An Integrated Wireline-Log and Biostratigraphic Appraisal of Olure-1 and Abigboro-1 Wells , Onshore Niger Delta

Studies of two wells within the coastal swamp onshore Niger Delta were carried out in order to determine the chronology, biozonation, and establishment of sequence stratigraphic framework for the representative wells. A total of twenty-eight (28) cutting samples were recovered and analysed from Abigboro-1 and Olure-1 exploratory wells for their foraminifera, calcareous nannofossils and palynomorphs’ compositions. The results indicated the presence of diverse species within the wells, which include Lenticulina grandis, Florilus ex. gr. costiferum, Hanzawaia concentrica, Hopkinsina bononiensis, Marginulina costata and pseudonodosaria sp., Bolivina mandoroveensis, Eponides eshira, Lenticulina grandis, Lenticulina grandis, Cibicorbis inflata, Heterolepa floridana, Florilus ex. gr scaphum, Poroeponides lateralis, Uvigerina sparsicostata, Uvigerina subperegrina, Bolivina ex. gr. scalptrata, Valvulineria sp. and Epistominella pacifica. An inner neritic (1694 m 2161 m), coastal deltaic (1557 m 1640 m) and shallow inner neritic (1347 m 1554 m) palaeoenvironment was suggested for Olure-1 well while a broad grouping of the intervals as inner to middle neritic was suggested for Abigboro-1 well. The ratio of species abundance/diversity and integration of log suite data predicted two Maximum Flooding Surfaces (MFS) and one Sequence Boundary (SB) for Olure-1 well. The MFS were encountered at depths 2728 m (MFS 1) and 1797 m (MFS 2), while the SB was predicted at depth 2602 m (SB1). Diverse systems’ tracts were identified and delineated from wireline logs of Olure-1 well and the implication for petroleum exploration was discussed. Late Oligocene early Miocene age has been assigned for the wells based on the recovered diagnostic species.


Introduction
The Niger Delta is situated in the Gulf of Guinea and extends throughout the Niger Delta province [1] (Figure 1).From Eocene to Present, the delta has prograded south-westward, forming depobelts that represent the most active portion of the delta at each stage of its development [2].These depobelts form one of the largest regressive deltas in the world with an area of 300,000 km 2 [3], sediment volume of 500,000 km 3 and a sediment thickness of over 10 km in the basin depocenter.The Niger Delta has been identified as an important sedimentary basin in Nigeria because of its reputation as one of the most prolific oil and gas provinces in the world, and it is known to contain one identified petroleum system [3] [4].With increasing exploration and exploitation of hydrocarbon in the Niger Delta, which in turn has made the search for oil and gas increasingly difficult, it has become pertinent to acquire knowledge and expertise in order to improve geological research and technology to field development and production of this vast hydrocarbon resource.This however has necessitated re-evaluation via the concept of high resolution biostratigraphy and sequence stratigraphy of the basin in order to obtain a clearer resolution of the subsurface stratigraphy of the Niger Delta sedimentary basin.This is made possible by the integration of biostratigraphic, seismic and wireline log data.For each of these data, a sequence stratigraphic model is developed via unconformity bounding surfaces, flooding  surfaces and systems tracts within sequence(s) and comparison of results is made across diverse data sets for accuracy and precision.

Location of the Study Area
The Niger Delta oil province of Nigeria is located on the southernmost end of Nigeria on the West African Continental margin.The study area is located onshore within OML 46.The basin lies between longitude 3˚E and 9˚E and latitude 4˚E and 5˚2'N (Figure 1).

Previous Work
The geology of Niger Delta in terms of age determination, depositional environment and biostratigraphic zonation has been discussed by many authors amongst which are Clement et al. [5] who reported the sequence stratigraphic study of "UNIABR Field" in northern delta depobelt of Niger Delta, and were able to identify five maximum flooding surfaces (MFS) and five sequence boundaries (SB) in which their average depths were tied to their absolute ages.
Late Eocene to early Miocene age was proposed for the studied area based on palynological zonation.Oloto and Promise reported on biostratigraphic study and palaeoenvironmental reconstruction indicated early-middle Miocene age for Bongo ST-1, Ngolo and Opukushi-25 wells [6].Early Miocene was proposed for Bongo-4 well based on the First Appearance Datum (FAD) and Last Appearance Datum (LAD) of diagnostic fauna and flora of Globorotalia obesa, Globigerinoides bisphericus, G. obliguus, Globigerinoides immaturus, Orbulina universa and Orbolina suturalis.The suite of early-middle Miocene benthic foraminiferal assemblage recovered include Bolivina miocenia, Lenticulina rotulata, Alveolephragmium crassium, Bolivina beyrichi, Saccammina complanata and Cyclammina minima.Also establish was the depositional environment and palaeobathymetry of the studied wells based on depth diagnostic benthics, floral and associated accessories like glauconite, mica, faecal pellets foraminiferal test linings, pediastrum and fungal spores within each wells.The depositional environment established range from marginal marine to shallow marine (littoral-middle neritic), corresponding to onshore to offshore within the continental plain to the continental shelf.Two microfossil biozones of Cyclammina minima and Haplophragmoides narivaensis have been established in the Oborduka-1 well, deep offshore of Niger Delta [7].They assigned late Miocene age and a nearshore and/or marginal marine environment for the sediments based on the recovered fauna and flora assemblages.Boboye and Adeleye documented the high resolution biostratigraphy of some early Pliocene-late Miocene strata using calcareous nannoplankton and foraminiferal of the deep offshore Niger Delta area of Nigeria.Four condensed sections for the foraminiferal assemblages and four zones for the calcareous nannofossils as correlated to the Global Cycle Chart (NN 13, NN 11, NN 10 and NN 9) were established [8].A tentative sequence stratigraphy framework was proposed and dated the Maximum Flooding Surfaces as early Pliocene to late Miocene which was associated with the 5.0 Ma, 7.4 Ma, 8.6 Ma and 9.2 Ma.Van Hoeken-Klinkenbers palynological studies on boreholes samples in the western part of the Niger Delta concluded a Palaeogene age in some part of the Niger Delta exhibiting poor palynomorphs preservation [9].A continuous temperature-climate optimum was demonstrated for the development of Globigerina bulloides and tropical climate for the development of Globorotalia foshi [10].It was concluded that Tertiary fauna provinces were distributed asymmetrically about the equator probably by major ocean currents.

Materials and Methodology
Data used in this study include log suites data and cuttings from two exploratory wells (Olure-1 and Abigboro-1) in Niger Delta.Systematic logging and detailed analysis and interpretation of the well logs, preparation and analyses for the fauna and flora contents were carried out.These were integrated to generate a sequence stratigraphic framework for the selected wells.The biostratigraphic analysis was carried out while the other analyses were achieved using Petrel and STRATABUG softwares.

Biostratigraphy Preparations
These cuttings were prepared for calcareous nannofosils, sporomorphs and foraminifera contents using standard preparatory techniques; The calcareous nannofossil analysis involves the standard simple smear preparation technique of Perch-Nielsen [13].About 5 g of the sample was gently crushed and dispersed in distilled water in a glass vial.It was dried at 50˚C and mounted on a glass slide with the Norland adhesive mounting medium.Species were identified using relevant reference slides, Chevron and Shell (SPDC) albums.
For the foraminiferal preparation, about 25 g of the sample were soaked in NaOH solution for 24 hours to ensure proper disaggregation of the samples and the liberation of any foraminifera or micro-fauna accessories within the cuttings.Open Journal of Geology The samples were then washed using a 63 micron sieve under a jet running water.The samples were then dried in an oven at 80˚C.The palynomorph analysis entails the addition of Hydrofluoric acid (HF) and was then treated with 10% hydrochloric acid in a fume cupboard to remove carbonates.10% HCl solution was then added and heated for 25 to 30 minutes.The samples were decanted thrice at an interval of 1 hour each through the Branson Sonifer, with the aid of 5 micron sieve to filter away the inorganic matter.The retrieved organic matter was stained with Safranin solution for easy identification of the dinocyst that are almost totally transparent.The Loctite impruv is used for permanent mounting of the slides.The slides were studied under a binocular light microscope and the identification was done via reference slides of Chevron and Shell albums.

Results and Discussion
The results showed that the biostratigraphy of Olure-1 and Abigboro-1 wells have fairly good microfossil recovery except for Abigboro-1 well which is barren in foraminifera and calcareous nannofossils (Figures 2-10 and Tables 1-5).

Lithostratigraphy
Base on the available well log (gamma ray) and cutting samples, the Olure-1 and Abigboro-1 wells penetrated a total depth of 3182 m (10,440 ft) and 2161 m (7090 ft) depth respectively.The lithology constitutes a mixture of sand and shale units, with the shale occurring in significant amounts in an environment that is wholly marine and in variable proportion for a paralic depositional set- ting.This was simulated by the gamma ray log signature.A coarsening upward or fining upward unit within a forestepping or backstepping successions of the log pattern defines stacking patterns of genetic units.This aided the prediction of the positioning of maximum flooding surfaces, and the identification of the systems tracts when integrated with biostratigraphic data (Figure 2 and Figure 3).

Biostratigraphy
Foraminifera study of the wells The biostratigraphic studies of the wells involved a total of eighteen ( 18  were prepared and analysed for their microfossils.The foraminiferal recoveries in Abigboro-1 well within the studied sections are poor and the absence of planktic species precludes a refined zonal delineation for the analysed interval using the Blow planktic scheme [14] [15].However on the basis of the recovered assemblages and the Niger Delta SPDC faunal zone scheme, the analysed intervals have been dated late Oligocene to early Miocene age corresponding to the N3 -N4 and F7800 -F9300 Zones for the two wells.The occurrence of Globigerinoides primordius at depth 2651 m within the Olure-1 well enabled the subdivision of the well section.The stratigraphic distributions of the recorded taxa are presented (Table 1, Table 3, Table 4 and Figure 4).

Foraminiferal Biozonation
The zonation of these study wells are based on the comparison of the recovered assemblages with that of the SPDC Niger Delta Faunal Zone; Abigboro  1 and Figure 5 and Figure 6).

Palynological Biozonation
This well falls within the broad Pan Tropical Magnastriatites howardi Zone of Germeraad et al. [17] and the P500 Zone of Evamy et al. [18].Only the P560 and P540 Subzones were identified within the P500 Zone; The top of this subzone is placed at 1749 m and it is defined by the increased occurrence of Retibrevitricolporites protrudens.The base is placed at the depth of 2161 m.This subzone is further characterized by the presence of Cicatricosisporites dorogensis, Gemmatriporites sp., Striamonocolpites rectostriatus, Arecipites exilimuratus and Grimsdalea poligonalis.

Calcareous nannofossil study of the wells
This entails a total of ten (10) cuttings from interval 1920 m -3182 m (Olure-1 well) and total of eighteen (18) cuttings selected from interval 1347 m -2161 m (Abigboro-1 well) were prepared and for nannofossils at various intervals.The study revealed rare to few and scattered occurrences of nanno-fossils in Olure-1 well while intervals have been found to be barren of nannofossil occurrences in Abigboro-1 well.The occurrence of few index markers within the analysed section of Olure-1 well facilitated the subdivision of the well sections into zones based on the zonation scheme of Martini [19].Interval 1920 m -2633 m has been dated early Miocene (NN2 -NN4 Zones) while interval 2633 m -3182 m is believed to be of late Oligocene to early Miocene (NP24 -NN1 Zones) (Table 2 and Figure 7).

Sequence Stratigraphic Framework
Case Study Appraisal for Olure-1 Well This was based on the gamma ray and resistivity log signatures of the well.The key surfaces and systems tract were identified (Figure 10).
Sequence Boundary (SB) and Maximum Flooding Surface (MFS) Three Maximum Flooding Surface (MFS) and Sequence Boundaries (SB) were identified respectively.These points corresponds to depths of 2728 m (MFS 1), (SB 1), 1661 (SB 2), and 825 (SB 3) for Sequence Boundary (Figure 10).MFS 1 and MFS 2 and SB 1 correspond to intervals where we have the peak and least abundance of biofauna respectively.

Lowstand Systems Tract (LST)
From the log signatures, the LST is bounded at the base by SB 1, SB 2, and SB 3 and at the top by TS 1, TS 2, and TS 3, showing the coarsening upward log motif.These system tracts represent sand packages that signify the basin floor fan, which is overlain by slope fan and capped by shaly lowstand wedge.The biostratigraphic study also showed progressive decrease in abundance and diversity of biofauna due to fall in water level and shallowing of the shore.

Transgressive Systems Tract (TST)
In Olure-1 well, the TST is bounded at the top by MFS 1, MFS 2, and MFS 3 and at the base by TS 1, TS 2, and TS 3. The stacking pattern showed a finning upwards log motif.Also the parasequences within the TST backstep in a retrogradational parasequence set.The progressive increase in biofauna indicated progressive increase in water level and that the systems tract progressively deepens upward as successively younger parasequences step farther landward.

Highstand Systems Tract (HST)
The HST is bounded at the base by MFS 1, MFS 2, and MFS 3 and at the top by SB 1, SB 2, and SB 3. The early highstand consists of an aggradational parasequence set as evidenced on the log signatures.However, the late highstand is composed of one or more progradational parasequence sets (Figure 10).The biofauna trend for this systems tract shows a peak in abundance and diversity of species followed by a progressive decrease.

Depositional Environment
The depositional milieu of the study wells were determined based on the sedimentological and biostratigrapical inferences (Figure 8).Based on the foraminifera recovered from Olure-1 well, the environment was suggested to have been deposited within the inner neritic environment with the occurrence of Lenticulina grandis, Florilus ex.gr.costiferum, Hanzawaia concentrica, Hopkinsina bononiensis, Marginulina costata and pseudonodosaria sp.Within the interval of 1557 m -1640 m which is predominantly sandy lithofacies indicated a coastal deltaic environment while the shaly upper section within 1347 m -1554 m suggested a shallow inner neritic environment with the presence of Lenticulina grandis at 1347 m -1356 m depth.

Exploration Implication
The application of sequence stratigraphic framework in the study wells have made prediction possible, the positioning of the key chronostratigraphic surfaces, such as flooding and erosional surfaces as well as the System Tracts (ST) and their parasequences stacking patterns.This allows for the identification of plays and potential prospect evaluations in exploration for oil and gas.The sequence stratigraphic study of the two wells indicated that the lowstand consist of sand packages that can serve as hydrocarbon reservoirs.It is suggested that during periods of lowering of the sea level, when the rate of sediment input far exceeded the subsidence rate (progradation), deposition of sandstone reservoir occurred in the form of incised valley fills, slope fan complexes and prograding complex within the lowstand wedges.As the sea level began to rise, the earlier formed reservoirs were sealed by the deposition of relatively organically rich shale prone sediments of the Transgressive Systems Tract (TST).This model produces a reservoir/seal relationship whereby hydrocarbon expelled by the organically rich lithofacies, upon maturation may be trapped within the sealed reservoir rocks either by structural, and/or stratigraphic trapping mechanism.

Conclusions
The integration of biostratigraphic data with wireline log data exemplified a sequence stratigraphic model to be generated for Olure-1 and Abigboro-1 wells.The presence of a Sequence Boundary at depth of 2602 m (SB 1) was also identified which also corroborated with the depth interval where there appeared to be the least abundance and diversity of species as shown in the biostratigraphic data of the well.The deposition environmental studies suggested an inner neritic to coastal deltaic setting for Olure-1 and Abigboro-1 wells.The presence of some diagnostic species such as Lenticulina grandis, Florilus ex.gr.costiferum, Hanzawaia concentrica, Hopkinsina bononiensis, Marginulina costata and Pseudonodosaria sp.(Olure-1 well) and Bolivina mandoroveensis, Eponides eshira, Lenticulina grandis, Lenticulina grandis, Cibicorbis inflata, Heterolepa floridana, Florilus ex.gr scaphum, Poroeponides lateralis, Uvigerina sparsicostata, Uvigerina subperegrina, Bolivina ex.gr.scalptrata, Valvulineria sp. and Epistominella pacifica (Abigboro-1 well) suggests late Oligocene -early Miocene age.A sequence stratigraphic model generated for exploration suggest the sand packages of the Lowstand Systems Tract can serve as reservoir rock while the fossiliferous shale units of the Transgressive Systems Tract could serve as the seals and source rock for hydrocarbon.

Figure 1 .
Figure 1.Map of Niger delta showing the location of studied area within OML 46.

Figure 9 .
Figure 9. Photomicrograph of some selected diagnostic pollen and spores, calcareous nannofossils and foraminifera.

Figure 10 .
Figure 10.Wireline log of the key chronologic surfaces for the Olure-1 well.
this subzone is placed at 1347 m and the base is defined by the increased occurrence of Retibrevitricolporites protrudens at 1749 m.This subzone corroborated with the abundant records of R. protrudens, Verrucatosporites usmensis, Praedapollis flexibilis, and Pteris sp.
was employed for the zonation of the wells; Interval: 1920 m -2633 m Zone: NN4 -NN2 Top: Placed at the first sample analysed at 1920 m Base: Top Helicosphaera truempyi Age: Early Miocene Description: This interval is characterized by rare to sparse occurrence of nannofossils.This interval has been delineated based on the presence of characteristic early Miocene (NN4 -NN2) taxa including Discoaster deflandrei, Coccolithus miopelagicus and Helicosphaera ampliaperta.Interval: 2651 m -3182 m Zone: NN1 -NP24 Top: Top Helicosphaera truempyi Base: Tentatively placed at the last analysed sample at 3182 m Age: Early Miocene to late Oligocene Description: This interval is characterized by very rare and sporadic occurrences of nannofossils.The occurrence of Helicosphaera truempyi at 2633 m depth is an indication that the well is not younger than the early Miocene NN1 Zone, while the other intervals stands for the total range (NN1 -NP24) of Helicosphaera truempyi Zonal marker.
Lithologic description from logs and cutting samples indicated a mixture of sand and shale units.The various biostratigraphic plots for Olure-1 well show the presence of Maximum Flooding Surfaces at depth intervals 2304 m -2487 m and 2652 m -2730 m, while Abigboro-1 well shows the presence of Maximum Flooding Surfaces at depth range between 1750 m -1788 m and 2115 m -2134 m.This was corroborated by the composite plot from biostratigraphical data and wireline log signatures which also indicated the presence of Maximum Flooding Surface at about 2728 m (MFS 1) and 1797 m (MFS 2) depths (Olure-1 well).
. They defined six foraminiferal zones (Assemblage/Partial range zones) for the middle to late Miocene Niger Delta namely: Globigerina cf ciperoensis Zone, Nonion centrosulcatum/Chiloguembelina victoria Zone, Eponides eshira Zone, Uvigerina sparsicostata Zone, Spirosigmoilina oligoceanica Zone, and Florilus ex.gr.costiferum Zone.The advantages of calcareous nannofossils in the recognition of marine flooding surfaces in the Niger Delta most especially in the late Miocene to late Pliocene time was discussed and recognized four delta wide flooding surfaces based on the Discoaster quinqueramus, Ceratholithus species and Gephyrocapsa species and Sphenolithus species.This corroborated the influx of Sphenolithus abies in the late Miocene of the Niger Delta [12].

Table 1 .
(a) Abundance and diversity of pollen and spores and (b) planktonic and calcareous foraminifera in Olure-1 well.

Table 4 .
Total diversity of palynomorphs and for aminiferal assemblages for Olure-1 well.

Table 5 .
Abundance and diversity of pollen and spores in Abigboro-1 well.