An overview of the prolific southeast offshore Niger Delta

Laboratory for understanding deepwater concession geology

Toyin Akinosho
Chevron Nigeria

• Eastern Niger Delta stratigraphic column.
• Eastern Niger Delta.

The southeastern offshore Niger Delta, as far out as the proximal edge of the continental slope, occupies some 6,000 square miles, roughly one-tenth of the entire basin. Yet it delivers 550,000 b/d oil, more than one quarter of Nigeria's 1.85 million b/d oil OPEC quota, as well as 110,000 b/d condensate, which is customarily not considered as part of the quota.

This small corner of Nigeria is also responsible for the major news coming out of the Nigerian segment of the oil patch in the last 15 years. Some major highlights:

• Nigeria's largest condensate field, Oso (1,000 MMBOE), which produces 110,000 b/d condensate for the Mobil-NNPC joint venture, was discovered in 1967. But it only just came on stream in 1989.

• Mobil's giant Edop Field (900 MMBOE) was discovered in this corner in 1981.

• Early last year, Mobil and Elf were embroiled in a struggle for who should operate the huge oil reserves in the Amenam-Kpono Field (800 MMBO), which is the largest discovery in the Niger Delta in the last 15 years. Elf drilled Amenam-1(OML 99) in June 1990, tested a cummulative 9,570 b/d oil in four zones with choke sizes between 28/64 and 36/64. API gravity ranged between 42.1 and 46.4. Two subsequent appraisals delivered pretty much the same result, with Amenam-3 encountering a deeper sand. In April 1995 Mobil reported via newswire, 17,000 b/d oil and 131, 604 Mcf (cummulative) flow rates from three sands in Kpono-1, which later turned out to be on the same structure as Amenam.

• The offshore field Zafiro, the first commercial oil discovery in Equatorial Guinea, is located barely 3.5 km south of Elf's OML 102. Mobil has commenced production of this field, but Nigeria and Elf are seeking joint production, based on claims that the Zafiro Field straddles Elf's OML 102 in Nigeria and Mobil's Block B in Equatorial Guinea.

• Nigeria and its south eastern neighbor, Cameroon are engulfed in a low intensity war caused by a 20-year old boundary dispute in the Bakassi Peninsula, in the extreme eastern corner.

• With a total acreage of a mere 3,273 sq km, in the main delta area, all located in the southeast offshore(excluding deepwater), Mobil compares poorly in sheer size with Shell's 31,777 sq km and Chevron's 8,936 sq km. Yet it ranks second only to Shell in production and currently outdoes every other Nigerian operator in activity, deploying eight rigs on an average of 13 locations in any given month since September 1996.

Petroleum geology

The southeast offshore Nigeria is, considered of a slightly different petroleum geology from the rest of the onshore and offshore parts of the basin. It is located in a complex structural environment resulting from deep seated shale deformation and associated faulting. Shale ridges, toe thrusts and diapirism are present in this corner. There is higher likelihood of stratigraphic traps here. To see the petrological uniqueness of the south east offshore, we must recall again the geology of the entire basin.

The Niger Delta is situated on the Gulf of Guinea on the west coast of central Africa. During the tertiary it built out into the Atlantic Ocean at the mouth of the Niger-Benue River system. Accumulation of marine sediments in the basin probably commenced in Albian time, after the opening of the South Atlantic ocean between the African and South American continents. True delta development, however started only in the late Paleocene/Eocene, when sediments began to build out beyond troughs between basement horst blocks at the northern flank of the present delta area. Since then the delta plain has prograded southward onto oceanic crust, gradually assuming a convex-to-the-sea morphology.

The sediments during the Paleocene/ Eocene were derived largely from the Benue-Niger drainage system which was - and still is - the most important supplier; and from the Cross River System, which supplied sediment to the eastern delta area, of which the south eastern offshore is a prominent portion.

Throughout the geological history of the delta, its structure and stratigraphy have been controlled by the interplay between rates of sediment supply and subsidence. In the Miocene/Eocene times, sediment supply into the delta increased significantly from the cross river system, which eroded mainly basement complex rocks and the Cretaceous embayment sediments which had been derived from basement complex crystalline rocks. Important influences on sedimentation rate have been eustatic sea-level changes and climatic variations in the hinterland. Subsidence has been controlled largely by initial basement morphology and differential sediment loading on unstable shale. The main sediment supply has been provided by an extensive drainage system, which in its lower reaches follows two failed rift arms, the Benue and the Bida basins.

The Niger Delta has prograded into the Gulf of Guinea at a steadily increasing rate in response to the evolving drainage area, basement subsidence, and eustatic sea-level changes. The submarine portion of the delta consists of a broad, shallow shelf which gradually merges into a long continental slope and rise extending as far as 250 km from the coast. The upper part of the slope is marked by a zone of faulted sediments, clay walls and diapirs, known as the distal part, as it represents the outer reaches of the most developed part of the delta.

Stratigraphy

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 an which the regressive sequence is demonstrated. The uppermost (shallowest ) part of the sequence is a massive nonmarine sand section, known as the Benin Formation, deposited in alluvial or upper coastal plain environments. This grades downwards into interbedded shallow-marine and fluvial sands, silts and clays, which form the typical parallic facies portion of the delta. It is known as the Agbada Formation; it has delivered most of the oil produced in the Niger Delta to date.

The base of the lithostratigraphic succession consists of massive and monotonous marine shales. This sequence, the Akata Formation, in general, contains few streaks of sand, possibly of turbiditic origin and were deposited in holomarine (delta front to deeper marine) environments.

The Niger Delta basin extends eastward offshore into the Rio Del Ray Basin in Cameroon, bounded on the east by the Cameroon-Annobon Volcanic axis which is part of the Fernando Po-Ascension Fracture Zone extending into the Atlantic.

The Miocene volcanicity along this axis may have affected the eastern part of the Niger Delta, leading to uplift and provided a ridge for future prolific infusion of shale diapirism. Tertiary tectonism in the eastern part of the delta has exerted a profound influence on the seismic stratigraphy of the South Eastern Offshore. It is a self evident truth that the strong diapirism offshore compared to onshore is primarily due to the pro-delta shales of the younger Akata formation offshore being less dewatered than the older shales onshore. Also, the basement high underlying the present coastline and the diapiric zone act as barriers to the up-dip migration of shales farther south. Besides, the shale type of the Akata formation changes from low Montmorillonite in pre-Miocene time to high Montmorillonite in post-Oligocene.

The Late Quarternary-Present Day Niger Delta is much smaller than the onshore area occupied by the Cenozoic Niger Delta. The present day subaerial delta limits extend from the eastern end of the Lagos-Lekki Lagoon Complex at the mouth of the Siluko River, via a series of sinuous arc shaped boundaries to Onitsha Gap, then along the valley of the Orashi river to near the mouth of the Imo River and eastwards to the Kwa Ibo mouth. Near the Kwa Ibo mouth marine estuarine conditions develop in the Cross River and Rio del Rey areas and extend into Cameroon (Arthur Whiteman, 1982).

These areas may be classed as belonging to High Energy Destructive Estuarine Delta Environment in contrast to the High Energy Constructive Lobate-Arcuate Delta Environment situated to the west. Whether in earlier times constructive deltaic conditions were developed in these areas is not clear but if the Cross River, but if the Cross River, Imo River and other main rivers were more active in transporting sediments than they are today, then constructive delta complexes may have developed. The presence of growth faults in the offshore of the Cross River Mouth may indicate that sand, silt clay differentiation may have taken place on a large scale in the past.

Whiteman argues that the amount of sediment discharge by the combined rivers entering the ocean east of the present day delta is unlikely to exceed a few percent of that carried by the Niger-Benue complex. Tidal ranges are over two times as high as they are at Lagos (3 ft) in the Calabar River area and tidal currents in the Cross River-Rio del Ray-Fernando Po area exceed 40 cm/sec and are able to keep sand on the move and keep silt and clay particles in permanent suspension, so preventing or retarding Lagoon-Barrier Bar- and Pro Delta Slope formation. Tidal ranges are likely to have been high in the past because of a restrictive geographic environment at the head of Gulf of Guinea and shallow water in between the island of Fernando Po and the mainland.

Agbada Formation divisions

In the southeast offshore, the hydrocarbon-rich Agbada formation is further subdivided into Biafra, Rubble and Qua Iboe members. Within these, there are type sections with intra and interformational relationships - the Biafra is Early to late Miocene. The Rubble bed is dated Late Miocene. The Biafra member progrades from thick continental/fluviatile sediments in the north into delta plain and delta front deposits in the south, eventually becoming prodelta deposits shaling into Akata farther south.

Although Omatsola argues that the Qua Iboe is one of the several early to Middle Miocene canyons (including Afam in eastern onshore) and Opuama (in Western swamp), now clay filled, which were cut into the shelf during periods of sea level fall, AAPG presentations by Mobil earth scientists insist that this very important shale member is Pliocene in age (Shanmugan et al AAPG Nice, 1996). Their thesis also discounts the canyon fill theory.

Mobil has identified erosional anticlines as well as slope dependent mass movement emplaced seismic facies along with evidence of the Qua Iboe as a transgressive deposit. Qua Iboe grades vertically upwards into the transitional shallow marine D1 sand which is named after a type section from the MD 1 well. D1 is overlain by the continental Benin Formation, which is expectedly so thin here. The top of the Qua Iboe is very shallow; (0.75 secs twt) on seismic profile. In any case, no one denies that Miocene shale tectonism and sea level fall have largely defined the unique stratigraphy of the south east offshore.

From literature, it would appear as if the Qua Iboe is the most significant of the local members of the Agbada in the south east offshore. The name itself, which is the same as one of the major river systems in these parts, confers some status. And Omatsola describes it simply as a fill which forms the top seal in Mobil's offshore fields. In reality, the other members are quite important. Some fields have been formed by the D1 sand. In several others, which are even very huge fields, the reservoir is a sequence within the Biafra member.

Within the Qua Iboe and the Biafra there have been distinct, horizons which have been correlated with log data. The Biafra is subdivided into Lower, Middle and Upper Biafra. Yet within these are visibly reworked sediments referred to as Rubble (should not be confused with the Rubble member). Thus we have Lower Intra Biafra Rubble, a horizon within the Biafra, which is different from Rubble bed, an independent stratigraphic member.

The importance of the Qua Iboe shale itself has been largely due to the Intra Qua Iboe reservoirs (e.g. Edop Field: See Hermance et al: Reservoir Compartment-alization of deepwater Intra Qua Iboe sand, Pliocene) Edop Field offshore Nigeria, AAPG, 1996. The Zafiro Field in offshore Equatorial Guinea is also a huge oil pool stored in sands within the Qua Iboe. A commonly, identified unconformity is the Base Qua Iboe BQI. The Lower, Middle, and Upper Biafra as well as Base Qua Iboe BQI and Rubble beds are attributed to the Miocene tectonism, said to be in three phases. Their typical traps include: rollover structure with landward dips onlapped by rubble beds simple erosional anticline; shale or mass supported simple roll over anticline; shale mass structure both on crest and flank.

Core examination of the Edop Field, as revealed in Mobil sponsored papers at the last AAPG conference in Nice, shows three Intra Qua Iboe reservoirs IQI 1, 2, 3, 4 with the following characteristics: sandy slump/mass flow, muddy slump mass flow, Bottom current reworking; non-channelized turbidity currents; channelized (coalesced) turbidity currents; channelized (isolated) turbidity currents; pelagic/hemipelagic H-levee; Reworked slope; wave-dominated and tide-dominated facies. With the exception of the reworked slope and wave dominated and tide-dominated facies, all these facies are of deepwater affinity. The paper said that the Intra Qua Iboe sands were deposited on an upper slope environment in close proximity to the shelf edge.

Through time, as the shelf edge migrated seaward, deposition began with a channel dominated deepwater system (IQI 1 &2), and progressed through a slump/debris flow dominated deepwater system (IQI 3, the principal reservoir) to a tide and wave dominated shallow water system. Compositional and textural similarities between the deepwater facies result in similar log motifs. Furthermore, these depositional facies are not readily apparent as distinct seismic facies. This study has resulted in seven defined reservoir units within the IQI.

The result of this study does not necessarily apply strictly to other fields, but it shows clearly that the Intra Qua Iboe sand package is not one homogenous package. It cautions against positions that place high emphasis on continuity of seismic character in delineating stratal surfaces. And because the sands are of deepwater origin, it has implications for deepwater exploration activity.

The four local members of the Agbada formation are not present everywhere in the south east offshore. They disappear northwards and westwards. They are absent in Ngo Field, which is a simple rollover downthrown to Chevron's Tubu Field. It is not clear if they are present in Ashland's Okwori South although one of them has been reported in Okpoho, in Shell's OML 71. Even in the very heart of Mobil's concessions, two or three members are sometimes absent. According to a published paper by Okonkwo et al, the Oso Field is a reservoir within the Biafra. None of the wells had encountered either the Rubble bed or the D1 sand. And the Qua Iboe is not productive here.

This is very instructive as far as oil exploration is concerned. In Equatorial Guinea, none of the wells drilled outside the limits of the Qua Iboe member have found oil. By the same logic, the very encounter of IQI in 1995 lead to the Zafiro discovery which in turn helped to draw the extent of the sequence as far as the present day "Delta toe".

The events happening in the southeast offshore Niger Delta compel our attention. The Amenam discovery in 1990 came at a time when the thinking in the Nigerian oil industry was that there were no elephants to be discovered anymore. "The Amenam discovery exploded a myth," said a high ranking executive of NAPE, the local affiliate of the AAPG. While there haven't been anything as profound as that since then, a rash of discoveries in the last three years has pointed up this corner as an increasingly important exploration area.

What's more, the few published seismic profiles in the southeastern offshore, as well its genetic origin as evidenced from the Hermance paper, have indicated that this is the best laboratory we have for the understanding of the deepwater concessions.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.