Foraminifera biostratigraphy and paleoenvironment of Well 5, OML 34, Niger Delta, Nigeria

Emmanuel C. Nwaejije, Enam O. Obiosio, and Ibrahim Hamidu

Article number: 20.3.51A
https://doi.org/10.26879/630
Copyright Palaeontological Association, October 2017

Author biographies
Plain-language and multi-lingual abstracts
PDF version

Submission: 19 January 2016. Acceptance: 6 October 2017

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ABSTRACT

​A full account of 67 ditch cutting samples collected from Well 5, OML 34 in the Niger Delta at depth intervals between 2441m to 3650m is presented. Lithologic description of the samples shows seven lithostratigraphic units composed of shale, siltstone and sandstone corresponding to the Paralic Agbada Formation. Foraminifera analysis was carried out on these samples, a total of 51 species were recovered; 37 benthonic and 14 planktonic. The planktonic index recovered from the well, Praeorbulina glomerosa, Praeorbulina sicana, Orbulina saturalis, and Catapsydrax dissimilis, revealed that the age of the penetrated well is Miocene. Three planktonic foraminifera zones corresponding to Blow, 1969 (N6 - N7, N8 - N9 and N9) and Berggren et al., 1995 (M4, M4 - M5, and M5) zones are proposed for the well. The planktonic zones are Catapsydrax dissimilis Partial-range zone, Praeorbulina glomerosa Interval zone, and Orbulina universa Taxon - range zone, respectively. Sediments of the well are considered to be of normal marine depositional environment based on shell type ratio and triangular plot of the foraminifera test type (arenaceous, porcelaneous and hyaline). The paleobathymetry of the well ranges from non-marine to middle neritic environment based on foraminifera distribution.

Emmanuel C. Nwaejije. Department of Geology, Ahmadu Bello University, Zaria, Nigeria. emmanuelcelestine01.ec@gmail.com
Enam O. Obiosio. Department of Geology, Ahmadu Bello University, Zaria, Nigeria. eoobiosio@abu.edu.ng
Ibrahim Hamidu. Department of Geology, Ahmadu Bello University, Zaria, Nigeria. hibrahim@abu.edu.ng

Keywords: biostratigraphy; lithostratigraphy; paleoenvironment; foraminifera; Niger Delta

Final citation: Nwaejije, Emmanuel C., Obiosio, Enam O., and Hamidu, Ibrahim. 2017. Foraminifera biostratigraphy and paleoenvironment of Well 5, OML 34, Niger Delta, Nigeria. Palaeontologia Electronica 20.3.51A: 1-17. https://doi.org/10.26879/630
palaeo-electronica.org/content/2017/2036-foraminifera-in-niger-delta

Copyright: October 2017 Palaeontology Association.
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INTRODUCTION

According to Evamy et al. (1978) and Reijers et al. (1997), the petroleum industry in Nigeria have described and adopted informal biostratigraphic zonations, which are largely unpublished. This lack of formal biostratigraphic zonations for the Niger Delta still persists even though foraminifera biostratigraphy of several wells from the Niger Delta have been extensively published (Petters, 1979, 1982, 1983; Adeniran, 1997; Ozumba and Amajor, 1999; Oluwatosin, 2010; Okosun et al., 2012; Obiosio, 2013; Oloto and Promise, 2014), however, the foraminiferal biostratigraphy of Well 5 OML 34 has not been documented.

Attempts to mutually validate and integrate the Shell Petroleum Development Company (SPDC) framework of Evamy et al. (1978) with published and unpublished works of other industry groups is hampered by the use of different alphanumerical coding systems for the foraminifera systematics. However, Reijers et al. (1997) reported that there have been concerted efforts within the scope of the stratigraphic committee of the Niger Delta (STRATCOM), to produce a generally acceptable delta-wide biostratigraphic framework, but not much has been accomplished after several data gathering exercises by the committee.

This study carried out a detailed biostratigraphic description of Well 5 OML 34, (Figure 1) in order to propose biozones and paleoenvironments for the well.

Geology of the Niger Delta

The Niger Delta is an oil province of Nigeria located on the West African Continental margin. The Niger Delta basin lies between latitude 4°00'00"N and 6°00'00"N and longitude 5°00'00"E and 8°00'00"E. It is bounded to the west and northwest by the Western African shield, which terminates at the Benin hinge line and to the east, by the Calabar hinge line. The Anambra basin and Abakaliki anticlinorium mark its northern limit. To the south, it is bounded by the Gulf of Guinea. The Niger Delta is a large arcuate delta of the destructive, wave-dominated type and is composed of an overall regressive clastic sequence, which reaches a maximum thickness of about 12 km in the basin centre. The shape and internal structure of the Niger Delta are controlled by fracture zones along the oceanic crust, such as the Charcot fracture zone, Chain fracture zone, and the Romanche fracture zone (Corredor et al., 2005).

The Niger delta is characterized by three formations that range in age from early Tertiary to Recent, showing an overall upward transition from marine prodelta shales (Akata Formation) through sand-shale paralic sequence (Agbada Formation) to continental sands and gravels (Benin Formation; Short and Stauble, 1967; Avbovbo, 1978). Murat (1972) described the three formations as being strongly diachronous, implying that the formations transit through several geologic time intervals.

METHODS

Biostratigraphic analysis was carried out on 67 ditch cutting samples obtained from Well 5, OML 34, Niger Delta. Samples were collected at 18m (60ft) interval with a total thickness of 1200 m. The procedure of analysis is as follows:

1. Lithostratigraphic analysis was carried out on the samples by visual inspection. Physical characteristics such as colour, texture, hardness, fissility, and rock type were noted taking into consideration published lithofacies description of the Niger Delta as well as lithofacies models of Webber and Daukoru (1975) and Whiteman (1982). Chemical test to determine the presence of calcareous materials was also carried out using 10% dilute HCl.
2. Biostratigraphic Studies: The procedures adopted for the microfossil extraction is in line with standard micropaleontological sample preparation technique (Pessagno, 1967; Zingula, 1968; Brasier, 1980). Ditch cuttings from the well were prepared for lithostratigraphic description and micropaleontologic analysis at 18 m (60 ft) intervals. The procedure is outlined as follows:
3. Prepared sample list, washed samples free from drilling mud and allowed to dry, treated a quantity (200 g) with one teaspoonful of anhydrous sodium carbonate for thorough disintegration, added enough water to cover the samples and allow to stand for few hours, washed the soaked sample using a 63 µ (230-mesh), sieve, dried the washed sample at a minimum temperature of 20°C, decanted the dried sample into coarse, medium and fine fractions, stored samples in well labeled sample bags.
4. In carrying out the detailed procedure outlined above, care was taken to avoid contamination with other samples.
5. All size fractions were examined individually on a picking tray, the grid lines in the tray helped to ensure that all parts of the tray was well observed. Foraminifera was picked with the aid of a sable brush 000 under a binocular microscope.
6. The various foraminifera taxa encountered in each sample during the picking exercise were grouped and mounted temporarily with gum on a micropaleontological slide cavity and covered with a cover slip. These slides were arranged serially for identification. The identification of the various foraminifera was done largely by comparison with forms that have been previously described by Sellier de Civrieux (1976), Petters (1982), Loeblich and Tappan (1987), and Bolli and Saunders (1985).

Biostratigraphic data yielded biofacies information for paleoenvironment and bathymetry. Dating of the key surfaces where possible was achieved by their calibration to the third order cycles of Haq et al. (1988).

Photomicrographs of foraminifera recovered from the studied well were taken using a Celestron 5 mp digital camera, mounted on a binocular microscope.

RESULTS

Lithostratigraphic Units

The lithostratigraphic section of the well is based on ditch cutting samples described and information gathered from wire line log. The thickness of analyzed well is 1209 m (2441- 3650 m interval). The lithologies are mainly sandstone, shale and siltstone, and the lithostratigraphic section revealed 7 distinct lithologic units (Figure 2).

Foraminifera Fauna

Foraminifera analysis was carried out on 67 samples obtained from the well (interval 2441- 3650 m). The foraminifera recovery was fair, the diversity, however, was high. The forms encountered include planktonic foraminifera, benthonic calcareous foraminifera, and also benthonic arenaceous foraminifera.

Planktonic foraminifera. Fourteen planktonic foraminifera species were recovered (Table 1, Appendix 1), which constitutes about 28% of the foraminifera population. The planktonic foraminifera are dominated by species of: Globigerina sp., Orbulina suturalis, Praeorbulina glomerosa, Globorotalia mayeri, and Planktic indet sp. Other important planktonic foraminifera species recovered include Catapsydrax dissimilis and Praeorbulina sicana.

Benthonic foraminifera. The benthonic foraminifera species in the well are made up of diverse and rich to occasionally abundant species. Preservation is fairly good all through the section. Thirty seven benthonic species was recovered, accounting for about 72% of the total foraminifera population, thirty-one calcareous species (Table 2, Appendix 2) and six that make up the benthonic arenaceous species (Table 3, Appendix 2). The benthonic foraminifera are dominated by species of: Uvigerina isidroensis, Brizalina mandoroveensis, Lenticulina grandis, Hopkinsina bononiensis, Hanzawaia stratonii, Ammonia beccarii, Heterolepa floridana, Cibicorbis inflata, Uvigerina sp. Valvulineria gasparensis, Haptophragmoides sp., and Spiroplectammina sp. Some of the other species have poor occurrences to single occurrences as in the case of Textularia laminata, Ammobaculites sp., Bolivina spinata.

DISCUSSION

Lithostratigraphy

The lithology (Figure 2) shows siltstone layers with the alternation of sandstone and shale, the shale is brown to grey, occasionally black to brown, moderately hard. The sandstone is fine-grained (occasionally coarse-grained), sub-angular to sub-rounded, well sorted and occasionally ferruginized. Figure 2 shows an alternation of sandstone with shale, suggesting that the studied well interval (2441 - 3650 m) penetrated the Agbada Formation of the Niger Delta. In general, it has been observed that the upper part of the formation has a higher sandstone percentage than the lower part, suggesting progressive, seaward advancement of the Niger delta through geological time.

Age Determination

Planktonic foraminifera are a microfossil group that play a vital role in biostratigraphic subdivisions and correlation of deep sea cores, hence, the importance of some planktonic foraminifera as index fossils have become increasingly recognized. Their abundance in marine sediments combined with the short life span of many species makes the planktonic foraminifera better suitable for time stratigraphic correlations, this is largely due to their wide geographical distribution combined with additional dispersal by ocean currents which makes them valuable index fossils for world-wide stratigraphic correlation.

The following index forms were identified: Praeorbulina glomerosa, Praeorbulina sicana, Orbulina saturalis, and Catapsydrax dissimilis. Based on the presence of these index forms, as defined by Bolli and Saunders (1985) and Petters (1983), the age of the studied well ranges from Early Miocene to Middle Miocene.

Early Miocene. The upper limit of the Early Miocene in the study well interval is marked at 3411 m based on the last downhole occurrence (LDO) of Praeorbulina glomerosa and the lower limit is marked at 3648 m based on the first downhole occurrence (FDO) of Catapsydrax dissimilis. This interval is characterized by the presence of Early Miocene planktonics (Globigerinoides quadrilobatus).

Early - Middle Miocene. The upper limit is marked at 3027 m based on the last downhole occurrence (LDO) of Orbulina universa and the lower limit is marked at 3374 m based on the last downhole occurrence (LDO) of both Orbulina saturalis and Praeorbulina glomerosa. Other planktonic forms present include Globigerinoides trilobus, Globigerinoides quadrilobatus, and Globigerinoides sacculifer.

Middle Miocene. The upper limit of the Middle Miocene is indeterminate in the sequence studied because of the absence of marker species, the lower limit, however, is marked at 3027 m based on the last downhole occurrence (LDO) of Orbulina universa. Other planktonic forms present include Praeorbulina sicana, Globigerina sp., Globigerina venezuelana, and Globorotalia mayeri.

Biozonation

Three zones have been proposed in this study, based on the International stratigraphic guide of Hedberg (1976) and the observation of the ranges of planktonic foraminifera (Figure 3) as follows: Catapsydrax dissimilis partial-range zone, Praeorbulina glomerosa interval zone and Orbulina universa taxon-range zone.

Catapsydrax dissimilis partial-range zone. Stratigraphic interval: 3648 - 3411 m

The zone is defined by the first downhole occurrence (FDO) of Catapsydrax dissimilis and Globigerinoides sp. at the base and the last downhole occurrence (LDO) of Praeorbulina glomerosa at the top (Figure 3). Other planktonic forms occurring within the zone are Globigerinoides sp. Globigerinoides quadrilobatus, Planktic indet sp. and Globorotalia mayeri. This zone is equivalent to M4 zone of Berggren et al. (1995) and the N6 - N7 zone of Blow (1969). The FDO of Catapsydrax dissimilis marks the M4 and N6/N7 boundaries of Berggren et al. (1995) and Blow (1969), respectively. Catapsydrax dissimilis is continuously present in the Early Miocene. The age of this zone is Early Miocene based on the presence of planktonic index forms.

Praeorbulina glomerosa interval zone. Stratigraphic interval: 3411 - 3027 m

The zone is defined by the LDO of Praeorbulina glomerosa at the base and the LDOs of Praeorbulina sicana and Orbulina universa at the top (Figure 3). Other planktonic forms occurring in this zone are Globigerinoides immaturus, Globigerinoides sacculifer, Globigerinoides trilobus, and Orbulina suturalis. This zone corresponds to the M4 - M5 and N8 - N9 zones of Berggren et al (1995) and Blow (1969) respectively. The zone is dated Early to Middle Miocene based on the presence of planktonic index forms.

Orbulina universa taxon - range zone. Stratigraphic interval: 3027 - 2760 m.

This zone is defined by the entire occurrence of Orbulina universa. The base of the zone is marked by the LDO of Orbulina universa, and Praeorbulina sicana, respectively, while the top is marked by the FDO of Orbulina universa, Globigerina venezuelana, and Planktic indet sp., respectively (Figure 3). Other planktonic forms occurring in the zone are Globigerina sp. Globigerina venezuelana, Globorotalia mayeri, Praeorbulina glomerosa and Praeorbulina sicana. The zone is equivalent to the M5 and N9 zones of Berggren et al. (1995) and Blow (1969), respectively. The zone is dated Middle Miocene based on the presence of planktonic index forms.

The planktonic foraminiferal preservation in the upper intervals of the well is poor. Stratigraphically important taxa (planktonic index forms) were not identifiable, therefore the undiagnosed stratigraphic interval above the Orbulina universa taxon - range zone is assumed to be M6 zone of Berggren et al (1995) and N9/N10 zone of Blow (1969), because of the stratigraphic position above the positively identified zone (Orbulina universa taxon - range zone). The age is assumed to be Middle Miocene for the same reason.

Paleoenvironment

Following the same principles as Bandy (1964), Funnel (1967), Boersma (1978), and Brasier (1980), the paleodepositional environment of the studied well was established based on the information interpreted from the evaluation of the benthonic foraminifera assemblages. This has been integrated with the lithologic description of the well and the planktonic/benthonic foraminifera ratio.

Paleobathymetry

Foraminifera data was most useful in the estimation of paleobathymetry, it involved the use of relative abundance and diversity of the foraminifera encountered as well as the occurrence of environmentally significant taxa. It is on these bases that the sediments of the well were interpreted to have fluctuated from non-marine to middle neritic as follows:

• Middle Neritic Environment
• Inner Neritic Environment
• Shallow Inner Neritic Environment
• Non marine Environment

The well interval possessed characteristics that are exhibited by the above environments.

Non marine environment (Figure 4). This inference is based on the following reasons:

The intervals are characterized by fine grained sandstone, siltstone, and shale. The presence of fine grained sandstone and carbonaceous detritus in these intervals suggests deposition in low energy environment, probably near-shore settings, and this condition is evident of paralic environments and reflects a marine transgression (Oboh-Ikuenobe et al., 2005).

The intervals are completely barren of foraminifera. The complete absence of fauna in this interval suggest a littoral (shore or coastal) settings.

Shallow inner neritic environment (Figure 4). This inference is based on the following reasons:

1. The intervals are characterized by medium to coarse grained sand (with very thin shale beds), suggesting deposition during progradational phase. The presence of coarse sandstone, ferruginous materials, and carbonaceous detritus in these intervals indicates deposition in high energy, probably near-shore settings.
2. The intervals contain very few benthonic and planktonic foraminifera (e.g., Ammobaculites sp., Valvullineria gasparensis and Globigerina sp.). Species abundance is low, and the number of species (diversity) is high. The planktonic types constitute from 20-30% of the total fauna, (Boersma, 1978).

Inner neritic environment (Figure 4). This inference is based on the following criteria:

1. The microfauna found here suggest inner neritic environmental settings with middle neritic influence, these include: Quinqueloculina sp. Lenticulina grandis, Uvigerina sp. Miogypsinoides sp. Cubicubis inflata, Brizaliana mandoreveensis, Spiroplectammina wrightii, Uvigerina isidroensis, and Uvigerina sparsicostata. The diversity range from zero to nine species.
2. The lithology of the intervals is composed of medium to coarse grained sand (smoky white to orange, sub-angular to sub-rounded, well-sorted and occasionally ferruginized), siltstone (white, fine-grained, micromicaceous, and carbonaceous plus traces of woody materials), and shale (brown to grey, fissile, moderately hard, micromicaceous, and occasionally carbonaceous).

Inner to middle neritic environment. This environment of the studied well (Figure 4) is inferred based on the following:

1. The occurrence of the typical forms from inner and middle neritic environments, including: Uvigerina sp. Heterolepa floridana, Valvulineria gasparensis, Cibicorbis inflata, Orbulina suturalis, Hanzawaia stratonii, Globigerinoides immaturus, Globigerinoides sacculifer, Globigerinoides trilobus, Hanzawaia concentrica, Hopkinsina bononiensis, and Spiroplectammina wrightii. The population of planktonics foraminifera increases as well as the species diversity (ranging from zero to 20 species).
2. The lithology is composed of sand (smoky white to orange, sub-angular to sub-rounded, well-sorted, and occasionally ferruginized), siltstone (white, fine-grained, micromicaceous and carbonaceous plus traces of woody materials), and shale (brown to grey, fissile, moderately hard, micromicaceous, and occasionally carbonaceous).

According to Okosun et al. (2012), the inner to middle neritic environment is characterized by the occurrence of the typical forms from inner, middle, and outer neritic environments like Uvigerina sp, Spiroplectamina wrightii, and Hanzawaia strattoni.

Middle neritic environment. This environment (Figure 4) is inferred based on the following characteristics:

1. The presence of indicator fauna like: Lenticulina inornata, Heterolepa pseudoungeriana, Lenticulina grandis, Uvigerina isidroensis, Brizalina mandoroveensis, Valvullineria gasparensis, Globigerina sp., Globigerina venezuelana, Orbulina universa, Globorotalia continuosa, Planktic indet sp., and Praeorbulina glomerosa. There is an increase in the number of planktonic specimens. The average planktonic/benthonic ratio is high, the simple species diversity also increased, ranging from zero to 23 species.
2. The lithology is composed of siltstone (white, fine-grained, micromicaceous, and carbonaceous plus traces of woody materials) and shale (brown to grey, fissile, moderately hard, micromicaceous, and occasionally carbonaceous).

This environment is recognized by the presence of indicator fauna like, Lenticulina inornata, and Heterolepa pseudougerina. Increase in the number of planktonic specimens. The average planktonic/benthonic ratio is high, the sample species diversity is also increased and the lithology of the middle neritic environment is composed of shale and silt (Okosun et al., 2012).

The occurrence of shell fragments, ostracods, gastropods, along with the occurrence of Textularina sp. suggest a shallow marine origin, the presence of Textularina laminata, Textularina panamensis, and Ammobaculites sp. suggest inner shelf environment of deposition (Nton and Esan, 2010).

Based on the paleoenvironmental interpretation provided above, it can be inferred that the paleobathymetry of the study well ranges from shallow to inner shelf environment.

Paleosalinity

Foraminifera live in all marine environments from the deepest ocean floor to the intertidal salt marshes found behind barrier islands or around the margins of estuaries.

Paleosalinity interpretations for the studied well were made based on Shell-type (morphogroup) ratio triangular plot (Figure 5). The proportion of the three shell type (agglutinated, hyaline, and porcelaneous; Figure 6) of foraminifera in a sample can be used to characterize a particular environment in seas and oceans.

The triangular plot above reveals the dominance of the hyaline calcareous shell type suggesting a normal marine shelf sea environment. Comparison with modern micro fauna suggests a normal marine neritic environment (Murray, 1973, 1991).

The dominance of the calcareous benthonics (FOBC) with over 60% of total forms present suggests an open marine condition (Nagy et al., 1988).

According to Murray (1991) and Sen Gupta (1999), the high diversity and dominance of calcareous taxa suggests transition from brackish marginal marine habitats to open neritic conditions.

CONCLUSION

Based on the lithology and foraminifera recovered from the well, it is inferred that the well penetrated the Agbada Formation and is of Miocene age (Early and Middle Miocene).

Three planktonic foraminifera zones: Catapsydrax dissimilis Partial-range zone, Praeorbulina glomerosa Interval zone, and Orbulina universa Taxon - range zone corresponding to Berggren et al. 1995 (M4, M4 - M5, and M5) and Blow 1969 (N6 - N7, N8 - N9 and N9) zones, respectively, have been proposed for the well. It is hoped that the three biozones proposed in this study will contribute to the findings of the stratigraphic committee of the Niger Delta (STRATCOM), to produce a generally acceptable delta-wide biostratigraphic framework.

The observed lithology and foraminifera assemblages especially the benthonics indicated that the sediments of the well were deposited in a non-marine to shallow inner neritic, inner neritic, inner to middle neritic, and middle neritic environments.

Further study of data sets from adjoining wells in the region will help in proper correlation of the zones, boundaries, and depositional environment in the study area.

ACKNOWLEDGEMENTS

The authors wish to acknowledge Shell Petroleum Development Company of Nigeria (SPDC) for providing ditch cutting samples used in this study. We also acknowledge the reviewers who took time to review and make amendments to improve this paper.

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