A new look at vintage data reinvigorates Cote d’Ivoire’s exploration potential

The PGS Ramforn Viking operating offshore West Africa. Image: PGS.

In 1996, Petroleum Geo-Services (PGS) acquired a 1,600-km², multi-client, 3D seismic survey in the waters offshore Cote d’Ivoire. This survey was planned to image the hydrocarbon prospectivity of four open blocks: CI-104, CI-515, CI-115 and CI-116, Fig. 1. Analysis and interpretation of the data highlighted the potential for commercial prospectivity, in particular:

• The geological similarities and proximity to oil and gas discoveries and fields
• That the blocks covered by the survey are unlicensed
• There is a presence of two play types within the blocks: structural and stratigraphic
• That there are numerous leads that can be mapped, associated with each of these plays.

Fig. 1. Map of central offshore Cote d’Ivoire, showing the location of the recently reprocessed, multi-client, 3D seismic survey (blue), a more recent PGS multi-client GeoStreamer 3D survey (pink), and PGS’ multi-client 2D seismic data. Also shown are the nearby oil and gas discoveries and fields (block, field and well data, courtesy of Wood Mackenzie PetroView.)

In light of the above, PGS took the decision in 2014 to reprocess the survey, to assist the exploration industry in realizing the apparent potential of this area. By applying a modern broadband workflow, PGS has been able to image the shallower sections in more detail, while improving the image resolution of the deeper section by enhancing the low-frequency content of the data. The results of this reprocessing effort have allowed further insight into the hydrocarbon potential of an under-explored area of the West Africa Transform
Margin (WATM).

REGIONAL EXPLORATION SUCCESS

Across the margin, important oil and gas discoveries have been made in similar geological settings to the acreage covered by the 3D survey. Offshore central Cote d’Ivoire, a trend of gas discoveries in Upper Cretaceous age stratigraphic traps (Foxtrot, Panthere and Marlin fields), and oil discoveries in Albian age structural traps (Lion, Espoir, Acajou, Baobab and Kossipo fields), has contributed to the regional success story. The recent Paon find, by Anadarko, also has proven the prospectivity of oil-bearing Upper Cretaceous stratigraphic plays. 

To the east, oil (Ivco, Belier, Addax and Ibex fields) and gas (Kudu and Eland fields) discoveries have been made in Upper Cretaceous reservoirs, mostly during the 1970s and 1980s. Meanwhile, offshore western Ghana, the shallow-water and deepwater Tano areas have witnessed the discovery of numerous oil fields (Tano, TEN and Jubilee field clusters).

In the under-explored offshore region of western Cote d’Ivoire, recent discoveries, such as Saphir (Total, 2014) and Morue (Anadarko, 2014), suggest that there may be considerable remaining petroleum potential to the west.

GEOLOGY AND PETROLEUM SYSTEMS

The offshore Cote d’Ivoire region is part of the WATM, a large tectonically constrained area bound by a series of major fracture zones formed during the separation of the African and American continents during the Late Jurassic period, Fig. 2.

Fig. 2. A satellite bouguer gravity map showing the major transform faults and the location of the 1,600-km2, multi-client, 3D seismic survey (yellow). High-gravity responses are shown in red, while low-gravity responses are shown in blue. (Sandwell et al, 2014)

Within the reprocessed multi-client dataset, three distinct tectonic phases can be recognized:

• Pull-apart syn-rift movement, forming a horst and graben topography with associated sediment fill; referred to as syn-rift.
• Strike slip syn-transform movement that resulted in the inversion of the syn-rift; referred to as syn-transform or inversion phase.
• Sediment input into the post-transform or post-inversion setting; referred to as post-transform.

The syn-rift phase saw the formation of extensional or pull-apart basins that have undergone considerable post-depositional inversion during the syn-transform or strike-slip phase. These large inversion structures not only provide numerous structural trapping opportunities, but they also bound the depocentres of the post-transform phase, which witnessed the input of large volumes of clastic sediment.

All three of these tectonic phases have resulted in the necessary petroleum system elements for prospectivity, and importantly provide the numerous combinations of play elements that make this area so prospective.

SYN-RIFT STAGE SETTING

Rift-controlled block faulting and graben filling characterized early tectonic activity offshore Cote d’Ivoire, followed by transform faulting and inversion across the region. The onset of pull-apart rifting occurred in the Upper Jurassic, resulting in the deposition of up to 2 km of continental conglomerates, sands and lacustrine shales, into deep syn-rift grabens. This was overlain by a further 5 km of continental and marginal marine sediments during the Lower Cretaceous.

During the Albian, the depositional environment became predominantly marginal marine, and a sequence of sands and shales were deposited with occasional black shales, conglomerates and calcareous deposits also present. Such sandstone reservoirs can be seen in a number of adjacent fields, such as Baobab and Espoir.

The onset of a major phase of inversion occurred during the syn-transform stage, soon after the end of the syn-rift stage and is marked by a major regional unconformity separating it from the overlying post-transform marine sediments. The inversion, which was active into the Neogene, was likely caused by strike-slip movement of the transform faults acting obliquely on the earlier rift basins.

SYN-RIFT STAGE PETROLEUM SYSTEMS

Plays within the syn-rift are typically related to the syn-transform inversion structures. The most prevalent source rocks for the syn-rift are Late Jurassic to Earliest Cretaceous, organic-rich muds deposited in restricted basins, together with black shales and siltstones of Albian age. These source rocks, where mature, have charged Albian age marginal marine sandstone reservoirs within inversion structures.

POST-TRANSFORM STAGE SETTING

The end of transform movement was superseded by thermal subsidence in the post-transform stage, contributing to a regional marine transgression in the Middle Cretaceous. The earliest post-transform sediments were likely to be sand-prone mass-flow deposits, interbedded with transgressive mudstones, ponded in topographic lows created by the break-up and inversion event structures. These deposits were overlain by organic-rich shales, associated with further transgressive events, succeeded by Coniacian to Maastrichtian marine sands, shales, calcareous deposits and minor conglomerates.

During the transgressive events of the Cenomanian to Turonian, organic-rich anoxic shales were deposited basin-wide, forming an important source rock for the region. These source-prone sediment packages deepen distally into the basin, where they are more likely to be within the oil window. Source maturity in the basin typically occurs where sediment overburden on Cenomanian and Late Albian source rocks reaches beyond 2,700 m (Brownfield, M.E, and Charpentier, RR, 2006).

An unconformity at the end of the Cretaceous marks the onset of deposition of Palaeocene and Eocene marine sandstones, shales and limestones that are overlain by Oligocene and Miocene shales.

Fig. 3. Section taken from the recently reprocessed, multi-client, 3D seismic dataset, showing the different play types: syn-rift structural plays, early post-transform plays and late post-transform plays, with syn-rift or Cenomanian source charge.

Significant sand input to the shelf during the post-transform stage suggests potential for large mass-flow, fan or channel sandstones in the offshore basin, creating important reservoir intervals charged by a post-transform Cenomanian source. Such Cretaceous sandstone reservoirs are seen in a number of discoveries, including Foxtrot, Tano and Paon, which also prove the proficiency of the source rock.

The Upper Cretaceous play types are analogous to Jubilee field in Ghana, with similar structural settings and sedimentation across the entire transform margin. On the South American margin, Zaedyus field was discovered in stacked Upper Cretaceous turbidite fans, proving the conjugate margin analogue. Similar stacked Cretaceous fan systems can be identified in PGS’ multi-client data in Cote d’Ivoire, in a range of untested leads, such as those shown in Fig. 3.

POST-TRANSFORM STAGE PETROLEUM SYSTEMS

Plays within the post-transform are typically stratigraphic, with a structural component related to the syn-transform inversion features. While the syn-rift source rocks demonstrate maturity in the proximal parts of the basin, the Cenomanian-Turonian regional anoxic event sediments are likely mature in the more distal parts of the basin. Both of these source rocks could therefore charge post-transform reservoirs that are up-dip from over overlie the source intervals. Reservoirs are interpreted to have resulted from mass-flow sands of Middle- to Upper-Cretaceous age, with evidence for channel and fan depositional geometries observed within the data, Fig. 6. Top seals for prospective features are provided by intra-formational transgressive mudstones, Upper Cretaceous deep marine shales and post-Cretaceous mudstones.

PETROLEUM PROSPECTIVITY

Fig. 4. Map of central offshore Cote d’Ivoire, showing the location of the recently reprocessed, multi-client, 3D seismic survey (blue), adjacent to a more recent PGS multi-client 3D survey (pink). Also shown are the five wells drilled on the area covered by the reprocessed dataset (block and well data courtesy of PetroView.)

A recent interpretation project undertaken by PGS on the reprocessed multi-client 3D survey has delineated three key plays. The first is an Aptian and Albian syn-rift structural play that has undergone inversion during the syn-transform. This play demonstrates shallow marine sandstone reservoirs charged by hydrocarbons from syn-rift source rocks. The second is an early post-transform shallow marine sandstone play that has also undergone inversion, and the third is a late post-transform stratigraphic play, with channel and fan sandstones charged by hydrocarbons sourced from a Cenomanian-Turonian source rock. Importantly, large structural and stratigraphic plays are commonly seen in close proximity, allowing their exploration from a common drilling location, Fig. 3.

Both the syn-rift and post-transform plays are clearly resolved by the data, particularly the Late Cretaceous channel and fan sedimentary systems that can be delineated and mapped throughout the dataset. Examples of these will be shown in the following sections.

The results of 10 wells drilled within the region were compiled and studied. The five wells that are covered by the 3D survey (Fig. 4) are Ocelot-1, K1-1X, K1-2X, East Grand Lahou-1 and South Grand Lahou-1, and the geological and fluid results of these wells were integrated into the interpretation of the dataset. Five further wells—Ivoire-1X, Saphir-1XB, Paon-1X, Hyedua-2 and GM-ES-1—within the region were also studied, and these results provided additional insight to the regional prospectivity story. 

EXAMPLES OF THE PLAYS IDENTIFIED

Fig. 5. An example of a syn-rift lead: a tilted fault block that has undergone inversion during the syn-transform phase. The East Grand Lahou-1 well, shown in the image, targeted the onlapping post-transform sediments.

Fourteen leads have been identified, characterized and mapped using the reprocessed dataset incorporating syn-rift, early post-transform and late post-transform plays. An example of a lead from each of these play types is described below.

SYN-RIFT LEADS

The prospectivity interpretation of the syn-rift section has identified four leads. Each of these leads is a structural closure within syn-rift fault blocks, which have mapped areal closures ranging from 14 km² to 100 km².

The example syn-rift lead shown in Fig. 5 is comprised of a tilted syn-rift fault block that has undergone inversion during the syn-transform. Associated with the structure are onlapping, early post-rift sediments that were targeted by the East Grand Lahou-1 well, drilled in 1999. It should be noted that as the well did not test the closure of the underlying syn-transform structure, it is presented in this study as a new lead.

POST-TRANSFORM LEADS

Fig. 6. An example of an early post-transform lead: a discreet early ponded sandstone deposit that overlies an inverted syn-rift structure.

The prospectivity interpretation of the post-transform section has identified and mapped 10 leads: six assigned to the early post-transform and four to the late post-transform. Each of these leads is stratigraphic, with varying degrees of structural control, and they have mapped areal extents of between 6 km² and 74 km².

Fig. 7. An example of a late post-transform lead: a system of stacked channels and ponded fans deposited within a post-transform inversion depocentre.

An example of an early post-transform lead is shown in Fig. 6. This lead is a ponded sandstone deposit that exhibits high seismic amplitudes. The lead has a mapped extent of 74 km² and overlies an inverted syn-transform structure that gives the potential for stacked syn-rift and post-transform leads.

An example of a late post-transform lead is shown in Fig. 7. This lead is a system of three or more stacked Maastrichtian channels with associated fans that have a mapped areal extent of 160 km². The channel sandstones are ponded within the palaeo-lows created by syn-transform inversion. The updip portions of these channels are shown in amplitude extraction maps to shale-out (Fig. 8), providing numerous intra-formational lithological seals.

CONCLUSIONS

Fig. 8. An amplitude extraction across the multi-client, 3D seismic survey, highlighting the bright amplitudes (yellow) of the stacked channels and ponded fans shown in Fig. 7.

Cote d’Ivoire is shown to be a continually promising area for petroleum exploration, with recently reprocessed PGS data demonstrating significant remaining prospectivity. This under-explored area of open acreage has similar geological characteristics to adjacent oil and gas finds.

The reprocessed 3D data help to understand both syn-rift and post-transform petroleum systems, and are essential to demonstrating the pinch-out nature of many of the Upper Cretaceous stratigraphic leads. A number of prospective features, such as Albian fault blocks and Cretaceous channels and fans, have been identified and mapped in the dataset.

Exploration risk is shown to be greatly reduced within these open blocks as a consequence of the stacking of differing play types, the presence of numerous successful analogues along the margin, and the imaging resolution of the recently reprocessed, multi-client, 3D seismic dataset. The presence of effective source rocks in the basin gives explorers the opportunity to develop play concepts in both syn-rift and post-transform, structural and stratigraphic regimes. Recent discoveries and increased exploration activity, along with newly reprocessed high-quality data, are sure to reignite interest in this under-explored area.      

REFERENCES

1. Brownfield, M.E., and R.R. Charpentier, “Geology and total petroleum systems of the Gulf of Guinea province of West Africa,” U.S Geological Survey Bulletin 2207-C, 32 p., 2006.
2. PetroView, http://www.woodmac.com