Received date: May 14, 2018; Accepted date: May 29, 2018; Published date: June 01, 2018.
A regional geological framework study has been carried out on the in the Central Swamp Depobelt of the Niger Delta using wells from the Odidi-Egwa area. Odidi and Egwa are two main producing fields located in the Central Swamp of the western Niger Delta. The objective of this study is to better understand the regional geology including regional stratigraphic and structural framework, reservoir depositional and lithological trend and stratigraphic variation cross the major growth fault between the two fields. It will provide a more informed knowledge of the necessary geological inputs for the different reservoir models in this project.
Two regional sequence stratigraphic correlations were made using wells from the Egwa, Odidi, Batan, Ajuju, Bamiene, Ubefan and Irigbo fields and wells, arranged as two panels. The sequences range in age from Early Miocene to Middle Miocene. The essential feature of the sequence stratigraphic correlation is that the wells in the area penetrated four maximum flooding surfaces and four sequence boundaries.
The conventional sequence trapped hydrocarbons mainly within the early Middle Miocene sequences. Biofacies interpretation shows that the hydrocarbon bearing sequences are deposited in nearshore to shallow water environments. Therefore, reservoirs in this area are mainly shoreface deposits. The sequence stratigraphic correlation shows that while the most active period of sedimentation in the Egwa fault block is between the 17.7 Ma and the 20.4 Ma, the major period of deposition switched to the 15.5 Ma and the 17.7 Ma in the Odidi fault block.
Most of the wells studied had their total depths in very good reservoirs giving hope to prospectivity below the present total depths of these wells. With better imaging of seismic data, it will be clearer to delineate structural features below the present total depths and thus properly evaluate the true potential of the deeper prospects. This is expected to open further exploration potential in these fault blocks.
A regional geological framework was, initiated for the area forming the central part of the western central swamp depobelt of the Niger Delta, (Figure 1). A total of eleven wells have been used for the study and these include, Egwa-1. Egwa-2, Egwa-34, Odidi-1, Odidi-2, Odidi-22, Batan-1, Ajuju-1, Bamiene-1, Ubefan-1, Ubefan-4 and Irigbo-1. The choice of wells has been made based on the availability of the biostratigraphical data (Palynological Zones, Foraminiferal Zones, Paleobathymetry data), good wireline log suits and as much as possible wells that did not penerate the multiple faults bounding the fields.
Four maximum flooding surfaces (15.0, 15.9, 17.4 and 19.4 Ma) and four sequence boundaries dated (13.1, 15.6, 16.7 and 17.7 Ma) have been identified and correlated within the Odidi- Egwa trend using the Niger Delta Chronostratigraghic Chart (Stacher et al, 1993). A total of eleven wells have been used for the study. The correlation was extended down to Irigbo-1 on the southern portion. The main hydrocarbon bearing intervals lie within major depocentres in each structure.
Geological background of the Niger Delta
Tectonic framework of the Niger Delta
The Niger Delta lies to the South and West of the shield areas of presumably mostly Precambrian to Early Paleozoic age  (Figure 2). The shield area contains depressions filled with sediments of Cretaceous, Tertiary and Quaternary ages. The Niger River flows through one such depression (Bida basin). In addition to these depressions, there exists a volcanic zone, the Fernando Polo-Cameroons volcanic zone, where volcanic rocks of Tertiary-age occur. Towards the southwest, an area of abyssal hills and the Guinea abyssal plane probably form the limits of the sub-aqueous part of the delta, which is on the flank of the mid-Atlantic Ridge.
The disposition of the basement blocks has affected the sites of the successive Tertiary depocenters. In particular, the Oligocene and Early Miocene depocenters seem to correspond to the low area between basement blocks (e.g. at the extension of the Benue Trough and in the low between the Onitsha and the Abakaliki blocks). The central area of erosion or non-deposition during Late Oligocene and Early Miocene times has been attributed to the positive effects of the underlying Abakaliki high. Basement control in the eastern parts of the delta is less obvious. Only the P520 depocenter corresponds to a prominent basement feature i.e., the subsidiary southwest - southeast trough cutting in the Calabar flank . Tectonic movement of the continental basement blocks in the northeast seems to have continued to some extent during sediment deposition in the Tertiary, particularly along the western fault boundary of the Onitsha block and its presumed southerly extension leading to the development of depobelts  and (Figure 3).
Regional Stratigraphy of the Niger Delta
The Tertiary Niger Delta is composed of an overall clastic succession that reaches a maximum thickness of 9 - 12,000 meters . Accumulation of marine sediments in the southern Nigerian 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 Middle Eocene, when sediments began to build out beyond the troughs between basement horst blocks at the northern flank of the present delta area. Since then, the delta has prograded southward onto oceanic crust gradually assuming a convex-to-the sea morphology [4-6].
The development of the Niger Delta depended on the interplay between sediment supply and subsidence as explained in the stepwise outbuilding of the delta by Knox and Omatsola . They define depobets as major fault bounded sequences about 40 km wide and up to 300 km in length. Large amounts of accommodation space are added during the early stages of depobelt development as the rate of sediment supply starts to exceed the rate of subsidence, the continental sands of the Benin Formation start to prograde over the depobelt leading to the deposition of the paralic sands and shales and the development of a younger depobelt further south (Figure 4-6).
Regional Setting of the Study Area
The Egwa and Odidi fields are located within OML 42 in the western part of the Central Swamp Depobelt slightly to the fringe of the Niger Delta, (Figure 1). Together the Central Swamp and the Coastal Swamp are the most prolific oil province in the whole of the Niger Delta as revealed by the shear number of fields, sizes, reserves and production levels. In the conventional sections, collapsed crests grading into the K-type and faulted rollover structures characterize this area. At depth, this trend is also characterized by mainly deep footwall closures with some hanging wall components in a limited sense (Figure 7). The Odidi and Egwa fields are dormant rollover structures in a typical hanging wall setting (Figure 8).
Sedimentation within the Central Swamp Depobelt was mainly wave dominated and the activity of the Opuama Canyon has been initiated on the north-western flank of the delta . The oldest sediments in the Central Swamp Depobelt are dated around the Latest Oligocene. Sedimentation in this area was mainly starved at this time as only shales of the Akata Formation were deposited . Deposition of the paralic facies (Agbada Formation) probably took place from the earliest Miocene times onwards (Figures 4 & 5).
Regional sequence stratigraphic correlations have been made using the following wells: Egwa-1. Egwa-2, Egwa-34, Odidi-1, Odidi-2, Odidi-22, Batan-1, Ajuju-1, Bamiene-1, Ubefan-1, Ubefan-4 and Irigbo-1 arranged as two panels. The sequences range in age from Early Miocene to Middle Miocene (P650 – P720), (Figure 9). The essential feature of the sequence correlation is that the wells in the area penetrated four maximum flooding surfaces namely, Bolivina-25 Shale, (15.0 Ma), the Chiloguembelina Shale, 15.9 Ma), the unnamed 17.4 Ma -Shale and the Ogara Shale (19.4 Ma) [10,11].
The conventional sequence trapped hydrocarbons mainly within the P680-P670 subzones, while the deep prospects are expected to trap hydrocarbons within the P650 and older pollen subzones. Biofacies analysis shows that the hydrocarbon bearing sequences are deposited in nearshore to shallow water environments. Reservoirs in this area should be mainly shoreface deposits.
The correlation shows that the most active period of sedimentation in the Egwa structure is between the 17.7 Ma and the 20.4 Ma, which includes the main reservoirs from the B8 to F7 sands (Figure 9,10). This interval is roughly correlatable to the interval from F3 to N sands in the Odidi structure. The major period of deposition switched to the 15.5 Ma and the 17.7 Ma in the Odidi structure, which include the reservoirs from C4 to F1 sands (Figure 11,12). The Stratigraphic variation across Egwa and Odidi is shown in Figure 13,14.
The general trend of sands shaling out towards the northwest can be observed in both Egwa and Odidi sections have been attributed to lack of sediment supply towards the distal part of the delta and is in conformity with the regional depositional pattern for the paleo Niger river .
The interesting feature in this area is that most of the wells stopped in very good reservoirs giving hope to prospectivity below the present total depths of these wells. With better imaging of seismic data, it will be clearer to delineate structural features below the present total depths. This is expected to open further exploration potential in these areas .
The main reservoirs are within the sequence from 20.4 to 17.1 Ma. Sands, generally shale out to northwest, a reversal of the main trend. This is a peculiarity in the field and is key to hydrocarbon development in the area.
The author is grateful to The Shell Petroleum Development Company of Nigeria especially the Exploration department for allowing the publication of the report.
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