Historically the Central Atlantic continental margin has shown much promise for the development of hydrocarbon plays and discoveries, since the discovery of small onshore gas fields in Senegal in the early 1960s, small offshore oil fields in Benin in the late 1960s, and larger, but sub-commercial heavy oil offshore Senegal, also in the late 1960s. Despite this early proof of viable petroleum systems and subsequent steady exploration activity, significant developments have been slow to arrive. However, recent exploration success means the outlook has changed dramatically.
Recent E&P activities have inevitably been cautious in the current economic climate but there are significant ventures that ensure that the area remains one of focus. Of note are: the start-up of the TEN development in Ghana by Tullow and partners, slated for mid-2016; appraisal of the Tortue West gas discovery by Kosmos in Mauritania/Senegal and proposed exploration of the same Albian-Cenomanian fairway in a nearby prospect; and further appraisal of the Albian SNE-1 & -2 oil discovery in Senegal by Cairn, with additional targeting of supplementary pools in superjacent Cenomanian reservoirs, for example by the recent BEL-1 well.
Following the release of key information by the relevant operators on the status of their exploration and development programmes, Gaffney, Cline & Associates (GCA) has undertaken a review of the Central Atlantic African margins to synthesise the available non-proprietary data and information to identify likely future exploration trends. The key questions are:
- Has the discovered resource base realised earlier expectations?
- What is the likely scale of potential resource growth?
- What are the key constraints and technical risks?
The area is divided into two zones (Figure 1);
• Passive Margin comprising the Senegal-Bove and Liberia sedimentary basins in the countries of Mauritania, Senegal, Guinea-Bissau, Guinea, Sierra Leone and Liberia
• Transform Margin comprising the Cote d’Ivoire and Keta-Togo-Benin sedimentary basins, in the countries of Cote d’Ivoire, Ghana, Togo, Benin and western Nigeria.
Although distinct in their geological histories, both areas have in recent years been rejuvenated by exploration programmes targeting novel structural and stratigraphic plays in Cretaceous and Tertiary sandstone reservoirs. Carbonate plays in the Jurassic have also been addressed by recent exploration on the Moroccan margin, and there is deeper upside in the syn- and pre-rift sections; although these remain of interest, they are not addressed here.
Past and Future Trends
The course of exploration in Central Atlantic Africa is depicted here via ‘pseudo creaming curves’ which show the historical growth of cumulative potentially recoverable resources assigned to each of the major ‘play groups’ (Figures 6 and 8), and the distribution of discovered pool sizes are shown in Figures 7 and 9, as a means of comparing the plays and their historical development. All data are shown with all hydrocarbon phases reported in MMboe . This is a simplification, but it is sufficient to show the general trends. Key discoveries in the plays’ development are identified on the graphs, and on the location map. They record the innovation in play concepts along with the growing capabilities to operate in the water depths of the continental slope; only six wells were drilled prior to 2000 in water depths greater than 500m, but recent drilling has exceeded 2,700m water depth in both Passive and Transform Margin areas.
Tertiary plays: Early exploration on the Passive Margin (Figure 6) was led by the discovery in the 1960s of the Dome Flore Field, containing a large undeveloped volume of biodegraded, heavy oil (10° API) in an Oligocene carbonate reservoir above a salt dome. This has been assigned a recoverable resource of 300 MMbo, but recovery factors remain very uncertain.
Chinguetti is a small oil discovery in Tertiary sandstones, again on the flanks of a salt dome (recoverable resource of 41 MMbo), but which demonstrated the reservoir potential of the Nouakchott delta and the potential for hydrocarbon generation in the underlying Upper Cretaceous kitchen. There has been some subsequent growth in the Tertiary play in larger pools with a greater component of stratigraphic trapping (Figure 6), but the main play fairway is likely restricted to the principal Tertiary delta system (the Nouakchott Delta). Potential outside of this core area may be limited due to localised reservoir distribution and vertical migration routes associated with salt piercements (e.g. Chinguetti) , reflected in the more recent tail-off and hiatus in this play’s development (Figure 6). Significant prospects of comparable size to the largest discoveries (Figure 7) are proposed by operators in both the northern and southern parts of the basin, but these are at shallow depths of burial, away from the principal deltaic depocentre. There may be significant seal and migration risks and/or potential for biogenic gas only.
Upper Cretaceous (Maastrichtian-Campanian) plays: Faucon was the first significant offshore discovery (recoverable resource assigned 100 MMbo) and demonstrated the trapping potential of salt domes in Upper Cretaceous sandstone reservoirs. The scale of this discovery is an order of magnitude greater than the earlier fault-related structural traps onshore (Figure 7). Additional prospects of this type are proposed by operators in the area, but are likely of high structural risk.
Of greater significance is the Maastrichtian to Campanian play demonstrated at the North Fan discovery (recoverable resource 282 MMbo). This has shown the potential of this reservoir immediately beyond the palaeoshelf edge in a trap apparently partly stratigraphic and partly controlled by structural drape over underlying drowned shelf edge highs. There is a string of other opportunities in the southern part of the area and growth of this play in this Casamance deltaic depocentre is likely, theoretically with future discoveries paralleling the resource growth in the older plays (Figure 6). This stratigraphic play is not proven more widely to the north in the Nouakchott delta which is predominantly a Tertiary feature, and if it exists, present day bathymetry may prove a constraint. It is invoked in the far north of the basin by operators in the Ras al Beida depocentre, with prospective pool sizes comparable to the discoveries further south. The risk here may be the presence of feasible hydrocarbon migration from areas of greater Tertiary burial to the south. In general the risks with this group of plays are in the viability of purely stratigraphic traps and the existence of migration paths in the absence of direct contact of the reservoir with source rock at the underlying unconformity and/or shelf edge faulting.
Lower to Upper Cretaceous (Albian-Turonian) plays: Much of the recent growth in recoverable reserve volumes for the basin is driven by the Albian to Cenomanian plays. Initial resources assigned to this play were at Pelican and adjacent Fregate which both proved the potential of Turonian channels at the palaeo-shelf and also viable hydrocarbon charge in the northern part of the basin. More recent growth has been as a result of combined stratigraphic (depositional and erosional ‘buried hill’) and structural traps at the palaeo-shelf edge, especially Cairn’s SNE-1 structure (median recoverable resource 330 MMbo) and further exploitation of this trend is planned, both within the proven Albian sandstones and in the extension into the Cenomanian, as for instance in Cairn’s recent Bellatrix-1 well. Additional prospectivity is defined in the Turonian sands within channels on the eroded slope, and within lowstand bodies in deeper water settings.
Most recent success (Figure 6) in this play group has been in gas discoveries including Tortue/Gembuel on the Mauritania/Senegal border, where Cenomanian age sandstone reservoirs have multi-TCF recoverable resources assigned in combined structural-stratigraphic traps. This discovery also implies deeper gas mature kitchens of Lower Cretaceous (pre-Albian age), that furthermore might be at oil window levels of maturity in deeper water areas on the fringes of the deltaic depocentre, with lesser burial. These developments strongly imply further petroleum system diversity beyond that associated with the Albian-Turonian source rocks. Public data provided by the operator would indicate that the wells drilled so far are at the optimal heart of the fairway in robust, essentially structural closures. These anticlines most likely relate to the underlying salt diapirism and/or gravitational structures detaching on the salt. It is inferred that future discoveries relying on more subtle structures may be smaller in scale than those made so far. The combination of seismic attribute and AVO analysis has proved successful in exploiting the gas play, but trap and reservoir definition may prove more challenging in exploiting smaller, oil-filled traps.
More generally, published prospect volumes for this group of plays suggest that discoveries may be made in the middle of the range pool volumes discovered to date (Figure 7) (100-200 MMbo), resulting in a typical future flattening of the ‘pseudo creaming curve’ (Figure 6).
This play has had some success on the Liberian margin, but published prospect volumes suggest that significant upside beyond the scale of what has been previously discovered (e.g. Montserrado with a recoverable resource of 160 MMbo) may be elusive.
Upper Cretaceous (Campanian- Maastrichtian) plays: Figure 8 shows steady growth of this component of the basins resources but with no large discoveries described by the available public data. The latest discoveries are of gas and condensate at Gye Nyama and of oil at Saphir, at the extreme east and west of the area respectively. The former field is the largest reported in the play (160 MMboe recoverable resource) and represents part of the fairway with clear migration links to underlying Turonian plays. The extent of this play component is not certain. There has been some recent growth in the resource associated with the Upper Cretaceous (Maastrichtian-Campanian) play but this may be restricted in extent by localised migration routes around significant reactivated fault trends and/or close stratigraphic proximity of overlapping sand systems, and the technical and commercial constraints imposed by water depth.
Additional potential is defined in significant prospects beyond Saphir to the west and, although this well is important in partly de-risking that part of the basin, sedimentary thickness and possibly kitchen maturity is expected to decline westwards as a key risk.
Upper Cretaceous (Cenomanian-Turonian) plays: In contrast to the later play, the Cenomanian to Turonian interval has shown extremely rapid growth from the early 2000s, following its more gradual earlier development (Figure 8). This is driven partly by the key discoveries at Jubilee, and the TEN area, both formed of composite Turonian channel and fan complexes draped over underlying structural highs, with closure updip provided by stratigraphic pinchout and/or faulting. In contrast to the Passive Margin basins, structuration relates to the modified and partially inverted structural fabric of the Lower Cretaceous syn-rift that has created a series of steeply down-stepping terraces on the continental margin which localise slope and basin floor channel and fan deposition.
In addition there are the Sankova East discoveries in the Cote d’Ivoire Basin, and Ogo-1 in the neighbouring Keta-Togo-Benin Basin structures near the fringes of the play fairway. Ogo is a structural trap over an underlying fault block. Resource volumes quoted are high, but part of the 775 MMbo recoverable resources attributed may possibly be more correctly considered in the Albian and other Lower Cretaceous reservoirs, a separate play group.
Lower Cretaceous (Valanginian-Albian) plays: Structural plays in the Lower Cretaceous are dominated by Baobab, a large anticlinal combined structural-stratigraphic trap, and this stands out when compared to other fields in this play (Figure 9). An interesting additional discovery may prove to be Dzata, which has demonstrated a structural trap directly associated with compression along the Romanche Fracture Zone which may possibly reinvigorate this play.
This play benefits from close proximity to source rock intervals. It is likely that further discoveries will be comparable to the main field size distribution rather than the outlier of unusual resource volumes at Baobab (Figure 9).
What Did the Study Conclude?
Resource growth can be compared to previous yet-to-find (YTF) estimates for these basins made by the USGS (2003, 2006). Definition of new potential over the last decade is generally in line with the predictions made by the USGS in YTF assessments for the Transform Margin, where median undiscovered (in 2006) recoverable resources were estimated to be 2,628 MMboe, compared to actual reported recoverable resource growth since that time of 3,163 MMboe.
For the Passive Margin, however, growth in the hydrocarbon volume base has greatly exceeded the USGS predictions (2003). Actual additions of 3702 MMboe since that time are more than 13 times the median estimate of undiscovered recoverable resource at that time. This improvement results from recognition of multiple hydrocarbon source kitchens, beyond the then established Turonian-Cenomanian mudstones, and of new reservoir-seal couplets as the basis for hitherto unrecognised plays. It is notable that recent exploration has been driven by independent exploration companies which have responded to the opportunities offered in an environment of relatively attractive fiscal and contractual terms.
A number of conclusions can be drawn from this study, which are summarised as follows:
- Overall the study area is one with a long exploration history, but where recent exploration successes have reinvigorated and increased play diversity with future prospects of resource growth.
- Relatively attractive fiscal and contractual terms appear to have played a part in making the area one of focus for a small group of independent oil companies.
- On the Passive Margin the resource base has greatly exceeded earlier YTF predictions, as a result of increased petroleum system and play diversity.
- On the Transform Margin the resource base is approximately in line with the YTF predicted previously.
- Future significant Passive Margin resource growth is expected in the Maastrichtian-Campanian basinal play, with potential for discoveries with a scale comparable to the largest of those currently existing. Principal risks are in those in definition of stratigraphic component of trapping and in the demonstration of viable migration pathways.
- There is potential for further exploitation of the Albian to Turonian plays on the Passive Margin, although scale is expected to be reduced from the recent major discoveries. The combination of seismic attribute and AVO analysis has proved successful in exploiting the gas play, but trap and reservoir definition may prove more challenging in exploiting smaller, oil-bearing traps.
- Growth in Tertiary plays is expected to be limited due to localised reservoir distribution and reliance on vertical migration routes associated with salt piercements.
- Additional significant growth on the Transform Margin is possible in all Cretaceous plays, but may be limited by the existing kitchen areas.
- For the Middle and Upper Cretaceous plays on the Transform Margin, controls may be additionally exerted by localised migration routes around significant reactivated fault trends and/or close stratigraphic proximity of overlapping sand systems.