Neogene Deep Water Agglutinated Foraminiferal Biostratigraphy and Biozonation in Niger Delta, Nigeria

This study entails the biostratigraphy of benthic agglutinating foraminifera of four wells in the deep water of the Niger Delta. A total of 453 ditch cuttings were analysed for wells A, B, C and D. Eighty-five and 164 ditch cuttings from the intervals of 4900–9920 ft. and 4530–14600 ft. of wells A and well B respectively, while 92 and 112 cuttings from 5760–11400 ft. and 6000–12750 ft. were analysed in wells C and D respectively. These samples were processed and analysed at 60 feet intervals. . The analysed sections of the wells are composed of shale, mudstones and sand deposited in the upper to lower bathyal environments. The agglutinating foraminifera recovered were fairly well preserved. Total foraminifera percentages recovered from wells A, B, C and D wells are 31%, 34%, 24% and 27% respectively. Marker species were identified and used to erect biozones. The following six agglutinated foraminiferal zones beginning from the oldest have been proposed, Eggerella scrabra Taxon Range Zone, and Ammobaculites strathearnensis–Eggerella scabra, Haplophragmoides narivaensis Eggerella scabra, Cyclammina cancellata–Ammobaculites strathearnensis, Cyclammina complanata–Glomospira gordiales, Cyclammina cf. minima–Ammobaculites strathearnensis Interval Range Zones. The agglutinated foraminifera assemblages were calibrated to the geologic timeframe by means of planktic foraminifera and calcareous nannofossils. The studied wells penetrated a sedimentary succession of Late Miocene to Early Pliocene age. Taxonomic notes on some of the observed species have also been provided. Citation: Ajayi EO, Agboneni OE (2016) Neogene Deep Water Agglutinated Foraminiferal Biostratigraphy and Biozonation in Niger Delta, Nigeria. J Earth Sci Clim Change 7: 371. doi: 10.4172/2157-7617.1000371


Introduction
Hydrocarbon exploration and production activities in the Niger delta which dates back to over five decades were mainly in the Eocene-Miocene onshore and shallow offshore sequences of the delta. But with the maturation of the onshore fields and availability of new technologies in the last decade, exploration and production of hydrocarbons have shifted to the deep offshore of the delta.
Deep water agglutinated foraminifera (DWAF) represent one of the most useful groups of microfossils for stratigraphical and paleoenvironmental interpretations of both high and low latitude areas [18][19][20][21]. They represent the most commonly recovered benthic organisms from strata deposited beneath the lysocline. DWAF find common applications for biostratigraphic work in the Deep Sea Drilling Project of the ocean basins and the Circum -Atlantic hydrocarbon basins [22,23] The main objectives of this work are to undertake the benthic agglutinated foraminiferal biostratigraphy and identify the biozones from four wells in the deep water of the Niger Delta of Nigeria ( Figure 1).

Geological setting
The Tertiary Niger Delta is located in the Gulf of Guinea on the west coast of central Africa. It lies between latitude 4° and 6°, and longitude 3° and 9° E in the southern part of Nigeria. The Niger Delta covers an area of about 7500 km 2 and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 9,000 to 12,000 meters [24].
Three main formation names have been assigned to correspond to the tripartite subsurface sequence of the Niger delta ( Figure 2). These are the Akata, Agbada and Benin Formations (Short and Stauble, 1967). The Akata Formation lies at the base of the deltaic sequence; the lithofacies is composed of shales, clays and silts with a few streaks of sand and sandstones possibly of turbidite origin. It is believed that the shale of this formation is the source rock for hydrocarbon in the basin and the shale outcrop offshore in diapers along the continental slope. This formation is overlain by the Agbada Formation which consists of alternations of sandstone and shale which forms the hydrocarbon prospective sequence of the Niger delta. The sandstones serve as hydrocarbon reservoirs and the shale contributing as source rocks and the seal. The Agbada Formation is underlain by The Benin Formation is composed predominantly of sand and sandstones with few shale intercalations. The overall sequence is strongly diachronous in the delta. The environment of deposition has been found to be of delta front, delta topset and fluvio detaic. Some parts of the delta experienced erosional truncation and resedimentation [25] The Agbada Formation was penetrated in the late Miocene to early Pliocene sequences of the four wells studied, piercing through the mobile shale, mud diapir and channelized turbidites.

Method of Study
A total of 453 ditch cuttings were analysed for the wells A, B, C

Results and Discussion
Forty-six agglutinating benthic foraminiferal species were identified from the four wells in this study. The foraminiferal assemblages were fair to moderately well-preserved. Six benthic agglutinating foraminiferal biozones were recognized based on their stratigraphic distribution in the four studied wells (Figures 3-7).
The ages of the identified zones were calibrated to the geologic timeframe by means of planktic foraminifera and calcareous nannofossils from the studied wells [26]. First downhole occurrences (FDO) which represent the highest downhole occurrences of chronostratigraphically significant agglutinating foraminiferal species were used to define the upper and lower boundaries of the IRZ while FDO and the lowest downhole occurrence (LDO) of the nominate taxon defined the upper and lower boundaries of the TRZ. This zonal characterization is in accordance with the work of Murphy and Salvador.

Eggerella scabra Taxon Range Zone (TRZ)
Stratigraphic range: This zone was identified in the four wells.     Programme, the various proceedings of the international workshops on agglutinated foraminifera since 1981 and other relevant journal papers.. No detail morphological descriptions are provided since they have been published in some of the above mentioned publications.

Haplogramiodes narivaensis, [Cushman and LeRoy, 1938 Plate I (Slides 6 and 7)]
Description: Test small planispiral and involute, composed of two volutions, the last of which is five to seven chambered. Umbilicus very deep and small, chambers tend to become elongate in axial direction and increase rapidly in size so that the end chamber is twice the size of the first chamber of the last volutions, aperture is interiomarginal, walls finely to coarsely arenaceous, surface smoothly finished.

Ammobaculites strathearnensis, [(Cushman and LeRoy, 1938) Plate I (Slide 5)].
Description: Test free, elongate, ovate to round in section, compressed with seven rapidly enlarging chambers in the last whorl; the last chamber only slightly uncoiled, sutures are flush and zall coarsely agglutinated, aperture terminal. Description: Test free, elongate, biserial, generally more or less compressed in plane of biseriality or rarely oval to circular in cross section; chambers numerous, generally closely appressed, wall agglutinated, simple; aperture single low arch at base of last chamber.

Remarks
Age: Late Miocene. Description: Moderately flaring triserial test with a strongly lobulate periphery, chambers expand fairly rapidly distorted pushed towards proloculus, thereby forming overlapping lobes, may be triangular in section, aperture wide and high on an excavated apertural face, wall fairly smooth, with angular quartz grains of uniform size set in an abundant matrix. Description: Three chambers in early whorls, later increasing in number or becoming uniserial aperture with valvular tooth, interiomarginal at least in early stage, may become terminal and modified to multiple aperture, test free, elongate, early chambers

Conclusion
The biostratigraphic analysis of the four wells A, B, C and D, showed the wells to be fairly rich and diverse in agglutinated benthic foraminifera. Six biozones comprising Eggerella scabra TRZ, Ammobaculites strathearnensis -Eggerella scabra, Cyclammina cancellata -Ammobaculitesstrathearnensis, Haplophragmoides narivaensis -Eggerella scabra, Cyclammina cf. minima-Ammobaculites strathearnensis and Saccammina complanata -Glomospira gordiales IRZ. The biozones were calibrated to the geologic timeframe by means of planktic foraminifera and calcareous nannofossils. The ages of the biozones range from the Late Miocene to the Early Pliocene. Three of the biozones have good stratigraphic correlation in the studied wells. The biozones could impact hydrocarbon exploration and production in the deep-water sector of the Niger Delta.