Geochemical Characterization of Agbada Formation, Osioka South Area, Western Niger Delta, Nigeria Omoboriowo A Oluwatoyin# 1, Soronnadi-Ononiwu C Godfrrey* 2 Geology Department, Faculty of Science, University of Port Harcourt, pp.mb 323, East West Road, Choba.Port Harcourt, Nigeria 1 dayoboriowo@yahoo.co.uk, Geology Department,Niger Delta University,Wilberforce Island,Nigeria # 1 Corresponding Author 2 chijiokesor@yahoo.co.uk, 118 Abstact-Ditch cuttings from a well in OML 6 in the Osioka area of Western Niger Delta were subjected to geochemical analysis to determine the TOC and values. The TOC values varied from 1.60w% - 1.80w% with an average of 1.66w%, while the values ranged from 0.018% - 2.848% with an average of 0.42%. The above results show that the TOC values fall above the minimum threshold for hydrocarbon generation potential. The minimum threshold value for TOC in the Niger Delta is 0.%. The average value of 0.42% is also indicative of good source rock potential for the studied samples. The transformation ratio which serves as a quantitative analysis to determine the level of maturity shows an average value of 0.2 against the minimum threshold value of 0.16. The implication, therefore, is that the sediments from the studied depth slice can be regarded as good source rock for hydrocarbon generation. Keywords: sourcerock,pyrolysis,ditch cuttings,formation. 1. Introduction Most people now believe that oil and gas are formed when the remains of dead animals and plants are mixed with sediments, buried and formed into rocks and then heated deep underground. The oil and gas then seep out through porous rocks where they may or may not collect in an oil or gas field. Geochemistry, particularly organic geochemistry tries to find if the rocks in an area are of the right sort and the right amount to form oil or gas. The mechanism of the transformation of the sedimentary organic matter into oil and gas is known as pyrolysis. These transformations take place in a sedimentary rock usually called a SOURCE ROCK. It is important, therefore, to recognize these rocks in the early stages of petroleum exploration, for their evaluation. The presence of more than one source rock in an area makes it more attractive. An estimate of how prolific the source has been and some indication of the nature of the hydrocarbon products (oil or/and gas) is valuable for effective exploration International Journal of Science & Emerging Technologies IJSET, E-ISSN: 2048-8688 Copyright ExcelingTech, Pub, UK (http://excelingtech.co.uk/) of petroleum. The Niger Delta stands among the world s best studied delta complexes. Three lithostratigraphic units are recognized in the Niger Delta namely Akata, Agbada and Benin Formations (Sort and Stauble, 1967). Source rock of the Niger Delta hydrocarbon has been a subject of controversy. Short and Stauble (196) and Frankl and Cordry (1967) proposed the shale of the paralic Agbada Formation as the source rocks, while Weber and Daukoru (197) and Ekweozor and Daukoru (1984) argued that in most parts of the delta, the Agbada Formation is immature and suggested the source rock to be the marine shale of Akata Formation which are more mature. It is in this regard, that the present work is aimed at evaluating the source rock qualities of the Akata Formation, with a view to further understanding the petroleum prospects of the Niger Delta region. 2. Location of Study Area The studied samples were recovered from an interval of an appraisal well within the Osioka area of the Western Niger Delta complex. (Fig1). ffffeeeeeeedd Figure 1:showing the location (Osioka) of the study Area
119 3. Aim of Study The aim of present study is to carry out geochemical characterization of a sedimentary section from a depth slice within OML 6, in the OSIOKA area of the Western Niger Delta. The characterization involves analysis and interpretation of source rock parameters in order to determine the hydrocarbon source potential of the studied sediments. 4. Methodology Sample Preparation: Selected sample interval (Table 1) the samples were oven dried properly after which they were ground individually. Table 1: Selected Sample Interval Sample Number Depth interval (meters) 1. 2780 278 2. 278 2790 3. 2790 279 4. 2800 280. 2810 281 6. 2880 28 7. 288 2890 8. 2890 289 9. 3030 303 10. 3040 304.Evaluation Techniques: The 10 drill cuttings were subjected to a geochemical analysis in order to characterize their petroleum generation potential. The analytical methods involves are: (a) Extraction and Fractionation of soluble organic matter () from the samples and (b) Determination of total organic carbon (TOC) content Total Organic Carbon (TOC): TOC determination is done to estimate the quantity of organic matter in each sample. The basic principle behind this is that organic carbon is determined by a mixture of hydrogen tetraoxosulphate (iv) acid and aqueous potassium dichromate (K 2 Cr 2 O 7 ).After complete oxidation from the heat of solution and external heating, the unused or residual K 2 Cr 2 O 7 (in oxidation) is titrated against ferrous ammonium sulphate. The used K 2 Cr 2 O 7, the difference between added and residual K 2 Cr 2 O 7 gives a measure of organic content of sediment. Soluble Organic Matter (): To determine source rock potential, maturity and depositional environment. The significance of this is that extraction and the determination of yield of soluble organic matter () allow for identification of hydrocarbon rich sediments, while the ratio of soluble organic matter () to the total organic carbon (TOC) gives an indication of the maturity status of hydrocarbon generative potential of the source rock.. Results The total organic carbon content (TOC) of the ten (10) analyzed samples varied from 1.60wt% - 180wt%, with an average of 1.66%. Table 2, shows the end point and TOC values obtained. Table 2 - Total organic carbon content of the study area. S/N0 Depth Interval End Point Values TOC (Wt%) Rating 1. 2780 278 1.92 1.80 Good 2. 278 2790 1.9 1.67 Good 3. 2790 279 1.79 1.70 Good 4. 2800 280 2.2 1.60 Good. 2810 281 2.2 1.61 Good 6. 280 28 2.23 1.61 Good 7. 288 2890 2.0 1.6 Good 8. 2890 289 2.02 1.66 Good 9. 3030 303 2.08 1.64 Good 10. 3040 3-4 2.0 1.6 Good The total organic carbon content of the Well samples analyzed varies from 1.60% - 1.80% with an average TOC value of 1.66wt%. These values are in agreement with the 0.4 4.4% range reported by Ekweozor and Okoye (198) for the Agbada Formation in the Niger Delta. It has been established from various studies that TOC of 0.% is the standard minimum threshold value for source rock to generate hydrocarbon. Therefore an average TOC value of 1.66% for samples studied is well above the minimum threshold for hydrocarbon generation. Ronov (198) states that the ability of a rock to generate and expel hydrocarbon is dependent on the quantity of organic matter present. The quantity of organic matter present in a rock can be evaluated and classified using the total organic carbon content (Philip et al, 1986) as indicated below. TOC (WT %) Grade of Source Rock < 0.% Poor 0.% - 1.0% Fair > 1.0% Good.
120 From the data above, it can be inferred that the analyzed samples which yield organic carbon values are greater than the threshold value (0.%). The organic carbon rating of the source rock can be said to be good to very good. Extractable Soluble Organic Matter (): The samples were subjected to extractable soluble organic matter analysis (),the values were shown (see Table 3) Table 3: Values obtained for the soluble organic matter () of Well samples by Soxtec extraction method. S/N O 1. 2. 3. 4.. 6. 7. 8. 9. 10. Dept h 2780-278 278-2790 2790-279 2800-280 2810-281 280-289 288-2890 2890-289 3030-303 3040-304 Wt of Sampl e (g) Wt of Extrac t 20 0.2187 20 0.1263 (w %) 0.643 0.631 (ppm ) 642 631 20 0.004 0.027 270 20 0.0036 0.018 780 20 0.0079 20 0.0043 0.003 8 0.021 380 21 20 0.0046 0.023 230 20 0.002 20 0.009 0.012 0.029 12 29 20 0.001 2.848 2848 The extractable soluble organic matter () showed a pattern similar to that of the total organic carbon content. The values increased with depth of burial (Table 3). The value of ranges from 0.018% to 2.84% with an average of 0.42%. Deroo et al (1977) have shown that soluble organic matter content in the range of 0.1% - 3.36% is high. The average value for analyzed well samples falls within this range, and is interpreted to be high. Transformation Ratio: The transformation ratio, which serves as a quantitative analysis is an index of maturity. It is investigated as a comparative measure between the values of and those of TOC. Table 4.Average transformation Ratio S/N 1. 2. 3. 4.. 6. 7. 8. 9. 10. Depth 278 2790 279 280 281 289 2890 289 303 2780-278- 2790-2800- 2810-280- 288-2890- 3030-3040- 304 TOC (w%) (w%) TR = /TOC 1.80 0.643 0.36 167 0.631 0.38 1.70 0.027 0.02 1.60 0.018 0.002 1.60 0.0038 0.01 1.61 0.021 0.013 1.6 0.023 0.014 1.62 0.012 0.008 1.64 0.029 0.018 1.6 2.848 1.72 Average transformation ratio is 0.2 According to Deroo et al (1988) values of transformation ratios between 0.002 0.016 indicate no hydrocarbon generation, that is, the standard threshold value of transformation ratio is 0.16. From the table, the average TR value for the samples is 0.2; this is far above the standard threshold value required for hydrocarbon generation. This implies that the sediments are good source rocks.
121 6. Discussion Hydrocarbon source rock evaluation of the samples from study area was carried out to determine whether they are good or poor source rocks. In the study, two criteria were used, namely,organic richness and degree of maturation. The determination of organic richness was based on the amount of organic carbon content and extractable organic matter. The total organic carbon content ranges from 1.60w% - 180wt%m with an average of 1.66%. This implies that they are very good, and fall within the range expected in the Niger Delta area (Ekweozor et al, 1984). The extractable organic matter also tends to increase as the depth of burial increases. The values obtained were interpreted to be high ( values of 0.018% - 2.848%, average of 0.402%). The level of maturity of the sediment to produce hydrocarbon was determined using the transformation ratio (TR). This is a ratio of the extractable soluble organic matter to total organic content (/TOC). The values tend to fluctuate. The highest value is obtained at 3040m-304m depth. The ratio of /TOC contained in sediments is a measure of the transformation of kerogen into hydrocarbon. It is low in immature sediments, but increases sharply in mature ones (Cavaliere, 1978). In this study, although initial high values at 2780m- 278m were recorded, this decreased and then rose sharply at 3040m-304m, suggesting that the sediment penetrated probably vary from immature at the top to mature at depth. Deroo et al (1988) has stated that values of 0.002-0.016 indicate no hydrocarbon generation. The average value of TR for the studied samples exceeds this threshold. Therefore, the samples can be said to be fairly mature and hence good source rock material. 7. Conclusion The result of the various geochemical analyses carried out within the studied sedimentary section shows that the samples exhibited the qualities of good source rock. The TOC values range from 1.60%-1.80% with an average of 1.66%. According to Bordenare et al (1993), the TOC of a sediment is the basic parameter which is required to interprete any other geochemical information obtained by other methods. Therefore, good source rocks have high TOC values. The average TOC value for the studied samples is in agreement with the 0.4-4.4% range reported by Ekweozor and Okoye (198) for the Niger Delta source rocks. It is generally accepted that good shaly source rock of liquid petroleum should normally have a minimum average TOC of 1 2wt%. Therefore, it is reasonable to conclude that the sampled section has the optimum Kerogen concentration to produce petroleum. This view is reflected in the extractable soluble organic matter values. The maturity status of the sediments show a fairly good agreement with the views of Short Sand Stauble (196), and Frankl and Cordry (1967) that the shales of the parallic Agbada Formation are source rocks in the Niger Delta. However, this differs with the views of Weber and Daukoru (197) and Ekweozor and Daukoru (1984) that in most parts of the delta, the Agbada Formation is immature as to be the source rock. References [1] Allen, J.R.,: Sedimentation in the Modern Delta of the River Niger, West Africa. Proc. 6 th Conference of Sedimentologists p. 26-34, 1963 [2] Bordenave, E., Ph. Betrand, M.L., Brosse, E., Espitalie J., Houzay, J.P., Pradier B., Vandenbroucke U. and Walgenwitz F., 1993. Geochemical methods and tools for source rock appraisal. In Applied Petroleum Geochemistry, ed, 1993. [3] Cavaliere, R.:Source rock evaluation of the Niger basin. Unpublished Agip report. Pp. 4 NAOC, 1978. [4] Deroo G., Herbin J.P., Roucache J. and Tissot B.,. Organic geochemistry of some Cretaceous black shales from sites 367 and 368,1977. [] Ejedawe J.E. and Okoh S.U.,. Prediction of Depths of Petroleum Occurrence in the Niger Delta. Oil and Gas Jour. V. 79, p190-207, 1981 [6] Ejedawe, J.E.,. Pattern of Incidence of Oil reserves in the Niger Delta Basin. Amer. Assoc. Petrol. Geol. Bull. V. 6, N0. 9 p.174-18, 1981 [7] Ekweozor C.M. and Daukoru, E.M., Petroleum Source Bed Evaluation of the Tertiary Niger Delta. Amer. Assoc. Petrol. Geol. Bull. V. 68, p. 390-394, 1984. [8] Ekweozor C.M. and Okoye N.V.,. Petroleum Source Bed Evaluation of the Tertiary Niger Delta. Amer. Assoc. Petrol. Geol. Bull V. 64, p. 121-129, 1980 [9] Evamy B.D., Haremboure J.W., Kameling P., Kaap W.A., Lolloy F.A. and Rowlands P.H., Hydrocarbon Habitat of Tertiary Niger Delta. Amer. Assoc. Pet. Geol. Bull. V. 62 p. 1-39, 1978 [10] Frankl E.J. and Cordry E.A., The Niger Delta Oil Province- Recent Developments Onshore and Offshore: Proc. 7 th World
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