NIGERIA - The Geology.

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.

With a surface of 923,769 square kilometres, Nigeria lies in a rich part of West Africa. It borders on Benin to the west, Niger to the north, Chad to the north-east and Cameroon to the south and east. Its coast lies on the Gulf of Guinea in the Atlantic Ocean.

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.

About one half of Nigeria's land area 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 49 years of intensive exploration, it is considered that the main prospectivity of Nigeria is restricted to the Niger Delta province and its adjacent offshore. Offshore areas now 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.

Most of the oil onshore lies at depths of 2,000 to 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 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 present 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 all the oil and gas fields of Nigeria. Total discoveries are estimated to be around 37.5 bn barrels of oil and 140 TCF of gas. Cumulative oil production is in the order of 17 bn barrels of oil and 20 TCF of gas. Much of the gas has been flared. Oil discoveries are split roughly two-thirds onshore including swamps and inland waterways, and one-third offshore in the submarine part of the delta.

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 Formation, locally 6,000 metres thick, comprises marine clays and shales together with deepwater sandstones.

The middle unit, the Agbada Formation, 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 Formation, up to 2,200 metres thick and comprising sands and gravels deposited mostly in very 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 interdigitating point bar and channel sands with intervening shales and mudstones.

The 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 N-W/S-E, all 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 the formation of ideal conditions for the 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 contain 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 and in non-associated gas fields. The gas fields usually 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 very uniform.

Despite the discovery of 37.5 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 formations, although 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 formations can explain the amounts of hydrocarbons that must have been generated.