The Niger Delta Basin is one of the rift-generated sedimentary basins on the continental margin of the Gulf of Guinea in Nigeria [1]. The study area lies approximately between latitudes 4°23′30″N and 6°24′00″N and longitudes 2°45′30″E and 6°46′30″E (Figure 1). As the Niger delta basin matures, most of its subsurface uncertainties lie at reservoir scale, and hence the need for application of biostratigraphy. Oil companies which invest hugely in this high-risk business of oil exploration have undoubtedly gained much from biostratigraphic studies. Apart from assigning ages to rocks, the prediction of water depths and palaeodepositional environments is vital for the understanding and deduction of depositional models with reasonable productiveness [2]. Previous studies include those on foraminifera biostratigraphy from offshore Western Niger Delta [3]. Fajemila [3] recognized five foraminiferal zones and inferred that the sediments were of normal salinity and belonged to early Pliocene to late Miocene age.
Sanuade [4] carried out calcareous nannofossil biostratigraphy of Well ‘K-2’, in the deep offshore, Niger Delta. Two major zones were identified, while one condensed section was believed to be associated with 2.0 Ma maximum flooding surface (MFS).
Aturamu [6] reported on integrated biostratigraphic studies using foraminiferal and palynomorph assemblages within the Niger Delta. They established two planktonic foraminiferal zones,
The present study gives an account of integrated foraminiferal, calcareous nannofossils and palynostratigraphy of the STEP-1 well, offshore western Niger delta, in order to deduce the age of the penetrated interval, and provide detailed information on the paleoenvironment of deposition and palaeoclimatic conditions of the sediments. Such information, no doubt, could serve as a sequence stratigraphic model, generally beyond the resolution of seismic stratigraphy.
The Niger Delta basin is situated between latitudes 3° and 6°N and longitudes 5° and 8°E in the Gulf of Guinea on the margin of West Africa, and is one of the largest deltaic systems in the world. Doust and Omatsola [8] and Short and Stauble [9] classified the subsurface Niger Delta into three stratigraphic units from the oldest to the youngest: Akata, Agbada and Benin Formations (Figure 2). The Akata Formation is the basal unit of the Tertiary Delta Complex and comprised shales, clays and silts at the base of the known delta sequence. They contain a few streaks of sand, possibly of turbiditic origin [10]. The Agbada Formation is the hydrocarbon-prospective sequence, a paralic clastic sequence which lies above the Akata Formation.
The upper part of the Agbada Formation often has sand percentages ranging from 50% to 75%, becoming increasingly sandy towards the overlying Benin Formation, while the basal part is more of shale sequence [8]. The Benin Formation is the freshwater-bearing formation in the Niger Delta. It comprises the top part of the Niger Delta Basin, from the Benin-Onitsha area in the north to beyond the present coastline [9].
Materials used in this study include ditch cuttings and gamma ray log. Because of the proprietary nature of the well, the exact location information was not provided, hence it was code-named STEP-1 well. The samples were selected and composited at intervals of 12 m and 24 m, respectively, which was then prepared and analysed based on fauna and flora contents. The biostratigraphic analyses were carried out while the other analysis was achieved using the STRATABUG software.
The lithologic description was carried out by examining the samples under a hand lens in order to identify the constituents, describe and name the lithology. This was supported by the GR log to complement the lithologic description based on its deflection away from the Shale Baseline (SBL).
The ditch cuttings were prepared for foraminifera, calcareous nannofossils and palynomorph contents using standard preparatory methods. Fifty samples were selected at 12 m intervals for foraminifera preparation; the standard weight (20 g) of each sample was soaked overnight to ensure proper disaggregation and liberation of microfossils present within the cuttings. The disaggregated samples were then washed under a shower of water over a 63 μm sieve, rinsed in liquid detergent to remove drilling mud and dried over a hot plate. The forms were picked with a picking needle under an Olympus binocular microscope. Preparation and identification of individual foraminifera were guided following the works of [11, 12, 13], among others.
Fifty samples were selected at 12 m interval for calcareous nannofossils; about 2 g of each of the samples were scraped onto a glass microscope slide. The slides were labelled sequentially and two blobs of Norland Optical Adhesive were affixed onto the cover-slip. The slides were dried and later studied under a transmitting light microscope in polarized light. This was done following Martini [14].
Twenty-five samples were selected and composited at 24 m intervals for palynological analysis. A constant weight (30 g) of each sample was initially given a 5% dilute hydrochloric acid treatment to remove carbonates prior to complete digestion in concentrated hydrofluoric acid (HF) under a fume cupboard. The samples were decanted thrice at an interval of 1 h each through the Branson Sonifier and with the aid of a 5 μm sieve to filter away the inorganic matter. A LOCTITE mounting medium was used for the residues, which are the palynomorphs. Identification and preparation of the specimen was done following Germeraad et al. [15].
The lithostratigraphic section of the studied well was produced from ditch sample descriptions and the deflection away from the shale base line on the gamma ray log. The total thickness of the analysed sample is 609.6 m (i.e. between 1,828.8 m and 2,438.4 m intervals). From the lithologic description, the samples are mainly shale with a little sand. The shale is fissile, greyish to black, while the sand is fine to medium grained. The observation revealed that the studied well is largely made up of a sequence of fine-grained shale alternating with fine- to medium-grained sandstone in the lower part while the upper part is mainly shale (Figure 3).
The well recorded fairly rich and diverse assemblages of planktonic and benthonic foraminifera at the upper part (1,902–2,109 m), with 55 species recorded. Of these, 22 (40%) species are calcareous, while 27 (49%) are arenaceous. Two foraminiferal “zones” were recognized in the studied section. The Cenozoic chronostratigraphic scheme of Berggreni et al. [13] and the Global Sequence Cycle Chart of Hardenbol et al. [16] were adopted for this study. The zones are characterized briefly below.
Stratigraphic interval: 1,841–1,987 m
Equivalent planktic foraminiferal zone: “Lower” N18–“Upper” N17 zone.
Age: Late Miocene (5.99–5.47 Ma)
Diagnosis: The top of this zone is placed at the 5.47 Ma MFS, recognized at 1,926 m, while the base is defined by the 5.8 Ma MFS, recognized at 1,987 m (Figure 4). The First Downhole Occurrence (FDO) of the zonal marker,
Stratigraphic interval: 1,987–2,438 m.
Equivalent planktic foraminiferal zone: “Middle-Lower” N17 zone.
Age: Late Miocene
Diagnosis: Undiagnosed
1,853 |
Early Pliocene | N18 | Indeterminate | FDO |
1,987 |
Late Pliocene | N17 and older | Peak fauna abundance 5.8Ma MFS |
The results of calcareous nannofossil analysis show high abundance and diversity of species (Figure 5). Biozonation and age determination of the well was based largely on calcareous nannofossils assemblages, abundance and diversity. The chronostratigraphic scheme adopted follows the usage of the worldwide zonation schemes of Okada and Bukry [17] and Haq et al. [18]. Considerable effort was made to identify and define zonal tops with the FDOs of diagnostic marker species, abundance and species diversity peak as these form the most reliable events [14]. The highest nannofossil peaks were dated using important marker species such as
Interval: 1,841–1,926 m
Zone: NN12
Age: Early Pliocene
Diagnosis: This interval is dated based on the presence of
Interval: 1,926–2,438 m
Zone: NN11
Age: Late Miocene
Diagnosis: This interval is characterized by an increase in nannofossil abundance and diversity. Its nannofossil peak at 1,963 m represents the 5.8 Ma MFS [18] late Miocene NN11 zone. This is confirmed by the FDO of
1,841 | First sample analysed | Late Miocene to early Pliocene | ||
1,902 | Presence of |
NN 12 | ||
1,914 | Presence of |
|||
1,926 | ||||
1,939 | FDO: |
5.8 | ||
1,963 | Maximum flooding surface | |||
2,292 | FDO: |
|||
2,316 | Maximum flooding surface, Presence: Discoaster quinqueramus | 7.0 | NN 11 | |
2,438 TD |
Twenty-five palynomorphs were selected at 24 m intervals. The palynomorphs are well preserved and fairly diverse (Figure 6). These include
The STEP-1 sediments are assigned to the section within the Pantropical
Zone: P800
Sub-zone: P840
Interval: 1,829–2,204 m
Discussion: The top of this sub-zone is placed at 1,829 m of the first sample analysed. The base is defined by the quantitative occurrence of
Zone: P800
Sub-zone: P830
Interval: 2,204–2,438 m
Discussion: The top of this sub-zone is placed at 2,204 m defined by the quantitative base occurrence of
1,829 | Miocene | Late Miocene | P 800 | P 840 | Quantitative base occurrence of |
|
1,981 | ||||||
2,134 | ||||||
2,204 | P 830 | |||||
2,286 | ||||||
2,438 |
Depth (m) | Epoch | Age | P zone | B zone | N zone | Pa zone | Pa zone and sub-zone | Bioevents | |
---|---|---|---|---|---|---|---|---|---|
1,900 | Early Pliocene | ZANCLEAN | N18 | NN 12 | P800 | P840 | FDO |
||
1,950 | Presence of |
||||||||
2,000 | Presence of |
||||||||
2,050 | FDO |
||||||||
2,100 | FDO |
||||||||
2,150 | Late Miocene | MESSIAN | N 17 and older | NN 11 | P830 | 5.8 Ma MFS | |||
2,200 | Peak faunal abundance (5.8 Ma MFS) | ||||||||
2,250 | 6.3 Ma SB | ||||||||
2,300 | Quantitative base occurrence of |
||||||||
2,350 | |||||||||
2,400 | 7.0 Ma MFS |
The ages for this well were established using three bioevents (Table 5): these include recorded peak faunal abundance (MFS), sequence boundary (SB) and the occurrence of some index fossils such as
1,926 | 5.0 | Foraminifera | FDO |
1,963 | 5.8 | Nannofossils | Maximum flooding surface (MFS) |
1,987 | 5.8 | Foraminifera | Peak faunal abundance (MFS) |
2,097 | 6.3 | Nannofossils | Sequence boundary |
2,316 | 7.0 | Nannofossils | MFS |
1 | Quaternary | Holocene | Taranian | |
2 | ||||
Pleistocene | Ionian | |||
3 | ||||
Calabrian | ||||
4 | Neogene | Pliocene | Gelasian | |
5 | Piacenzian | |||
6 | Zanclean | |||
7 | Miocene | Messinian |
STEP-1 well | |
8 | ||||
9 | Tortonian | |||
10 |
The presence of some benthonic foraminifera such as
With land-derived palynomorphs such as
Palynofossils are preserved mainly in continental basins. The characteristics of great quantity, wide distribution and different preserved lithofacies are the unique advantage of these fossils [19]. For this reason, palynomorphs are now very important for reconstructing palaeoclimatic conditions at the time of sediments deposition. The climate of an area is reflected by its vegetation type [20]. The three important palynomorphs used for this study are
From Figure 7, the dominance in abundance of
From Figure 8, the dominance in abundance of
Foraminifera, calcareous nannofossils and palynomorph integrated biostratigraphic studies of the STEP-1 well in the offshore of Niger Delta Basin has resulted in the identification of biostratigraphic zones, determination of ages as well as reconstruction of the palaeoenvironment and palaeoclimatic conditions. The presence of some benthonic foraminifera such as