NIGERIA - The Geology Of Nigeria.

Nigeria is on the west coast of Africa, on latitudes 4[degrees] north of the Equator and latitudes 3[degrees] and 14[degrees] on the east of the Greenwich Meridian. It shares boundaries with the republics of Benin and Niger in the west, Cameroon in the east, Niger and Chad in the north and the Gulf of Guinea in the South. Area: 923,768.64 sq km. Capital: Abuja. Population: About 140m, consisting of 374 ethic groups with Hausa, Igbo Yoruba constituting the major languages. Official language: English

To oil explorers, Nigeria has good prospects. The complex nature of multi-reservoired fields suggests there can be ample scope for adding oil and gas reserves in the existing proven areas, either by detecting small undrilled structures or by adding new reservoirs to existing fields.

The country is divided into three sections and is defined by the Y-shaped formation of the River Niger and its tributary, the Benue. To the north of the Niger-Benue confluence lies a savannah country leading towards the Sahara. On the south-western side of this Y lies an area in which there existed a number of clearly defined states in the country's history. The region stretches along the forest tracks of the West African coast, between the Atlantic to the south and the savannah states to the north.

Half of Nigeria's land is composed of three main basement complexes located in the west, the north-west and the south-east. Between these massifs, sedimentary basins are strung out along a north-east/south-west axis from Lake Chad to the Niger Delta, i.e. along the length of the Lower Niger-Benue river systems. The north-west/south-east trending Bida Basin follows the course of the Upper Niger River.

After more than 55 years of intensive exploration, it is considered that the main prospectivity of Nigeria is restricted to the Niger Delta and its adjacent offshore. This part of West Africa's offshore is one of the most prospective petroleum provinces in the world.

The Nigerian offshore areas being explored and considered highly prospective are at least 1,000 metres below sea level and then another 4,000 metres underground. There are only two other parts of the world where offshore oil is produced at such depth: the Gulf of Mexico and offshore Brazil. These areas have proved to be rich in oil and gas.

Most of the oil onshore lies at depths of 2,000-3,000 metres. Oil there is usually found associated with gas and water in the porespaces between the grains of sand which make up the reservoir. Oil is found in areas where columns of sedimentary formations (Fms) are more than 2,000 metres thick: in sand, sandstones, limestones, evaporites and shales. They are mostly of marine origin, as in the Niger Delta, Anambra and Chad Delta Basins.

The Niger Delta has three major depositional prisms: (1) a Cretaceous deltaic complex developed 600 miles inland from the mouth of the Niger Delta, (2) a Late Cretaceous to Palaeogene "proto Niger" delta complex which was formed seaward of the earlier delta, and (3) the younger Cenozoic Niger Delta complex which was built out over newly formed oceanic crust as the South Atlantic opened during the separation of the South American and African tectonic plates. Seaward of the latest delta system are deepwater sediments forming the pro delta slope.

The young Cenozoic Delta hosts most of the oil and gas fields of Nigeria. Total discoveries were estimated at about 40 bn barrels of oil and over 187 TCF of gas.

Following the opening of the South Atlantic, vast quantities of clastic sediments were poured into the delta giving rise to three main rock units which are partially time equivalents and were deposited as prograded wedges. The lowest unit, the Akata Fm, 6,000 metres thick, comprises marine clays and shales together with deep-water sandstones.

The middle unit, the Agbada Fm, is up to 4,600 metres thick locally and consists of shallower water marine sands and lagoonal sands and shales. The Agbada is the main reservoir unit of the delta.

The top unit is the Benin Fm, up to 2,200 metres thick and comprising sands and gravels deposited mostly in shallow seas or in the delta's top environment. As the delta built out to the south-west constantly, changes in water depth and migration of the distributary channels produced a very complex set of inter-digitating point bar and channel sands with intervening shales and mudstones.

A younger Cenozoic Delta complex is notable for its development of "growth faults" which have produced the traps for its oil and gas fields. Growth faults are not present in the two earlier delta complexes, a factor which may partially explain their lack of prospectivity.

Growth faults are so called because they are active during the deposition of the sediments which they intersect. They are commonly cuspate in plan view, are steep-sided near the surface and flatten out to parallel bedding with depth. They frequently have a "roll-over" anticline with four-way closure on the downthrown side of the fault and antithetic faulting in the downthrown block. They are of limited length.

The delta is cut by a homogenous sequence of down-to-the-basin faults trending north-west and south-east, similar in length and remarkably equidistant from each other. The coincidence of movement on the faults and the growth of the anticlines as the sediment was deposited resulted in ideal conditions for subsequent entrapment of hydrocarbons.

Oil and gas are found on the upthrown side of the main fault and in the anticlinal structure on the downthrown side of the fault. Numerous subordinate traps are formed by the antithetic faults affecting the rollover anticline. In addition, the shales associated with the rollover faults are commonly over-pressured and diapiric, adding to structural development in the overburden and increasing the seal on faults.

Niger Delta fields have multiple reservoirs. Oils are commonly gassy and have high-solution gas content. Reservoirs have primary and secondary gas caps. Free gas reservoirs are present in oilfields. Gas fields have high condensate yields.

The size and distribution of Niger Delta fields is remarkable. In most other oil provinces, there is a noticeable skewness in the distribution of field sizes, e.g., the bulk of reserves in the province is contained in a relatively small number of fields. In contrast, the distribution of field sizes in the Niger Delta is uniform.

Despite the discovery of about 50 bn barrels of oil, few - if any - fields contain more than 1 bn barrels. All the fields are located in similar types and sizes of structures. They are about 5 km long and 1 to 3 km wide. However, the smallness of the fields means that the channel sand reservoirs can usually be correlated, and thus likely to be in communication, across the whole field.

Analysis of potential source rocks in the Niger Delta has failed to identify any single rich source. Shales in the Akata and the Agbada Fms, though generally poor in organic matter, are likely to be the sources of both the oil and gas. Only the vast quantities of dispersed organic material in the large bodies of shale in the Fms can explain the amounts of hydrocarbons which must have been generated.

Future potential for Nigeria lies in deep waters in front of the Niger Delta, as in the deep-water areas along the whole coast of West Africa. The prospects there are somewhat similar to those of the Gulf of Mexico and the Campos Basin off Brazil.

In the late Jurassic, the Gondwana land mass began to rift and slowly break apart. Some 50m years later the South Atlantic ocean was in place, separating South America and Africa. In early stages of the Gondwana breakup, an important development was the intermittent breaching by marine waters of a southern land barrier - Walvis Ridge - when Brazil and Nigeria were still joined at the top. Waters remained trapped when breaching ceased and evaporated to form thick layers of salt and salt Fms.

The area where this happened is called Aptian Salt, or Angola Basin. Now the Angola Basin extends north from the under-water Walvis Ridge, which runs to the west into the South Atlantic from Namibia, up to a second under-water barrier, the Guinea Ridge, which runs out in a westerly direction from Cameroon.

The West African offshore contains over 20 bn barrels of oil to be explored. Shell, producing about half of Nigeria's oil output and a leading developer in the Gulf of Mexico, is exploring deep and ultra-deep areas off Angola, Congo and the Ivory Coast, as well as in the offshore sector of Nigeria. Total, Chevron and ExxonMobil, all big operators in Nigeria, are among majors targeting most of these areas as well, and have made important discoveries (see background in Vol. 61, Gas Market Trends No. 5).