Depositional Environment and Petrophysical Characteristics of "LEPA" Reservoir, Amma Field, Eastern Niger Delta, Nigeria

Headnote

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 30 and 60 and N and Longitude 50 and 80E. It ranks among the world's 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.

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.

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 7000ftand 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 depositional environment 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.

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 driftwhich 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).

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 offshore (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.

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.

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 of planktonic 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 of deposits 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 structure

b. Structure with multiple growth fault

c. Structure with antithetic fault

d. Collapsed crest structure

The structural configuration of the Amma is a large collapse crest roll over anticline trending eastwest 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).

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.

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.

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 depositional environment. 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.

A blocky or cylindrical shape indicates massive or thickly bedded sandstone which is lithologically uniform or with very little thin non-sandy interbedded. This type of sands is characteristics of tidal channel, barrier bars, and fluvial channel sand in the delta plain

A funnel shape curve indicates a coarsening upward trend. This is typical of beach sand, barrier bar sands and stream bars, which are characteristic of shore line deposits and deltaic environment (figure7).

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 of sediment/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,wave rippled 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 Reservoir C are within the cored interval as shown by X which lie the within 12936-13441ft. (Figure 8)

The stacking of the above listed lithofacies aided the reconstruction of the sub-enviroment of deposition of reservoir sand within the cored intervals.

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:

Lithofacies: Bioturbated Sandy Heterolith: Dominantly medium-fine grained, poorly sorted grayish 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 heterolithic crosstratification.

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

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.

Sedimentary Structures: Weakly to moderately bioturbated with heterolithic cross stratification and 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.

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).

Lithofacies: Planar/Parallel Laminated Sandstone

Characterized by coarse grained light brown sandstone,

Sedimentary Structures: Consist of multi directional trough cross bed-sets but also include low angle 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.

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

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

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

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

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.

Sedimentary Structures: Small scale stratification with parallel,nearly horizontal laminated beddings, strongly bioturbated with heterolithic stratification.This section on the Gamma ray log shows a serrated to complex funnel shape indicating lower shoreface sands.

Lithofacies: Laminated Mudstone

Consist of fine - very fine dark grey siltstone with significant proportion of mud and clay

Sedimentary Structures: Predominantly large scale, nearly horizontal planar, laminated beddings strongly 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 bell shape indicating fine grained siltstone at the upper part of tidal channel grading into laminated tidal flat mudstone at the top.

Lithofacies: Bioturbated Sandy Heterolith

Well 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 and suggest probably a Tidal Channel.

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 silts intercalations.

Sedimentary Structures: Consist of bidirectional ripple lamination, low angle planar cross bed sets,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 shape indicating middle to upper shore sands.

Depth Shift

The Depth shift(table 2) is the difference in depth between the core and the wire line log (GR) this shiftwas 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 2 for the core depth, log depth and their corresponding depth shift.

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 curve characteristics 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. The curve 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 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 seen in 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 seawards into 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 and associated 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 strong bidirectional 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 serrated sequence with a sharp (erosional) lower contact marked by coarse lag sands and gravels and a sharp to gradational 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 and clays). The Gamma ray log shape displays a serrated bell (sometimes symmetrical) log curve with gradational top.

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 were labeled 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 is 22.2%, permeability average is 25.37, it is interpreted that the reservoir has good to excellent porosity and good permeability.

Well 3

Reservoir A

This occurs at the depth range of 3642.36 - 3733.8m. it has a gross thickness of 91.44m and net sand 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.

Reservoir C

This occurs at the depth range of 3909.6 - 3985.26m. it has a gross thickness of 76.2m and net sand 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 is interpreted 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 sand thickness 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 lithologic character. 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.

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.

Discussion, Conclusion and Recommendation

By going through the process of core description/interpretation and calibrating with other well data reservoir 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 good reservoir quality. The reservoir quality of the cored section is highly variable.

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 be cored in Amma reservoir.

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

References

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