Calcareous Nannofossil Biostratigraphy of A , B , C , D Wells , Offshore Niger Delta , Nigeria

The calcareous nannofossil biostratigraphy of four deep water wells, A, B, C, D, offshore Niger delta has been studied in order to document their biostratigraphic distribution, establish biozonation and stratigraphic correlation. Ditch cutting samples were collected at 30 ft intervals. They were processed according to the standard calcareous nannofossil sample preparation technique. A total of forty-two nannofossil species were identified in the four wells. Most parts of the wells are characterized by abundant and diverse nannofossil assemblages which permitted the subdivision of the sections into zones. A total of five zones and two subzones were identified from this study based on diagnostic marker species and notable nannofossil events. The zones are Catinaster coalitus zone (NN8), Discoaster hamatus (NN9), Discoaster loeblichi which occupy the base of Discoaster berggrenii and the top of Discoaster bollii was used to identify the NN10 zone. The main marker for this zone which is Discoaster calcaris is missing in the study wells; this may be due to dissolution of the calcitic wall during sedimentation process. Other zones encountered in this study are Discoaster berggrenii subzone (NN11a), Discoaster quinqueramus subzone (NN11b), Amaurolithus tricorniculatus zone (NN12) and Ceratolithus rugosus zone (NN13). The age of the strata ranges from the middle Miocene to the early Pliocene. The zones were based on the first and last appearances of maker species as well as their relative abundances. The occurrence of Amaurolithus delicatus in wells C and B implies that the well intervals containing the species cannot be younger than early Pliocene age. The four wells show good correlation with each other based on the identified zones. The biozones identified in this study will be useful for subdivision and correlation work in the deep offshore Niger delta Neogene sequence.


Introduction
As the world class hydrocarbon matures, most of its subsurface uncertainties lay at reservoir scale.The need for well site biostratigrapher to monitor the drilling and correlate the strata is essential to avoid abortive and unprofitable exercise.Calcareous Nannofossil therefore is a major tool used by the biostratigrapher in the characterization of the reservoir strata and correlation in the well site operation.The importance of biostratigraphic correlation is critical in the construction of accurate time slice and play fairway analysis in Petroleum exploration.The aim of this study is to document the distribution of calcareous nannofossils, determine their Biostratigraphy and delineate biozones, correlate the wells and interpret their geological history.

Location of Study Area and Geology
The wells are located in the offshore Niger Delta, Nigeria, which is situated in the Gulf of Guinea.(Figure 1) Wells D and C are located in the western arm of the Charcot fracture zone, while Wells B and A are located in the eastern section of the Charcot fracture zone.The materials used for this study were obtained from deep offshore field owned by Shell Nigeria Exploration Petroleum Company (SNEPCo), Lagos.
The Niger Delta is subdivided into three diachronous lithostratigraphic units.These are the Benin Formation.(mostly continental sands), the Agbada Formation (alternation of sands and shales) and the Akata Formation.While 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.The Agbada Formation was penetrated in the middle Miocene to late Pliocene sequences of the four wells studied, piercing through the mobile shale, mud diapir and channelized turbidites.

Methodology
The materials include soft and paper copies of seismic sections and base map, ditch cutting samples from four wells A, B, C and D and digital well log data.A total of 453 ditch cuttings were analysed for wells A, B, C and D. Eighty-five and 164 ditch cutting samples from the intervals of 4900-9920 ft. and 4530-14600 ft. of wells A and well B respectively, while 92 and 112 samples from the intervals of 5760-11400 ft. and 6000-12750 ft. were analysed for wells C and D respectively.These wells were logged and sampled at 30 ft. intervals and processed for nannofossil analysis using the standard smear slide preparation method.The prepared slides were examined under transmitted light microscope.Detailed identifications (to species level where possible) were made.Fossils were recorded in the analysis sheet with other relevant information.The species name and abundance of each species with depths were used as input data into the Stratabug biostratigraphic software.The data were utilized and integrated in the age dating of critical horizons.From this data set, plots of population abundance and species diversity were made using the prepared checklists.The coincidence of these peaks guided the selection of the candidate maximum flooding surfaces, the positions of which were subsequently confirmed on the log.

Discussion of Results
Calcareous nannofossils species and abundance have been analysed and interpreted based on the assemblages of diagnostic species and notable biostratigraphic events in the A, B, C and D wells.(Figures 10-13) A total of five zones and two subzones were identified from this study based on the assemblages of diagnostic species and notable nannofossil events namely: Ceratolithus rugosus zone (NN13) (CN10c), Amaurolithus tricorniculatus zone (NN12) (CN10a-CN10b) (duly represented by Amaurolithus delicatus), Discoaster quinqueramus subzone (NN11a) (CN 9b ), Discoaster berggrenii subzone (NN11b) (CN 9a) Discoaster calcaris zone (the missing marker species represented by Discoaster loeblichii) (NN10) (CN 8a-8b), Discoaster hamatus zone (NN9) (CN7a-7b), and Catinaster coalitus zone (NN8) (CN6), (Tables 1-4).The missing marker species might be as a result of dissolution of the skeletal calcite or sedimentation processes.The four wells correlate reasonably, as they show similarities in their nannofossil species (Figures 6-9).It was observed in the correlation of the biozones in the well sections that the wells on the eastern wing of the Charcot fracture zone are down thrown with a drop of about 2,490 ft to 4,350 ft.(830 to 1318 meters) relative to the wells on the western wing of the fractured zone (Figure 9).The biozonation and correlation of the wells A, B, C, and D have helped to subdivide the deep offshore Niger delta Neogene sequence into easily recognizable biostratigraphic units that will enhance prospectively of hydrocarbons.The relative abundance and short stratigraphic ranges of nannofossils make them an excellent group for the biostratigraphic subdivision of the Mesozoic and Cenozoic strata (Figures 2-6).Their planktic habit and thus relatively rapid dispersal over large areas enhance their usefulness as tools for regional and inter-regional correlations.Zoning and correlation were established in A, B, C, and D wells.The wells were zoned using the standard nannofossil zonation schemes.These are the NN (Neogene Nannofossils) zones of Martini (1971)

Figure 1 .
Figure 1.Simplified geological map of the study area in the Niger Delta (KEY: A -Well A; B -Well B; C -Well C; D -Well D and the CN (Calcareous Nannofossil) zones of Okada and Bukry (1980).Based on the work of Berggren et al. (1995), absolute ages have been assigned to important bioevents.The zones encountered in this study ranged from the Middle Miocene to the Early Pliocene.Well A lies within the Late Miocene while Wells B, C, and D penetrated strata from Middle Miocene to Early Pliocene age.Catinaster coalitus zone of Middle Miocene represents the NN8 (CN6) zone of Martini (1971), it occurs only in Well C. The Discoaster hamatus zone NN9 (CN7a-7b) occurs across the studied wells representing the Middle/Late Miocene.The occurrence of Discoaster leoblichii and the top occurrence of Discoaster bollii were used to mark the NN10 (CN 8a-8b) zone in place of the zonal marker species, Discoaster calcaris which was missing.Discoaster berggrenii, NN11a (CN9a) subzone and Discoaster quinqueramus NN11b (CN9b) sub zone are the marker species for the Late Miocene.All the above named zones fall within the Middle Miocene to Late Miocene.The last zones encountered in this study are the Amaurolithus tricorniculatus zone, which has been represented by Amaurolithus delicatus in Wells B and C. Ceratolithus rugosus zone was identified in Well C where it has been represented by Ceratolithus acutus.These zones collectively fall within the NN12 and NN13 (CN10c) representing the Early Pliocene.

Table 1 .
Nannofossils biozonation of Well D

Table 2 .
Nannofossils biozonation of Well B

Table 3 .
Nannofossils biozonation of Well A

Table 4 .
Nannofossils biozonation of Well C