Palynostratigraphy and Paleoclimate of the Sequences Penetrated by Meren 31 Side Tract-2 Well , Offshore Niger Delta

Palynological study of an offshore well in the Niger delta was carried out to document the palynomorphs assemblage and subsequently date and establish paleoclimatic conditions in a portion of the Niger delta. One hundred and thirty seven cutting samples provided by Chevron Nigeria Plc were composited at 27 m interval for the study. The samples yielded forty six palynomorphs, which include Racemonocolpites hians, Zonocostites ramonae, Monoporites annulatus, Botryococcusbraunii, dinocyst Lycopodiumsporites spp., Pachydermitesdiederixi and Psilatricolporitescrassus. A Middle-Late Miocene age belonging to P800 and P700 zones of Germeraad was established for the interval studied. There were clear crests and troughs in the microfloral abundance indicative of alternations of dry and wet climatic conditions during the deposition of the Agbada Formation.


Introduction
Meren 31st-2 is one of several development wells drilled in Meren field in oil mining lease (OML)-95 of Chevron Nigeria Limited (Figure 1).The well was drilled to a total depth of 2758 m and located on Longitude 4˚50' and Latitude 5˚58' E (N 193335.92;E 273519.3) in the Western part of Niger Delta, West coast of Africa.Several publications directed towards exploration and exploitation of hydrocarbon abound on this prolific oil field.Recent publications particularly [1] on flow units, connectivity, and reservoir characterization of the field revealed that Meren field consists of complex reservoir architecture characterized by shoreface clinoforms and a history of progradation and retrogradation cycles.A high resolution sequence stratigraphic and reservoir characterization of some sand units in the field had been carried out [2].No previous investigation on miospore assemblage from Meren field has been made public.However, some miospores have been reported to occur in the Niger delta and used for both stratigraphic and climatic studies.For example Podocarpus milanjianus marked the boundary between Benin and Agbada Formations of the Niger Delta [3].Twenty nine informal palynological zonations of the Niger Delta using alphanumerical coding system [4] appear to form the background information for in-house zonal scheme of Shell Petroleum Development Company.Using detailed morphological analysis of pollen groups which were assigned to the genera Praedapollis, Striamonocolpites, Arecipites, Spirosyncolpites, Racemonocolpites, Verrutricolpites, Retibrevitricolporites and Belskipollis, a refinement of the palynostratigraphic zonation and correlation of the Neogen of the Niger Delta was established [5].The pollen grains of a 36 m deep core from Niger Delta were analyzed [6] to reach a conclusion that in the Quaternary, there were shifts in the extent of rain forest and savanna with the latter replacing the formal.Paleoenvironmental analysis of a section Niger Delta Basin (Ogbomotoru-1 well) (Figure 1) was used to infer transitional environment of deposition with marine interference based on the abundance of Zonocostrites ramonae, and occurrence of Crassoretitriletes vanraadshooveni, Magnasriatites bowardii and Pachydermites diederixi savannah [7].
The primary focus of this work is to establish the palynomorph contents, their palaecology and paleoclimate as well as the environment of deposition of the strata penetrated by Meren 31st-2.It is envisaged that the result shall add to information on the miospores recoverable from the western portion of the Niger delta and assist in the ongoing refining of the zonal scheme for the Niger delta.

Geological Setting
The Niger delta is the most significant hydrocarbon province on the West African continental margin.It started to evolve in Eocene times and deposition is still ongoing offshore.Three main formations have been recognized in the subsurface of the Delta [8]- [13].These are the Benin, Agbada and Akata Formations.These formations were deposited in continental, transitional and marine environments, respectively; together they form a thick, overall progradational passive-margin wedge [2].
The Akata Formation is the basal unit composed mainly of marine shales believed to be the main source rock within the basin.The Agbada Formation is made up of alternating sandstone, siltstone and shale sequences that constitute the petroleum reservoirs of the basin.On the other hand, the Benin Formation largely consists of non-marine sands with a few shaly intercalations [2].

Materials and Methods
A total of one hundred and thirty seven ditch cutting samples (which were composited to forty six samples) used for this study was provided by Chevron Nigeria Plc.From each sample, about 25 gm was weighed and thoroughly washed/cleaned.The pretreatment of the samples with various acid combinations include removal of unwanted carbonate material by washing with 10 ml diluted hydrochloric acid as well as further treating the residue with 60% hydrofluoric acid and boiling hydrochloric to dissolve/remove all silicates and silicofluoride gel respectively.The ultrasonic centrifuge machine was used to further separate out the dissolved material (i.e.dirt, clay, mud etc.) from the organic matter residue for 2 minutes.Subsequently, three drops of Canada balsam in Xylene on the slide and mixed thoroughly allow for proper mixing and then pipette into a cover slip glass slide on top of the hot plate until dryness and was ready for palynological microscopic evaluation.
The prepared slides for palynomorphs were studied under DP12 Olympus microscope, identification and description were attempted for as many palynomorphs as possible using relevant literature.Photographing of the well preserved palynomorphs was done using attached DP12 Olympus microscope camera.

Results
The stratigraphic distribution of fifty-six (56) pollen and spore species used for the palynological biozonation and palaeoclimatic interpretation of Meren 31 is shown in Figure 2 and Table 1, Table 2, Table 3, and Table 4 1).Common records of microforaminiferal wall linings, dinoflagellate cysts and fresh water algae Botryococcus braunii were occasionally recorded within the well section (Table 2).This assemblage is indicative of shallow marine environment of deposition.The ratio of Monoporites annulatus to Zonocopotites romanae is shown in Table 3, while the numerical and percentage of pollens and spores are shown in 4. Table 5 is a summary of the biozonaton proposed for the sequences encountered in the studied well.The distribution chart of all the palynomorphs encountered in the studied well and the inferences drawn from them is as displayed in Figure 2.   Diagnosis: The top of this subzone is marked by the top regular occurrence of Racemocolpites hians at 1698 m while the base is defined by the quantitative top occurrence of Verrutricolporites rotundiporus at 2082 m.This subzone is further confirmed by the regular and abundant records of Verrutricolporites rotundiporus, Psilatricolporites crassus and Acrostichum aureum.

Systematic
(iii) Subzone P770 Interval: 2082 m -2758 m Age: Middle Miocene Diagnosis: This is last and oldest subzone recognized in Meren-31st-2 well.The top of this subzone was defined by the quantitative top occurrence of Verrutricolporites rotundiporus.

Palaeoclimate
The climatic fluctuation in Meren 31st-2 well is discussed according to the "floral" zones.
Depth 2758 m -2082 m At the total depth of the well (2758 m), there was a very low to zero percentage of grass pollen, Monoporites annulatus, with an observed increase in the percentage ratio of Monoporites annulatus to Zonocostites ramonae from 0% to about 7.1% from 2713 m up to 2576 m and peaked at 2219 m (about 28.6%) (Table 3).This increase was accompanied by a rather more consistent record of mangrove pollen, Zonocostites ramonae, with sharp increase in numerical quantities of this wet climate indicator at 2356 m and between 2493 m and 2521 m where (it)Cyperaceapolis spp., Pteris attained the highest quantitative occurrence within this "Floral" zone with 65 counts (Table 3).Within this zone, the rare occurrence of marine element, dinocyst Lycopodiumsporites spp was also recovered (Table 3).A wet condition would have been suggested for this zone due to the regular occurrence of mangrove pollen and abundance of fresh water alga, Botryococcus braunii along with brackish water swamp species, Pachydermites diederixi and Psilatricolporites crassus, which were abundant within this zone coupled with the rare occurrence of Gramineae pollen, but for the rare record of Cyperraceaepollis spp., which is a humid climate indicator [23] [24].The zone was probably experiencing a humid climate during this time.According to [25], the abundance of Rhizophora pollen above 40% (Table 4) in sediments indicates a good representation of mangrove swamp, suggesting a humid tropical lowland climate during the deposition of the mentioned interval.Some of the savannah pollens recorded in the above named zone include Corylus spp., etc while the wet climate indicators are freshwater swamp rainforest species like Sapotaceae, Verrutricosporite rotundiporus, Gemmamonoporites spp., Striatricolpites catatumbus and a few others.The presence of the dinocyst species, Lycopodiumsporites spp suggests sediment deposition in a shallow marine environment with frequent freshwater incursions and lowered sea levels following glacial maxima [26].

Depth 2082 m -1698 m
There was a sharp quantitative (numerical) increase in mangrove pollen Zonocostites ramonae from about 65 counts at 2493 m to 520 counts at 1698 m, 250 counts at 1862 m and 138 counts at 1807 m, with a gentle downhole increase in Monoporites annulatus (Table 3) within this zone.This observation could be hardly visible when considering it in terms of percentage composition of the total mangrove pollen and Gramineae pollen, where Zonocostites ramonae attained 100% in some depth within this floral zone (Table 3).There were also noticeable fluctuations in the occurrence of ferns, Laevigatosporites spp., Verrucatosporites spp.and Stereiosporites spp.within this interval.
This zone is probably a dry climate phase with warm temperatures.In a study by [27] in SW Turkey high percentages of Gramineae pollen types in sediment was taken to point to drier local conditions.A noticeable presence of savanna species Echistephanoporites echinatus, further confirms the prevalence of a dry climate within this interval.Other important savanna species occurring within this zone include Concentricytis spp., Cyperaceaepollis spp., Corylus spp., Pteris spp.and more.However, a few rainforest/fresh water swamp species were recorded within this zone.These include Canthium spp., Retibrevitricolporites spp., Psilatricolporites onitshaensis etc., (Table 1) and a rare to common record of Botryococcus braunii.The occurrence of small quantities of mangrove pollen in this zone was probably due to minor local short-lived transgressions of the sea, thereby allowing only very limited extension of mangrove vegetation.
Depth 1698 m -1524 m By the time of deposition of the sequences within this interval, the pollen record indicates that mangrove swamp forest vegetation was now well established, with the quantitative occurrence of Zonocostites ramonae reaching its highest numerical value (520 counts) at 1698 m (Table 3).So also was a high occurrence of fresh water swamp species, Sapotaceae, Retitricolporites irregularis, Retibrevitricolporites protudens etc., with rare occurrences of other fresh water sporomorphs of Verrutricolporites rotundiporus, Gemmamonoporites spp., Verrutricolporites microporus, Magnastriatites howandi etc.Also recorded were rare to common occurrences of brackish water swamp species of Pachydermites diederixi and Psilatricolporites crassus and fresh water algae, Botryococcus braunii.It is worthy to note the increase in abundance of pteridophytes (fern spores), Laevigatosporites spp., Verrucatosporites spp.and Stereiosporites spp.coupled with an almost consistent occurrence of Acrostichum aureum and rare occurrences of lowland rainforest species like Nympheaepollis clarus, Echistephanoporites echinatus, Racemonocolpites hians, Canthium spp., etc. (Table 1 and Figure 2).There is also an overall increase in the recorded quantities of grass pollen, Monoporites annulatus, both numerically and in ratio to mangrove pollen.There was a visible common to abundant occurrences of Corylus spp., Cyperaceaepollis spp.and Pteris spp., and rare to common occurrences of Ctenolophondites costatus, Charred Gramineae cuticle, Chenophondipollis spp., Elaeis guineensis, Numulipollis neogenicus, Peregrinipollis nigericus etc., which are all savanna species (Table 1).
An initial rise in sea level with the mangrove vegetation increasing in extent is suggested for this zone.The fluctuations in percentage occurrence of Zonocostites ramonae may probably be a result of variations in the intensity and extent of the tidal streams thereby causing fluctuations in the extent of mangrove forest.This rise and fall of the tides may also bring about drier conditions resulting in a reduction of forest vegetation and subsequently promoting expansion of the savanna [26].This agrees with the reports of [23] which revealed cyclic fluctuations in the vegetation and continental climatic condition of North-West Africa in the Pliocene, when river discharge ceased, wind transport of pollen grains prevailed over fluvial transport.
Based on the top regular occurrence of Racemonocolpites hians and Racemocolpites hians, the P820 and P780 subzone, respectively were recognized within the section of the well studied.The top of subzone P770 was defined by the quantitative top occurrence of Verrutricolporites rotundiporus, which enabled assignment of middle Miocene to the deepest part of the well.
Paleobathymetric analysis based on microfloras indicated that the majority of the spores and pollen species showed minor variations in abundance and diversity within the studied section of the well.Abundant occurrence of Monoporites annulatus in some sections of the well enabled delineation of the sequences that were deposited under wet conditions during the Miocene.In most cases, such interval is characterized by the dominant occurrence of pollen grains of Rhizophora spp., Zonocostites ramonae, and spores of pteridophytes and fungi.The sequences within interval 2082 m -1698 m were probably deposited under a dry climatic phase with warm temperature based on the occurrence of Echistephanoporites echinatus Concentricytis spp., Cyperaceaepollis spp., Corylus spp., and Pteris spp.