Structural styles in deep offshore West Africa

Deepwater geology not extension of inshore basins

Fred Akanni
Contributing Editor
Lagos

PART I: This feature on West Africa's deepwater basins is Part I of a two-part series on deepwater geology off West Africa. Part II, scheduled for the April issue, will delve into drilling and geological specifics.

The race for oil and gas in deepwater, which resumed in earnest off western and southwestern Africa at the turn of this decade, continues to get keener. Some five billion bbl of oil have been discovered since 1994, most of it off Angola and Nigeria. Also, there is evidence that the depths of the Atlantic Ocean hold prospects larger than those areas that have so far yielded relatively large discoveries.

The West Africa deepwater trend refers to the deep offshore portion of the six basins that line the coast of the South Atlantic, from Cote D'Ivoire in the west to Angola in the south-southwest.

These six basins are members of the Passive Margin and Delta Sag basin families, which are the two most productive of the five major families of sedimentary basins in Africa. The six are all part of a large linear trough, which has been subsiding, probably since late Jurassic time. It was then that the super-continent Gondwanaland divided into West Africa and South America, forming the Atlantic Ocean between them.

The Gondwanaland breakup was probably initiated in the Gulf of Guinea area, hence a thick and extensive miogeoclinal wedge of sediment accumulated along the continental margin of West Africa in the Gulf of Guinea area. The basins, in rough but intelligible definitions, include the following:

Abidjan Basin: This basin extends from Cape Palmas Escarpment (St Paul Fracture Zone) in the west to Romanche Fracture Zone in the east. It straddles Cote D'Ivoire, Tano in Ghana, and the meta sub-basin in Ghana.
Offshore Benin Basin: This basin is structurally defined by the Romanche Fracture zone in the west and the Okitipupa High in the east. Its maximum development is in offshore Benin Republic. The flanks extend to Togo in the west and Nigeria in the east.
Cenozoic Niger Delta: This complex includes the Rio-Del-Ray-Cameroon-Fernando Po Basin. It is located between the Okitipupa High in the West and the Cameroon Volcanics to the east.
Gabon Coastal: This basin extends from the southern part of the Cameroons, through Equatorial Guinea to Gabon. It is bounded on the south by the Gabon Fracture Zone.
Lower Congo Basin: This basin is bounded to the north by the Gabon Fracture Zone and extends into northern Angola.
Kwanza Basin: This basin is located squarely on the edge of Angola, from north to south.
Congo Fan: This basin lies parallel, on the seaward side of the Lower Congo basin. It looks like the ultra deepwater portion of the Lower Congo Basin. In simplest terms, it is a dump of sediments from the Congo River, which cuts perpendicular to the trend of the Lower Congo Basin. The Congo Fan, essentially a Tertiary development, is younger than the Gabon Coastal, Lower Congo and Kwanza Basins, all of which are Cretaceous in age.

Previous work has characterized the Gabon Coastal, Lower Congo and Kwanza as mere sub-basins of the Aptian Salt Basin, as each of them share common structural and stratigraphic characteristics. In this article, unless otherwise stated, we shall group them all as the Aptian Salt Basin.

Deepwater history

The deepwater portions of these basins have been the sites of exploration since the early 1970s. In fact, when Chevron begins to produce the Cuito field, in the Lower Congo Basin, in 1999, it will be 25 years since Shell plugged the first deepwater well in offshore Africa as a dry hole.

Astarte Marine-1, drilled in 644 meters water depth off the coast of Gabon, was followed by V1, in 588 meters water depth off Mauritania, drilled by the same company. Despite the fact that V1 encountered only oil shows, Shell maintained its status as the world's leading operator in frontier exploration by drilling Aulica Marine-1 in 698 meters water depth off Gabon. This was another dry hole.

In the meantime, Esso plugged a dry hole, Haute Mer-1, in 520 meters of water offshore Morroco in February 1975. Things were quiet until May 1978, when Elf spudded A-A1 in 236 meters water depth off the South African coast. It was abandoned (dry) in September 1979 at a total depth of 3,802 meters.

The period 1980-1982 marked the peak period of drilling activity. In the 1990's renewed interest began with stronger commitment.

The first deepwater oil discovery came in 1980, off Cote D'Ivoire. Phillip's A-01X was spudded in 363 meters water depth around Christmas of 1979 and completed in May 1980.

Shelf profile plays

All the basins along the West African coast were formed as a result of the continental division. Their structural profiles exhibit the history of the crustal adjustments and rearrangement that ensued from this breakup.

Seismic profiles from the oldest basins, which for our purposes include the Aptian salt and the Abidjan Basin, indicate distinct uncomformities that correspond to age and sedimentary buildup from the rift stage to the present.

In the Abidjan Basin, a strong breakup uncomformity in the Middle Cenomanian overlies and truncates the underlying Albo-Aptian sediments, which are truncated to the north by the underlying sand-rich rift sequence. The standard pre-rift tilted blocks underlie and sometimes push up the rift sediments.

The overlying Turonian and Senonian sequence is that of a marginal sag basin containing turbidites both proximal and distal to prograding deltaics. Most cross-sections suggest the basin does not have sand in the right framework for hydrocarbon production, which requires the right trapping mechanism, seal, and burial. This is why it has not delivered huge fields. The biggest oil and gas discoveries here have been in turbidites (eg. Belier field), which are neither extensive nor laterally connected.

Aptian Salt Basin

In the Aptian Salt Basin (Gabon Coastal, Lower Congo and Kwanza), a Jurassic to Lower Cretaceous non-marine rift sequence is capped by a transgressive sheet sand, above which is deposited the Aptian salt.

Above the salt, the Albian is generally characterized by a carbonate platform. The overlying Upper Cretaceous to Tertiary is mostly clastic. Oil has been found in pre-salt and post-salt sands and in carbonates.

In one spectacular profile off Angola, large pools of oil are encountered along the same hole in reservoirs situated in four way closures. The traps lie stacked between two back-to-back faults in sequences of Eocene sands. Upper Cretaceous carbonates occur in the post-salt and Lower Cretaceous sand and is present in the pre-salt.

Niger Delta Basin

In between the Aptian Salt basin and the Abidjan basin, lies the Cenozoic Niger Delta basin. This is a much younger basin than either the Aptian Salt or the Abidjan Basin, having evolved essentially in the Tertiary.

The Niger Delta is a regressive sequence of clastic sediments developed in a series of offlap cycles. All deep wells in the basin document a tripartite lithostratigraphic succession in which the regressive sequence is demonstrated.

The uppermost (shallowest) part of the sequence is a massive non-marine sand section, known as the Benin Formation, deposited in alluvial or upper coastal plain environments. This formation grades downward into the Agbada Formation, a series of inter-bedded shallow-marine and fluvial sands, silts and clays. These form the typical parallitic facies portion of the delta. The Agbada Formation has delivered most of the oil produced in the Niger Delta to date.

The Akata Formation forms the base of the lithostratigraphic succession and consists of massive and monotonous marine shales. This sequence, in general, contains a few streaks of sand, possibly of turbiditic origin that were deposited in holomarine (delta front to deeper marine) environments (Omatsola, 1982). It is important to note that most of the hydrocarbon traps of the Niger Delta are structural and were developed as a result of synsedimentary deformation.

Limiting potential rewards

It is tempting for the explorationist to frame a deepwater exploration strategy around the distal extensions of known plays. Companies have been known to walk away from concessions if they can't find in one, some prospective sequence that is related to those that have been proven shoreward. This was one of the reasons Chevron refused to participate in the bidding for deepwater concessions in Nigeria in 1991.

Similarly, Elf, Marathon, and Occidental Petroleum were all looking for distal extensions of known plays when they acquired some of the deepwater leases in Gabon in 1992. To the contrary, Shell went for deepwater Gabon, planning to test structures that were analogous to deepwater structures.