sign in sign up
<<
Home , Palynology

Palynological interpretation of the palaeoenvironments of Miocene strata of the well Igbomotoru-1, Niger delta

F.E. OBOH M.B. SALAMI ~~ J.L. CHAPMAN De~~nrtrneiifof GecJ/Ogynrrd Geophysics, fipnrtincwt of Geoli& De~mtrtrerrtof Enrtlr Scieirc~s, Uriir1ersity of Missoirri, Obofriiii Azt~olorcwUirim7rsit!y, Uiriz~ersityof Cninbrirfp,, Rollr?, IIe-lfr, Nigrrin. Dozor?ii~gStrcrt, Missoriri 65401, U.S.A. Cnttrbridgr, CB2 3EQ, U.K.

ABSTRACT Palynological and lithologicalresults have been employed in interpreting the depositional environment of strata penetrated by the Igbomotoru-1 well at the interval 1123-3583 metres. The environment of deposition is largely transitional with marine influence occurring only at the base of the studied sequence. Zonocostitesramonae is frequent to abundant in most samples: this indicates a high influence of mangrove swamp vegetation in the environmentat the time of deposition.The absence or rarity of this species at some horizons within the studied interval is taken as evidence of deposition in a more continental environment or of a swamp forest area composed of non-Rhizophora type trees. The nature of the organic matter points to a source area near the basin of deposition for the . I. ttiicropalaeoritol., 11 (1): 1-6, June 1992.

INTRODUCTION SAMPLE SITE AND PREPARATION Studies on various aspects of the structure, sedimentology, Well Igbomotoru-1 is located at latitude 431'40.3"N and and of the Niger delta have longitude 603'29.6"E within the Niger delta (Fig. 1). It was been published by several workers including Allen (1964,1965), drilled to a total depth of 4325 metres by the Shell Petroleum Frank1 & Cordry (1967), Short&Stauble(1967),Adegoke(1969), Development Company of Nigeria. The sediments vary from Weber (1971), Burke et a[. (1971, 19721, Merki (1972), Murat coarse clastics with occasional lignite streaks (0-1590m) to (1972), Oomkens (1974), Weber & Daukoru (19761, Asseez finer clastics (15904325m).They have been grouped into four (1976), Evamy et a/. (1978), Petters (1979), Ejedawe (19811, informal lithostratigraphical units designated A to D (Fig. 2) Lambert-Aikhionbare (1982), Lambert-Aikhionbare & Shaw which were delineated on the basis of gamma ray log and (1982),Ogbe (1982),Odigi (1987)and Bustin (1988).Despite the sidewall sample descriptions (Oboh & Salami 1989). These fact that exploration and exploitation have been going on in this units correspond to the Benin and Agbada Formations which petroliferous basin for several decades, there is relatively little consist of continental sands and transitional sand and shale published information on the of the sediments. This sediments described by Short & Stauble (1967).Three informal lack of accessible information is probably due to the palynological subzones which are subdivisions of the confidentiality of oil company reports. Crassoretitriletes uunruudshooueni zone of Germeraad rf 01. (1968) The most comprehensive published work to date on the have been established.These subzones arePeveguinipollis iiigericiis palynology of the Niger delta was carried out by Germeraad et (1123-1950m), Retitricolpites sp. 1 (1950-2480m) and d.(1968). The work involved the study of palynomorph Clauamonocolpites sp. (2480-3583m ) lFig.2; see also Oboh & assemblages of Tertiary sediments in some parts of tropical Salami 1989, for further details and photomicrographs of South America, Africa and Asia. Its primary purpose was to palynomorphs]. arrive at a better interpretation of the stratigraphy of those Forty-one sidewall samples were collected from slialy sedimentary basins where other means of correlation have horizons from 1123-3583m depth which has been dated as failed. Other published works on the palynology in this area, Middle to Upper Miocene on the presence of cetain covering the Late Cretaceous to Recent, are largely concerned and palynomorphs (Oboh & Salami 1989).Samplesof about 258 with descriptions of palynomorphs and palynological zonations. were prepared by standard palynological techniques, including However, some of them incorporated environmental oxidation in nitric acid for a few minutes, and the unsieved interpretation in their studies (see van Hoeken-Klinkenberg, residues were strew-mounted on slides. The total area under 1964, 1966; Clarke, 1966; Clarke & Frederiksen, 1968; Knaap, the cover slip of each slide was systematically studied under a 1971; Sowunmi, 1973, 1981; Legoux, 1978; Jan du Ch@ne& Leitz Orthomat - W binocular and identification Salami, 1978; Jan du Ch@neet a/., 1978 and Biffi & Grignani, was attempted for as many forms as possible. Colouration and 1983). size of organic matter present were also determined. This study of sidewall samples from Igbomotoru-1 well in the Niger delta, aims at using both palynological and lithological ENVIRONMENTAL RECONSTRUCTION data to interpret environmental conditions in the area of A. Limitations toreconstruction of environment of deposition deposition as well as highlighting the problems often The main limitations which arise when reconstructing encountered in environmental reconstruction. environments of deposition in deltaic sediments, like those in

1 Oboh, Salami & Chapman

has remained sufficiently close to the equatorial region, the polar advances of the Pleistocene have had a minimal effect on the flora, but the eustatic drop in sea level has resulted in the advance of the coastline (Fig. 3). This minimal effect of the Pleistocene is substantiated by the nearly uniform nature of the palynofloras in thedeltaic sediments since Miocene and Pliocene times (Germeraad et a/., 1968).Several and types found in these sediments appear similar to those produced by the present vegetation zones and can therefore be used as indicators of past environments based on recent ecological information.

B. Methods of interpretation Four features have been employed in this environmental interpretation. They are: 1. Association of some environmentally restricted marker species such as Cvnssoretitriletes uniiraadslzooz)eiii, Magnnstriatites hozcrardi and Pachydermites diederixi. 2. Percentage of Zonocostites mmonue (Rhizophorn type) in the total palynomorph sum. 3. Natureof organicmatter in thesediments. 4. Lithological characters of the strata.

C. Results of environmental study The distribution of some of the palynomorphs from the Igbomotoru-1 well samples is shown in Fig. 2. The various parameters employed in the Fig. Map of the Niger delta showing the location of the well Igbomotoru-1 interpretation of the environment of deposition 1. are discussed as follows: the Niger delta, come from the problems of contamination, and 1. Environmental marker species. Many of the palynomorphs the interpretationof the data collected. Palynologists work with are closely similar in morphology to recent species so that two types of samples: surface and subsurface samples. The assumptions can be made about their ecological requirements latter can be further divided into three classes: (a)conventional (Germeraad et a/., 1968). For example, species indicative of cores, (b)sidewall cores and (c) ditch-cuttings. Both surface and coastal plain environments include Cvnssoretitriletes subsurface samples are susceptible to contamination during zmirandshoozwii (a climbing fern of coastal swamp forest), collection, which in turn has negative effects on stratigraphical Mnpnstrintites hozclnrdi (a small fern of open freshwater habitats), interpretations. Contamination may also occur during sample Pnclz!/dermites rliederiii (an angiosperm of coastal swamps) and preparation from air- or water-borne pollen and or Zoiiocostites rnmoiine (the mangrove group).All four species are human error. Of the three sample types derived from oil wells, present throughout most of the sampled sequence although M. conventional cores are the most reliable because they are the Iiozmrrliis rather intermittent and rare. This last species, according least likely to be contaminated by drilling mud or caving of rock to Germeraad et 01. (1968),is dispersed mainly by water and the strata, whilst the ditch-cuttings are the least reliable for these spore is likely to occur most frequently in inland environments reasons. In a situation in which ditch-cutting have to be used, whereas the distribution of Z. rniiioiiae tends to be coastal and the sampling interval may have to be increased to eliminate the offshore marine. Therefore, the fairly high levels of Z. rniiioiine problems due to caving. A sampling interval of 20-30m is compared with the infrequent occurrenceofM. hozinrdisuggests usually appropriate in such cases. Adequate care must be taken that the sequence recorded here represents mainly a coastal to see that sidewall cores are not contaminated by extraneous environment of deposition or a marine situation where Z. matter. rniiioiine has been transported offshore before deposition. It is important to bear in mind whilst making palaeoecological deductions that climatic conditions and latitudes have changed 2.Percentageof Zonocostites ram0nae.Z.rniiioiincisadistinctive through geological time (Germeraad et nl., 1968, Smith rt nl., pollen type found in extant genera of mangroves notably 1981).Despite the factthat theNigerdelta hasadvanced seaward Rhizophorn (Germeraad et d.,1968). Muller (1959),after study of since its inception, its palaeocoastline in the Middle to Upper recent deposition of pollen in and around the Orinoco delta, Miocene was not substantially different from the present position showed that R/iizophorn produced abundant pollen which can (see Fig. 3), and the present climatic conditions are considered be blown by wind over a wide geographical area. High similar to those of the Miocene (Weber, 1971).Since this delta perceiktges of NiizoyIiorn pollen occur close to the source and

2 Miocene palaeoenvironment, Niger delta

01 m W !? a az 0 z 0' a m n 0 m; 0 m W i 3 W 0 0 m a W W t I- P at ta m P 0 t!a Pm 0 0 ? 5 0 0 0 A z~ 3 P 0,X c< a aW -J I- 0 > 0 f 0 a I-- *A

A W z-

W B 0

0- - s C

a

W RETiTRlCOLPlTES SP.1 SUBZONE a - a a - -- .- CLAVAUONOCOLPITES

W sp. SUBZONE z Y rare ((28) OD 0 o c c a s ion 8 I (45 8, - occaseonal 10 I freauent (5- 101 Y lreauent lo a abundanl 0 abundant 0101 0- -I Fig. 2. Stratigraphic distribution of selected species. Figure also shows the depths at which samples were taken from the well.

3 Oboh, Salami & Chapman

IW ... -_--. ,- YILlS 0 --__ a0 40 00 .- ..---I. \ -._ 4- f . I' a.) Fig. 3. I'alaeogeography of the Tertiary Niger delta showing the stages of delta growth (after Short & Stauble 1967). at some distance offshore in marinedeposits. Inshore these high indicates that marine influence is very slight. This is further percentages (>50%) are due to high concentrations of the evidence that the high percentages of Z. ranioiine mentioned Rhizophora pollen; further offshore they are due to low total abovearedue todepositionclose tosource, i.e., coastal deposition pollen counts. Therefore, its quantitative distribution makes it rather than the offshore transportation and concentration found a useful species for environmental interpretation (Muller & by Muller (1959). Caratini, 1977). Z. ranionae is frequent to abundant throughout most of the 3. Organic matter. Theaccumulationand degradationof organic sequence described here, suggesting the presence of Rhizopkorn matter is related to surface conditions in the depositional type mangrove swamp in the area. In the studied interval, the environment and to diagenetic changes. Hence the presence or percentage varies from about 2% to more than 40% (Fig. 2). The absence and abundance of different organic components can be absence or rarity of this species from some horizons indicates directly related to lithofacies (Batten, 1981).It is also important that deposition of those sediments occurred somedistance from to separate the characteristics that derive from the nature of the the mangrove edge. primary material of the sediments from those induced by Above approximately 1600m the lithological analysis subsequent diagenesis, differential migration and indicates a more continental influence which would fit with the metamorphism (Staplin, 1969, p.50). advance of the delta front suggested by Short & Stauble (1967). The organic matter in the samples recorded here consists of This is also suggested by the intermittent presence of the plant cuticles and wood fragments of a complete range of size freshwater fern M. hnzcnrdi. from small to large. A long distance of transportation would Between 2680m and about 3000m the decrease in Z.ramrim have effected the sorting of the sizes as finer fractions tend to be could also indicate a more inland environment. However, deposited further offshore (Nwachukwu & Barker, 1985). The Crassoretitriletes zmrandshooveiii and Verrircii tosporites irsiiierisis varied range in these Nigerian samples indicates short distance are very common over this interval. Both species are considered transport, i.e. the source area for the sediments is the surrounding tobeclimbingferns (Cermeraad etnl., 1968)and their abundance environment, and deposition in an area of variable energy indicates a swamp forest area composed of non-Rhizopliorn type where both coarse and finer particles settle out. trees. The occurrence of M. hozonrdi again suggests some The colours of the organic matter types were determined, freshwater influence and the presence of more open vegetation. although organic matter colouration was not used in Unfortunately the sampling intervals in this lower part are too environmental interpretation,. The fragments of cuticles and great to determine the extent of this vegetation change very wood are a mixture of colours, ranging from yellow to black. accurately . Normally, organic matter colour gets darker with increasing Marine organisms are rare in the studied interval. This depth of burial. Therefore, the wide range of colour of organic

4 Miocene palaeoenvironment, Niger delta fragments in these samples probably suggests that there has marsh habitats (Germeraad et al., 1968).Such changes could be been reworking and subsequent oxidation in the paralic due to fluctuation in the species making up the coastal swamp sequence. Additionally, the use of nitric acid for oxidation may forestoranadvanceof thedelta frontcreatinga more freshwater have contributed to the wide range of colours in our samples. influence in the area of deposition. Towards the top of the Reworking waseasily established for someof thepalynomorphs studied sequence, in the Upper Miocene, Z. ranlome and the studied, especially when their ages were known. For example, other pollen types become intermittent and the sedimenary a specimen of Cingdatisporites cf. ornatus, an Upper Cretaceous environment more continental. This supports the view of Short spore, was found in one assemblage. However, the occurrence & Stauble (1967) that the delta front advanced towards the end of Cicatricosisporites dorogensis in these Miocene sediments, of the Miocene as deltaic sediments built up. above the Oligocene range reported by Germeraad et al. (1968), was moredifficult todismissasreworkingas thesporesoccurred ACKNOWLEDGEMENTS in several samples (see Fig. 2) and wereextremely well preserved. The authors wish to thank the Shell Petroleum Development A common feature observed on many palynomorphs was Company of Nigeria Limited for permission to use their material corrosion; this results from oxidation and may indicate a degree from well Igbomotoru-1 and to publish these results. We are of reworking of deltaic sediments. The plant debris in the also grateful to M.A. Sowunmi, J.I. Nwachukwu and K.P.N. sediments are terrestrially-derived types I1 and 111 organic Kumaran for their comments and suggestions on an earlier matter (Bustin, 1988). version of the manuscript, Cambridge Earth SciencesSeries No. 2300. 4. Lithology. Lithological studies of the borehole have dealt with the nature of the accessory associated with the Manuscript received August 1991 sediments (Oboh, 1987; Oboh & Salami 1989). Pyrite, mica Manuscript accepted October 1991 flakes, limonite stains, lignite streaks and heavy minerals were occasionally encountered in the sand bodies. The shales REFERENCES commonly contained pyrite and mica flakes and occasionally Adegoke,O.S. 1969.EocenestratigraphyofsouthernNigeria.Mem.Bur. lignite, carbonaceous materials and glauconite. Of these Rech. Geolog. Min., 69, 23-49. accessory minerals, pyrite and glauconite are environmentally Allen,J.R.L. 1964.Sedimentationin themoderndelta of theRiver Niger, significant. West Africa. Sedimentology,l, 26-34. Allen, J.R.L. 1965. Late Niger delta and adjacent areas: The common occurrence of pyrite in the shale bodies confirms sedimentary environments and lithofacies. Am. Assoc. Petrol. Geol. the reducing conditions under which they were deposited. Bull., 49, 547-600. Glauconite is a typically marine and although it was Asseez; L.O. 1976. Review of the stratigraphy, sedimentation and present in small amounts within unit D, its occurrence decreased structure of the Niger delta. In Kogbe, C.A. (ed.),Geology of Nigeria, from the base of the sequence. This shows that the sediments at 259-272, Elizabethan Press, Lagos. the base of the succession are of transitional nature from marine Batten, D.J. 1981.Palynofacies,organicmaturationand source potential influenced conditions. for petroleum. In Brooks, J. (ed.), Organic riinturntioii studies and fuel explorntion, 201-223, Academic Press, London. Biffi, U. and Grignani, D. 1983. Peridinioid cysts from the CONCLUSIONS Oligocene of the Niger delta. Micropalaeontology, 29 (2), 126-145. Palynological and lithological evidence has beenused to interpret Burke, K.C., Dessauvagie, T.F.J. and Whiteman, A.J. 1971. Opening of the environment of deposition of the studied interval (1123- the Gulf of Guinea and geological history of the Benue depression 3583 metres). The range of size of organic matter (especially and the Niger delta. Nnture, 233,51-55. cuticles and wood fragments) indicates that the source of Burke, K.C., Dessauvagie, T.F.J. and Whiteman, A.J. 1972. Geological is near the basin of deposition, although the colour history of the Benue valley and adjacent areas. 111 Dessauvagie, T.F.J. range and presence of corroded palynomorphs show a degree and Whiteman, A.J. (eds), African Geology, 187-218, University of of reworking. The marine influence, indicated by small amounts Ibadan Press. Bustin, R.M. 1988. Sedimentology and characteristics of dispersed of glauconite within the shales of unit D, is strongest towards organic matter in the Tertiary Niger delta: origin of source rocks in a the base of this unit. The rarity of in the sequence, deltaic environment. Am. Assoc. Petrol. Gml. Bull., 72,277-298. however, shows the lack of marine depositional environments Clarke, R.T. 1966. Peregrinipollis nigericus, a new palynomorph from the throughout the rest of the sampled section. Upper Tertiary of Nigeria. Grana Pnlynol., 6 (3), 545-546. The frequent to abundant occurrence of Z. ramoiiae in most Clarke, R.T. and Frederiksen, N.O. 1968. Some new sporomorphs from parts of the sequence suggests the presence of Rhizophora type the Upper Tertiary of Nigeria. Gram Pnlynol., 8 (l),210-224. mangrove swamp in the area. However, the percentages of Z. Ejedawe J.E. 1981. Patterns of incidence of oil reserves in Niger delta rainonae obtained in the present study seldom reach the very basin. Ain.Assoc. Petrol.Geol. Bull., 65,1574-1585. Evamy,B.D.,Haremboure, J., Kamerling, P., Knaap, W.A., Molloy, F.A. high levels quoted by Muller (1959) for the modern vegetation and Rowlands, P.H. 1978. Hydrocarbon habitat of Tertiary Niger of the Orinoco delta environment. delta. Am. Assoc. Petrol. Geol. Bull., 62, 1-39. The absence or rarity of Z. ramonae at some horizons in the Frankl, E.J. and Cordry, E.A. 1967. The Niger delta province - recent Peregrinipollis iiigericus and Clauamoriocolpites sp. subzones developments onshore and offshore. P roc. 7th World Petroleum suggests two periods of long term variation in the mangrove Corigress, l(b), 195-209. vegetation during the time of deposition. The interval of low Germeraad, J.H., Hopping, C.A. and Muller, J. 1968. Palynology of frequency of Z. rnmoiiae in the Middle Miocene corresponds Tertiary sediments from tropical areas. Rev. Pnlaeobot. Palyiiol., 6,189- with high percentages of climbing ferns indicative of swamp or 348.

5 Oboh, Salami & Chapman

Jan du Ch@ne,R.E., Onyike, M.S. and Sowunmi, M.A. 1978. Some new Oboh, F.E. and Salami, M.B. 1989. Lithostratigraphical and Eocenepollen of theogwashi-Asaba Formation, southeastern Nigeria. palynostratigraphical studies of Igbomotoru-1 well, Niger delta. 1. Rev.Esp. de Micropal., 10(2),285-322. African , 9, (3-4), 531-540. Jan du Ch@ne, R.E. and Salami, M.B. 1978. Palynology and Odigi, M.I. 1987. Mineralogical and geochemical studies of Tertiary of the Upper Eocene of the well Nsukwa-1 (Niger sediments from the eastern Niger delta and their relationship to delta, Nigeria). C.R. des Siances, SPHN Geneve, 13(1), 5-9. petroleum occurrence. /. Petrol. Geol., 10(1), 101-114. Knaap, W.A. 1971. A montane pollen species from the Upper Tertiary Ogbe, F.G.A. 1982. The of the Niger delta, Nigeria. of the Niger delta. Niger. /. Mining and Geology, 6(11-2), 23-29. Niger. 1. Mining and Geology, 18, 59-75. Lambert-Aikhionbare, D.O. 1982. Relationship between diagenesis and Oomkens, E. 1974. Lithofacies relations in the Quaternary Niger delta pore fluid chemistry in Niger delta oil-bearing sands. /. Petrol. Geol., complex. Sedirnentology, 21,195-222. 4(3), 287-298. Petters, S.W. 1979. Nigerian Paleocene benthonic foraminifera1 Lambert-Aikhionbare, D.O. and Shaw, H.F. 1982. Significance of clays biostratigraphy, and paleobiogeography. Marirzt, in the petroleum geology of the Niger delta. Clay Minerals, 17,91-103. Micropaleontology, 4,85-95. Legoux,O. 1978.Quelquesespecesdepollen caracteristiquesdu NeogPne Short,K.C. andstauble, A.J. 1967,Outlineofgeologyof Nigerdelta. Am. du Nigeria. Bull. Cent. Rech. Explor. - Production ElfAquitaine,2(2),265- Assoc. Petrol. Geol. Bull., 51(5), 761-779. 317. Smith, A.G., Hurley, A.M. and Briden, J.C. 1981. Phnnerozoic Merki, P. 1972. Structural geology of the Cenozoic Niger delta. In Palmcontinental World Maps. 102p. Cambridge University Press, Dessauvagie, T.F.J. and Whiteman, A.J. (eds), African Geology, 635- Cambridge. 645, University of Ibadan Press. Sowunmi, M.A. 1973. A preliminary palynological study in the Rivers Muller, J. 1959. Palynology of the Recent Orinoco delta and shelf State, Nigeria. Odutna Magazine, 1(1), 13-14. sediments: Reports of the Orinoco shelf expedition, volume 5. Sowunmi, M.A. 1981. Late Quaternary environmental changes in Micropaleontology, 5(1), 1-32. Nigeria. Pollen et spores, 23(1), 125-148. Muller, J. and Caratini, C. 1977. Pollen of Rhizophora (Rhizophoraceae) Staplin, F.L. 1969.Sedimentary organic matter, organic metamorphism as a guide fossil. Pollen et Spores, 19(3), 361-389. and oil and gas occurrence. Bull.Can. Petrol. Geol., 17(1), 47-66. Murat, R.C. 1972. Stratigraphy and paleogeography of the Cretaceous Van Hoeken-Klinkenberg, P.M.J. 1964.A palynological investigation of and Lower Tertiary in southern Nigeria. In Dessauvagie, T.F.J. and some Upper Cretaceous sediments in Nigeria. Pollen et Spores, 6,209- Whiteman, A.J. (eds), African Geology, 251-266, University of Ibadan 231. Press. Van Hoeken-Klinkenberg, P.M.J. 1966. Maestrichtian, Paleocene and Nwachukwu, J.I. and Barker, C. 1985. Organic matter: size fraction Eocene pollen and spores from Nigeria. Leidse Geol. Meded., 38,37-48. relationshipsfor recent sediments from theOrinoco Delta, Venezuela. Weber, K.J. 1971. Sedimentological aspects of oil fields in the Niger Mar. Petrol. Geol. 2,202-209. delta. Geologie en Mijnbouw, 50(3), 559-576. Oboh, F.E. 1987. Biostratigraphy and ~ialeoenr~ironmentnlanalysis of strata Weber, K.J. and Daukoru, E. 1976. Petroleumgeology of theNiger delta. penetrated by Igbomotoru-1 well, Niger delta. 106p. Unpublished MSc. Proc. 9th World Peetroleifni Congress, 2, 209-221. thesis, University of Ife, Nigeria.

6