AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

© 2014, Science Huβ, http://www.scihub.org/AJSIR

ISSN: 2153-649X, doi:10.5251/ajsir.2014.5.2.73.80

Hydrocarbon reservoir characterization of “Keke” field, Niger Delta using 3 seismic and petrophysical data

* P.I. Edigbue1; A.A. Komolafe2; A.A. Adesida3 and O.J Itamuko4

1 Department of Earth Sciences, King Fahd University of Petroleum & Minerals, Saudi Arabia 2 Department of Remote Sensing and Geoscience Information System, Federal University of

Technology, Akure, Nigeria 3Shell Petroleum Development Company, Portharcourt, Nigeria

4Blue Ocean Technical Services Ltd, Abuja, Nigeria Corresponding Author’s E-mail Address: paul_edigbue@yahoo.com; phone: +2347038684403

ABSTRACT

An integrated 3D seismic and petrophysical data was employed to evaluate hydrocarbon reservoir potentials of “Keke” field in the Niger Delta. Two sands units (S1 and S2), which exists between 9127ft and 11152ft were correlated and mapped using gamma ray log while petrophysical parameters were obtained from other well logs. The sand units were further mapped on the seismic sections. Seismic structural attribute (variance) was used to delineate six major faults in the field. Time and depth maps were generated for both top and base of the horizons from which the isopach map was derived. Hydrocarbon in-place was calculated from the obtained seismic and petrophysical parameters in order to unveil the potentials of the reservoirs. Delineated sand units, S1 and S2 are characterized by hydrocarbon saturations of 65% and 81.8% respectively. Isopach map show thicker sediments in the central and southern parts of the field ranging from 70ft to 90ft, isochron and isodepth revealed structural high (anticlines) at the central fault block of the field sandwiched between the growth faults, which are responsible for possible hydrocarbon accumulation. The trapping mechanisms are fault-assisted and rollover anticlines. Hydrocarbon in-place for Sand S1 was estimated to be 14.6million ft3 of gas and 6,011.3 bbl of oil while Sand S2 showed 512.3764million ft3 of gas and 6041.94 bbl of oil. The results show that the trapping mechanisms and the obtained Petrophysical parameters in “Keke” field are favourable for hydrocarbon accumulation. Estimated reserves are satisfactory for further exploration work. Keywords: 3D seismic, Petrophysics, Reservoir and Reserves

INTRODUCTION

Petroleum resources remain very vital to the economy of several nations of the world. The enormous cost of exploration for this all-important resource makes it necessary for the attainment of high level of perfection in the methods adopted for its detection and quantification. Since cost effectiveness is the driving factor in oil and gas industry, interest in reservoir evaluation is channel towards the need to quantify the reservoir with reduced level of uncertainty associated with geological models. Drilling of an oil well is very costly venture coupled with the fact that hydrocarbon reserve are depleting. The deposits yet undiscovered are in more complex geological environments and hence it is important to exploit new development with higher resolution seismic reflection methods. Analysis of seismic data

is the key to reservoirs evaluation and subsurface pore fluid monitoring. While there have been great advances in 3D seismic data processing, the quantitative interpretation of the seismic data for rock properties still pose many challenges. Quantitative seismic interpretation according to Avseth (2005) shows the application of rock physics in reservoir’s parameters’ prediction through integration of seismic data sedimentological information and stochastic methods. In seismic reservoir characterization and/or evaluation, detailed characteristics of reservoir using seismic data are analyzed and described both in quality and quantity. This is done by delineating reservoir parameters such as porosity, permeability, water saturation, pore fluid etc from non- reservoir parameters (Avseth, 2005). The Agbada Formation is a major hydrocarbon reservoir and producer in the Niger Delta Basin

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KEFE6

KEFE4

KEFE2

KEFE1

479000 480000 481000 482000 483000 484000 485000

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1:12952

Gas ZoneOil ZoneWater ZoneUndifferenciated ZoneFluids

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PAUL

09/09/2008

Map

(Tuttle et al., 1990). This Formation consists of sandstone and shale intercalation. The structural setting of the ‘Keke’ field is majorly growth faults and rollover anticlines. Due to its complex internal architecture and structural complexity as a result of growth faults development, there is a possibility of having some bypassed reservoirs or reserves compartment. The use of advanced technological tools in 3D seismic data interpretation and integration with other geological data have yielded great results in mapping several structural and stratigraphic prospects (Srivastava et al., 2005). In this research, 3D seismic surveys, together with well log were adopted to develop a detailed structural interpretation of the ‘Keke’-Field. This research shows how 3D seismic data acquired on the ‘Keke’-Field, Niger Delta Basin images subsurface structure which could not to fully interpret on 2D seismic and well logs. It also aims at delineating the economic reservoir unit(s) within the lithologic sequence and characterize reservoirs both in quality and quantity. STUDY AREA AND GEOLOGY The field under consideration, identified as “KEFE” for the purpose of this study is an onshore oil field in the Niger Delta region, located in the southern part of Nigeria (Figure 1).

Fig. 1: Study Area Location

Niger Delta according to Klett et al.(1997) is situated within the Gulf of Guinea with extension throughout the Niger Delta Province. It is located in the southern part of Nigeria between the longitude 4

O – 9

o East

and latitude 4o

- 6o North. It is situated on the West

African continental margin at the apex of the Gulf of Guinea, which formed the site of a triple junction during continental break-up in the Cretaceous (Doust, 1990). From the Eocene to the present, the delta has prograded southwestward, forming depobelts that represent the most active portion of the delta at each stage of its development (Doust and Omatsola, 1990). These depobelts form one of the largest regressive deltas in the world with an area of some 300,000km

2 (Kulke, 1995) a

sediment volume of 500,000 km3 and a sediment

thickness of over 10 km in the basin depocenter (Michele et al., 1999) . Niger Delta Province contains only one identified petroleum system (Ekweozor and Daukoru, 1994; Kulke, 1995) referred to as the Tertiary Niger Delta (Akata –Agbada) Petroleum System. Extended research by Tuttle et al. (1990) confirmed this one petroleum system with the delta, which was formed at the triple junction related to the opening of the southern Atlantic beginning in the late Jurassic and continuing into the Cretaceous. The delta, based on Ekweozor and Daukoru (1994) and Tuttle et al. (1990) began its development in the Eocene with the accumulation of sediments that are now about 10 kilometers thick. The maximum extent of the petroleum system coincides with the boundaries of the province (Figure 2). The area is geologically a sedimentary basin, and consists of three basic Formation: Akata Agbada and the Benin Formations. The Akata is made up of thick shale sequences and it serves as the potential source rock. It is assumed to have been formed as a result of the transportation of terrestrial organic matter and clays to deep waters at the beginning of Paleocene (Tuttle et al., 1990). According to Doust and Omatsola (1990), the thickness of this formation is estimated to about 7,000 maters thick, and it lies under the entire delta with high overpressure. Agbada Formation is the major oil and gas reservoir of the delta, It is the transition zone and consist of intercalation of sand and shale (paralic siliciclastics )with over 3700 meter thick and represent the deltaic portion of the Niger Delta sequence (Doust, 1990; Tuttle et al., 1990). Agbada Formation is overlain by the top Formation, which is Benin. Benin Formation is made of sands of about 2000m thick (Avbovbo, 1978).

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Fig. 2: Map of the Niger Delta showing the province outline (maximum petroleum system) bounding structural features; minimum petroleum system as defined by oil and gas field center points 200, 2000, 3000 and 4000m bathymetry contours; and 2 and 4km sediment thickness [data from Petroconsultants (1996)]

. METHODOLOGY

The materials used for this research work include the following: 3D stacked depth migrated seismic sections comprising of 400 inlines and 200 crosslines, base map of the study area, check shot survey curve, continuous velocity logs, gamma ray and resistivity logs. Integrated methods involving 3D seismic interpretation and petrophysical data analytical methods were employed to meet the objectives of this research. Interpretation of seismic sections was done interactively in Petrel software from where two (2) sand units were mapped and evaluated by generating various indicator maps such as structural, Isopach, Isochron and isodepth maps. Information is useful in determining appropriate locations for drilling exploratory, appraisal or development wells within a prospect. This study involves the interpretation of 3-D seismic data, which in turn involves the analysis of seismic sections, and through this, the generation of maps for interpretation of anomalies in an attempt to locate structures favourable for accumulation of hydrocarbon. To do this, we bear in mind the following important geological information in seismic traces: i. Seismic method measures only physical properties and their changes, geological properties can be obtained only by inference, ii. reflection evident on a seismic section has time and character,

iii. the reflection time as an expression of the depth and dip of a geological surface and of its velocity. Studies of reflection time allowed us to see folding and faulting of a geological surface and also to recognize structural traps. Further, they allow us to see angular unconformity; between two surfaces and so to recognize truncation and erosions. They also allow us to see patterns of surfaces and so to recognize certain depositional mechanisms; from these, in turn we can make inference about facies. The reflection character is an amalgam of amplitude waveform and polarity. It is an expression of physical properties above, below the reflection surface, and of interferences between reflections from closely spaced reflection surfaces. Studies of reflection character allow us to map variations in the nature of the rocks and so to supplement our interference about their type and condition. Well logs were correlated and appropriate reservoirs bodies were characterized in terms of their petrophysical parameters (Figure 3). Reservoirs were calculated using volumetric method from the integrated information from the 3D seismic interpretations and petrophysical analysis.

Fig. 3: Correlation Panel Showing the Delineated Reservoir

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RESULTS AND DISCUSSIONS

This research defines the structural, petrophysical and volumetric characteristics of the interpreted horizons using 3D seismic and well log data (Figures 3 and 4). It therefore examines the hydrocarbon reservoir in terms of its geologic structures, reservoir properties and reserve estimates. The study integrated a suite of 600 seismic section (400inlines and 200 crosslines) and 5 composite well log data acquired over ‘Keke’-Field, onshore Niger Delta Basin. The seismic section was mainly used for structural interpretation while the logs are essential for petrophysical evaluation of the delineated reservoir. The area obtained from the seismic section when combined with the thickness of the reservoir and other petrophysical parameters such as porosity (Ø), water saturation (Sw) is used in estimating for the volume of hydrocarbon in place of the reservoir. Two sands units (S1 and S2), which exists between 9127ft and 11152ft were delineated from the correlation of four wells using well logs (Figure 3). The lithologic units are consistent across the wells, meaning that most lithologic units are present. Increasing trend of the thickness of the shale units with depth indicate that the sequence is approaching the Akata formation. The two horizons were also delineated on the seismic section as shown in Figure 4(a and b).

Fig. 4 Seismic horizons for (a) Sands 1 and (b) Sand 2

Figure 5 (a, b and c) shows the fault structural map. Six faults labeled F1-F6 (4 major and 2 minor) are interpreted. The map shows the various positions and orientations of the faults. The presence of these faults in the study area is an indication that there is possibility of hydrocarbon accumulation. Weber and Daukora (1975) described faults as good migration path for hydrocarbon into the reservoir rocks. The trend and dip direction of each fault are depicted as interpreted on inline 1300. The two synthetic faults F1 and F2 extend throughout the field and trending west-east; they dip northeast – southwest and southeast – northwest respectively. Another major synthetic fault is F3. It trends northwest – southeast and dips southwest. Fault F4 is another antithetic fault trending northeast – southwest and dips northwest. Fault F5 is probably an extension of fault F3, having the link outside the seismic survey. The Time Structural Maps (Figure 6) shows the two-way-traveltime of the mapped horizons S1 and S2 and highlights the geometry of the reflector (Kearey and Brooks, 1984). Examination of this map shows the presence of structures (growth fault and anticline) that can possibly harbour hydrocarbon in the study area. An anticlinal structure could be observed about the central portion of the study area closing on a

a a

b

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major fault, F1. This shows that the trapping mechanism is a fault-assisted anticlinal structure. Fig.5: (a) Variance Attributes time slice (b) Variance attributes Section and (c). Fault Map Showing the Mapped Faults.

Fig. 6: Time Structural Map Showing the Delineated Structures in horizons (a) S1 and (b) S2

The depth structural map (Figure 7) depicts area where there is structural high. Commercial oil accumulations occur predominantly in the structurally highest part of a given macrostructure in the strike sense, despite viable trapping conditions down plunge (Evamy et al, 1978). In “Kefe” field, the trapping systems are anticlines and fault assisted closures (Figure 7). Fig. 7 Depth structural maps for (a) Sand 1 and (b) Sand 2

a

b

b

c

F

2 F

3

F

4

F

5

a

a b

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Reservoir Properties: Hydrocarbon potentials of the delineated sands were revealed in their Petrophysical properties. Table 1 shows reservoir properties that were calculated for reservoirs S1 and S2 from wells 1 and 4. From this table, it can be deducted clearly that the sands are well sorted with low values of water saturation (Sw). Sand 2 shows more of hydrocarbon potential than Sand 1 in both wells. The effective porosities are satisfactory for a reservoir to be adjudged a producible reservoir. Table1: Petrophysical Properties of Delineated Sands in Wells 1 and 4

The average values obtained for the petrophysical parameters of the delineated reservoirs show that the reservoirs S1 and S2 are highly prolific with maximum values of 65.48% and 81.8% hydrocarbon saturation with 30.20% and 28.19% porosity respectively (Table 1). These two petrophysical parameters are essential in computing hydrocarbon in place. It is worthy of note that the two delineated reservoirs have high potentials for hydrocarbon in well 4 than well 1

Hydrocarbons in Place and Recoverable Resource: The estimation method used for this study is volumetric estimation of the reserve. This volumetric estimation of hydrocarbon reserves involves the integration of various geological parameters obtained from both surface (seismic) and subsurface (well log) geophysical data. Petrophysical parameters such as porosity and water saturation have already been calculated as in the last section. The reservoir for estimation of hydrocarbon in place, which can be located on the structural maps as delineated. The average of the petrophysical parameters for the mapped reservoir was computed to get the corresponding values used in computing for the hydrocarbon in place. Calculations of original hydrocarbon in place were done using the following standard volumetric estimation formula in equation (1) -------------- (1) Where: OHIP = Original Hydrocarbon (Gas/oil) in Place (m

3)

A = Areal extent of the accumulation (ha * 10000) H = Average Net Pay for the reservoir zone (m) Φ = Average Effective Porosity (Fraction) Sw = Average Water Saturation (Fraction) The depth structural maps (Figure 6) displays the gas oil contact (GOC) and oil water contact (OWC) from which the area extent of the reservoir was estimated and hence the eventual hydrocarbon in place. The hydrocarbon – water contact (HWC) of Sands 1 and 2 were found approximately on the good closures of 613m and 2953m depth while oil water contact (OWC) were found to be at 200m and 2934m respectively. Computer cube cell estimation was adopted to calculate the value. The hydrocarbon could be quantified into specific value of gas and / or oil due to availability of neutron and density logs. The results obtained are presented in Table 2.

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Table 2: Reservoir Volumetric Parameters

From the estimation, it can be concluded that the value of hydrocarbon (either gas or oil) in place for both reservoirs, as revealed by this analysis could be said to be in commercial quantity and the profitability may be high which could cater for the expenses of carrying out the exploration and exploitation of both oil and gas. Fig.8. Depth Structural Map showing fluids contacts and Delineated Prospects

CONCLUSIONS

Information derived from 3D seismic data volumes resulted in more understanding of the structural styles and architecture, and in accurate delineation of reservoir blocks in the study area. Within the scope of this work, it has been established that the trapping mechanisms in the study field are fault assisted and rollover anticline. The Petrophysical properties of “Keke” field reflect the ability of the formation to store and produce hydrocarbon. Hydrocarbon saturation parameter concentrates generally at the central and north-eastern parts. Faulting and folding play a prominent role in the definition of the structural setting. These structural features constitute the main structural traps detected in the study area. Sands 1 and 2 have good hydrocarbon potential that made them economically viable. The major anticlinal ridge in the mapped horizon is favorable to the accumulation of hydrocarbon. The work reveal that the central fault block of the field where there are existing wells have structural highs (anticlines) that are sandwiched between the growth faults, which are responsible for possible hydrocarbon accumulation. The isopach map revealed that the reservoir very thick and favourable for hydrocarbon accumulation. The isopach map shows thicker sediment in the central and southern parts of the field. The estimated hydrocarbon in place of the reservoir is satisfactory. Since all the available wells were located consequent to a 3D seismic survey, high resolution 4D seismic survey should be carried out within the field in order to cater for the by passed hydrocarbon in the area. More rigorous stratigraphic framework should be built and integrate more data/information to develop the Prospect. The surrounding fields should also be developed because the reservoirs delineated extend outside the studied field.

ACKNOWLEDGEMENTS

The Authors appreciate the Exxon Mobil for donating workstations and Petrel software used for this research. Our gratitude goes to Chevron Nigeria for providing the seismic and log data. REFERENCES Avbovbo, A.A. 1978. Tertiary lithostratigraphy of Niger

Delta: American Association of Petroleum Geologists Bulletin, v. 62, p. 295 - 300.

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Avseth, P.M.T.M.G. 2005. Quantitative seismic interpretation : applying rock physics tools to reduce interpretation risk: Cambridge, UK; New York, Cambridge University Press.

Doust, H. 1990. Petroleum geology of the Niger Delta: Geological Society, London, Special Publications, v. 50, p. 365.

Doust, H., and Omatsola, E. 1990. Niger Delta, In, Edwards, J. D., and Santogrossi, P.A., Eds.,Divergent/Passive Margin Basins: AAPG Memoir 48: Tulsa, American Association of Petroleum Geologists, v. 48, p. 239-248.

Ekweozor, C.M., and Daukoru, E.M. 1994. Northern delta depobelt portion of the Akata-Agbada(!) petroleum system, Niger Delta, Nigeria, in, Magoon, L.B., and Dow, W.G., eds., The Petroleum System—From Source to Trap American Association of Petroleum Geologists Bulletin, v. AAPG Memoir 60, p. 599-614. .

Kearey, P., and Brooks, M. 1984. An Introductory to Geophysical Exploration: Oxford, London, Blackwell Scientific Publications, 171 -199 p.

Klett, T.R., Ahlbrandt, T.S., Schmoker, J.W., and Dolton, J.L. 1997. Ranking of the world’s oil and gas provinces by known petroleum volumes: . U.S. Geological Survey Open-file Report, p. 97-463 CD-ROM.

Kulke, H. 1995. in, Kulke, H., ed., Regional Petroleum Geology of the World. Part II: Africa, America, Australia and Antarctica: Berlin, Gebrüder Borntraeger, p. 143-172.

Petroconsultants. 1996., Petroleum exploration and production database: : Houston, Texas, Petroconsultants, Inc., [database available from Petroconsultants, Inc., P.O. Box 740619, Houston, TX 77274-0619].

Srivastava, A. Singh, V., Vijayakumar, V., Singh, B., and Gupta, S. 2005. Identification and delineation of subtle stratigraphic prospects by advanced interpretation tools: A case study: The Leading Edge, v. 24, p. 792-798.

Tuttle, M.L.W., Charpentier, R.R., and Brownfield, M.E. 1990. Tertiary Niger Delta (Akata-Agbada) Petroleum System (No. 701901), Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa, in USGS, ed., The Niger Delta PetroleumSystem: Niger Delta Province,Nigeria, Cameroon, andEquatorial Guinea, Africa: Denver, Colorado, U.S. Department Of The Interioru.S. Geological Survey.

Weber, K.J., and Daukoru, E.M. 1975. Petroleum geology of the Niger Delta, Ninth World Petroleum Congress, Volume 2: London, Applied Science Publishers, Ltd, p. 210- 221.