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Int. J. Pure Appl. Sci. Technol., 10(2) (2012), pp. 38-61

InternationalJournal ofPure andApplied Sciences and Technology

ISSN 2229 - 6107

Available online atwww.ijopaasat.in

Research Paper

Depositional Environment and Petrophysical

Characteristics of LEPA Reservoir, Amma Field,

Eastern Niger Delta, Nigeria

A.O. Omoboriowo1, *

,K.C. Chiadikobi

2and O.I. Chiaghanam

3

1Department of Geology, University of Port Harcourt, Port Harcourt, Nigeria

2, 3

Department of Geology, Anambra State University, Uli, Nigeria

* Corresponding author, e-mail: ([email protected])

(Received: 18-5-12; Accepted: 9-6-12)

Abstract: The LEPA reservoirs penetrated by five wells drilled in Amaa field,Niger Delta, were investigated for its depositional environments and

petrophysical characteristics. The porosity ranges from very good to excellent and

the permeability vary from good to excellent. Assessment of the depositional

environments is based on the integration of well logs and core data. Lithofacies

analysis is grouped into facies association comprising tidal channels, Upper

shoreface, and Lower shoreface. A mixture of marine reworked sands and

subordinate fluvial sands, marked by erosion base characterises the tidal channel.

The Upper shoreface facies consist of coarsening upward sequence, sandstone

succession of fine to very fine sand facies associations and these were caliberated

with selected logs to allow the field correlation. The result available from

integration of wireline log and core data reveal that the environment of deposition

of LEPA reservoir sands lies within the marginal marine environments.

Keywords: Reservoir, Lithofacies, Depositional environment, Wireline log,

Shoreface, Porosity, Permeability.

Introduction

The Niger Delta Basin occupies the Gulf of Guinea continental margin in equatorial West Africa

between Latitude 30and 6

0and N and Longitude 5

0and 8

0E. It ranks among the worlds most prolific

petroleum producing Tertiary Deltas (Selley, 1997). The stratigraphy, Sedimentology, structural

configuration and paleo environment in which the reservoir rocks accumulated have been studies by

various workers. These include (Short and Stauble, 1967; Weber, 1971; Weber and Daukoru, 1975;Evamy et al;, 1978; Rider;, 1996; Selley,1997 and many others.

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Int. J. Pure Appl. Sci. Technol., 10(2) (2012), 38-61. 39

Once an accumulation of petroleum has been discovered, it is better to characterize the reservoir as

accurately as possible in order to calculate the reserves and to determine the most effective way of

recovering as much of the petroleum as possible. Tinker,(1996) defined reservoir characterization as

the quantification, integration, reduction and analysis of geological, petrochemical, seismic and

engineering data. This research work aims at determining the various depositional environments and

creating a conceptual depositional model for the Amma reservoir sand based on sedimentological

studies using core and log data (Figure 1). However, increase confidence in reservoir characterization

and architecture is provided by integration of a large number of well data. The goal of this study is to

provide a better understanding of the distribution of reservoir properties (porosity, permeability) and

other sedimentological features likely to have an impact on fluid flow.

Figure 1:Data Requirement for Reservoir characterization.

Study Location

The Amma -Field is located in OML-XYZ of the swamp region of the Niger Delta Nigeria. See figure

2. A total of 5 well have been drilled in the Amma structure encountering 19 reservoirs between

depth of 7000ft and 12000ftss,13 of these reservoirs are oil bearing while 6 are gas bearing two of the

oil bearing reservoirs are planned for further development, no hydrocarbon bearing reservoirs were

logged in well one. There are 7 completed drainage point in the four wells all producing under

primary recovery techniques. Total cumulative oil production as at 1-12-2005 is 3MMstb.

Objectives

1. To use well log data to study the distribution of Petrophysical properties and depositionalenvironment for better understanding of reservoir properties in the study area.

2. Study and established a relationship among Lithofacies.3. Carry out detailed reservoir correlation of the reservoir sands.

SequenceStratigraphy

Engineeringdata

StructuralMap Reservoir

Biostratigraphy

Core DataWirelineLog

DepositionalEnvironment

Depositionalmodel

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Figure 2:Showing the Base Map of the Study Area

Stratigraphic Setting

The stratigraphy of the Niger Delta is a direct product of the various depositional processes prevalent

in the area. The Delta displays a concentric arrangement of terrestrial and transitional depositional

environment (Selley, 1997). The environment can be broadly categorized into three distinct facies

belt. These are (1) Continental Delta top facies (2) The paralic Delta front facies and (3) Pro-Delta

facies. Fluvial process control sedimentation in the lower flood plain of the delta top environment,

while from the mangrove swamp coastward, tidal influence prevail (Figure 3). The dominant process

which construct the beach ridge and barrier complexes along the delta coast are a combination of

large swell waves which approach from the south west and the vigorous longshore drift which the

wave generate. Offshore, the warm Guinea current prevails operating as four independent cells, under

the influence of the convex, seaward coastline of the Niger Delta and the predominant NE-directed

trade wind (Selley, 1997).

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Fig. 3:Recent Depositional environments in the Niger Delta Complex

The above depositional processes from fluvial, coastal, marine including turbidity current coupled

with the rise and fall of sea-level have determined the stratigraphic fill of the Niger Delta.

The Niger Delta basin consists of a series of depocenters or belts (Stacher, 1995). Major structure

building growth fault determine the location of each depobelt. The entire sedimentary wedge was laid

down sequentially in five major depobelt each 30-60km wide, with the oldest lying further inland and

the youngest located off shore (Fig 4) (Reijers 1996). Due to the continuous deltaic progradation

which commenced since in Early Tertiary, the stratigraphic unit in the Niger Delta is strongly

diachronous and difficult to subdivide and correlate using marine biostratigraphic criteria. Hence

sequence stratigraphy is applicable in the delta in that the fundamental building block of the Niger

Delta succession is well defined cyclic offlaping parasequence set.

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Figure 4: Niger Delta Depobelts, Sequence Stratigraphic model and Relations to hydrocarbon

Occurrence (After Selly, 1997)

Each parasequence set consist of a marine clay that represent marine flooding surface, that change

upward into proximal fluviomarine interlaminated silt, sand and clay, usually followed by various

types of lower and upper shoreface sand and coastal plain continental deposit (Selley, 1997). Three

main subdivisions have been recognized in the subsurface of the Niger Delta complex. (Short and

Stauble, 1967; Frankl and Condry, 1967; Weber and

Daukoru, 1976). The basal unit is the Akata Formation, overlain by the Agbada Formation, with the

topmost unit as the Benin Formation.

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Akata Formation

The Akata Formation is the under compacted, over pressured, marine prodelta megafacies of the

Niger Delta basin. It is composed mainly of marine shale with occasional turbidite sandstone and

siltstone (Short and Stauble, 1967). The thickness ranges from 600m to over 6000m and depends on

the shale diapirism. It is thought to be the sources rock of the Niger delta complex. Abundance ofplanktonic foraminifera assemblage indicates deposition of the Akata shale on a shallow marine

environment (Whiteman 1982).

Agbada Formation

The Agbada Formation underlies the Benin formation and consist of interbeded fluviomarine sands,

sandstones and siltstone of various proportion and thickness representing cyclic sequence of offlap

unit (Weber, 1971). Texturally the sandstone vary from coarse to fine grained, poorly to very well

sorted, unconsolidated to slightly consolidated. Lignite streak and limonite coating occur with some

shell fragment and glauconitic (Short and Stauble, 1967). The shale are medium to dark grey, fairly

consolidated and silty with localized glauconitc. Shaliness increases sownward and the formation

passes gradually into the Akata formation. The Agbada Formation constitute a complex series ofdeposits laid down under at least five sub-environments of deposition including holomarine, Barrier

bar, barrier foot, Tidal coastal plain and lower deltaic flood plain (Whiteman, 1982). The thickness

arranges from 0-4 500m.

Benin Formation

The Benin Formation is the Topmost unit, composed of fluviatile gravel and sands. It is described as

the coastal plain sands which outcrop at Benin, Onitsha and Owerri province and elsewhere in the

Delta area (Reyment, 1965). The deposit is predominantly continental in origin and consist of

massive, highly porous, fresh water bearing sandstones with little shale intercalation which increases

toward the base of the formation.Texturally, it consists of fine grained sand and commonly granular. The grains are sub-rounded to

well rounded, poorly sorted and partly unconsolidated. The sand are white or yellowish brown due to

limonitic coat. Plant remains and lignite streak occur in places, with haematite and feldspar grain

(Weber, 1971). It ranges from Miocene-Recent in age although lack of faunal content makes it

difficult to date directly. The thickness ranges from 0-2100m (Short and Stauble, 1967). It is thickest

in the central area of the delta where there is maximum subsidence. The Benin formation is partly

marine, partly deltaic, partly estuarine and partly lagoonal or lay down in a continental upper deltaic

environment (Short and Stauble, 1967; Reyment 1965). To date, very little oil have been found in the

Benin formation

Structural Setting

Growth fault triggered by penecontemparaneous deformation of deltaic sediment are the common

structures in the Niger Delta, (Merki, 1972; Evamy et al, 1987). They are generated by rapid

sedimentation and gravitational instability during the accumulation of the Agbada deposits and

continental Benin sands over the mobile undercompacted Akata prodelta shale. Lateral flowage and

extrusion of the Akata prodelta shale during growth faulting also account for the diapiric structure on

the continental slope of the Niger Delta in front of the advancing depocentre of paralic sediment

(Selley, 1997). (Weber and Daukoru, 1975), recognized four main types of oil field structure

(Figure5)

a. Simple rollover structureb.

Structure with multiple growth faultc. Structure with antithetic fault

d. Collapsed crest structure

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The structural configuration of the Amma is a large collapse crest roll over anticline trending east-

west bounded to the North by the major XX boundary fault, it forms part of the large Baristo

structural trend, the hydrocarbon found at shallow depth are trapped against the southernmost

antithetic fault while at deep levels the hydrocarbon are dip closed in of this same antithetic fault

(figure 5c).

Fig 5:Principal Oil Field Structure of the Niger Delta (from Webber and Daukoru, 1975)

Methodology and Data Source

Different methods of study have been adopted in this research for the evaluation of the Amma

reservoir sands. Various research materials were provided by shell Petroleum Development Company

of Nigeria.

Data Available

Base map showing the structural element and location of wells. See figure 2. Wireline logs (GR, FDC, and CNL). Core photographs.Procedures

The core photographs provided were those of well four. Core photographs were studied and described

from bottom upwards.

The procedure for the description is as follows:

1. Close observation of the core photos noting the general characteristics and geological succession.

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2. Boundaries of each core section were noted.3. Study of sedimentary structures was carried out noting features like crossbedding, lamination

e.t.c. The degree of bioturbation was indicated.

4. Based on the descriptions, lithology and grain size, dominant sedimentary structures, the

lithofacies types were determined and interpreted using the lithofacies classification scheme

(Table 1)

5. Core/log Calibration was carried out by using core information to characterized the well the well

logs.

Table 1: Tabulated Lithofacies Scheme (After S.P.D.C, Nigeria)

Dominant Grain Size Dominant sedimentary structure Secondary sedimentary structure

S C-coarse

M medium

F fine

>90% sand

S(sandstone

Dominant)

H Heterolithic>50% sand

>50% mud

M (mudstone dominant)

>90% mud

M (mudstone0

C (coal)

M

X(Cross bedded)

P (Planar, parallel bedded)

H(hummocky)

W (wave rippled)

C (current Rippled)

B (Bioturbated)

R(Rooted)

F (fossiliferous)

O (organic carbonaceous)

C(cement)

S(siderite)

Id(soft sediment deformed

Slumped, slide, micro-faulted)

Log Shapes

In recent times, the shapes of gamma ray are becoming more important as these have been found to be

very variable, show greater detail and are related to the sediment character and depositionalenvironment. The Gamma ray log is frequently an indicator of shale content. This is related to the clay

content. A bell shaped log with gamma ray value increasing upwards to a lower value indicates

increasing clay content (Figure 6). A funnel shape with the values decreasing regularly upwards

shows a decrease in clay content. The decrease in clay content is correlated to an increase in sand

content and grain size. Shapes on the Gamma ray log can be interpreted as grain size trends and by

sedimentological association as cycles. A decrease in gamma ray value will indicate and increase in

grain size. Small grain size will correspond to higher gamma ray values. The sedimentological

implication of this relationship leads to a direct correlation between facies and log shape.

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Figure 7: Facies indication from Gamma Ray, the idealized examples of both log shapes and

sedimentological facies. (Serra and et al., 1975.)

Result and Interpretation

Facies Analysis

The depositional environments have been inferred for the Amaa reservoir sand. Reconstruction of the

depositional environment is the main aim of facies analysis. Lithofacies can be defined as a body ofsediment/rock with specific lithologic and organic characteristics.(grain size, sorting, sedimentary

structure) which are impacted by a particular set of energy.

Lithofacies can be distinguished in cores but cannot always be distinguished from logs because the

resolution of the logs (minimum 2ft) does not allow subtle difference between some lithofacies types.

Observation from the cored well 4 was used in the analysis of the Lithofacies type. This classification

is based on four descriptors or facies elements (Rider, 1996). They are lithology, grain size, and

dominant sedimentary structure.

Lithology:This is the first and highest order descriptor. It is grouped into:Sandstone (S), Heterolithic (H), Mudstone (M)

Grains Size:This is the second descriptor. Sandstone lithofacies are differentiated into coarse (C),medium (M) and fine grained. Heterolithic lithofacies are differentiated into sandstone (S)

mudstone(m).

Dominant Sedimentary Structure:This is the third descriptor. It can be cross bedded,waverippled e.t.c.

Geological Core Analysis

Up to three reservoirs were identified in Amma well-4 which was correlated across five wells and

they are labelled Reservoir sand A to C. However only the ReservoirCare within the cored interval

as shown by Xwhich lie the within 12936-13441ft. (Figure 8)

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The stacking of the above listed lithofacies aided the reconstruction of the sub-enviroment of

deposition of reservoir sand within the cored intervals.

Figure 9: Correlation of Reservoir sands in well L01 to LO5 in Amma Field,Niger Delta.

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The interpretation of the stacked lithofacies is tied to the interpretation of the wireline logs shapes of

the sand units [i.e matching of the cores and logs. (Figure 10)]

Therefore, the details description of the core samples based on lithofacies (lithology, grainsize, and

colour), sedimentary structures` are presented below:

Core 1: Aama Well 4 (Depth 12863.4-12866.4 ft) Reservoir C

Lithofacies: Bioturbated Sandy Heterolith: Dominantly medium-fine grained, poorly sortedgrayish brown sandstone with vertical gradation to dark colour ripple laminated shaley sands on top

showing a finning upwards sequence: resulting from low energy offshore sediments.Bioturbation is

intense.

Sedimentary Structures: Medium to fine grained sand stones,highly bioturbated with heterolithiccrosstratification.

Depositional Environments: This section on the Gamma ray logs shows serrated bell shape whichis diagnostic of offshore transgressive sands associated with the lower shoreface.

Core 2: Aama well 4(12906.0-12909.0 ft) Reservoir C

Lithofacies: Bioturbated Cross Bedded Sandstones: Dominantly medium-fine-grained,moderately sorted light-Brown sandstones with gradation to dark colour. With graduation to dark

colour rippled laminated shaley sands on top. Bioturbation degree is very high.

Heavy Bioturbation

Bioturbation

Cross Beds

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Sedimentary Structures: Weakly to moderately bioturbated with heterolithic cross stratificationand shale ripple horizontal laminar overlain by medium-fine-grained sands with intense bioturbation

which tent to obliterate the sedimentary structures.

Depositional Environments: This section on the Gamma Ray log shows serrated bell Shaped

signature typical of Tidal channel.

Core 3: Aama well 4 (12936-12951ft) Reservoir C

Lithofacies: Cross Bedded Medium to Fine Sandstone: Medium to fine sandstone,bioturbation level very low, cross bedded, with minor parallel lamination (Depth 12946.5-1295ft).

Grain size increases upward into light brown sandstone with thinner shale intercalation (Depth 12936

12946ft).

Core 4:Aama well 4 (12965-12971ft) (Reservoir c)

Lithofacies: Planar/Parallel Laminated SandstoneCharacterized by coarse grained light brown sandstone,

Sedimentary Structures: Consist of multi directional trough cross bed-sets but also include lowangle bidirectional cross bed and sub horizontal plane plane beds i.e hetrolithics planar cross

stratification. Sporadic to weakly bioturated base overlain by parallel to planar, laminated sands.

Bioturbation very low.

Cross beds

Parallel/Planar Lamination

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Depositional Enviroment: This section on the Gamma ray log shows serrated funnelshape,indicating laminated uppershoreface.

Core 5: Aama well 4 (13022.0-13028.0 ft) (Reservoir C)

Lithofacies: Cross Bedded Bioturbated Sandstone: Characterize predominantly grey fine tomedium- grained sands with erosional base and shale intercalation.

Sedimentary Structures: Consist of trough cross-Bed that displays a general finning upwardstextural trend. The strong bioturbation tend to obliterate the sedimentary structures.

Depositional Enviroment: This section on the Gamma ray log shows serrated bell shapeindicating a heavily bioturbated Tidal channel capping laminated tidal channel.

Core 6: Amma well 4 (13054-13060ft) (Reservoir c)

Lithofacies: Parallel Laminated Sand Stone.It is characterized by fine to very fine grained, very well sorted sandstone interbedded with dark to

pale grey siltstones and mudstones. Sandstone is dominated by planar to nearly horizontal laminated

bedding lamination. The degree of Bioturbation is very high.

Parallel

Lamination

Bioturbation

Cross Beds

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Sedimentary Structures: Small scale stratification with parallel,nearly horizontal laminatedbeddings, strongly bioturbated with heterolithic stratification.This section on the Gamma ray log

shows a serrated to complex funnel shape indicating lower shoreface sands.

Core 7: Aama well 4(13235-13241ft) (Reservoir C)

Lithofacies: Laminated MudstoneConsist of fine very fine dark grey siltstone with significant proportion of mud and clay

Sedimentary Structures:Predominantly large scale, nearly horizontal planar, laminated beddingsstrongly bioturbated with horizontal burrows. The mud layers in the mixture occurs as continuous

drapes forming flaser and wavy bedding.

Depositional Environment: This section on Gamma rays log shows serrated symmetrical to bellshape indicating fine grained siltstone at the upper part of tidal channel grading into laminated tidal

flat mudstone at the top.

Core 8: Aama well 4 (Depth13313ft-13319ft) Reservoir C

Bioturbation with

O hiomor ha Burrows

Planar Lamination

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Lithofacies: Bioturbated Sandy HeterolithWell to moderately sorted brownish sandstones with weak gradation to dark brown coloured ripple

laminated shaley sands on top.intensly bioturbated.

Sedimentary Structures: Weak graded bedding with large traces of Ophiomorpha Burrows,

bioturbation increases upwards visible clay occurs as linning to the Ophiomorpha Burrows.

Depositional Enviroment: These sections on the Gamma log shows blocky serrated shape andsuggest probably a Tidal Channel.

Core 9: Aama well 4(13435.0-13441.0 ft) Reservoir C

Lithofacies: Cross Bedded Medium to Fine Sandstone Characterized by moderately well

sorted to well sorted, medium to coarse grained light brown sandstones with minor amount of siltsintercalations.

Sedimentary Structures: Consist of bidirectional ripple lamination, low angle planar cross bedsets,with heterolithic planar cross stratification which tends obliterate the sedimentary structures at the

lower section. Bioturbation decreases upwards.

Depositional Environment: This section on the Gamma ray log shows serrated funnel shapeindicating middle to upper shore sands.

Cross Beds

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Figure 9: Integration of logs and cores in the determination of depositional environments.

Depth Shift

The Depth shift (table 2)is the difference in depth between the core and the wire line log (GR) this

shift was carried out because cores are cut during drilling so that their total length are calculated by

adding all the lengths of drill string together.

This is made by matching the core and log and picking out on the logs the missing interval on the core

taking the gamma ray log as reference, the depth on the core is subtracted from that on the

corresponding depth of the gamma ray log. A negative or positive value is achieved based on whether

the depth of the core is higher or lower than the depth on the gamma ray log. See table 2for the coredepth, log depth and their corresponding depth shift.

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Table 2: The Log Depth, Core Depth and their Equivalent Depth Shift

DEPTH SHIFT

Plate Core Wire Line Ft M

1 12863.4 12866.4 128893 12886 20 6.06

2 12906 12906 12925 12928 19 5.76

3 12953 12951 12997 13012 44 13.33

4 12965 12971 13018 13024 51 14.06

5 13022 13028 13053 13059 31 9.39

6 13054 13060 13078 13084 24 7.27

7 13235 13241 13246 13252 11 3.33

8 13312 -13319 13313 13319 0 0

9 13435 13441 13452 13458 17 5.15

Facies Association and Interpretation

Facies associations are groups of facies that occur together and are considered to be genetically or

environmentally related (Reading, 1979).These associations are related to a range of energy level

within an environment of deposition. Due to the resolution of the log data, it is necessary to carry out

some grouping or simplification of lithofacies association in order to get a consistent march with logs

and reservoir property data. The Lithofacies described from well are described in terms of lithology,grading feature, sedimentary structure, and then lithofacies association are interpreted in terms of

depositional environment.

Thus interpreting a facies is in reference to its neighbour. It reflects combination of processes and

environment of deposition, which is the result of the co-occurrence of a set of lithofacies arranged in a

particular order .Log interpretation only was used to infer the environment of deposition of reservoir

sands not within the cored interval (characterization of sands based on geometrical shape of the

curve).

The Marine Enviroment

Marine Shelf: The upper contact is gradational while the lower contact is abrupt. The curvecharacteristics are serrated while the curve shape is bell (fining upwards sequence), resulting from

lower energy offshore sediments being progressively deposited over higher near shore sediment.

Regressive Marine Shelf: The upper contact is sharp while the lower contact is gradational. Thecurve characteristics are serrated while the curve shape is funnel (coarsening upward

sequence).Resulting from higher energy near shore sediments being progressively deposited over

lower energy offshore sediment.

Marginal Marine Enviroment

Barrier Bar Sands: This lies between the continental and the marine depositional realms (Boggs,1987). This is a narrow zone dominated by fluvial, wave and tidal processes This include dunes

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beach, foreshore and shoreface as well as the Tidal channel and Tidal flat which generally constitute a

delta.

On the Gamma ray logs, it shows less serrated funnel shape with gradational base and abrupt contact

i.e coarsening upwards sequence.

Lower, Middle and Upper Shorface Sands: The stacking pattern here indicates a coarsening,thickening upwards sequence and upwards shallowing of water depth in prograding shoreface as seenin the Gamma ray logs

This generally has funnel shape gamma ray logs signature, serrated curve characteristics due to

intermittent alternation of sand and shales the contact could be gradational or sharp at the base though

mostly gradational.

Lower Shoreface: This deposit form under relatively low energy conditions and grade seawardsinto open marine self; composed predominantly of fine to very fine sand but may contain intercalation

of silts and mud. It has small scale cross stratification formed by predominantly landwards migrating

ripples bud planar, nearly horizontal laminated beddings as sedimentary structures. Lamination tends

to be obliterated with intense bioturbation.

Middle Shoreface: Deposit form under higher energy condition owing to breaking of waves andassociated long-shore and rip currents; sediments consist of fine to medium-grained clean sands with

minor amount of silts and shell materials. Sedimentary structures can be highly complex including

ripple cross lamination and trough cross stratification. Trace fossils vertical burrows such as skolithos

and ophiormopha.

Upper Shoreface: Deposits forms within the surf zone in an environment dominated by strongbidirectional translational wave and longhore currents; sediment textures range from sands to gravel.

Sedimentary structures are predominantly multidirectional trough cross-bed sets, but may include

low-angle bidirectional cross-beds sub-horizontal planar beds. Trace fossils such skolithos are not

abundant.

Beach: It is an intertidal zone between extending from mean low tide level to mean high tide level,corresponding to the wave swash: deposits are composed predominantly of fine-to medium-grained

with heavy mineral concentration, well sorted sand that displays sub-horizontal parallel lamination

and low angle seaward-and landward and long shore dipping crossbeds. Bioturbation structures are

rare.

They appear as thick mass of clean sands on with blocky to funnel shape log signature.

Tidal Channel and Tidal Flat

Tidal Channel: Consist predominantly of sands. The gamma log shape displays cylindrical serratedsequence with a sharp (erosional) lower contact marked by coarse lag sands and gravels and a sharp togradational upper contact (within the blocky profile, there are some weak fining upwards trends

exhibiting a hybrid of marine and fluvial origin).Sedimentary structures may include bidirectional

large to small scale planar and trough cross-beds may display a general finning-upwards textural trend

(Boggs, 1987).

Tidal Flat: Deposits comprise of trough flaser and lenticular bedded fine sand muds (silts andclays). The Gamma ray log shape displays a serrated bell (sometimes symmetrical) log curve with

gradational top.

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Figure 10: Vertical profile of the Tidal Channel of well 4

Figure 11: Vertical Facies Profile of the Lower Shorefacace of well 4

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Petrophysical Evaluation

Five wells were provided. The Petrophysical characteristics were determined from wells 2, 3, and 4.

the determination of Petrophysical characteristics were not possible for well 1 and well 5 as a result of

the fact that there was no restivity log in 1 and 5.

A close look at correlated logs shows that there are three reservoir sand bodies for each well and werelabeled as reservoir A, B and C.

Well 2

Reservoir A

This occurs at the depth range of 3657.6 3771.9m. it has a gross thickness of 114.3m and net sand

thickness of 72m. The porosity values of the reservoir range from 17 44 with an average value of

25.5%. The permeability values rages from 73-2055.81 md with average value of 169.36.md.

This indicates that the reservoir has good to excellent porosity and excellent permeability.

Reservoir B

This occurs at the depth range of 3825.24 3901.44m. It has a gross thickness of 76.2m and net sand

thickness of 64.2m. the net to gross is 84.252%. The porosity values of the reservoir ranges from 24

33% with an average value of 30%.

The permeability value from 36.22 299.64md. With average value of 197.02md. it is interpreted that

the reservoir has very good to excellent porosity and has good to very good permeability.

Reservoir C

This occurs at the depth range of 3924.3 4015.74. it has a gross thickness of 106.68m and net sand

thickness of 64.44m. the net to gross is 69.38%. The porosity value of the porosity average value is22.2%, permeability average is 25.37, it is interpreted that the reservoir has good to excellent porosity

and good permeability.

Well 3

Reservoir A


thickness of 51.44m. the net to gross is 56.26% the porosity values of the reservoir ranges from 25.4%

with an average value of the permeability value ranges from 69.231.56md with average value of

172.85md. it is interpreted that the reservoir has very good porosity and has good permeability.

Reservoir B

This occurs at the depth range of 3817.62.6 3886.2m. it ha a gross thickness of 68.52.2m and net

sand thickness of 45.52m. The net to gross is 66.43% the porosity values of the reservoir ranges from

25.52% with an average value of 25.2%.

The permeability value ranges from 69.231 136.56md with range value of 69.23md. it is interpreted

that the reservoir has very good porosity and has good permeability.

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Reservoir C


thickness of 59.2m. the net to gross is 77.69% the porosity values of the reservoir ranges from 23.29%

with an average value of 25.9%.

The permeability value ranges from 25.61 136.56md with average value of 60.08md. it isinterpreted that the reservoir has very good porosity and has good permeability.

Well 4

Reservoir A

This occurs at the depth range of 3672.84 3746.28. it has a gross thickness of 91.44m and net sand

thickness of 63.44m. the porosity values of the reservoir ranges from 30 37% with an average value

of 32.1%.

The permeability value from 159.86 628.75md with average value of 269.76md. it is interpreted

that the reservoir has excellent porosity and has good to very good permeability.

Reservoir B

This occurs at the depth range of 3840.48 3919.68m. It has a gross thickness of 76.2m. The net to

gross is 60.62%. The porosity values of the reservoir ranges from 24 33% with an average value of

30%,

The permeability value from 32.22 229.64md with average value of 179.02md. it is interpreted that

the reservoir has very good excellent porosity and has good to very good permeability.

Reservoir C

This occurs at the depth ranged of 2929.54 4046.22m it has gross thickness of 106.86m and net sandthickness of 90.68m. The net to gross is 85%. The porosity values of the have the average value of

30.1%, permeability has the average value of 188.84md, it is interpreted that the reservoir has the

excellent porosity and very good permeability.

Well Correlation

Well correlation entails determination of the continuity and equivalence of lithologic units,

particularly reservoir sands or marker sealing shale across a region of the subsurface (Tearpock And

Bischke, 1991).

Correlation was done by matching of patters from one log to the other. The lithologic units were

represented in vertical succession by distinct surfaces which represented changes in lithologiccharacter. The GR logs were used for the correlation

Reservoir Architecture and Depositional Model

Reservoir architecture determination requires detailed sedimentological analysis. Based on the core

description and interpretation of well reservoir sand is believed to include shoreface faices running

East West directions crossed by channels of possible tidal or fluvial origin in North South

direction. The L1 reservoirs of the Amma field represent part of a progradational shoreface

environment. It consists mainly of more proximal deposit of distributory channel, tidally influenced

distributory channel and then shoreface facies. As the distributory channel deposit moves seaward, it

encounters tidal current, deposited as tidally influenced sediments. These tidally influenced deposits

cut the shore face.

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Following subsequent transgression, the shoreface deposits are moved into the lagoon forming wash

over fan deposits. As the tidal current settles gradually, sediment carried by it settles forming

suspension fallout deposits.

Based on the classification of sand bodies and their related geomorphologic feature (table 3), the

general depositional environment of the Amma field lies within the marginal marine depositional

enviroments.

Table 3: Classification of Depositional environments of Sand Bodies and their related

Geomorphology features

Discussion, Conclusion and Recommendation

By going through the process of core description/interpretation and calibrating with other well datareservoir architecture is nearly accurately conceptualized. Detailed core analysis shows that the

lithofacies are sandstone of fine to medium-grained texture and different sedimentary structures like

cross and planar bedding, heterolithic stratification and so on as well as trace fossils like

ophiormorpha burrows. Environment of deposition was interpreted by the use of cores and inferred by

comparing the shapes of the gamma ray logs signature with standard log motif of Schlumberger

(1985) to determine whether it is a bell, funnel or block shape. It was deduced that the study area is

within the marginal marine depositional environment and comprise of tidal channel sands, distributary

mouth bars, barrier island (lower, middle and upper shorefaces) and near offshore (the shelf).The

lithofacies are stacked in an upward coarsening succession and fining upward succession and

interpreted to represent progradational shoreface deposits.

The general depositional environment of the Amma reservoir lies within the marginal marine

enviroment. Depositional system and their component facies form the primary building block of goodreservoir quality. The reservoir quality of the cored section is highly variable.

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Based on the findings in this research, the following recommendations are made to guide further oil

exploration in the Amma field and other related nearby oil fields.

1. In order to reduce uncertainties associated with field development, more wells should becored in Amma reservoir.

2. The number of representative wells in the field should be increased.References

[1] K.C.B. Burker and A.J. Whiteman, Uplift, rifting and the breakup of Africa, Proc. Nato. Conf.

on Continental Drift, Newcastle. Academic Press. In: Implications on Continental Drift to

Earth Sciences, D.H. Tarling and S.K. Runcorn, (Eds), 1973.

[2] B.D. Evarmy, J. Haremboure, P. Kamerling, W.A. Knaap, F.A. Molloy and P.H. Rowlands,

Hydrocarbon habitat of tertiary Niger delta,Amer. Assoc. of Petrol Geol. Bull.V., 61(1978), 1-

39.

[3] E.J. Frankl and E.A. Cordry, The Niger delta oil province: Recent development onshore,

offshore, Proc.7thWorld petroleum Congress, 2 (1967), 125-209.

[4] P. Merki, Structural geology of the cenozoic Niger delta, In T.F.J Dessauvagie and A.J

Whiteman (eds), African Geology, Ibadan University Press, Ibadan, 1972.[5] H.G. Reading, Sedimentary Environment Landfacies, Oxford Blackwell Science Berlin,

London, 1978.

[6] R.A. Reyment, Aspects of the Geology of Nigeria, University of Ibadan Press, 1965.

[7] M.H. Rider, Well Geological Interpretation of Well Logs: Interprint Ltd, 2002.

[8] Schlumberger, Well Evaluation Conference, Schlumberger International Houston, Texas.

1985.

[9] P. Stacher, Present understanding of the Niger delta hydrocarbon habitat, In: M.N Oti and G.

Postman eds., Geology of Deltas, Rotterdam, 1995.

[10] S.W. Tinker, Building the 3-D jigsaw puzzle: Application of sequence stratigraphy to 3-D

reservoir characterization, Permain Basin, Amer. Assoc. Petrol. Geol. Bull., 80(1996), 460-

482.

[11] K.J. Whiteman, Nigeria: Its Petroleum Geology, Resources and Potential, Graham andTrotman, 1982.

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