HIGH HYDROCARBON PROSPECT IN NIGER DELTA COMPLEX, NIGERIA

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1 The Journal of MacroTrends in Energy and Sustainability MACROJOURNALS HIGH HYDROCARBON PROSPECT IN NIGER DELTA COMPLEX, NIGERIA Abatan O.A*.,Ojo, R.K ** *Physics/Electronics Unit, SLT, Moshood Abiola Polytechnic, Abeokuta, Ogun State, Nigeria **Federal College of Education, Osiele Abeokuta, Nigeria Abstract Seismic reflection records comprising twenty lines together with well information acquired from a field in the Niger Delta Complex, Nigeria were studied and interpreted for their geologic setting and possible hydrocarbon prospect in the area. The procedures include location, evaluation and classification of the structural traps in the study area that are favourable to hydrocarbon trappings and accumulations. The major and minor faults are delineated showing the tectonic activity in the area. Time isochrones prepared for the picked horizons, gives a structural setting, which favours hydrocarbon trapping. For an area or well to be hydrocarbon prospective, it should be located within a closure because closure is required for trapping and accumulation of hydrocarbon. The structural information obtained from the area under shows that three well EE1, EE2, and EEW2 drilled there are believed to be hydrocarbon prospective with EEE1 and EEW1 due to their not been closure may be unproductive. Availability of more information on the east side of the seismic location map shows that a well ARI is highly hydrocarbon prospective. Keywords: Seismic reflection records, the Niger Delta Complex 1. INTRODUCTION Exploration for hydrocarbon is capital intensive. The cost for drilling a well is so much that only the services of good imaginative and experienced explorationists are required in the oil industry. Before embarking on drilling, genuine and convincing information on interpreted seismic reflection data of any field of interest cannot be over emphasized. Experience over the years has shown that areas classified as been fully explored for hydrocarbon sometimes give useful additional information when the seismic reflection records 59

2 from those areas are subject to reviewing. Seismic reflection prospecting is a very useful tool in hydrocarbon exploration. The subsurface is mapped by measuring the times required for a seismic wave (or pulse) generated from a near surface explosion of dynamite, mechanical impact or vibration to return to the surface after reflection from interfaces between formation having different acoustic impedances. Variation in the reflection times from place to place may indicate structure features. Such features include anticlines, faults, salt domes and reefs. There are two main approaches to the interpretation of seismic reflection sections. These are: structural analysis and stratigraphical analysis both of which are assisted by seismic modelling. 2. REVIEW OF THE NIGER DELTA COMPLEX ORIGIN The Niger Delta is of the tertiary age and occurs at the Southern end of Nigeria bordering the Atlantic Ocean and extends from about longitude E and N covering about 75000km 2 and composed of an overall regressive elastic sequence reaching a maximum thickness of 9-12km (Evamy et al, 1976) as indicated in Fig.1. Being wave dominant in its build up, its subsurface sedimentary sequence was found to have continental trasitional and marine sediments with sand/shale and shale as 3-fold subdivision (Weber et al. 1975). 60

3 Fig. 1. Sketch geological map of Nigeria showing the Niger Delta Complex 3. EXPERIMENTAL DETAILS In order to interpret the seismic lines and to re-appraise all the reported drilled wells in order to ascertain whether the drilled well(s) were productive or not and to proffer probable reasons, both major and minor faults and seismic sections were identified and marked and transferred to the base map. Dip and strike duration were used to map horizons as represented by seismic events over the area by correlations and timing. Structural contoured maps for the chosen horizons were produced and their classification based on the nature and structural disposition of the faults and their closures and the comparison of the drilled wells 4. STRUCTURE MAPS IN TIME (ISOCHRON) For a first hand appraisal of the surface of the reflecting horizons identified, seismic time isochron maps were prepared. The key requirement is the base or the seismic situation map; the map shows the location of the seismic lines with small circles indicating the major shot points Fig

4 Fig. 2.Isochron Map showing Seismic location Map Faults which were identified on the seismic sections were drawn on the base map, and the time values entered at their appropriate shot points contoured. 5. WELL LOGS ANALYSIS The well logs data were interpreted quantitatively. The gamma-ray (GR) log and the spontaneous potential (SP) curve were used in lithologic identification of the interesting layers. The two most important formations of sedimentary rocks in oil exploration are the sandy and shaly formations. While the earlier contains the oil due to its porosity and permeability, the latter entraps the oil from escaping. Thus the two could be differentiated from the sharp difference in their gamma-ray count. This is fundamental principle and objective of the gammaray log. The SP curve records the electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and a certain ion selective rocks (Shale). 62

5 Both the SP curve and GR log are generally recorded in Track 1 (left track) of the log. They are usually recorded in conjunction with some other log-such as the resistivity or porosity log. Indeed, nearly every log now includes a recording of the SP curve and or GR log. Although relatively simply in concept, the SP curve and GR logs are quite useful and information. Among their uses are the following Differentiate potentially porous and permeable reservoir rocks (sandstone, limestone, dolomite) from non- permeable clays and shales Define beds boundaries and permit correlation of beds Give qualitative indication of beds shaliness Aid the lithology (mineral) identification In the case of the SP curve, permit the determination of formation water resistivity, WR. In the case of GR logs, detect and evaluate deposits of radioactive minerals. Based on the lateralog the area can be divided into four zones or formations, as X, Y, Z and Z 1. Due to its high gamma-ray count, zone Y is automatically counted out since this zone is most likely to be of shally formation. Although zone X has low gamma-ray count, implying sandy formation, it also has low reading on the deep resistivity reading, thus its hydrocarbon prospects is highly improbable. Most likely, this is a water bearing zone. Zone Z and Z 1 show similar characteristics of high readings of the deep resistivity as well as low gamma-ray count. This in the absence of any counter indication such as positive SP curve is a good pointer to hydrocarbon presence. Positive SP curve indicates shally formation, while negative, indicates sandy formation. Zone Z 1 differs from Z by its very high deep resistivity reading, much higher than that of Z, this could possibly be an indication of higher gas-oil ratio. 6. RESULT/DISCUSSION Three reflecting horizons were picked from the seismic sections for structural information. The horizons are fairly horizontal but the lateral continuity were broken by the existence of growth faults. This is observable in both the dip and some of the strike lines. The basis objectives of this paper are to determine the structural setting of the study area, so as to evaluate the geologic feasibility of location hydrocarbons, if any and to re-appraise any drilled wells in order to search for probable new test drilling areas. The structural setting of study area is similar to those discussed by Evamy et al, (1973). Short and Stanble (1967). Growth faults are regarded as a product of gravity sliding during the course of deltaic sedimentation which according to Evamy et al (1977) are caused by the structural instability resulting from the greater rate of deposition over rate of subsidence. This causes the mobile marine shale to flow, making the structure to fracture and slide downwards. In the study area this fault is the main synsedimentary structure and can be seen in all the lines. One important observation worth mentioning is that all the lines, the growth faults show the flattening nature, characteristics of the growth faults in the Niger Delta. Also, the downthrown side is on the inner curvature of the fault as expected. 63

6 The rollover anticlines are observed near the major growth faults in all the lines and they also show some of the major rollover anticline but broken by crustal faults. Those anticlines are the result of a rotational effect caused by gravitational imbalance, thus making the heavier side, which corresponds to the thicker part of the structures, to slide downward in a curvilinear path. The part that slides down is the downthrown side of fault. 7. HYDROCARBON POTENTIALS The structures that favour hydrocarbons accumulation in the Niger Delta need be considered in order to understand the hydrocarbon potentials of the study area. According to Doust et al (1970) most of the hydrocarbon traps found in the Niger Delta are structural and were as a result of synsedimentary deformation. Avbovbo (1978) also stated that hydrocarbons in Niger Delta are trapped by rollover anticlines and faults closures. Doust et al (1990), in describing areas that have structures similar to the one under study, stated that in such simple structures, in which there are few faults, the northern faults blocks commonly contain hydrocarbons and they show a graduation from dip closure at shallow levels to a partial fault closure at intermediate levels, to complete fault closure at deep levels. From the above facts therefore, one can conclude that structurally, the area understudy is a good hydrocarbon potential zone. However, this is possible only if the same rocks have attained the required level of maturity and the reservoir rocks have enough porosity and permeability for the storage and flow of the enclosure hydrocarbon. Also, the presence of an impermeable structure is needed in order to keep the entrapped hydrocarbon in place. 8. ISOCHRON MAPS Isochron maps were prepared for each of the three horizons mapped in the study area (Fig. 2-4). The aim is to monitor the structure trend with depth. A close look of the major faults on each of the maps, shows a unidirectional migration. 64

7 EEW1 EEE1 Fig. 3.Isochron Map showing Seismic location Map A r1 Fig. 4.Isochron Map showing Seismic location Map However, the most interesting feature of the isochron maps is the central part of the area (along line X22 and X23). In all the three horizons, it can be seen that a closure is formed at his location. The presence of closure is necessary for a trap to be formed (Telford et al 1976). Apart from the closure of the central part of the area, in isochron maps of horizons I and III there are other closures. There are some wells drilled in these closure in isochron of horizon one, three closures can be seen and wells EE1, EE2, EEW2 and AR1 have been drilled there, so also in isochron two where wells EE1, EE2, and EE2 were drilled and wells EE1, EE2, AR1 and EEW2 were drilled in the closures of isochron three. 9. WELL LOG RESULTS The shaly formations were delineated from sandy formation by the use of gamma-ray (GR) log and SP curve. The sandy formation was further broken down into sections in order to sift the areas that are likely to be hydrocarbon prospective. This was achieved by the use of the lateralog. Based on the structural setting in the study area and the results of the well logs, the following conclusions are arrived at: i. The wells EE1, EE2 and EEW2 drilled in closures conforms with the results of the present study and are believed to be the hydrocarbon prospective and therefore may have high gas-oil ratio 65

8 ii. Wells EE1 and EEW1 are structurally unproductive and therefore may not be favourable to hydrocarbon trappings and accumulations iii. Availability of data on the East-side of the seismic location map helps in determining the structural pattern of the area which shows that well AR1 is hydrocarbon prospective. 10. CONCLUSION Seismic data analysed consists of twenty seismic sections, six Time-depth (TZ) curves and composite logs for six wells. The seismic sections were interpreted for fault mapping and three horizons were identified, delineated and correlated, throughout the entire sections to represent the, three reflecting surface of interest, the horizons were named I, II, and III. Major growth fault was mapped through the horizons. It was observed that the faults throw increases with depth, thus the first horizon (I) had the smallest throw while the third horizon III had the largest throw. This is in conformity with the behaviour of growth faults found in the Niger Delta. For an area or a well to be hydrocarbon prospective, it should be located within a closure because closure is required for trapping and accumulation of hydrocarbons. A closure is formed at the intersection of lines X24 and X37, X22 and X43 and X22 and X41 in the first horizon (I), thus structurally; the wells EE1, EE2 and EEW2 drilled there respectively would be hydrocarbon prospective. These three wells EE1, EE2 and EEW2 were also located inside closure in horizons II and III which buttress the fact that they are prospective in hydrocarbons. Availability of more data on the East side of the seismic location map helps in determining that well AR1 is hydrocarbon prospective. REFERENCES Akande, S. O. and Erdtmann, B. D. (1998) Burial metamorphism (thermal maturation) in Cretaceous sediments of the Southern Benue Trough and Anambra Basin, Nigeria. AAPG Bulletin 82(6), Akande, S. O. and Ojo, O. J. (2002) Organic petrology and Rock-Eval studies on source rocks facies of the Campanian- Maastrichtian Patti Formation, Southern Bida Basin, Nigeria. NAPE Bulletin 16, Akande, S. O., Ojo, O. J., Erdtmann, B. D. and Hetenyi, M.(1998) Paleoenvironments, source rock potential and thermal maturity of the Upper Benue rift basins, Nigeria: implications for hydrocarbon exploration. Organic Geochemistry 29, American Association of Petroleum Geologists Memoir; Vol. 67, pp Avbovbo A.A. (1978) Geology and Hydrocarbon Productive Trends of Southern Nigeria basin. The Oil and Gas Journal,. Pg Avbovbo, A. A., Ayoola, E. O. and Osahon, G. A. (1986) Depositional and structural styles in the Chad basin of Northeastern Nigeria. AAPG Bulletin 70, Barker, C., Wang, L. and Butler, E. B. (1989) Distribution of bitumens in shales and its significance for petroleum migration.15th International Meeting on Organic Geochemistry, Paris, France. 66

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