Department of Geology, University of Ibadan, Ibadan E-mail:[email protected]
Tel: +2348023417013; +2348072544692
Abstract
Sequence stratigraphy, a recent tool for better understanding of stratigraphic distributions and prediction of souce beds and reservoirs, has been employed in Emi Field, offshore eastern Niger Delta. The study aims at deducing key bounding surfaces, depositional sequences and their corresponding systems tracts. In addition, the papaeoenvironment of deposition and potential stratigraphic traps for oil and gas are highlighted. These are based on integration of results obtained from seismic profiles, composite wire-line logs and biofacies data of four wells within the field of of study.
Varying propotions of sand and shale lithologies obtained from composite logs indicate the presence of two lithostratigraphic units, notably; the Agbada and benin Formations. Seismic reflection configurations, well-logs shapes and faunal abundance/diversities, reveal inner neritic to supra-littoral palaeoenvironments of deposition characteristic of a phase of delta progradation. Also , two sequences boundaries and three maximum flooding surfaces are recognized and used to subdivide the stratigraphic succession into depositional sequences and their corresponding system tracts. Hightstand and transgressive systems tracts are recognized in each of the three depositional sequences. However, the absence of Lowstand systems tract can be inferred to have resulted from erosion by succeeding Transgressive systems tracts.
Marker shales, characterized by Haplogmoides-24 ( 6.0 Ma) and Bolivina-48 ( 5.5Ma) were used to date the key bounding surfaces with the aid of the Niger delta chronostratigraphic chart. Integrated analysis reveals sediment deposition to be from Late Miocene-Late Pliocene. Arising from the seismic profiles, it can be observed that the area is dominated by growth faults showing a depositional model characterized by expansional fault system. Keywords: Sequence Stratigraphy, Emi Field, Niger Delta
Introduction Exploration for oil and gas has been an ongoing work in the Niger Delta basin. Various tools have been used by past workers to study its sedimentology, stratigraphy and economic prospects (Short and
Stauble, 1967; Weber and Daukoru, 1975; Azeez, 1976). Sequence stratigraphy is no doubt a recent tool for these purposes.
Sequence stratigraphy is a multidisciplinary approach to the study of genetically related facies within chronostratigraphically significant surfaces (Van Wagoner et al., 1990). It provides a potential unifying framework for interpreting much of rock records, and has considerable economic significance as it helps in identifying exploration prospects and predicting source rocks, seals and potential reservoir traps.
This study therefore utilizes its predictive power to enhance an understanding of the stratigraphy and economic potential of Emi Field, eastern offshore Niger delta, with a view of reducing exploration risks. Also, an accurate framework for laterally extrapolating depositional environments and lithologies away from the well sites will be provided. Interpretations will be based on evidences from seismic reflections, wireline log responses and high-resolution biostratigraphy relevant to the area of study. Location of Study Area and Geology The four (4) wells, used in this study are located in the offshore depobelt of the eastern Niger Delta, Nigeria, and lie within the concession of ExxonMobil (Fig. 1). The base map for the location of the study field is shown in Fig. 2. The Niger Delta basin is located on the continental margin of the Gulf of Guinea in equatorial West Africa and lies between latitudes 4o and 7oN and longitudes 3o and 9o E (Whiteman, 1982). It ranks among the worlds’ most prolific petroleum producing Tertiary deltas that together account for about 5% of the worlds’ oil and gas reserves. It is one of the economically prominent sedimentary basins in West Africa and the largest in Africa (Reijers, 1996).
Figure 1 :
117 M.E. Nton and T.B. Esan
Figure 2 :
Detailed studies on tectonics, stratigraphy, depositional environment, petrophysics , sedimentology and hydrocarbon potential are well documented in the literature (Short and Stauble, 1967; Weber and Daukoro, 1975; Evamy et al., 1978; Knox and Omatsola, 1989; Doust and Omatsola, 1990; Reijers and Nwajide, 1996, Nton and Adebambo, 2009; Nton and Adesina , 2009) among others.
Three lithostratigraphic units have been recognized in the subsurface of the Niger Delta (Short and Stauble, 1967; Frankl and Cordy, 1967 and Avbovbo, 1978). These are from the oldest to the youngest, the Akata, Agbada and Benin Formations (Fig. 3). The Akata Formation (Eocene – Recent) is a marine sedimentary succession that is laid in front of the advancing delta and ranges from 1,968ft to 19,680ft (600- 6,000m) in thickness. It consists of mainly uniform under-compacted shales with lenses of sandstone of abnormally high pressure at the top (Avbovbo, 1978). The shales are rich in both planktonic and benthonic foraminifera and were deposited in shallow to deep marine environment (Short and Stauble, 1967).
The Agbada Formation (Eocene-Recent) is characterized by paralic interbedded sandstone and shale with a thickness of over 3,049m (Reijers, 1996). The top of Agbada Formation is defined as the first occurrence of shale with marine fauna that coincides with the base of the continental-transitional lithofacies (Adesida and Ehirim, 1988). The base is a significant sandstone body that coincides with the top of the Akata Formation (Short and Stauble, 1967). Some shales of the Agbada Formation were thought to be the source rocks, however; Ejedawe et al., (1984) deduced that the main source rocks of the Niger Delta are the shales of the Akata Formation.
The Benin Formation is the youngest lithostratigraphic unit in the Niger Delta. It is Miocene –Recent in age with a minimum thickness of more than 6,000 ft (1,829m) and made up of continental sands and sandstones (>90%) with few shale intercalations. The sands and sandstones are coarse-grained, subangular to well rounded and are very poorly sorted. Materials and Methods of Study The different datasets employed in this study were provided by the Exxon Mobil Producing Nigeria Unlimited, Lagos, Nigeria. These include; 3-D Seismic profiles, composite well logs comprising mainly spontaneous potential, gamma ray, resistivity, density and neutron logs of the four wells (Emi-1, Emi-2, Emi-5 and Emi-6). Others are high resolution biostratigraphic data, consisting microfaunal abundance and diversity charts of Emi-6 well, lithological descriptions of ditch-cuttings recovered from Emi-6 well and a Base map showing well locations in the field.
Niger delta chronostratigraphic chart, an adaptation of the global chronostratigraphic chart of Haq et al., (1988) (Fig.4 ) was used with the biostratigraphic data to assign ages to the bounding surfaces.
119 M.E. Nton and T.B. Esan
Figure 4 :
Seismic Stratigraphic Analysis The application of seismic stratigraphic techniques to a grid of seismic lines allows for grouping seismic reflections into units. These units correspond to depositional intervals, which are chronostratigraphically significant, and called depositional sequences and systems tracts.
The first objective of seismic stratigraphic analysis is to interpret the depositional sequences on seismic sections by identifying unconformities on the basis of seismic reflection terminations. Fig.5 shows a simplified seismic section highlighting the different types of lateral terminations and the various unconformities related to them (Vail, 1987).
Identification of System Tracts The various system tracts which make up each depositional sequence could be identified on the basis of objective geometrical criteria. According to sequence stratigraphic model, four types of systems tracts can be identified; lowstand, transgressive, highstand and shelf margin. Each of these systems tracts could be defined on the basis of the criteria listed in Vail (1987). High Resolution Biostratigraphy Palaeontological and palynological data in the form of high-resolution biostratigraphy have provided information on depositional environments and paleobathymetry (Vail and Wornardt, 1991; Mitchum et al. (1993).
Recognition and verification of the occurrence of climatic cycles (warm and cold periods) that appear to coincide with the physically defined sequences is also possible with the use of high-resolution biostratigraphic data. Sequence boundaries that are associated with eustatic falls occur at the onset of fauna representing cold periods. High or rising eustatic sea level, indicating warm periods characterizes fauna in condensed sections of the highstand and transgessive systems tract. The prograding complex of the lowstand systems tract is typically indicated by an upward change from cold to warm periods.
In this study, high resolution biostratigraphic data consisting of microfaunal abundance and diversity chart, aided in the delineation of the Maximum Flooding Surfaces and paleobathymetric interpretation. The locally recognized cycles were correlated with the globally recognized eustatic cycle chart (Haq et al., 1988). Well Log Interpretation The gamma ray, spontaneous potential and resistivity logs were mainly used in this study with support from density and neutron logs. The gamma ray log used in this study has a shale reference line of 75 API, chosen from the range of 0-150 API values, which respond to the natural radioactivity of the formation. The spontaneous potential ( SP) log, measured in millvolts (Mv), records the electrical potential ( voltage) produced by the formation which result from differences in salinities between resistivity of mud filtrate (Rmf) and that of the formation water ( Rw). At positions where shales are encountered, the SP curve usually defines a more or less straight line on the log known as the shale
121 M.E. Nton and T.B. Esan
baseline. Opposite sandstone or any other permeable formation, the curve shows deflection from the shale baseline. If Rw >Rmf, deflection is to the left and vice versa (Schlumberger, 1989).
Resistivity logs measure the resistance of rock unit to electric current, which is determined by voltages across the electrodes. Porous and permeable sands contain fluids, which increase the resistivity while shales are compacted low resistivity rocks.
Log shapes are interpreted to predict lithology, lithofacies, depositional environment and most importantly, the depositional sequence. Results and Interpretation Seismic Sequence and Systems Tracts
Analyses of strata termination patterns present on the seismic profile of Emi field have resulted in the recognition of seismic sequence bounded above and below by sequence boundaries (SBI and SB 2) (Fig.6). These sequence boundaries were identified by characteristic onlap and erosional truncation patterns.
Figure 6 :
SFCLPC
TSTMFS
HST SB2
SB1
SB1- Lower Sequence boundarySB2- Upper Sequence BoundaryMFS- Maximum Flood SurfaceSFC-Slopefan ComplexLPC- Lowstand Prograding ComplexTST- Transgressive SystemTractHST- Highstand System tract
LEGEND
The nonconformity surface characterized by a downlap at the top and an apparent truncation at the bottom, represents the main condensed section of the depositional sequence. This coincides with the maximum flooding surface (MFS) as shown on Fig.6 and in agreement with the work of Vail (1987).
Various systems tracts present in the recognised sequence were identified on the basis of objective geometrical criteria. The Lowstand prograding complex (LPC) was identified by its offlap configuration and it is bounded below by a sequence boundary (SB1) and above by a ravinement surface (RS). Directly overlying the ravinement surface is the Transgressive Systems Tract (TST) which is bounded above by MFS. However, the retrograding configuration characteristic of the TST
was not clearly observed because of the chaotic nature of the reflection. Above the TST is the characteristic prograding sigmoid to oblique offlap configuration of the Highstand Systems Tract (HST). It is bounded above by a sequence boundary (SB2). Depositional Environment
The seismic profile of Emi Field (Fig. 7) shows the areal association of various seismic facies within two reflector terminations. Of note is the oblique prograding configuration as it turns to chaotic configuration.
An oblique prograding configuration has three zones; the upper area termed topset or undaform zone, an intermediate one (foreset or clinoform zone) and lower zone referred to as bottom set or fondoform (Brown and Fisher, 1980). The topset zone can be said to correspond to delta plain, and the upper part of the foreset will generally contain sand. The bottom set zone usually consists of shale but there can also be siltstone-sandstone intercalations. The oblique clinoforms are typically associated with delta progradation; high energy depositional environment with prevalent sands (Brown and Fisher, 1980). According to Vail,(1987), the mounded onlap fill and chaotic fill facies correspond to high energy sediments, deposited during various re-sedimentation stages. Structural Interpretation
In this study, the seismic section of Emi Field (Fig. 7) shows the effect of growth fault in an expanded fault system, which is in agreement with the work of Mitchum, (1977). Below the SB1, seismic sequences have been highly displaced such that apparent truncation of beds is common. However, this is expected since the Niger Deltas’ tectonic setting is dominated by growth faults.
Figure 7 :
Continous
TIM
E(S
eco
nd
s)Oblique
Chaotic
SB2
Sigmoid
SB1
SB1- Lower Sequence Boundary
SB2- Upper Sequence Boundary
LEGEND
123 M.E. Nton and T.B. Esan
Well Data Interpretation
In this study, well-log sequence analysis proposed by Vail and Wornardt (1991) and Mitchum et al. (1993) has been adopted. Lithology and Depositional Environment
The gamma ray and resistivity logs of the four wells studied were interpreted for lithology and palaeodepositional environments. Within the logged intervals, the lithology is dominated by alternating sand and shale, occurring approximately in a 60:40 ratio.
Based on varying proportion of sand and shale with few occurrences of silt, two major lithostratigraphic units were identified . These are; the continental sands of the Benin Formation, and the paralic Agbada Formation. Four lithofacies sequences, namely; the transitional /upper paralic, Qua Iboe member (upper Marine), Biafra member (Lower paralic) and lower marine, were identified in the Agbada Formation (Fig 8).
The lower marine lithofacies sequences are predominantly shales with occasionally 5-25ft thick sand intercalations. These characteristics, coupled with the presence of pyrites, suggest deposition in a low energy, slightly deep anoxic marine setting . Micropaleontological evidence from Emi-6 well such as presence of Haplophragmoides sp, Textularia sp, Nonionella stella, Bolivina sp, Trochamimina sp among others, suggest deposition in inner neritic environments with appreciable middle neritic influences (Fig. 8). Also, palynological evidence demonstrated by the distinct abundance of Pediastrum, suggest strong fluvial activities characteristic of deposition during a dry palaeo-climatic phase. Such a view has been expressed by Germeraad et al, (1968).
The lower Biafra subunit consists predominantly of sands interbedded with shales/siltstones of varying thicknesses. Presence of slight ferruginous materials in the analysed ditch-cuttings within this interval suggest some level of oxidation, shallow to intermediate water depths, in high to medium energy settings. Also the presence of carbonaceous detritus and pyrites indicate communication with the shoreline and intermittent anoxic conditions. On the GR log, the sands exhibit serrate to multi-serrate cylinder, upward fining and occasional upward coarsening motifs, representing sub aqueous channel and sub aqueous mouth bar deposits. Available palaeoenvironmental indications from associated micro faunal from the well such as; Bolivina sp, Textularia sp, Ammonia beccarii, Quinqueloculina sp, Eggrerella scabra among others, suggest inner neritic environmental settings with middle neritic influence (Hallock and Peebles, 1993) (Fig. 8). Some rainforest/ fresh water palynomorphs identified in this study include Psilate phanocolporites, Retitricolporites irregularis and Canthium ( Muller, 1981)
The middle Biafra sub unit is monotonously shaly, except for the serrate- cylinder shaped subaqueous channel sand occasionally noted on the GR logs of the studied wells. Palynofacies recorded in Emi-6 well such as Zonocostites ramonae, Botrycoccus and Monoporites annulatus at this interval indicate a deep water environmental setting within a distal fluvio-marine realm. Microfaunal assemblage for this subunit is similar to those mentioned earlier, thus implying middle to outer neritic paleo-environment (Fig. 8).
The upper Biafra subunit is predominantly a sandy section with alternating thinner shale beds. The predominantly medium to fine grained nature of sands suggest deposition in shallow to intermediate water depths. The presence of occasional coarse sands, shell fragments, ferruginous materials and carbonaceous detritus in the analyzed ditch cutting of Emi-6 well indicates deposition in high energy, probably near-shore settings. The presence of rare pyrites also in the analyzed ditch cuttings of this interval in Emi-6 well suggests slight anoxity, while glauconite pellets indicate marine influences, particularly towards the upper parts of this sub unit. The sands exhibit hybrid units, consisting of a buildup of multi-serrate cylinder-shaped, upward fining, as well as upward coarsening units. These are interpreted as subaquaeous channel and barrier bar deposits in shallow water shelf settings.
The Qua-Iboe member has a predominantly shaly nature, which coupled with the presence of glauconite pellets and pyrites, suggest deposition in a low energy, slightly anoxic, marine setting. The presence of rare to few ferruginous materials suggests occasional oxic conditions. Available paleo-environmental conditions reflect deposition in inner neritic settings with littoral influences (Fig 8).
The transitional /Upper paralic lithfacies sequence is monotonously sandy with the lower part of the interval having interbedded sands and silty shales. The predominantly sandy nature of the sequence suggests deposition during progradational phase of a delta out-building in a lower coastal plain setting. The medium to granule-sized nature of the sediments also indicates high-energy conditions. On GR log, the sands exhibit serrate cylinder shapes, occasional upward fining and upward coarsening motifs characteristic of delta distributary/fluvial channel and barrier bar deposits of lower coastal plain settings. The complete absence of fauna in this sequence is characteristic of supralittoral settings. The Benin Formation is dominated by continental sands. The sediments of this interval were however not analysed for their microfauna and microfloral content.
125 M.E. Nton and T.B. Esan
Depositional Sequences and Systems Tracts
Depositional sequences, systems tracts, sequence boundaries and maximum flooding surfaces were identified based on their diagnostic characteristic log patterns in all the studied wells and are reported below. EMI-1 Well This well was logged from 500ft to 11,300ft (TD) and three maximum flooding surfaces at 10,290ft, 6,330ft and 2,670ft; and two sequence boundaries at 7460ft and 3090ft were recognized. Consequently, three depositional sequences were delineated (Table 1). Table 1: Sequence Stratigraphic framework of EMI-1 Well
SEQUENCE DEPTH(FT) SYSTEM TRACTS IMPORTANT KEY BOUNDING SURFACES
LEGEND HST – Highstand system tract TST – Transgressive system tract SB -- Sequence Boundary MFS – Maximum Flooding Surface EMI- 3 Well This well was logged from 500ft to 7100ft (TD). Two maximum flooding surfaces at depths 5982ft and 2490ft; and two sequence boundaries at depths 6700ft and 2860ft were delineated. Three depositional sequences were identified as shown in Table 2. Table 2: Sequence Stratigraphic framework of EMI-3 Well
SEQUENCE DEPTH(FT) SYSTEM TRACTS IMPORTANT KEY BOUNDING SURFACES
LEGEND HST – Highstand system tract TST – Transgressive system tract SB -- Sequence Boundary MFS – Maximum Flooding Surface EMI-5-Well This well was logged from 500ft to 8500ft. Three MFS were identified at 8265ft, 5880ft and 2745ft; and two sequence boundaries were delineated at 6300ft and 2900ft. Consequently, three sequences were identified as shown in Table 3.
LEGEND HST – Highstand system tract TST – Transgressive system tract SB -- Sequence Boundary MFS – Maximum Flooding Surface EMI-6-Well This well was logged from 970ft to 10796ft. Three MFS were identified at 10,490ft, 7900ft and 4130ft. Also two SBs were delineated at 8545ft and 4470ft. The three sequences recognized within the logged interval are shown in Table 4 . Table 4: Sequence Stratigraphic framework of EMI-6 Well
SEQUENCE DEPTH(FT) SYSTEM TRACTS IMPORTANT KEY BOUNDING SURFACES
LEGEND HST – Highstand system tract TST – Transgressive system tract SB -- Sequence Boundary MFS – Maximum Flooding Surface Biostratigraphic Data Interpretation
Palynological analysis of Emi-6 commenced from 1,970ft down to 10,796ft. Within this depth range, 98 ditch cutting samples composited at 90ft were processed to generate the database for palynostratigraphic interpretations. The palynological zones and subzones inferred for this well are referenced against the zonal schemes of Germeraad et al. (1968), Evamy et al. (1978), Legoux (1978) and the in-house scheme currently used by ExxonMobil, Nigeria. Comparative correlation of the various palynological zones, subzones and palyno-cycles established for Emi-6 well is shown in Table 5
Palyno-cycles inferred for sediments in the analysed section were based on the succession of the peak occurrences of spores, palmae, mangrove, herbs and Gramineae. The peaks of these flora groups are indicative of an ecological phase during which a particular group was dominant (Poumot, 1989). The succession of these peaks is repetitive and each succession represents a palyno-cycle. These palyno-cycles may be useful for regional correlation. In Emi-6 well, twelve (12) palyno-cycles were identified (Table 5).
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Table 5: Palynological zonation of Emi Field DEPTH(Ft) GERMERAAD EVAMY etal P & M PALYNOCYCLE AGE
et Al (1968) -1978 FLORAL ZONE
1970 ITHE THE 2390
P1 II3000 2930 LATE
IIIECHITRICO 3830 3470
4000 P2 IV PLIOCENE4320 4010
LPOLRITE P3 VP880 4370
5000 5230 VIP4-P6 4550
SPINOUS 6220 4910 EARLY6000 P7 VIII
6860 6820 6270 PLIOCENE7000 IX
P860 7440X UPPER
8000 ZONE ZONE 7880 LATE 8810 XI MIOCENE
900010050
10000 XII MIDDLELATE
10796 10976 MIOCENE11000 RF-4 RF-4
In foraminiferal biostratigraphic analysis, one hundred and forty eight (148) composite samples
obtained from Emi-6 well were used for this study. The benthic foraminiferal zones established for Emi-6 are presented in Table 6. Apart from arenaceous, calcareous (benthic) and planktic foraminiferal species and their respective indeterminate forms, gastropods, shell fragments, echinoid remains, fish teeth and ostracods were also recovered. Table 6: Alontological Zonation Of EMI 06
DEPTH (FT) BENTHIC ZONES AGE 2,000 3,000 ZONE II PLIOCENE 4,000 (VALVULINA FLEXILIS) 5,000 6,000 6.810 6.810 7,000 8,000 ZONE II LATE MIOCENE 9,000 10,000 10.796 10.796 11,000
Identification and Dating of Key Bounding Surfaces The maximum flooding surfaces and sequence boundaries are key bounding surfaces identified in the study area. The maximum flooding surfaces identified in the study area correspond to the transgressive marker shales of the Niger Delta chronostratigraphic chart and they are marked by Haplophragmoides-24 (6.0Ma) and Bolivina-48 (5.5Ma). Both marker shales were identified in Emi-6 well. Two sequence boundaries were identified and dated with the aid of the Niger Delta chronostratigraphic chart. The ages assigned to them are 5.6Ma and 4.1Ma. The sequence boundaries are represented in all the wells. Table 8 shows the age and depths of the key bounding surfaces as present in all the wells after correlation of all the studied wells. Sequence Stratigraphy of Emi Field
The Transgressive Systems tracts (TST) observed in the data used, is bound below by a Type 1 sequence boundary and above by a maximum flooding surface. It is characterized by dipping stratal geometries on seismic data. This defines the retrogradational accumulation of sediments on the sequence boundary. Transgression indicates either a decrease in sediment supply or an increase in accommodation space associated with a relative rise in sea level. Sedimentary facies within the TST tend to fine upward, as a result of progressive deepening of depositional environments and the landward shift of the shoreline. Thus, the TST unit encompasses environments from littoral to outer shelf neritic.
During transgression, older deltaic complexes, built up and out across the shelf during the previous LST phases, are eroded or overstepped, a process which extensively redistributes sands as sheet across the shelf. This is the probable cause for the absence of the lowstand systems tract in the study area.
The maximum flooding surfaces (MFS) delineated in the study area are interpreted to have developed during the highest point of sea level rise and thew maximum landward incursion of the shoreline. They exhibit pelagic deposition and sediment starvation on the shelf and slope, and separates phase of shoreward retrogradation (transgression) from trhose of basinward progradation (regression). Condense sections, marked by diversity and abundance peaks of flora and fauna, formed with the MFS deposits at depths; 4130ft, 7900ft and 10490ft in Emi-6 well. The highstand systems tract ( HST) ovelies the preceeding TST phase and is capped by a Type 1 sequence boundary. The HST was recognized on the seismic profile by clinoforms downlapping onto the MFS ( Fig. 6). It occurs when the sediment supply rate exceeds the accommodation space, causing parasequence deposition to either aggrade upwards or prograde basinwards. Deltaic progradation observed from this systems tract is characterized by an upward coarsening of sediments as seen on well log patterns of Emi 1, 3, 5 and 6, and oblique clinoforms on the seismic profile.
Table 7 and Fig. 9 show the correlation of the four wells studied using the key bounding surfaces. Table 7: Age and Depths of Key Bounding Surfaces Key Bounding Surfaces Depositional Seque NCE EMI -- 1 EMI -- 3 EMI -- 5 EMI – 6 SB -- -- -- -- MFS (Bolivina - 48
3 2670 ft 2490 ft 2745 ft 4130 ft
SB (4.1 Ma)
3090 ft 2860 ft 2900 ft 4470 ft
MFS (Bolivina – 48
2 6330 ft 5982 ft 5880 ft 7900 ft
SB (5.6 Ma)
1 7460 ft 6700 ft 6300 ft 8545 ft
MFS (Haplophragmoides24)
1090 ft ---- 8265 ft 10490 ft
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Figure 9:
Implication for Exploration and Development It has been widely reported that growth fault -related structural traps form the dominant traps in the petroliferous Niger Delta. However, a major reason sequence stratigraphy was advanced is to discover subtle stratigraphic traps that result from the rapid facies changes occurring between successive systems tracts. The cyclic pattern of the alternating TST and HST in the studied wells is indicative of a good environment for organic matter accumulation and generation. The pelagic shales of the transgressive systems tract could form good source rocks and cap rocks for the underlying and overlying HST given the right conditions. Reservoir quality sands within the HST could serve as good reservoir while faults, active in this area, could serve as conduits for upward migration of hydrocarbon. Summary and Conclusions Two major lithofacies sequences were delineated based on their characteristic features. These are notably; the continental sands of the Benin Formation and the paralic Agbada Formation. The Agbada Formation was further divided into transitional/paralic Qua Iboe member (Upper marine), Biafra member (Lower paralic) and the Lower marine unit. The sediments were deposited in environments ranging from supralittoral through inner neritic for the highstand system tract and middle neritic to outer neritic for transgressive systems tract.
Cyclical depositional patterns, marked by regionally isochronous transgressive surfaces identify index fossils notably Haplophragmoides-24 (6.0Ma) and Bolivina-48 (5.5Ma) in marker shales. These marker shales have high faunal content and correspond to the maximum flooding surfaces. Also, the
ages assigned to the sequence boundaries are 5.6Ma and 4.1Ma. The key bounding surfaces were used for regional correlation of the wells. Late Miocene-Late Pliocene age, corresponding to the P800 zone of palynological zonation, and the zones I and III of the palaeontological zonation have been assigned to the sediments. The Miocene-Pliocene boundary was established at approximately 6,860ft.
Three major sequences (1, 2 and 3) were identified in all the studied wells, with sequence 3 recognized in the seismic section. Transgressive systems tracts were recognized as fining upward (retrogradational) parasequences, and (progradational) parasequences, in the depositional sequences. The alternation of highstand sands and transgressive shales is inferred to provide the desired combination of reservoir and source rock required for hydrocarbon generation.
From the results obtained, it can be concluded that the application of sequence stratigraphy to Emi field has enhanced the interpretation of the stratigraphic build-ups, recognition of isochronous surfaces and identification of prospects and leads. The correlation of isochronous, laterally persistent transgressive marker shales across fault blocks permits the recognition of the thickening or expansion of sedimentary sequences on the down-thrown blocks.
It is however recommended that further work be carried out in the study area with more biostratigraphic data for more wells as well as synthetic seismogram to help balance the discrepancies in results obtained between seismic and well data. Acknowledgement We appreciate the assistance of the management and staff of ExxonMobil, Lagos, Nigeria , for providing the data used in this study. We are grateful to the Nigerian Association of Petroleum Explorationists (NAPE) for the opportunity given to present this paper at the International conference and exhibition in Abuja, Nigeria. References [1] Adesida, A. and Ehirim, B.O. 1988. Cenozoic Niger Delta: A guide to its lithosedimentary
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List of Figures 1. Map of Niger Delta showing the location of study field 2. Base map of study area 3. Stratigraphic column showing the three formations of the Niger Delta ( After Doust and Omasola,
1990) 4. Niger Delta Cenozoic Chronostratigraphic Chart ( After Haq et al., 1988) 5. Stratal termination patterns ( After Vail., 1987) 6. Seismic stratigraphic interpretation of Emi Field 7. Seismic reflection configuration and structural features in Emi Field 8. Microfaunal Distribution and abundance chart of Emi-6 well 9. Sequence stratigraphic framework of Emi-Field
List of Tables
1. Sequence stratigraphic framework of Emi -1 well 2. Sequence stratigraphic framework of Emi -3 well 3. Sequence stratigraphic framework of Emi – 5 well 4. Sequence stratigraphic framework of Emi -6 well 5. Palynological zonation of Emi- 6 well 6. Palaeontological zonation of Emi- 6 well 7. Age and depths of key bounding surfaces
