Paleoclimatic Cycles, Sea Level History and Sequence Stratigraphic Elements in Eocene–Oligocene Sediments of BIMOL-1 Well Northern Niger Delta Basin, Nigeria

: Lithofacies succession and palynomorph data trends in BIMOL-1 well in the north-western Niger Delta Basin have been investigated in order to unravel paleoclimatic influence on paleo-sealevel change and facies evolution. Eight lithologic cycles composed of sand and shale were identified. Miospore speciation revealed forty two miospore form species and ten dinoflagellate cyst species. Miospore and dinocyst trends revealed six climate driven sea level cycles that influenced sedimentation and facies distribution and characteristics. Miospore age determination of the succession revealed L-Eocene to L-Oligocene age range. Nine wet and eight dry climate driven transgressive and regressive events were identified, corresponding to sea level rise and to sea level fall correspondingly. Dry climate occasioned continental progradation that generated thick sand intervals, while wet climate triggered sea level rise, generating thinner sand bodies as transgressive sand reservoirs. Seven high stands (HSTs), eight transgressive (TSTs) and eight lowstand (LSTs) systems tracts, distributed within nine sequences were identified. Candidate MFSs, the 50.0 Ma, 48.9 Ma, 46.1 Ma, 43.2 Ma, 41.0 Ma, 34.0 Ma MFSs were identified. Candidate SBs identified include the 50.7 Ma, 48.4 Ma, 47.2 Ma, 44.4 Ma, 42.7 Ma, 40.1 Ma and 32.4 Ma SBs. Early Rupelian sequence boundaries were identified. Erosion/non-deposition of the Priabonian and parts of the Bartonian stage were revealed that inferred erosion/non-deposition of about 7 Ma of sediments in the well area. A synthesis of results of the evaluated proxies revealed that Paleoclimate-driven sea level and paleovegetation trends acted as key facies generators in the well area.

Ancient sedimentary successions host a variety of records which may be important in unravelling past sea level and climatic fluctuations.Eocene-Oligocene sediments of the Niger Delta Basin are known to register varied sea level regimes reflected in the cyclic lithofacies pattern of the formational units (Reijers, 2011).The abundance and presence of palynotaxa preserved in sediments has been used as paleoclimatic and paleocenographic proxies for many decades (Jan du Chene and Adediran, 1984;Gregory and Hart, 1992;Lucas, 1992;Elsik, and Yancey, 2000;Adeonipekun, et al., 2012) and the influence of climate on paleovegetation cyclicity and sea level change is well documented (Van Der Hammen, 1957, 1961;Rull and Poumot, 1997;Rull, 2000).Studies have shown that paleoclimatic trends and fluctuations can be revealed by biosignals stored in sedimentary successions.The study of palynotaxa provides beyond age determination, fundamental contributions of our understanding of ancient climatic regimes.The biogeography and diversity of palynotaxa through geologic time varied depending on latitudinal zonation as well as climatic dynamics and variability (McIntyre and Bè, 1967;Findlay and Giraudeau, 2000;Boecked and Baumann, 2008).Changing climate as experienced in the present is observed to drive vegetation trends and global sea level change and patterns.The imprint of this process generates various sedimentary signatures registered as facies depending on the location where such process operates.Tertiary sediments of the Niger Delta Basin display various lithofacies components distributed within different lithostratigraphic units (Short and Stauble, 1967;Avbovbo, 1978, etc.), known to be products of oscillating sea level regimes through geologic time (Reijers, 2011).Several studies on the palynology, sedimentology, paleodepositional environment and sequence stratigraphy etc., on the Niger Delta have been carried out to contribute to the knowledge base of the geology and history of the Delta, but few have been directed towards paleoclimatic and paleooceanographic factors that can be related to sequence stratigraphic framework.In this study, we seek to use biosignals to unearth paleoclimatic cycles locked in Eocene-Oligocene sediments of the Niger Delta accessed through well cutting samples retrieved from BIMOL-1 well in the northern Niger Delta Basin.This study is aimed at investigating the paleoclimatic cycles, sea level history and sequence stratigraphic elements in Eocene -Oligocene sediments of BIMOL-1 well northern Niger Delta basin, Nigeria.It is envisaged that the cycles can be related to and used to distinguish sedimentary packages that would reflect systems tracts.In the Niger Delta sedimentary pile, individual sea-level cycles are reflected in the various sedimentary sequences and interferences of cycles with different periods result in megasequences that are chronostratigraphically confined and sedimentologically characterised (Reijers, 2011).(Allen, 1965;Whiteman, 1982) Geological Setting of the Niger Delta Basin: The Niger Delta Basin (Fig. 1) stands as one of the most prolific hydrocarbon provinces in the African continent.With a clastic wedge of just over 10 km thick, (Fig. 2), a very high concentration of petroleum per unit volume of rock is key note.known estimates of the ultimate recoverable hydrocarbons from the Niger Delta province range from 35 x 10 9 BBL of oil and 120 x 10 12 SCF of gas (Ekweozor and Daukoru, 1994) to 66 x10 9 BBL of oil equivalent (BOE; Saugy and Eyer 2003).For many decades, exploration and production has been concentrated mostly on land or in shallow water, although since 1996 licensing rounds, major deep-water discoveries, of the order of 10 9 BOE, have been made (Agbami, Bonga, Bonga SW, Chota, Erha, Usan, Ukot, Zafiro, Ikija, Etan and Bobo).The sedimentary pile of the Niger Delta Basin has built out over the African Atlantic continental margin and adjacent oceanic crust since Eocene times (Evamy et al., 1978).The sedimentary pile of the basin is subdivided into a three-fold age diachronous stratigraphic subdivision, into the marine Akata, shallow marine Agbada and delta plain Benin formations, which reflects the distinguishing sedimentary environments of a regressive megasequence (Short and Stauble, 1967;Doust and Omatsola 1989;Morgan 2003).The Akata Formation which consists of parallellaminated mud has formed in deep water and, basinfloor, pro-delta setting.Generally the Akata Formation is viewed as the main source sediments for hydrocarbons in the Niger Delta.It is 3-4 km thick and overpressured (Short and Stauble, 1967;Doust and Omatsola 1989;Haack et al., 2000).The Agbada Formation consists of mixed clastic sediment, and is about 3 km thick or more.The Agbada Formation was formed in a paralic environment.Sediments of the Agbada Formation act as the main reservoir rocks for hydrocarbons generated in the basin.Capping the delta is the Benin Formation which is largely continental and fluvial in origin and consists of mainly sands.

MATERIALS AND METHODS
Sampling: A total of fifty non-composited ditch cuttings of sand and shale from a near-surface depth of 486 -3216 m of the BIMOL-1 well located in the western reaches of the north-western area of the Niger Delta Basin (Fig. 1), were subjected to whole rock visual grain textural analyses using stereomicroscopic description to determine gross grain characteristic (mineralogy, morphology, sorting, presence of accessory materials, size distribution, and colour).The well under study belongs by Shell Petroleum Development Company (SPDC), but coded BIMOL-1well in this study for confidentiality reasons.For lithofacies description, a sampling range of 18 -78 m (av.= 56.5 m) for the upper section of the well (depth range of 486 -648 m), a sampling range of 18 -108 m (av.= 66.71 m) for the mid-section (depth range of 828 -1674 m) and a sampling range of 18 -84 m with an average of 52.50 m for the lower section (1782 -3216 m) of the well was achieved, with a general sampling range of 48.82 m for the well.

Palynological sample Preparation and Analysis:
Thirty five (35) samples from the well section were subjected to series of acid treatment for palynological slide and analysis in accordance with techniques described by Traverse (1988), while age determination is based on earlier works by Muller (1959), Gemeraad (1968), Adegoke (1969), Legoux (1978), andSowumi (1981b).

RESULTS AND DISCURSION
Lithostratigraphy: Whole rock lithologic sample description revealed eight distinct lithologic units/successions in the well section as detailed below (Fig. 3  The palynological constituents recovered from the well were initially classified according to their biological origin as continental and marine forms and further sub-classified into Miospore, fungi which together constitute the allochthonous forms and dinocyst (Fig. 5) which makes up the authochthonous fraction.Quantitative miospore counts in the well ranged from thirty seven (37) to one thousand four and sixty nine (1469), while dinoflagellate cyst species count ranged from one (i) to forty nine (49) at various depth intervals and complete absence in some depth intervals.Quantitative depth distribution of the various form species are presented in Table 1.A total of forty two (42) miospore species and ten (10) dinoflagellate cyst species were identified in this study.

Sea level history, Paleoclimate and Climatic cycle:
The integration of sedimentological and palynological results from the well enabled valid inferences to be made regarding the age of the sediments, changes in sea level through time and paleoclimatic regime.From graphical analysis of miospore and dinocyst abundance (Fig. 4), inferences concerning sea level history, paleoclimatic changes and cycles were made.This analysis revealed a climate driven cyclic sea level pattern that influenced sedimentation and facies distribution and characteristics.
Sea Level History And Paleoclimatic Changes: Seal level history which is the record of the rise and fall of global sea level above and below its mean datum in past geologic times and driven by paleoclimatic indices which refers to past climatic conditions that prevailed in a given region is defined in this work.
The importance of this record is seen as a rise in sea level is marked by transgression of the sea, during which a landward shoreline shift is recorded and vice versa, thus important in erecting sequence stratigraphic framework for sedimentary columns.
Clues about sea level history and paleoclimate in the well area have been obtained from miospore and dinocysts abundance as proxy indicators, such that a climate driven rise in sea level is marked by wet climatic conditions leading to lush vegetation and high preservation of forms, while the reverse is obtained for seal level fall.
The result of the graphical analysis of the plot of miospore and dinocysts quantitative signatures with depth (Fig. 4) reveals changes in the paleo sea level and paleoclimate in the well section in which sea level rise alternates with seal level fall at various depths, revealing a cyclic pattern.In this signature, a rise and fall in sea level corresponds to wet and dry climatic conditions respectively (Table 2, Fig. 7).An age evaluation of these forms revealed a Lower Eocene to Lower Oligocene age range for the sediments as detailed below: Lower Eocene 3216 -2832 m: The low abundance and subsequent disappearance of Cyathidites sp., at this depth interval indicate Lower Eocene.This age is also confirmed by the interval being immediately below the first appearance datum of Middle Eocene markers -Cleistophollis patens and Pachydermites diederixi (Fig. 6).Other forms that occur within this interval include Zonocostites ramonae, Laevigatosporites sp. and Monolete spores.
Middle Eocene 2832 -1782 m: The interval is characterized by the first appearance of Cleistophollis patens, Praedopollis Africans and Pachydermites diederixi (Fig. 6), which indicate Middle Eocene (Legoux, 1978).The occurrence of Grimsdalea polygonalis within this interval further confirms s this age.Other palynomorphs that characterize this interval include Retibrevitricolporites obodoensis, R. triangulates, Retitricolporites irregularis, and Laevigatosporites sp.These forms, according to Adegoke, (1969) are common in the Middle Eocene but not endemic to it.(Reijers, 2011), include the 50.0Ma Late Ypresian MFS which marks the close of the Ypresian stage; the 48.9 Ma, 46.1Ma and 43.2 Ma MFSs of Lutetian stage (Figs.7 and 8).Others include the 41.0 Ma Bartonian and the 34.0 Ma Rupelian MFSs.The Priabonian and parts of the Bartonian stage in the well seems to have experienced some form of erosion/nondeposition.This observation is made visible by the systems tracts and the age of the sediments formed within that section of the well, as four maximum flooding surfaces (MFS 39.4,38.6,36.8 and 35.9 Ma) are absent in the well section, thus indicates the nonexistence of the upper Bartonian and the entire Priabonian stage of the Late Eocene which accounts for the erosion/non-deposition of about 7 Ma of sediments in the well area and probably marking a major sequence boundary dated 40.1 Ma in this well (Figs.7 and 8).Seven candidate sequence boundaries (SB) dated 50.7 Ma and of Ypresian stage, 48.4 Ma of Early Lutetian, 47.2 Ma and 44.4 Ma of Mid Lutetian, and 42.7 Ma Late Lutetian were recognized (Figs. 7  and 8).Others include the 40.1 Ma Bartonian and the 32.4 Ma Early Rupelian sequence boundaries.

Conclusions:
Palynotaxa record reveals paleovegetational trends which reflect paleoclimatic records and consequent sea level cycles.These correlated with paleovegetational change signatures, indicating that paleovegetational change is occasioned by paleoclimatic change in ancient geologic history that correspondingly affected sea level cycles.Acting as key facies generating elements, in that the thicker sand intervals/successions are closely related to dry climatic periods (sea level fell) and progradation which heralded the sedimentation pattern and vice versa.Sea level trends and climatic conditions correlated with different systems tracts depending on the climatic regime, thus this technique would enable tracking of significant events in the sedimentary history.

Fig. 2 :
Fig. 2: Schematic subsurface cross section showing the main stratigraphic units of the Niger Delta Basin (afterEkweozor and Okoye, 1980) ): Unit 1 (3216 -2970 m): This unit is composed of 246m thick black fossiliferous fissile shale and forms the basal unit of the succession.Unit 2 (2970 -2856 m): This unit is composed of 114m thick subangular to subrounded coarse grained sand, and unconformably overlies Unit 1. Unit 3 (2856 -2574 m): This sequence is composed of about 282m of black carbonaceous fossiliferous shale.Unit 4 (2574 -2484 m): This interval is composed of subrounded -rounded medium to coarse grained sand, sandwiched between thick shale of unit 3 below and unit 5 above.Unit 5 (2484 -1674 m): This unit is composed of 800m of black fossiliferous fissile shale.This unit occur as the thickest unit in the well section.Unit 6 (1674 -1404 m): This interval is composed of 730m thick subangular -rounded medium to coarse grained sand unconformably overlying shale of unit 5. Unit 7 (1404 -648 m): This unit is characterized by 756m of shale and sand interbeds.The shale interbeds are fossiliferous and displays medium the dark grey colour.The sand interbeds are characteristically composed of subangular -rounded very fine to medium grained sand.Unit 8 (648 -486 m): this unit caps the well section and is also composed of sand and shale interbeds.It is distinguished as a different unit based on the morphological characteristics of the sand components which displays subrounded to well-rounded fine to medium grains and light to medium grey shale interbeds.

Fig. 3 :
Fig. 3: Lithologic of BIMOL-1 well showing the two main lithologies, sand and shale, in the sections.QuantitativeDistribution of Palynological Constituents:The palynological constituents recovered from the well were initially classified according to their biological origin as continental and marine forms and further sub-classified into Miospore, fungi which together constitute the allochthonous forms and dinocyst (Fig.5) which makes up the authochthonous fraction.Quantitative miospore counts in the well ranged from thirty seven (37) to one

Fig. 7 :
Fig. 7: Quantitative Plots of miospore and dinocyst reflecting changes in sea level, paleoclimate, paleoenvironment and sequence stratigraphic elements recognized in the well section

Table 1 :
Quantitative depth distribution of miospores and

Table 2 :
Sea level and climatic index versus depth recognized in the well section

Depth (m) Sea Level History
The geologic age of the sediments was determined by index forms identified among the many pollen and spores recovered from the well.These include: Cleistopholis patens, Pachydermites diederixi, Grimsdalea polygonalis, Grimsdalea magnaclavata, Cyathidites sp., Verrucatosporites sp., PaleoclimaticCycles, Sea Level History and Sequence…..Twenty three systems tracts made up of seven high stands and eight transgressive and eight lowstand systems tracts, distributed within nine sequences have been identified.Six candidate maximum flooding surfaces (MFS) based on age dates defined by age significant palynotaxa and constrained with the Shell Company in Nigeria (SCiN) Chronochart