5.1 Palynological Biostratigraphy of Well BB-3
Moderately rich, well preserved and fairly diverse palynomorph assemblage were recorded within the analyzed interval. Zonocostites ramonae, Monosporites annulatus, fungal spores/hyphae, species of Sapotaceoidaepollenites and Laevigatosporites dominated the microfloral assemblage of the analyzed section. Few specimens of marine indicator palynomorphs recorded include Operculodinium centrocarpum, Lingulodinium machaerophorum, Achomosphaera ramulifera, Spiniferites ramosus and indeterminate dinoflagellate cysts. Some of these forms recovered from the studied wells are illustrated in photomicrographs plates 6,7 and 8, while palynomorph and palynofacies distributions chart of well BB-3 is shown in Fig. 4.
Belskipollis elegans zone – taxon range zone
Stratigraphic Interval: 10740–12480 ft
Description
Subzonal top is tentatively placed at 10740ft, top of the analyzed section while base rich occurrence of Belskipollis elegans at 12480ft defines the subzonal lower boundary.
Diagnosis
Moderately rich abundance and fairly diverse of miospores characterize this interval. Zonocostites ramonae, Monoporites annulatus, fungal spore/hyphae, species of Psilamonocolpites, Sapotaceoidaepollenites, Laevigatosporites, Polypodiaceiosporites and Leoitriletes dominated the microfloral assemblage of this interval. Sparse to common recoveries of Racemonocolpites hians, Verrutricolporites rotundiporus and Belskipollis elegans are other diagnostic feature of this zone.
Age
The zone is dated Middle Miocene because taxa such as Verrutricolporites rotundiporus and Racemonocolpites hians are diagnostic of Middle Miocene (Morley, 1997).
Remark
This is equivalent to P700 zone and P740 subzone of Evamy et al. (1978)
Crassoretitriletes vanraadshooveni zone – taxon range zone
Stratigraphic Interval: 12480–14580 ft
Description
Base rich occurrence of Belskipollis elegans at 12480ft defines subzonal top while Last Downhole Occurrence (LDO)/base occurrence of Crassoretitriletes vanraadshooveni at 14580ft marks the subzonal lower boundary.
Diagnosis
Zonocostites ramonae and Laevigatosporites spp were abundant while Monoporites annulatus, fungal spore/hyphae, species of Sapotaceoidaepollenites, Psilamonocolpites, Polypodiaceiosporites and Leoitriletes were appreciable or fair within the interval.
Age
First Downhole/Top Occurrence of Spirosyncolpites bruni at 13020ft and Last Downhole/Base Occurrence of Belskipollis elegans at 14220ft confirm penetration of Middle Miocene.
Remark
This is equivalent to P700 zone and P720 subzone of Evamy et al. (1978)
Pachydermites diederixi zone- taxon range zone
Stratigraphic Interval: 14580–15620ft
Description
Base occurrence of Crassiretitriletes vanraadshoveni defines the subzonal upper boundary at 14580ft while the subzonal lower boundary is tentatively placed at 15620ft, depth of last analyzed sample.
Diagnosis
Appreciable recoveries of miospore were recorded within this interval. The earlier mentioned miospores in the overlying subzone still dominated the microfloral assemblage. Zonocostites ramonae, fungal spore/hyphae, species of Sapotaceoidaepollenites and Laevigatosporites dominated the microfloral assemblage of this interval. Slight improvement in the recoveries of Racemonocolpites hians, Magnastriatites howardi and Echiperiporites icacinoides are diagnostic of the interval.
Age
Quantitative top occurrence of Praedapollis flexibilis at 14700ft confirms the penetration of this zone and dated Early-Middle Miocene.
Remark
This is equivalent to P600 zone and P680 subzone of Evamy et al. (1978).
5.3 Sequence Stratigraphy
Sequence stratigraphic analysis of well BB-3 is presented in Fig. 9. Sequence stratigraphic analysis and interpretation of well BB-3 was carried out by the integration of biostratigraphic, lithologic and wireline (Gamma Ray and Resistivity) log data. Well log sequence stratigraphic technique proposed by Vail and Wornardt (1991) was used for this study bearing in mind the possible pitfalls commonly associated with log facies analysis, ie downhole cave-ins and broad sampling intervals.
Sequence stratigraphic interpretations of the well relied on the fair to good recoveries of palynomorphs and occurrences of key marker species. The sequence stratigraphic summary is presented in Fig. 10. Details of the interpreted systems tracts are presented below for the wells.
In this study four depositional sequences were recognized in BB-3, comprising transgressive system tracts (TST), highstand systems tracts (HST), low stand system tracts (LST), Maximum flooding surface (MFS) with their corresponding Sequence Boundaries (SBs).
Highstand Systems Tract (HST)
The Highstand Systems Tract is chareacterised by stratigraphic interval deposited during a relative decrease in sea level between the MFS and the overlying SB (Boggs, 2006), a set of prograding coarsening upward and shallowing upward parasequence that terminates at SB (Reijers et al., 1996), a shift in palynomorph associations from dinoflagellate and marine organic matter dominant assemblages (TST) to spore-pollen and terrestrial organic matter dominated assemblages owing to increased freshwater influx and the progradation of the coastline (Sancay, 2005). Phytoclasts in HST are not degraded (Tyson, 1995). Three HST were encountered within TD- 15460 ft,14950 − 13690 ft, and 12750 − 11330 ft in the studied well.
Transgressive Systems Tract (TST)
TST is deposited during the relative rise of sea level and persists until the maximum relative rise of sea level is released at the MFS. TST is characterized by parasequences backstep in a retrogradational parasequence set, progressively deepen upwards as successively younger parasequence step further landward (Reijers et al., 1996). The palynofacies association is more of marine taxa, progressive decrease in the abundance of phytoclasts with paticles becoming more rounded. TST is usually composed of basal shore-face sands overlain by fossiliferous pelagic shales which displays a fining upwards sequence and it is bounded below and above by Transgressive Surface (TS) and the downlap surface of the MFS respectively (Reijers et al., 1996). The shales and mudstones of TST form an excellent seal for hydrocarbon reservoirs. TST in the Niger Delta is commonly made up of thick shales occasionally punctuated by relatively thin and poorly developed reservoirs sand bodies. TST were encountered within 15370 − 14920 ft, 13500 − 12800 ft and 11240 ft in the studied well.
Lowstand Systems Tract (LST)
LST is deposited as lowstand prograding complexes during the slow relative rise of sea level at or near shelf margin (Vail and Wornardt, 1991). LST is characterised by is overall coarsening upward parasequences sets which show sedimentation rate in excess of the accommodation space. The Transgressive Surface (TS) separates the Lowstand System Tract and the Transgressive System Tract. Palynofacies association within LST contains abundant large phytoclasts (black, charcoalified woody detritus) and poorly preserved pollen and spores indicative of partial oxidation (Tyson, 1995; Batten and Stead, 2005). LST consists of shoreface sands, with shale bands and have been considered among the best exploration targets since they form good reservoir. 15450 − 15350 ft, 13680 − 13500 ft and 11340 − 11300 ft were the intervals at which LST was encountered.
Maximun Flooding surface
The Maximum Flooding Surfaces (MFS) represent the fullest areal development (apex) of a major condensed section where marine influence has encroached the greatest distance beyond the shelf. Maximum Flooding Surface is a correlatable physical surface that commonly represents the maximum shale point between the fining upward (backstepping) transgressive system tract and the coarsening upward (forestepping) highstand system tract (Reijers et al., 1996). Maximum Flooding Surface is a downlap surface and is usually associated with major faunal and floral abundance and diversity peaks. Maximum Flooding Surface is further marked or identified by Highest Gamma Ray Log values, Lowest Resistivity Log values, High Spontenous Potential Log values and Low Sonic Log peak. These are the characteristics of the shaliest part of the major condensed sections (Reijers et al., 1996). MFS was encountered at 14930 ft and 12780 ft.
The surfaces were dated 15.9 Ma and 15.0 Ma by correlation to the Niger Delta Cenozoic Chronostratigraphic Chart which contains the eustatic curves of Vail (1987) and the global sea level cycles of Haq et al. (1988). The MFS falls between P680 zone and P730 subzone in the Niger Delta pollen zones contained in the Niger Delta Cenozoic Chronostratigraphic Chart and also corresponds to Chiloguembelina 3 shale marker species.
Sequence Boundaries (SB)
Sequence Boundaries refer to the erosional (unconformity) surfaces bounding depositional sequence. They are formed during the relative lowering and during lowstand of sea level. They are also recognised on the log as sharp lithologic contacts within progradational sequences or as the points of change between coarsening upward (forestepping) HST and fining upward (backstepping) TST. SB is further characterised by a major shift in the biofacies assemblages from deeper below the Sequence Boundary to shallower above the boundary (Armentrout, 1991). SB was encountered at 15470 ft, 13690 ft and 11350 ft. It is dated 15.5 Ma and 13.1 Ma because of its stratigraphic position in the sequence. The SB immediately above 15.9 Ma and 15.0 Ma MFS is 15.5 Ma and 13.1 Ma (Fig. 10) in the correlation of palynostratigraphic zones to the Niger Delta Cenozoic Chronostratigraphic chart.
Hydrocarbon Exploration Implications
The established palynostratigraphic zones and systems tracts find their usefulness in directing well trajectory. The sandstone units within the HST and LST interpreted as channels deposits form good potential reservoirs within the well BB-3. The shale units of HST and TST serve as excellent top and bottom sealing rock for hydrocarbon in the reservoir sand. These shale units could equally serve as good source rocks. The shale units (seals) of TST and HST, and resrvior rocks of LST and HST combined to form the stratigraphic traps for hydrocarbon accumulation in the studied well that could be targeted during hydrocarbon exploration. The Condensed Section together with the associated MFS serve as regional seals and source sediments in the field.