Well Logs 3 D Seismic Sequence Stratigraphy Evaluation of Holu Field, Niger Delta, Nigeria

Transcription

1 Well Logs 3 D Seismic Sequence Stratigraphy Evaluation of Holu Field, Niger Delta, Nigeria John O. Amigun 1, Olumide Adewoye 1, Temitope Olowolafe 1 and Emmanuel Okwoli 2 1 Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. 2 Department of Physics, Kogi State University, Anyigba, Nigeria. ABSTRACT Three Wells, check-shot and 3-D seismic data were used to evaluate the lithology, lithofacies, sequence stratigraphy, seismic facies and depositional environments of Holu field, Niger delta. Its well logs sequence stratigraphic analysis revealed five depositional sequences with associated six sequence boundaries (SB1, SB2, SB3, SB4, SB5 and SB6 ) occur at respective depths of (1905 m, 1680 m, 1550 m, 1385 m, 1345 m, and 1275 m), five maximum flooding surfaces (MFS1, MFS2, MFS3, MFS4 and MFS5) each associated with depositional sequences at respective depths of (1755 m, 1575 m, 1405 m, 1360 m and 1305 m) and five system tracts. The 1 st depositional sequence consist of two system tracts (TST and HST), 2 nd depositional sequence is made up of three system tracts (LST, TST and HST), 3 rd depositional sequence consist of two system tracts (TST and HST), 4 th depositional sequence is made up of three system tracts (LST, TST and HST) and the 5 th depositional sequence also possessed three system tracts (LST, TST and HST). The well logs also revealed dominant lithologies (sand and shale). Log facies shows heamipelagic shale, marine shale, fluvial channel fill sands, transgressive sands and crevasse splay sands. Characteristics seismic facies involving amplitude, continuity, frequency and reflection configuration deduced Holu field environment of deposition to be fluvial systems to marine environments. Keywords: seismic sequence stratigraphy; system tracts; lithofacies; Niger delta; well logs and seismic facies. 1. INTRODUCTION It has been observed over the years that hydrocarbon exploration and exploitation attention has been on structural traps. At present most of the identified structural closures on the shelf and upper slope have been drilled and the search for hydrocarbon is becoming increasingly more difficult and expensive [1]. In a country like Nigeria where oil has been the backbone of her economy, combined geophysical well logs and 3-D seismic stratigraphy approach has not been a doubt an effective exploration tools to delineate lithology, lithofacies, sequence stratigraphy, depositional environment and hydrocarbon reservoirs. Sequence stratigraphy tremendous ability to decipher the earth s geological record of local to global changes has help greatly to improve the predictive aspect of hydrocarbon economic exploration and production. It analyses the sedimentary response to changes in sea level and the depositional trends that emerge from the interplay of accommodation i.e. space available for sediments to fill as well as sedimentation [2]. The success and recognition of sequence stratigraphy stems from its applicability in both mature and frontier hydrocarbon exploration basins, where data-driven and model-driven predictions of lateral and vertical facies changes can be formulated, respectively. Therefore in this study, existing well logs and 3-D seismic data were analyzed and interpreted using the predictive models capability of sequence stratigraphy which has proven to be effective in reservoir characterization and in reducing lithologyprediction risk for hydrocarbon exploration in order to evaluate the Holu field. 2. LOCATION AND GEOLOGY OF THE STUDY AREA Holu field is located within onshore Niger Delta, Nigeria (Figure 1). The Niger Delta is situated in the Gulf of Guinea and extends throughout the Niger Delta Province. It is located in the southern part of Nigeria between the longitude East and latitude North. 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. These depobelts form one of the largest regressive deltas in the world with an area of some 300,000km 2, a sediment volume of 500,000 km 3 and a sediment thickness of over 10 km in the basin depocenter. Niger Delta is divided into three formations, (Figure 2) representing prograding depositional facies that are distinguished mostly on the basis of sand-shale ratios [3] [4]. The Benin formation is the uppermost unit, it consists of massive freshwater bearing continental sands and gravel deposited in an upper deltaic plain environment and extends from the west across the whole Niger Delta area and southward beyond the existing coastline. The thickness of the formation ranges from 305 m in the offshore to 3050 m onshore. 26

2 The Agbada formation forms the hydrocarbon-prospective sequence in the Niger Delta [5]. It is composed of sands, silts and shales in various proportions and thicknesses, representing cyclic sequences of off-lap units. It reaches a maximum thickness of more than 3050 m. The Akata formation composed of shales and silts at the base of the known delta sequence. They contain a few streaks of sand, possibly of turbiditic origin and were deposited in holomarine environment. The thickness of this sequence is not accurately known; but may reach 7000 m in the central part of the delta. Figure 1: Location and Base Map of the Study Area showing Seismic Lines and Wells Figure 2: Niger Delta Stratigraphy. (Modified from [3] [4]). 27

3 3. METHODOLOGY Wireline logs (gamma ray, resistivity, density and neutron), 3D seismic data and check-shot data were examined, analysed and interpreted following a procedure in line with the specific objectives of this study. 3.1 Well logs Analysis Well logs which represent the geophysical recordings of various rock properties in boreholes were employed in this study for facies analyses i.e. lithostratigraphy Identification and sequence stratigraphy. Lithostratigraphy Identification Lithostratigraphy involves the correlation of similar lithology that are commonly diachronous. Gamma ray (GR) log was used to delineate the lithology in the study area (Sand and shale bodies). The sand bodies were identified by the deflection to the left of the GR log due to the low concentration of radioactive minerals in sand while deflection to the right signifies shale which is as a result of high concentration of radioactive minerals in it. Conventionally, GR log is set to a scale of API, central cut off of 65 API units in which less than 65 API is interpreted as sand while greater than 65 API is interpreted as shale. Well log Sequence Stratigraphy Well logs allow the identification of stratigraphic sequences by analysing the stacking patterns of the genetic units. The strata patterns in the sedimentary record of an area are the results of tectonics, eustasy and climate hence, stratigraphic surfaces which signify depositional changes become the key to establishing sequence stratigraphic units of such study area. A depositional sequence is the basic unit of sequence stratigraphy which can be explained as a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities. According to [6] [7], in vertical succession all depositional sequences are composed of the following elements in this order: sequence boundary; lowstand systems tract; transgressive surface; transgressive systems tract; maximum flooding surface; highstand systems tract. The concept of systems tract as applied in this study; define a linkage of contemporaneous depositional systems forming the subdivision of a sequence [8]. The interpretation is based on strata stacking patterns, position within the sequence and types of bounding surfaces. The timing of systems tracts is inferred relative to a curve that describes the base-level fluctuations at the shoreline. In a summary, Table 1 describes the various forms of depositional sequence elements of system tract. From the explanation in Table 1, log patterns are therefore diverse and generally indicative of changing energy regimes through time. According to [9] [10], the ranges of log motifs related to different environment of deposition are shown in (Fig. 3). Hence for Holu field well log analysis carried out in this study, sequence boundaries, maximum flooding surfaces and the system tracts were delineated based on the stacking patterns and log motif and as well as lithologic correlation within the three wells i.e. holu 1, holu 2 and holu 3 shown in Figure 4 and 5. Table 1 : The Depositional Sequence Elements of System Tract and their Description 28

4 Figure 3: The Different Types of Log Motif as related to the Environments of Deposition D Seismic Interpretation To ensure the continuity of events both on the seismic section and well log, a well to seismic tie was done using check shot data. In Figure 6, wells holu 2 and 3 with the sequence boundaries were tied and displayed on the seismic section. The sequence boundaries identified on the wells were mapped directly on the seismic section i.e. where surface is identified by distinctive reflection pattern observed over a layer with relatively large extent (Figure 7). Also, three major faults observed from the seismic section were mapped as well. Seismic Stratigraphy Interpretation The concept of seismic stratigraphy is the deduction of stratigraphy and depositional facies from seismic data. In its application, seismic reflection terminations and configurations are interpreted as stratification patterns, and are used for recognition and correlation of depositional sequences, interpretation of depositional environment and estimation of lithofacies. Seismic sequence analysis subdivides the seismic section into enclosures of concordant reflections, which are separated by surfaces of discontinuity defined by systematic reflection terminations. These enclosures of concordant reflection (seismic sequences) are interpreted as depositional sequences consisting of genetically related strata and bounded at their top and base by unconformities or their correlative conformities. As described in Figure 8(ia), reflection terminations interpreted as strata terminations include erosional truncation, toplap, onlap, and downlap [7]. Afterward the definition of seismic sequences, the environment and lithofacies within the sequences are interpreted from both seismic and geologic data of the study area. Seismic facies analysis which is a geologic interpretation of seismic reflection parameters i.e. configuration, continuity, amplitude, frequency, and interval velocity was further carried out. It involved the recognition of the seismic facies units, definition of their limits and mapping of their areal associations. They are interpreted to express certain stratification, lithologic and depositional features of the deposits that generated the reflections within the units. Major units of reflection configurations and that of prograding configurations are described in Figure 8(ib) and 8(ic) respectively [7]. 29

5 Figure 4: Well Log Sequence Stratigraphy showing the Delineated Lithology, Systems tract and Lithofacies. 30

6 Figure 5: Well Log Sequence Statigraphy showing the Stacking Patterns within holu 1 and 3 Figure 6: seismic-well tie using check shot data at Inline

7 Benin formation Agbada formation Akata formation Figure 7: Seismic Section showing the Sequences and the associate Sequence Boundaries. Figure 8: (ia) Seismic stratigraphic reflection terminations within idealized seismic sequence. (ib) Various seismic reflection configurations and modifications. (ic) Seismic reflection patterns interpreted as prograding clinoforms [7]. (ii) seismic facies units based on amplitude, frequency, continuity and reflection geometry [13]. 32

8 4. RESULTS AND DISCUSSION The lithostratigraphic interpretation of well logs shows that the lithology within the area of study is mainly sands and shale. In Figure 4, the well logs sequence stratigraphy analysis revealed five depositional sequences with associated six sequence boundaries namely; SB1, SB2, SB3, SB4, SB5 and SB6 which occur at respective depths of 1905 m, 1680 m, 1550 m, 1385 m, 1345 m, and 1275 m. Also in Figure 4, five maximum flooding surfaces (MFS1, MFS2, MFS3, MFS4 and MFS5) each associated with depositional sequences were delineated at their respective depths of (1755 m, 1575 m, 1405 m, 1360 m and 1305 m). The first depositional sequence consist of two system tracts (TST and HST), second depositional sequence is made up of three system tracts (LST, TST and HST), third depositional sequence consist of two system tracts (TST and HST), fourth depositional sequence had three system tracts (LST, TST and HST) and the fifth depositional sequence also possessed three system tracts (LST, TST and HST). Figure 5 shows the stacking patterns within the wells, this is interpreted as the depositional patterns of the sediments where MFS represents the turning point from finning upward into coarsening upward. Lithofacies analysis was done across the three wells (Figure 4); log motifs described in Figure 3 were used to classify the facies. The following lithofacies namely; fluvial channel fill sands, marine shale, heamipelagic shale, transgressive sands and crevasse splay sands were revealed. Its detail across the three wells is summarised in Table 2. The depositional environments were mapped across the wells using log signatures and stacking patterns the results were summarised also in Table 2. The environment of deposition varies from one depositional sequence to another i.e. from the top is fluvial / shore face to fluvial / estuarine at the bottom. In seismic sequence stratigraphy, seismic reflection is inferred to represent an isochronous surface except where the reflection surface is an unconformity identified by toplap, baselap, onlap or truncation. In this study, the depositional sequences mapped from the wells were also mapped on the seismic section (Figure 9). These depositional sequences are surfaces of principal unconformities within the basin and the boundaries associated with them are erosional boundaries. The Seismic facies analysis involves critical recognition of reflection characteristics such as continuity, amplitude, frequency and configuration (Figure 10). The Figures 9 and 10 show seismic facies analysis where seismic reflection configurations reveal gross stratification patterns from which lithology, depositional processes and environments were interpreted. Table 3 summarized the result of the facies analysis carried out. Benin formation Agbada formation Akata formation Figure 9: seismic interpretation showing depositional sequences and associated seismic facies 33

9 Figure 10: seismic facies characteristics for seismic facies interpretation Table 2 : Sequence Stratigraphy, Depositionan Environments and Lithofacies Analysis from Wells 34

10 Table 3: Summary of Seismic Facies Analysis 5. CONCLUSION Five depositional sequences within Holu field Niger Delta have been mapped both on well logs and 3D seismic data using sequence stratigraphy approach. Well logs revealed dominant lithologies (sand and shale). Log facies shows heamipelagic shale, marine shale, fluvial channel fill sands, transgressive sands and crevasse splay sands. Majority of the sand bodies are those of the channel-fills. These are sand deposits occurring underneath the paralic sandy sediments of Agbada formation. These same sands corresponded to the lowstand system tract. Similarly majority of shale body are those of marine shale / heamipelagic shale and they fall within late to early HST and late TST of Agbada formation. Characteristics seismic facies involving amplitude, continuity, frequency and reflection configuration deduced the environment of deposition to be fluvial systems to marine environments REFERENCES [1] Oyedele K.F., Oladele S.,Ogagarue D.O. and Bakare K. (2012) Sequence stratigraphic approach To hydrocarbon exploration in Bobsaa field, onshore Niger Delta Journal of Petroleum and Gas Exploration Research Vol. 2(6) pp , [2] Catuneanu O. (2006), principles of sequence stratigraphy, pp , Elsevier Science Ltd., Amsterdam. [3] Shannon, P. M., and Naylor N., (1989): Petroleum Basin Studies: London, Graham and Trotman Limited, p [4] Doust H., Omatsola O., (1990): Niger Delta: In Divergent and Passive margin Basin (Edwards J.D. Santoyrossi P.A. Eds.) American Association of Petroleum Geologists. Memoir 48: [5] Weber K. J., and Daukoru, E. M., (1975): Petroleum geology of the Niger Delta: Proceedings of the Ninth World Petroleum Congress, volume 2, Geology: London, Applied Science Publishers, Ltd., p [6] Sloss, L. L., Krumbein, W. C., and Dapples, E. C. (1949). Integrated facies analysis. In Sedimentary facies in geologic history (C. R. Longwell, Ed.), pp Geological Society of America Memoir 39. [7] Mitchum, R. M. Jr., and Vail, P. R. (1977). Seismic stratigraphy and global changes of sea-level, part 7: stratigraphic interpretation of seismic reflection patterns in depositional sequences. In Seismic Stratigraphy Applications to Hydrocarbon Exploration (C. E. Payton, Ed.), pp [8] Brown, L. F. Jr., and Fisher, W. L. (1977). Seismic stratigraphic interpretation of depositional systems: examples from Brazilian rift and pull apart basins. In Seismic Stratigraphy Applications to Hydrocarbon Exploration (C. E. Payton, Ed.), pp [9] Cant, D. (1992). Subsurface facies analysis.in Facies Models: Responseto Sea Level Change (R. G. Walker, and N. P. James, Eds.), pp Geological Association of Canada, GeoText 1. 35

11 [10] Posamentier, H. W., and Allen, G. P. (1999). Siliciclastic sequence stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology No. 7, p. 210 [11] Serra, O., Fundamentals of Well-Log Interpretation. (2. The Interpretation of Logging Data). Developments in Petroleum Science, 15 B, 684 pp., Elsevier (Amsterdam). [12] Vail, P. R., Mitchum, R. M. Jr., and Thompson, S., III (1977). Seismicstratigraphy and global changes of sea level, part four: globalcycles of relative changes of sea level. American Association of Petroleum Geologists Memoir 26, pp [13] Veeken, P.C.H., (1983), seismic Stratigraphy, basin analysis and reservoir characterization (Klaus Helbig and Sven Treitel, Eds), vol. 37, pp Elsevier Science Ltd., Amsterdam. 36