sequence stratigraphic implication to hydrocarbon ... · sequence stratigraphic implication to...

DOI:10.23883/IJRTER.2018.4052.II9ND 405

SEQUENCE STRATIGRAPHIC IMPLICATION TO

HYDROCARBONEXPLORATION IN “BETA FIELD”, NORTHERN

DEPOBELT OF THE NIGER DELTA BASIN

Ukpong, A. J.1, 2

and Anyanwu, T. C.2

1Department of Geology, Gombe State University, Gombe, Nigeria (Sabbatical),

2Department of Geology, University of Calabar, Calabar, Nigeria

Abstract : Seismic and checkshot data, geophysical data and biostratigraphic data extracted from

ditch cutting samples, were used for the sequence stratigraphic analysis of Beta-24 well within the

Beta field, Northern depobelt of the Niger Delta basin. The results revealed three Maximum

Flooding Surfaces and three Sequence Boundaries. Systems tracts; Highstand Systems Tract,

Transgressive Systems Tract and Lowstand Systems Tract were identified within the well.

Foraminiferal bioevents: Last Downhole Occurrence (LDO) of Globigerina ampliapertura at

2928m, First Downhole Occurrence (FDO) of Bolivina ihuoensis at 2768m and the LDO of Bolivina

imperatrix at 2648m delineated from the well were used to date the interpreted surfaces with the aid

of the Niger Delta chronostratigraphic chart. The ages assigned to the Maximum Flooding Surfaces

(MFS: 1, 2, 3) were 38.0Ma, 36.8Ma, 35.9Ma respectively and sequence boundaries (SB: 1, 2, 3)

were 37.3Ma, 36.3Ma and 35.4Ma respectively. The depositional environment of the sediments

penetrated by the well were inferred based on well log signatures and biostratigraphic information to

have fluctuated from non-marine through coastal deltaic to marine. The Maximum Flooding

Surfaces and Sequence Boundaries identified from the foraminiferal-well log sequence stratigraphy

were tied to seismic using checkshot data. Seismic stratigraphic interpretation revealed two key

bounding surfaces, two systems tracts and two major faults in the field. The faults (F2 and F3) were

responsible for the structural truncations that limited the recognition of strata terminations in the

vicinity of the well. An observation of the horizons within the systems tracts revealed that they may

have potentials to serve as excellent reservoir rock and seals.

Keywords: Sequence stratigraphy, Seismic stratigraphy, Biostratigraphy, Northern depobelt, Niger

Delta.

I. INTRODUCTION

The concept of sequence stratigraphy has emerged as an important exploration tool for hydrocarbon

in the Niger Delta basin fills. Sequence stratigraphy is the analysis of rock relationships within a

chronostratigraphic framework of repetitive, related genetic strata bounded by unconformities or

their correlative conformities (Posamentier et al., 1988; Van Wagoner, 1995; Onyekuru et al., 2012;

Nton and Ogungbemi, 2011). Sequence stratigraphy is achieved through the integration of data sets

such as petrophysical well logs, biostratigraphic and seismic data; it involves the delineation of

stacking patterns and the key surfaces that bound successions as defined by their strata stacking

patterns (Catuneanu et al., 2011). Sequence stratigraphic analysis is very important in hydrocarbon

exploration as it gives companies competitive edge and reduces their risk of bidding on offshore

fronts (Oyedele et al., 2012). This technique has been widely accepted in the oil and gas industry

because it provides logical and powerful tool for the analysis of chronostratigraphic related

sedimentary packages as well as serving as useful guide in predicting facies relationship (Van

Wagoner et al., 1988).

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The Niger delta is considered one of the highest hydrocarbon producidiscoveries of giant oil fields in the deepThe delta has prograded in southwest direction from the Eocene to the present, forming depobelts which represent the various active stages of its development (Doust and Omatsola, 1990). The five depobelts that constitute the delta are: Northern, Greater Ughelli, Central Swamp, Coastal Swamp and Offshore depobelt (Fig. 1). The Niger Delta is one of the largest regressive

with an area of 75,000 km2 and estimated 9000

2012; Oresajo et al., 2015). The study area is located in the Northern depobelt, considered the oldest productive hydrocarbon depobelt in6°N and Longitudes 5°E 6°E (Fig.1). The depobelt has high hydrocarbon potential because of the reported high discovery of oil and condensate (Esedo and Ozumba 2005; Avuru Oladotun et al., 2016).

Figure 1: Map of the Niger Delta depobelts showing the location of “Beta field” in the northern depobelt (modified from Ejedavwe et al. 2002).

Previous studies have investigated the concept of sequence stratigraphy in relation to hydrocarbon

exploration in different parts of the Niger Delta. Olusola and Olusola (2014) analyzed the clastic

sedimentary succession in the Olay Field, Niger Delta usin

which involved integrating 3D seismic data, checkshot data for six wells and suites of well logs from

5 wells. Oresajo et al., (2015) subdivided the stratigraphic section within the Emi Field; eastern

Niger Delta into genetically related packages bounded by significant chronostratigraphic surfaces


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2017, All Rights Reserved

The Niger delta is considered one of the highest hydrocarbon producing basins in the world, with the discoveries of giant oil fields in the deep-water areas making it a key target for exploration activities. The delta has prograded in southwest direction from the Eocene to the present, forming depobelts

various active stages of its development (Doust and Omatsola, 1990). The five depobelts that constitute the delta are: Northern, Greater Ughelli, Central Swamp, Coastal Swamp and Offshore depobelt (Fig. 1). The Niger Delta is one of the largest regressive

and estimated 9000–12,000m of clastic sediments thickness (Ojo

., 2015). The study area is located in the Northern depobelt, considered the oldest productive hydrocarbon depobelt in the Cenozoic Niger Delta that lies between Latitudes 5°N and 6°N and Longitudes 5°E 6°E (Fig.1). The depobelt has high hydrocarbon potential because of the reported high discovery of oil and condensate (Esedo and Ozumba 2005; Avuru

Figure 1: Map of the Niger Delta depobelts showing the location of “Beta field” in the northern depobelt

(modified from Ejedavwe et al. 2002).



sedimentary succession in the Olay Field, Niger Delta using the sequence stratigraphic principles


., (2015) subdivided the stratigraphic section within the Emi Field; eastern

o genetically related packages bounded by significant chronostratigraphic surfaces


- 2018 [ISSN: 2455-1457]

406

ng basins in the world, with the water areas making it a key target for exploration activities.

The delta has prograded in southwest direction from the Eocene to the present, forming depobelts various active stages of its development (Doust and Omatsola, 1990). The five

depobelts that constitute the delta are: Northern, Greater Ughelli, Central Swamp, Coastal Swamp and Offshore depobelt (Fig. 1). The Niger Delta is one of the largest regressive deltas in the world

12,000m of clastic sediments thickness (Ojo et al.,

., 2015). The study area is located in the Northern depobelt, considered the oldest the Cenozoic Niger Delta that lies between Latitudes 5°N and

6°N and Longitudes 5°E 6°E (Fig.1). The depobelt has high hydrocarbon potential because of the reported high discovery of oil and condensate (Esedo and Ozumba 2005; Avuru et al., 2011;

Figure 1: Map of the Niger Delta depobelts showing the location of “Beta field” in the northern depobelt



g the sequence stratigraphic principles


., (2015) subdivided the stratigraphic section within the Emi Field; eastern

o genetically related packages bounded by significant chronostratigraphic surfaces


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@IJRTER-2017, All Rights Reserved 407

based on wireline logs from six wells, 3D seismic data, checkshot and biostratigraphic data.

Onyekuru et al., (2012) employed the sequence stratigraphic methods to reveal four 3rd order

depositional sequences bounded by three Sequence Boundaries, three 3rd order Maximum Flooding

Surfaces dated 15.9, 17.4 and 19.4 Ma, respectively and Systems Tracts in the “XB Field”, Central

Swamp Depobelt, Niger Delta Basin. Nton and Ogungbemi (2011) employed the concept of

sequence stratigraphy in the K-Field, within the western Niger Delta by integrating wire line logs of

four wells and high resolution biostratigraphic data. Adegoke (2012) integrated well logs and

biostratigraphic data of three wells within the western offshore Niger Delta for the sequence

stratigraphic analysis of sediments penetrated by the wells.

The application of the concept of sequence stratigraphy is believed to improve the knowledge of

stratigraphic framework, hydrocarbon distribution and facies geometry across several fields in the

Cenozoic Niger Delta sedimentary basin as many studies have provided baseline for stratigraphic

interpretation of these strata (Onyekuru et al., 2012; Oyedele et al., 2012; Olusola and Olusola

2014). This present study employs the concept of sequence stratigraphy in order to delineate

sequence boundaries, maximum flooding surfaces and systems tracts for prediction of hydrocarbon

reservoir, seal and source rock in “Beta-24 well” field, Niger Delta basin using integrated well logs

data, 3D seimic volume, check shot data and biostratigraphic data.

II. GEOLOGICAL SETTING AND STRATIGRAPHY

The Niger Delta basin is located on the Gulf of Guinea in equatorial West Africa (Fig. 2). It is one

of the largest regressive deltas in the world (Doust and Omatsola,1990) considered a classical shale

tectonic province (Wu and Bally, 2000). The delta is situated at the southern end of the Benue

Trough exactly at the point where the three arms of a rift triple junction met. This triple junction was

formed during the split of the African and South American plates which also resulted in the opening

of the South Atlantic in the Early Jurassic–Cretaceous (Corridor et al., 2005). The Niger Delta is

bounded to the east by the Cameroon volcanic line, to the west by the Dahomey basin, and the

4000m(13,100-ft) bathymetric contour (Fig. 2). The delta is characterized by coarsening upward

regressive sequences with thickness of 30,000 to 40,000 feet at its maximum and deposited in a

series of offlap cycles truncated by sea level changes (Evamy et al., 1978). The Charcot fracture

zone as well as other fracture zones expressed as trenches and ridges formed during the opening of

the South Atlantic along the oceanic crust control the shape and internal structure of the

delta(Corridor et al., 2005).


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Figure 2: Location Map of the Niger Delta region showing the main sedimentar

The stratigraphy of the Niger Delta basin consists of Cretaceous to Holocene marine clastic strata

that overlie oceanic and some continental crust fragments (Corridor

Cretaceous section is yet to be penetrated beneath the Niger De

nearby Anambra basin (Reijers et al

Niger Delta is divided into three formations representing prograding depositional environments (Fig.

3); the Paleocene to Recent pro-

Eocene to Recent paralic facies of the Agbada Formation which is overlaid by the Oligocene to

Recent fluvial facies of the Benin Formation(Short and Stauble, 1967; Reijers

et al., 2005).


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2017, All Rights Reserved

Figure 2: Location Map of the Niger Delta region showing the main sedimentar

features (Corredor et al., 2005).


that overlie oceanic and some continental crust fragments (Corridor et al

Cretaceous section is yet to be penetrated beneath the Niger Delta basin, it has been inferred from the

et al., 1997; Corridor et al., 2005).The Cenozoic stratigraphy of the


-delta facies of the Akata Formation at the base, overlain by the


Recent fluvial facies of the Benin Formation(Short and Stauble, 1967; Reijers


- 2018 [ISSN: 2455-1457]

408

Figure 2: Location Map of the Niger Delta region showing the main sedimentary basins and tectonic


et al., 2005). Though the

lta basin, it has been inferred from the

., 2005).The Cenozoic stratigraphy of the


delta facies of the Akata Formation at the base, overlain by the


Recent fluvial facies of the Benin Formation(Short and Stauble, 1967; Reijers et al., 1997; Corridor




Figure 3: Stratigraphic column showing the three formations of the Niger Delta (After Tutlle et al., 1999).

III. MATERIALS AND METHODS Materials provided for this study include seismic volume in SEG-Y format, geophysical well logs,

base map, check shot data and ditch cutting samples (intervals 2008 – 3396 m) from Beta field. The

standard micropaleontological sample preparation method involving sample disaggregation and

washing through a 63 micron mesh sieve, drying and picking of the foraminifera and accessory fauna

were used (Armstrong and Brasier, 2005; Ukpong et al., 2017). The foraminiferal statistical data was

imputed into Strata- Bugs (Biostratigraphy Data Management software) for data processing and

integration with the well logs data. Seismic analysis was achieved on Petrel window (version 2014).

The workflow plan involved: loading of seismic and well data, review of seismic and log data after

loading, integration of biostratigraphic data and their calibration with seismic data, identification of

sequence boundaries, maximum flooding surfaces and system tracts. Faults were picked on the

seismic sections along the Dip sections. The time-depth relationship was determined using the check

shot data provided for the study (Fig 4).




Figure 4: Check Shot Chart for the study well

The following methods were adopted for the sequence stratigraphic interpretation:

(a) Foraminiferal biozonation and paleobathemetric interpretation. The P16/17 and P16/17 – P18/19

foraminiferal Zones of Blow (1979) which corresponds to the Late Eocene to Early Oligocene

age was delineated. Paleobathymetric interpretation shows that the environment ranged from

non-marine through shallow inner neritic, inner neritic, middle neritic and outer neritic (Fig. 5).

(b) Geophysical well log interpretation (gamma-ray and resistivity logs) was confirmed from the

lithologic information obtained from the analyses of ditch cuttings using a binocular microscope.

The well log characteristics were used to delineate the different systems tracts (Fig. 5). Sequence

boundaries were delineated at points of inflection from progradation to retrogradation of

parasequences in shallowing upward sand sections, and at the fluctuation points of lowstand

systems tract. Points of high gamma ray and lowest resistivity log response were used for the

identification of maximum flooding surfaces (Vail and Wornardt, 1990;Onyekuru et al., 2012).

(c) From the foraminiferal abundance/diversity curves and well log signatures, three Maximum

Flooding Surfaces (MFS) were identified at 2,900m, 2,768m and 2,528m and dated 38.0Ma,

36.8Ma and 35.9Ma respectively and Sequence boundaries (SBs) at 2840m, 2728m and 2408m

dated 37.3Ma, 36.3Ma and 35.4Ma respectively.Lowstand Systems Tract (LST), Transgressive

Systems Tract (TST) and Highstand Systems Tract (HST) were also identified (Table 1).

(d) The MFSs and SBs identified from the well log-foraminiferal sequence stratigraphy were

integrated and displayed on seismic. The well intersected inline 1740 and crossline 6170 on the

seismic volume (Fig. 6). Structural truncation by faulting and the subsequent attendant strata

displacement around the area were the well intercepted the seismic, made it difficult for stratal

terminations to be identified on the section that intersects the well and as such the surfaces were

traced into sections where reflections are seen to lap out at their depositional limit. Seismic

analyses delineated two key bounding surfaces: maximum flooding surfaces, sequence

boundaries and two systems tracts: Highstand systems tract and lowstand systems tract.

y = 0.848x

R² = 1

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 500 1000 1500 2000 2500 3000 3500

Dep

th (

m)

Time (ms)

Checkshot plot




Figure 5: Foraminiferal Distribution and Stratigraphy of Beta-24 Well within the Beta Field




Figure 6: Siesmic Base Map of Beta Field, Showing well intersected by Inline 1740 and Crossline 6170.




Figure 7: Niger Delta Cenozoic Chronostratigraphic Chart (Reijers 2011)

IV. RESULTS AND DISCUSSION

4.1 LITHOSTRATIGRAPHY

Lithostratigraphic analyses were based on detailed description of ditch cutting samples and gamma

ray log signatures. The result shows an alternation of sand and shale in a progressive pattern. The

sandstone sequences in the well studied were mostly light grey to smoky white, coarse to fine

grained, moderately to well sorted and sub angular to subrounded while the shale units are dark grey,




subfissile to fissile, hard to moderately hard, calcareous and micromicaceous. The variation of sand

and shale in different proportions as well as the foraminiferal information from the ditch cuttings was

used to identify the Lower Agbada Formation as the major lithostratigraphic units in the well field

(Fig 5). The Agbada Formation is comprised mostly of sands and minor shales in the upper section,

and an alternation of sands and shales of equal proportions at lower levels (Reijers et al. 1997). Short

and Stauble (1967) studied the Agbada Formation and described it as being characterized by the

alternation of sandstone and sand units with shale layers. The Agbada Formation has been the major

target for hydrocarbon exploration in the Niger Delta.

4.2 SEQUENCE STRATIGRAPHIC ANALYSES

DATING OF KEY BOUNDING SURFACES

The maximum flooding surface (MFS) and sequence boundaries (SB) are the key bounding surfaces

identified in the well with their corresponding depths. The ages and bioevents were obtained based

on the zonal correlations and cycles of Blow (1969, 1979) and by correlation with the Niger Delta

Chronostratigraphic Chart (Fig. 7). The first Maximum Flooding Surface (MFS) identified in the

well (Fig. 5) was dated 38.0 Ma using the LDO of Globigerina ampliapertura at 2928m

corresponding to a regional marker Uvigerinella-8 and occurred within the P16/P17 foraminiferal

zone of Blow (1979).The second MFS was dated 36.8 Ma. using the FDO of Bolivina ihuoensis at

2768m, and occurred within the P16/P17 foraminiferal zone of Blow (1979) while the third MFS was

dated 35.9Ma based on the LDO of Bolivina imperatrix at 2648m corresponding to a regional marker

(Orogho shale) of the Niger Delta Chronostratigraphic chart (Fig. 7). The surface occurred within

P16/17-P18/19 foraminiferal zone of Blow (1979).

Three sequence boundaries (SB) were equally identified and dated in Beta-24 well, the first was

dated 37.3 Ma. at the depth of 2840m within the P16/17 foraminiferal zone of Blow (1979) while the

second and the third SBs were dated 36.3Ma at the depth of 2728m and 35.4Ma at depth of 2408m

respectively, both occurring within the P16/17-P18/19 foraminiferal zone of Blow (1979) (Fig.

5).The dating of these surfaces was done with the aid of the Niger DeltaChronostratigraphic

Chart.Table 1 shows the age and depths of the key bounding surfaces and systems tracts as present in

the studied well.

SYSTEMS TRACTS

Systems tracts are components of depositional sequences. They are divided into three groups

according to relative sea level at the time of deposition: Lowstand at low relative sea level,

Transgressive as shoreline moves landward (retrogradational) and Highstand at high relative sea

level. The shape and content as well as the stratigraphic order of systems tract can be predicted.

The Lowstand Systems Tract

The Lowstand Systems Tract (LST) was defined at intervals of barren fauna in the upper sections

while the lower section of the well is composed of few benthics and the planktonic foraminifera

Globigerina sp. (Fig. 5).The section is characterized by massive blocky sands. The shale units in this

intervals contained reworked materials of terrestrial origin. The lowstand deposits are derivatives of

gravity movement due to sediments depositions by rivers as they traverse the shelf and upper slope

proceeding towards the incised valleys and canyon along the continental slope (Armstrong and

Braisier 2005). The Lowstand Systems Tract is characterized by interbedding of shale and sandstone

that follow a coarsening upward pattern with initial progradational stacking pattern and a subsequent

aggradational stacking pattern. There is shallowing upward trend of foraminiferal assemblages from

regions with high foraminiferal abundance to complete barren regions (Fig. 5).




Transgressive Systems Tract

Transgressive Systems Tract (TST) identified showed an upward increase in foraminiferal

abundance and diversity, it represents the well sections with the highest foraminiferal abundance and

diversity often culminating in the Maximum Flooding Surfaces. The TSTs identified exhibits

retrogradational geometries. These deposits are composed of sand units characterized by fining and

thinning upward trends which are overlain by the Maximum Flooding Surfaces and underlain by

Sequence Boundaries and Transgressive Surfaces (TS).The TST contains rich preserved microfossils

(Fig. 5). This has been reported to be due to the progressive supply of sediments in the process of

transgression as turbidity current reduces leading to the high occurrences of clear water microfauna

(Armstrong and Braisier 2005).

Highstand Systems Tract

Highstand Systems Tract (HST) are deposits that accumulates when sediment supply exceeds the

rate of accommodation space at the onset of relative sea-level rise (Catuneanu et al. 2011). The HST

deposits are capped by surfaces of erosion or non deposition and their correlative conformity

(Posamentier and Allen 1999). The HSTs delineated were characterized by progradational sequences

with an alternation of sands and shale, occurrences of sparse benthonic foraminifera and complete

absence of the planktic assemblages (Fig. 5). The prograding HSTs consists of gravity flow deposits

with high rate of microfossils reworking with shallowing upward trend of benthic fauna (Armstrong

and Braisier 2005).

Table 1: Sequence Stratigraphic Summary of Beta- 24 Well

Depth Interval (m) Systems Tract Key Surfaces

2008 – 2408 LST -

2408 SB (35.4Ma)

2420 – 2528 HST -

2528 - MFS (35.9Ma)

2528 – 2728 TST -

2728 - SB (36.3Ma)

2728 – 2768 HST -

2768 MFS (36.8Ma)

2768 – 2840 TST -

2840 - SB (37.3Ma)

2840 – 2900 HST -

2900 - MFS ( 38.0 Ma)

2900 - 3180 TST -

3180 – 3396 LST -




4.3 SEISMIC SEQUENCE STRATIGRAPHY

An interpretation of the nature of strata terminations on the seismic profile assisted in the delineation

of systems tracts and key bounding surfaces within the field. Structural truncation by faulting as well

as the resultant stratal displacement hindered the mapping of Maximum Flooding Surfaces (MFSs)

and Sequence Boundaries (SBs) on the inline section that intersects the well (Fig. 8). These surfaces

were however mapped and traced into sections where reflections lapped out at their depositional

limits as seen on the seismic profile. The mapping of these surfaces was on bases of reflection

terminations. The Maximum Flooding Surface (MFS) was mapped as a downlap surface whereas the

Sequence Boundary (SB) was mapped as both onlap and downlap surfaces. The Highstand Systems

Tract (HST) reflectors are found downlaping on the MFS while the Lowstand SystemsTract (LST)

reflections are seen as onlaping and downlaping surfaces on the Sequence Boundary (Fig. 9).Three

types of reflection terminations, two systems tracts and key bounding surfaces respectively were

identified. The Maximum Flooding Surface which defined the lower limit of the Highstand Systems

Tract (HST) formed the basal surface. It is seen as a gently dipping downlap surface along which the

more steeply dipping and prograding facies of the HST lapout at their lowest limit and having a

width of more than 3,900m in its progradational direction.The Sequence Boundary(SB)forms the

lower surface and marks the upper limit of the HST.The surface is more extensive, extending beyond

the area covered by the seismic profile.The SB has a variable inclination, changing from gentle to

steep geometries.This is contrary to the MFS whose inclination is more or less uniform.The SB at its

most shallow part has a gentle disposition while the lowstand facies lapout on the surface in an on

lap pattern.

The Highstand Systems Tract (HST) identified is composed of two facies units, the upper unit

characterized by intermediate frequency, moderate amplitude and moderate to low continuity

progradational unit, and the lower discontinuous, low amplitude and high frequency clinoform unit

which downlappedon the MFS. The presence of the topset unit within the prograding facies indicates

that the reflection pattern is sigmoid and that the interpreted lapout could be an apparent downlap.

The Lowstand Systems Tract (LST) identified overlies the Sequence Boundary, comprising of high

facies continuity units. The reflections onlaping and downlaping on the Sequence Boundary are of

variable amplitude; low frequency high continuity and parallel reflection pattern. The prograding

facies with a wedge shaped external form has an approximate length of 3,900m and width of 1,150m.

The internal reflection is high while the amplitude is high to medium. This facies also display topset

reflections which is an indication that the shape is sigmoid and terminate downward on the

seismically resolved and the delineated MFS in a lapout pattern. This facies unit occurred within a

Highstand Systems Tract because it is bounded at the top by a Sequence Boundary and at the base by

a Maximum Flooding Surface.




Figure 8: A structurally smoothed inline section of the seismic cube showing the position of the well within

the seismic volume. F2 and F3 are interpreted faults, top of VOI and base of VOI represent the top and

base of the interval of interest as mapped on the well data, while the SBs and MFSs are mapped formation

top data from log signatures.




Figure 9: Reflection Pattern, Sequence Boundary and Maximum Flooding Surface Identified onSeismic

Cross Sections in Beta Field

4.4 IMPLICATIONS TO HYDROCARBON EXPLORATION

An observation of the shape and contents as well as stratigraphic distribution of the identified

systems tracts in “Beta-24” well shows they could have good reservoir potentials suitable for

exploration. The Lowstand systems tracts (LSTs) identified contains thick sand units which are

bounded by shales which could act as sealing rocks. The sands could act as good reservoirs while

active growth faults seen in the vicinity of the well could serve as pathways for hydrocarbon upward

migration. The shale unit in the overlying Transgressive Systems Tract and the underlying Highstand

Systems Tract could form excellent seals assuming the right prevailing conditions. Olusola and

Olusola (2014) identified hydrocarbon occurrences within Lowstand Systems Tracts and

Transgressive Systems Tracts reservoirs where the major traps are structural and shales within the

Transgressive Systems Tract. Sands of reservoir quality are also found within the Transgressive

Systems Tracts, especially those occurring within the non-marine to shallow inner neritic

environment (Mitchum et al, 1993). The transgressive sands constitute good potential reservoirs.

These sands are capped by the overlying and underlying shales of the Highstand Systems Tracts and




Lowstand Systems Tract which provided good stratigraphic traps. The hemipelagic - pelagic shales

found in the Transgressive Systems Tract, and the levees of the leveed channel of slope fans are

possible excellent source rock in the well field. Also the Highstand Systems Tracts with identified

thick sandy unitcan serve as important reservoirs as they are also capped by pelagic shales within the

systems tract.

V.SUMMARY AND CONCLUSIONS Sequence stratigraphic analysis of “Beta-24” well has enabled the subdivision of the stratigraphic

succession of the well into packages of genetically related strata bounded by sequence boundaries

(major erosion surface) and maximum flooding surfaces. Foraminiferal information extracted from

ditch cuttings and the Niger Delta chronostratigraphic chart were used for dating the key surfaces

identified. The ages assigned to the Maximum Flooding Surfaces (MFS: 1, 2, 3) were 38.0Ma,

36.8Ma and 35.9Ma respectively and sequence boundaries (SB: 1, 2, 3) were 37.3Ma, 36.3Ma and

35.4Ma respectively. The depositional environment of the sediments penetrated by the well

fluctuated from non-marine through coastal deltaic to marine. The Maximum Flooding Surfaces and

Sequence Boundaries identified from the foraminiferal-well log sequence stratigraphy were tied to

seismic using checkshot data. Seismic stratigraphic interpretation revealed two key bounding

surfaces (Maximum Flooding Surface and Sequence Boundary), two systems tracts (Lowstand

Systems Tract and Highstand Systems Tract), and two major faults. The faults (F2 and F3) were

responsible for the structural truncations that limited the recognition of strata terminations in the

vicinity of the well. The horizons within the systems tracts revealed that they may have potentials to

serve as excellent reservoir rock and seals.The transgressive sands constitute good potential

reservoirs. The pelagic shales of the transgressive systems tract form seals which represent the

(potential) stratigraphic traps in the well field. Hydrocarbon migration pathways within the field

could be through structural truncations by faults and carrier beds.

ACKNOWLEDGMENTS The authors are grateful to the management and staff of the Department of Petroleum Resource

(DPR) and the Nigerian Agip Oil Company (NAOC), for providing the data set for this study. We

are equally grateful to the Departments of Geology University of Calabar, Calabar Nigeriaand

Gombe State University, Gombe, Nigeria for providing conducive environments for the research. We

appreciate South Sea Petroleum Consultants, Port-Harcourt, Nigeriafor their useful inputs.

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