South Atlantic margins. Evidence from wells drilled throughout the Gulf of Guinea suggest that this source rock has generated the hydrocarbons that have charged numerous successful Mesozoic plays and furthermore allow us to conclude that the C1, C2 and C3 gas encountered at the Sazue well, mentioned previously, is likely derived from this source rock.
A shallowing of depositional environments, subsequent to the deposition of the Cenomanian-Turonian shales, resulted in the deposition of tidal-dominated deltaic and shallow marine sands of the Turonian Abeokuta Formation and continental sands of the Coniacian-Santonian Agwu Formation. These Turonian Sands form the main reservoir in the Aje and Ogo Fields on the neighbouring Nigerian Shelf and also contained oil shows when encountered by the onshore Bodashe-1 exploration well, drilled close to the border with Benin. Additionally, the sands are seen to outcrop some 100 km north of the coastline. In-country fieldwork, undertaken to sample and photograph out-crops of the Mesozoic wedge, permitted the analysis of Turonian sandstones, which are interpreted as having been deposited in a fluvial environment.
The uppermost Cretaceous reservoirs are formed by the Maastrichtian to Danian Araromi Formation sands, again deposited in a fluvial-deltaic to shallow marine environment. The seal to these reservoir sands is provided by thick Paleocene and Eocene marine shales emplaced by a regional Danian transgressive event.
Within the Tertiary succession, the only sands sufficiently deeply buried to form reservoir rocks are the Lower Miocene Afowo Formation sandstones, deposited within a shallow marine environment and again sealed by intraformational transgressive mudstones.
A Bright Future
With deepwater exploration in West Africa inherently risky and incurring unpalatable capital costs, exploration of the shallow water shelfal and onshore plays of the Gulf of Guinea has been reinvigorated. The Dahomey Embayment presents explorers with the opportunity to drill a number of Mesozoic structural plays with clastic reservoirs deposited in marine environments, as well as large Palaeozoic structural plays with marginal marine reservoir sands and proven source rocks, both within shallow water and onshore acreage and with a domestic market and rapidly increasing demand for energy.
The operators of these blocks are now progressing to drill the first exploration wells in the area since 1993, either located on structures derived from interpretation of the existing high resolution gravity gradiometer data or more accurately positioned with the acquisition of new 2D seismic data focused to reveal the interpreted structures.