Palynological Study of OSE 1 Well in Offshore Niger Delta Basin: Implications for Age, Paleoclimate and Depositional Paleoenvironment

Palynological study of 45 ditch samples from sedimentary successions penetrated by OSE 1 well located offshore western Niger Delta Basin was undertaken to determine the age of the sediment and reconstruct the paleoclimate and depositional paleoenvironment. All the analyzed samples were compo-sited at 30 ft from stratigraphic depth interval of 6330 to 9010 ft. The litho-logic composition comprised mainly of brownish grey or light grey, fine-grained shales with a gypsiferous horizon near the bottom. Palynological slides were prepared following standard palynological preparation procedures of sample maceration with hydrochloric (HCl) and hydrofluoric (HF) acids. The samples yielded moderately well-preserved, low to high diversity palynomorph assemblages of 58 taxa comprising 29 pollen grains, 9 pteridophic spores, 17 dinoflagellates, one acritarch, one freshwater algae and a fungi spore. The stratigraphic distribution of microflora assemblages was highly variable. The assemblage abundance was dominated by pollen and spores. The retrieved pollen grains included Monoporites annulatus, Sapotaceoidaepollenites sp. Psilatricolporites crassus, Psilatricolporites sp., Zonocostites ramonae, Retitricolporites irregularis, Monocolpites marginatus, Monocolpites sp., Pachydermites diederixi, Proxapertites cursus, present in the assemblage. The freshwater algae was Botryococcus braunii. Fungi spore was present but not in significant amount. The stratigraphic interval was dated Early to Late Miocene (Burgidalian-Messinian) based on the occurrence and distribution of some stratigraphic fossil marker species including Magnatriatites howardi, Crassoretitriletes vanraadshooveni and Retistephanocolpites gracilis The abundant occurrence of pollen and spores typical of rain forest and freshwater swamp communities in the assemblage indicated tropical paleoclimatic conditions. The distribution pattern of Monoporites annulatus (Poaceae) and Zonocostites ramonae (Rhizophora) suggested seasonal fluctua-tions between dryer and wetter climatic conditions. The sediments were deposited in a wide range of environments including brackish lagoons, open salt water swamps, freahwater and nearshore to inner neritic settings.


Introduction
The Niger Delta Basin is of great economic importance because of its petroleum resource. Since 1956 when petroleum was discovered in commercial quantity in the basin, Nigeria's economy has gradually shifted from Agro-based to oil dependent. Today, roughly 80% of the nation's foreign exchange earnings derived from the sales of oil and gas. Exploration efforts for petroleum in the basin which began in 1937 were not rewarded until 1956 when the first producing well was drilled in Oloibiri field by Shell-BP. Nigeria joined the league of oil producing countries in 1958, with a modest daily production of 5100 barrels. The 1956 discovery of commercial quantity of petroleum in the Niger Delta Basin attracted the attention of many multinational oil companies and researchers to the region. Niger Delta petroleum province is ranked 11 globally in terms of petroleum resources, containing roughly 2.92% and 2.98% of the world-discovered oil and gas respectively [1]. Today, Nigeria's crude oil reserve, much of which occurs in the Niger delta Basin has remained stagnant at roughly 37 billion barrels of oil with a daily oil production of about 2.4 million barrels for nearly a decade as a result of ebbing exploration activities. The Federal Government in 2010 set a national target of 40 BBO in reserve and a daily production of 4 MBO per day by 2020.
There have been considerable exploration activities in the basin and other sedimentary basins of Nigeria, especially since the discovery of oil in the delta. The delta has been covered with a dense grid of 2-D and 3-D seismic data and over 5000 wells have been drilled and developed [2]. Much of the crude oil is being mined from the conventional structural traps such as rollover anticlines, fault and diapiric traps within the Agbada Formation. This implies that much of the hydrocarbon trapped in subtle stratigraphic traps has yet to be discovered. If the national target is to be achieved, exploration efforts should be shifted to finding to be an indispensable tool in petroleum exploration because of its ability to provide chronostratigraphic framework for the analysis and correlation of the lithic fills in basins that are deposited in response to sea level fluctuations, sediment supply and subsidence [3].
Although recent advances in seismic data acquisition techniques such as 3D seismic technology have led to the discovery of new giant hydrocarbon fields across the globe, yet the usability and effectiveness of seismic stratigraphy in petroleum exploration is hampered not only by its poor resolution in deep sedimentary basins but also by the high cost of interpretation of seismic data. As pointed out by [4], the science of petroleum exploration is multi-tool based, involving the integration of palynology with other geological disciplines such as geophysics, sedimentology, geochemistry and petrophysics with the ultimate goal of generating a geological model and elucidating on the petroleum system.
The main applications of palynological analysis in petroleum exploration are chronostratigraphic correlation, evaluation of potential source rocks, reservoir rocks and sealing rocks, paleoenvironmental and sequence stratigraphic studies.
Palynology is a useful stratigraphic tool in petroleum exploration applicable to sediments deposited in a wide range of depositional settings ranging from continental through coastal to shallow marine. [5], referencing [6] emphasized the use of fossil Rhizophora to Camptostemon ratio for sequence stratigraphic interpretations. According to them, [6] identified an inverse relationship in the occurrence trends of the two genera in relation to sequence stratigraphy, noting that the ratio of Rhizophora to Camptostemon is high during highstand system tract and low in lowstand system tract and transgressive system tract as a result, the sequential record of fossil pollen assemblages. Also, [4] demonstrated how the sequential record of fossil pollen assemblages can be used to study sea level oscillations and their phases, which can be related to particular depositional systems tracts. Although some of biostratigraphic studies have been carried out by several workers in the Niger Delta Basin including [7]- [14], the information they provide cannot be extrapolated over a long distance because of the very complex nature of stratigraphic architecture of the delta occasioned by the numerous synsedimentary faults which mainly deformed and compartmentalized the stratigraphic interval of interest, the Agbada lithostratigraphic unit. The availability of biodata from well to well will guarantee more accurate stratigraphic analysis thereby leading to the discovery of subtle hydrocarbon traps in the basin. It is the goal of this study therefore to utilize palynological analysis to erect biozones and reconstruct the depositional environment of the sedimentary succession penetrated by OSE-1 well (see Figure 1).

Tectonic Setting and Stratigraphy of Niger Delta Basin
The Niger Delta Basin is located in the Gulf of Guinea on the West African continental margin in the coastal area of Nigeria. It marks the southern limit of the  Benue Trough hence its evolution has been linked with that of this larger sedimentary complex [15]. The Benue Trough has been recognized as the failed arm of the three radial rift systems that met at an R-R-R triple junction in the Gulf of Guinea which was initiated in Early Cretaceous as a result of crustal doming [16]. The Niger Delta Basin represents the third phase in the evolution of the trough and its associated basins. The commencement of sedimentation during Aptian led to the evolution of the trough as the failed arm of a rift triple junction associated with the separation of South American and African plates. The second cycle from Santonian to Eocene began after the Santonian folding and thermo-tectonic event, leading to the formation of Anambra Basin and Afikpo Syncline.
The third phase from Eocene-Recent resulted in the formation of the Tertiary Niger Delta. The main depocentre is thought to have been located at the triple junction between the continental and oceanic plates [2]. The lithic fill of the basin consists of Paleocene to Recent siliciclastic deposits accumulated on both the faulted crustal blocks of the African continental margin and the oceanic crust [5] [17]. The delta has prograded southwestwards since the Eocene leading to the formation of major developmental stages including Central swamp I, Central Swamp II, Coastal Swamp II and Offshore depobelts. The accumulation of paralic sediments in each depobelt occurred in response to eustatic sea level oscillations active within the basin during the development of a depobelt [14]. The delta complex is deformed by well-developed growth faults and large-scale mud diapirs and its growth is closely related to the development of the diapirs [17].
The Niger Delta Basin contains an upward-coarsening regressive sequence of siliciclastic rocks roughly 12 km thick [18]. The lithic fill of the basin has been divided into three lithofacies namely Akata, Agbada and Benin Formations  [21]. These streaks of sand are thought to represent turbidites. The formation was deposited in prodelta to deeper marine settings. [22] stated that the formation was deposited in shallow to deep marine environments based on foraminifera evidence The formation is diachronous, ranging from Paleocene to Holocene. It outcrops offshore as diapirs along the continental slope, and onshore in the northeastern part of the delta, where it is characterized as Imo Shale [20].
The Agbada Formation is marked by alternating sandstone, siltstone and shale sequence with a thickness of over 3000 m [2]. Petroleum reservoirs in the basin occur as fine-to coarse-grained sandstone. These alternating clastics are the truly deltaic portion of the sequence that deposited in a series of deltaic environment such as the delta-front, delta-topset, and fluvio-deltaic environments [17]. The top of the formation is marked by the first occurrence of fossiliferous marine shale bed at the base of the continental Benin Formation. The base is an expansive sandstone body that overlies Akata Shale. Interdedded shales of the Agbada Formation are possibly source rocks. However, [23] opined that Akata Formation was the main source rock of the Niger Delta. Its age ranges from Eocene to Pleistocene.
The Benin Formation (Miocene-Recent) is the youngest unit in the basin.
With a minimum thickness of over 6000 ft, the formation consists mainly of sand, gravels and back-swamp deposits [24]. The shallowest part of the sequence is composed almost entirely of continental sand. The sands and sandstones are coarse-grained, sub-angular to well-rounded and are very poorly sorted. It also contains streaks/lenses of shale, clays, etc. The sands and sandstones are white or yellowish-brown because of limonitic coatings [17]. It was deposited in alluvial or upper coastal plain environments following a southward shift of deltaic deposition into a new depobelt. The formation thins towards the basin and pinches out near the shelf edge. Sporomorphs were identified to the lowest taxonomic level possible. Identification of fossil taxa was done by comparison with published literature such as [8] and other published albums. Photomicrographs of the most vital palynomorphs were taken at ×100 oil immersion objective. Palynomorphs were classified into five groups based on ecological and morphological affinities with modern taxa namely, pollen, spores, marine, fungi spore, and freshwater algae. Marine group made up of dinoflagellates, foraminifera inner linings and archritarch were also documented.

Materials and Methods
Chronostratigraphic inferences were deduced based on traditional graphic correlation method which is premised on the relative abundances, first appearance datum (FAD) and Last appearance datum (LAD) of the fossil palynomorphs present in the stratigraphic section to erect palynological zonations [8].
The biozonation of OSE-1 well is based on palynological zonation schemes of [9] and [8] for the Niger Delta stratigraphic successions.

Results
All the samples analyzed were fine-grained, brownish gray to light gray shales  were moderately well preserved. The stratigraphic distribution of the palynoflora was highly variable. Some palynomorphs were common (e.g Monoporites annulatus and Laevigatosporites sp) while others (e.g Foveotriletes margaritae and Belskipollis elegans) had rare occurrence. Pollen grains accounted for the highest counts, followed by spores, dinocysts, algae acritarch and fungi element which had a single count. The assemblage distribution chart and photomicrographs of some important taxa are shown in Figure 3 and Plate 1.

Phytoecological Characterization
Following [26], the total palynomorphs assemblages were categorized into five phytoecological classes since majority of the microfloras could be linked to extant plant families (see Table 2). The comparison was based on both morphological similarity and ecological affinities with modern plant relatives. The botanical affinity of some palynomorphs could, however, not be ascertained. The fossil pollen Peregrinipollis nigericus, a tricolpate, was considered to have unknown botanical affinities even though the pollen was morphologically similar  to that of modern genus Brachystegia [27]. The identified phytoecological groups were mangrove elements, freshwater swamp and water edge elements, rain forest elements, savannah elements represented mainly by Gramineae pollen, Monoporites annulatus, as well as marine elements represented by dinoflagellate cysts. Other groups were freshwater algae represented by Botryococcus braunii and fungal spore (see Table 3) The identified mangrove elements were represented by Acrostichum aureum, Zonocostites ramonae, Psilatricolporites crassus, Pachydemites diederixi, Proxapertites cursus and Verrutricolporites sp. Of these, the first two were the most dominant, accounting over 65% of the total mangrove elements, The freshwater swamp and water edge elements were characterized by Laevigatosporites sp., Retitricolporites irregularis and Striatopollis catatumbus, with Laevigatosporites accounting for more than half of the group frequency. The marine element comprised a wide spectrum of dinoflagellate cysts already listed in this work. They included Spiniferites sp. and Polysphaeridium zoharyi with a good representation of both Peridnioids (e.g. Lejeunecysta sp. and Paleocystodinium) and Gonyalacoids (e.g. Polysphaeridium zoharyi and Tuberculodinium vancamporae) genera. Savannah element was restricted to the occurrence of a single taxon Monoporites annulatus, a well known Gramineae (Poaceae) or grass pollen. Its distribution in the studied stratigraphic interval was more consistent in the upper half of the well interval, from 720 -6330 ft.
The coastal plain elements were presented by Magnastriatites howardi, Crassorettriletes vanraadshooneni, Echiperiporites estalae, and Retitricolporites sp. while the rain forest elements were Polypodiaceoisporites sp., Sapotaceoidaepollenites sp., Canthiumidites sp. and Nymphaepollis sp. [28] reported the abundant presence of Zonocostites ramonae in the mangrove communities of Venezuela with a wide geographic spread in the Early Miocene. Z. ramonae is traceable to the fossil pollen of Rhizophora, the predominant genus in

Chronostratigraphy
Palynomorphs, because of their abundant occurrence in wide spectrum of facies and rapid evolutionary changes are veritable tool for biostratigraphic and peleoecological studies (Traverse, 2007). Palynological chronostratigraphic zones were erected dependent on the identification of some key parameters of fossil palynotaxa like first appearance datum (FAD), last appearance datum (LAD), peak development and their absence. Some stratigraphic marker palynomorphs such as Crassoretitriletes vanraadshooveni, Racemonocolpies hians and Verrutricolporites rotundiporus retrieved from the well section facilitated zonation.
The studied interval from 6330 to 9010 ft was subdivided into six informal zones namely OS-1 to OS-6. The zones corresponded to P600 -P800 t0 of [9]. The recognized zones were as discussed below.  (1978). The zone is characterized by the Peak occurrence and maximum diversity assemblage of marine elements dinoflagellates cysts, including Achomosphaera sp., Paleocystodinium sp., Homotryblium plectilum and Nematosphaeropsis sp. at 8740 -8770 ft. Also, the zone was characterized by low occurrence of Monoporites annulatus. Other pollen occurring within the zone included Verrutricolporites rotundiporus, Psilatricolporites crasus and Psilatricolporites sp. [33] reported that the FAD of the Homotryblium plectilum was

Paleoclimate
The understanding of the ecology of the parent plants of fossil palynomorphs was deployed to aid identification of temporal variations in ecology and climate.
Environment of deposition and climate exert primary controls on lithofacies (e.g sandstone or shale) and well as on the composition and diversity of palynofloras. The vegetation of any province was determined largely by the geology of the area (soil and parent rocks) and climate [30]. Therefore climate exerts primary control on vegetation. in conjunction with the Gramiineae/Rhizophora ratio was knowledge of the ecological preferences of the fossil pollen parent plants, e.g. [10] and [30]. The phytoecological categorization of some of the forms encountered in this study is shown in Figure 4 and Table 2 [34]. It has been emphasized by [32], referencing [8], that increased occurrence of Gramineae pollen (represented by M. annulatus) indicated prevalence of open vegetation and development of extensive grass land in a generally dry climate with marked rainy seasons. Further evidence corroborating this inference was the presence of Polysphaeridium zoharyi within the interval. The dinocyst P. zoharyi is adapted to high salinity waters. [33] posited that Polysphaeridium zoharyi was a taxon adapted to very saline waters and therefore indicative of arid or dry climate. The freshwater fern, Laevigatosporites sp. despite having a good representation in the interval, was intermittent in its occurrence, indicating that freshwater bodies frequently dried up because of extended dry climatic conditions. The record of Botryococcus braunii in this zone was high which might suggest frequent to intermittent influx of freshwater. The algae however had been known to be adapted to even saline brackish water so that its occurrence did not necessarily indicate persistent freshwater influx into depositional site [35]. The percentage pollen recorded for the interval appeared cyclical, falling from 78% at 8920 -8950 ft to 00.00% at 8200 -8230 ft and peak- The upper half of the well, the interval 7420 -7450 ft to the highest level 6330 -6360 ft was marked by increased occurrences of both Monoporites annulatus and Z. ramonae (Figure 4). The interval was also characterized by higher frequencies and diversities of pteridophytic spores. This observation implied the persistent presence of expanded mangrove communities during the interval.
There was higher and more consistent occurrence of freshwater and water edge elements, especially Laevigatosporites sp. (Figure 3). The percentage spores or pollen/spore ratio did not vary much (Appendix 1). This interval probably   [26]. This deduction was corroborated by [33] who posited while referencing [36] that abundant records of the mangrove pollen species Zonocostites ramonae occurring in association with abundant freshwater swamp, brackish water swamp, aracaceous pollen alongside freshwater algae was indicative of wet and cold climate.

Depositional Paleoenvironment
The depositional environment was interpreted by considering the ecological af- [37] posited that the presence of poorly preserved dinocyts Spiniferites together with foraminiferal test linings indicated close proximity to marginal marine setting. The same interpretation was applicable to Zone III (8200 -8230 to 7540 -7570 ft) because of the common occurrence of abundant marine elements.
Zones II and IV fell within the intervals 8530 to 8260 ft as well as 7510 to 6330 ft. The zones were considered to be similar because of the absence of marine elements except for the minor record of one count of Tuberculodinium vancampoae and one count of Leiosphaeridia sp., an acritarch together with 2 counts of undifferentiated dinocysts. The two intervals were marked by abundant record of pollen, spores, freshwater and water edge elements including Laevigatosporites sp. there were also abundant occurrence of mangrove elements including Acrostichum aureum and Z. ramonae. The association most likely indicated sedimentation in swamps and fluvial settings. The few counts of marine element in the Zone IV might be the effect of minimal marine influence as a result of tide. [34] reported that tide-dominated delta usually developed extensive mangrove vegetations.

Conclusion
Palynological study of OSE 1 well in offshore Niger Delta Basin revealed that the stratigraphic interval contained variable low to high diversity assemblages. The  However, further studies such as foraminiferal biostratigraphy and log analysis would be needed to establish this deduction.