Oil production in Nigeria started in 1958 after the discovery of Oloibiri oil field in 1956. Today Nigeria is ranked sixth in OPEC and fourteenth in the world in the league of oil producers, and has a reserve base of about 70 bboe.
All of Nigeria’s productive capacity is from the Tertiary Niger Delta Basin, in which some 1,300 exploration wells have been drilled. While most of the onshore and shallow offshore concessions are held by multinationals under a joint venture (JV) agreement with a memorandum of understanding that guaranteed a profit to the operator depending on the price of oil, the deepwater concessions are held under various production sharing contract agreements.
Exploration in the Niger Delta has waned over the years, primarily because of difficulties in onshore operations due to community-related disturbances, including militancy and industry unwillingness to invest as a result of perceived poor operational terms.
In the past several years, the government’s stated objectives of building local E&P capabilities in the areas of operatorship – as well as increased local content, encouragement of new entrants to the exploration scene, expropriation of inactive assets of JV license holders and creation of co-ventures and partnerships through award of marginal fields or blocks to indigenous operators – are additional operational elements to enhance exploration activities.
The Petroleum Industry Bill currently is being discussed in the Nigerian Senate and House of representatives.
After 54 years of oil production and failure to achieve the 2010 aspiration of four million bopd, it is pertinent to examine past performance of exploration drilling to gain some insight into future potential.
Of the Niger Delta’s 1,300 exploration wells, 822 have been drilled onshore, 412 in the shallow offshore and 82 in the deepwater (see figure).
For this column, the Middle Miocene Play will be examined in order to illustrate the application of the well look-back as an indicator of the basin’s future potential.
The Niger Delta is a self-contained petroleum system with source rocks described at discrete levels: the Eocene, the Oligocene and Lower Miocene. The Paleocene and the upper Cretaceous may also constitute potential source rocks, but their contribution to the Niger Delta oils is not proven.
Onshore, the delta is divided into five extensional depobelts: Northern Delta, Greater Ughelli, Central Swamp, Coastal Swamp and Shallow Offshore depobelts. Sediments contained in these depobelts become progressively younger seaward.
In deep water, the delta is divided into three structural belts: Inner Thrust Belt, Fold Belt and Outer Thrust Belt, developed in a compressional regime.
Some nine plays have been penetrated in the Niger Delta.
Onshore, the plays are developed in association with conventional delta shoreface and shallow marine reservoirs, while in the deepwater the plays are developed in turbidite sands (figure 4).
Each well penetrates no more than two or three productive plays, principally because of the progradational nature in the evolution of the delta. In the deep water however, the plays are vertically stacked, with several plays capable of being reached in each well.
Recognition of this play distribution also determines, to a large extent, the target objective plays in each structural belt. Thus, in the Northern Delta depobelt the primary objective play is the Middle Eocene, while in the Coastal Swamp depobelt the primary play is the Middle Miocene.
In each case, the younger section is developed in predominantly continental facies that lack seals and traps, and may have been shielded from charge by regional seals below.
Middle Miocene Play Results
The Middle Miocene play is one of the Niger delta’s most prolific, and is developed in the North and Central belts of the Coastal Swamp depobelt and in the eastern part of the Shallow Offshore depobelt.
To date, 331 exploration wells have been drilled to test the Middle Miocene as primary or secondary objective, with a success rate of 67 percent. The level of full penetration onshore is only about 18 percent, an indication of remaining potential yet to be explored in the deep play.
In contrast, in the deep water, the level of penetration is almost always full. Here, the Middle Miocene has been penetrated in almost all the wells, as it is often targeted as part of the vertical stack of plays.
While the Middle Miocene is primarily restricted to the Coastal Swamp depobelt, it also is highly productive in the eastern part of the Shallow Offshore depobelt. This part of the Shallow Offshore is adjacent to a very narrow shelf area further updip, and this may have created favorable conditions for development of shelf collapse features feeding reservoirs into a slope environment down dip.
Also, this is a zone of development of complex duplex thrust structures bringing these reservoirs to shallower levels. Other departures from the norm may exist elsewhere in the older depobelts.
Overall Play Elements
Reservoir presence is usually not an issue in the Niger Delta’s Middle Miocene, where onshore reservoir facies are developed in shelf sands and, in deep water, in turbiditic and slope facies sands.
There also is ubiquity of traps of various types. Here again, a distinction is made between the onshore, where most of the traps are structural (growth fault related rollover anticlines), and the deep water, where the traps almost always have a stratigraphic component in association with compressive structures.
Seal development is represented by top seal and fault seal (juxtaposition and shale smear) components, depending on the structural type and the depth of occurrence.
Examination of data for the Niger Delta shows that well failure is attributed to the following factors:
- Fault seal failure – perhaps the most prevalent form of failure in the Niger Delta.
- Lack of a valid trap, which may be due to several factors:
- Poor trap definition, especially if drilled on 2-D seismic.
- Well positioned off structure.
- Change in structural culmination with depth (for wells drilled before deviation drilling).
- Structural complexity and inability to reach target with a vertical well.
- Absence of stratigraphic pinch-out of the target.
- Poor reservoir development – more prevalent in the deep water and in deeper parts of the onshore.
- Migration by-pass.
Figure 2 is the play analysis summary for the Middle Miocene – a shorthand depiction of the combination of the four elements of reservoir, seal, charge and trap for each well in the middle Miocene Play of the Niger Delta.
This summary and the underlying database in ArcGIS provide a one-stop location of legacy data on well results for any play and any block in the Niger Delta.
The compilation includes the usual IHSE well data, the post drill analysis evaluation and the various data sources. It forms the foundation on which future more detailed analysis can be based, and it will require regular updates as more wells are drilled.
The low level (18 percent) of full penetration of the Middle Miocene onshore is an indication of the potential in the onshore deep play. This conclusion is true for the other eight plays in the onshore Niger Delta, and clearly indicates a potential for a deep play exploration frontier.
Accessing these levels depends on technological advancement, both in imaging these depths and in drilling over-pressured intervals. Conceptual models of the depositional model for these depths need to be tested in order to incrementally de-risk the opportunities.