GEOCIENCIAS-VOL 14-1 2010.vp EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 14, No. 1 (June 2010): 88-99 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA Ekine, A.S.1 and Onuoha, K. M.2 1 Department of Pure and Applied Physics, University of Port Harcourt, PortHarcourt 2 Department of Geology, University of Nigeria, Nsukka, Nigeria asekine2001@yahoo.co.uk ABSTRACT The methods of seismic geohistory analysis and differential interformational velocity analysis have been applied using data from some interpreted seismic sections within the Anambra basin. These two techniques for basin analysis have been applied in the basin in an attempt to aid the identification of anomalous velocity zones. Results from the seismic geohistory analysis in- dicate that some faults arising probably from compressional stresses, due to the upliftment of the Abakaliki anticlinorium, were observed to be significant mostly in the northern and southeastern regions of the basin. These faults which originated in the Santonian are probably related to the first folding episode in the evolution of the Benue Trough. The Maestrichtian / Campanian faults observed towards the north of Anambra River-3 well may also have been influenced by these stresses or are directly related to the Post-Maestrichtian folding episode in the Benue Trough. The fault system KP identified on the seismic section is a Pre-Eocene event which cuts across from the south of Okpo-1 and Nzam-1 to the north of Iji-1 well. Sediments to the north of Nzam-1 well site have experienced more faulting during the Lower-Maestrichtian times. Fault systems that oc- curred after the Paleocene times are more prominent in the southern parts of Nzam-1 towards Iji-1 well sites. Generally most of the major faults occurred in the Paleocene. The major ‘DIVA’ anomaly observed southeast of Anambra River-1 well corre- lates with the zone of overpressures within which liquid hydrocarbon, water and gas have been discovered in the basin. The Maestrichtian to Paleocene sediments in the southern and mainly in the southwestern sector of the Anambra basin should be the major sedimentary strata with liquid hydrocarbon potentials, whereas the Lower Cretaceous and particularly the Santonian sediments exhibit the highest potentials for gaseous hydrocarbons. Key words: seismic geohistory; seismic section; differential velocity; Anambra basin. RESUMEN Los métodos de Análisis Sísmicos histórico y análisis de velocidad interformacional diferencial se han aplicado usando datos de algunas secciones sísmicas interpretadas de la cuenca Anambra. Estas técnicas se han aplicado en un intento de ayudar a la identificación de zonas de velocidad anómalas. Los resultados del Análisis Sísmico histórico indican que algunas fallas probablemente surgen de esfuerzos de compresión, debido a la elevación del Anticlinorio Abakaliki, importantes sobre todo en las regiones norte y el sureste de la cuenca. Estas fallas que tuvieron su origen en el Santoniano, están probablemente relacionados con el primer episodio de plegamiento en la evolución de la depresión de Benue. Las fallas maestrichtiense / 88 Manuscript received: 17/12/2009 Accepted for publication: 15/05/2010 Campaniano observadas hacia el norte del pozo Anambra River -3, también pueden haber sido influenciadas por estos esfuerzos o están directamente relacionados con el episodio de plegamiento post-maestrichtiano de la depresión de Benue. El sistema de fallas KP identificado en la sección sísmica es un evento pre-Eoceno que atraviesa desde el sur de Okpo-1 y Nzam 1-al norte del pozo Iji-1. Los sedimentos al norte de la ubicación del pozo Nzam-1 han experimentado mayor fallamiento en el maestrichtiano temprano. El sistema de fallas producido después del Paleoceno es más prominente en las partes meridionales desde Nzam-1 hacia el pozo Iji-1. En general la mayoría de las principales fallas se produjeron en el Paleoceno. La gran anomalía ‘DIVA’ observada al sudeste del pozo Anambra River -1 se correlaciona bien con la zona de sobrepresiones en la que hidrocarburos líquidos, agua y gas han sido descubiertos. Los sedimentos que van del maestrichtiano al Paleoceno en el sur y principalmente en el sector suroccidental de la cuenca Anambra deberían ser los principales estratos sedimentarios con potencial de hidrocarburos líquidos, mientras que los del Cretácico Inferior y en particular los sedimentos Santonianos exhiben el más alto potencial de hidrocarburos gaseosos. Palabras clave: Geohistoria sísmica, sección sísmica; velocidad diferencial; cuenca Anambra. Introduction The Anambra Basin has been identified as one of the major in-land sedimentary basins in Nigeria. It is bounded on the east by the Abakaliki anticlinorium and on a south-westerly direction by the Benin hinge-line, while the southern ex- treme is marked by the upper limits of the Eocene growth faults of the Niger Delta (Merki, 1972). The basin is about 300 km long in a northeast – southwest direction, extending between the Onitsha environs in Anambra State to the Loko area in Benue State of Nigeria. Its southwestern tip is about 160 km wide, while the northeastern extreme is about 48 km wide (Whiteman, 1982) (Fig. 1). The geologic history of the basin has been that of nearly continuous subsidence and sedimentation, and had therefore remained largely unaffected by major tectonism. The strati- graphic successions in the Anambra Basin and environs 89 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA Cratonic Stable or block foulted Basinal Orogenic Hinge line Site of shoal carbonate Oil, tar seapage Ameki Fm. Mamu, Ajali Nsukka Transgressive Phose Regregressive Phose Imo Shale Nkporo Shale Lower Campanian - Lower Eocene Figure 1. Tectonic map of Southern Nigeria (adapted from Murat 1970). have been discussed by several authors which include (Reyment, 1965; Murat, 1972; Adeleye, 1975; Peters, 1978; Whiteman, 1982; Hoque and Nwajide, 1984; Agagu et al, 1985). The reconstruction and the analysis of the geologic his- tory of a sedimentary basin are very significant for the analy- sis of the subsidence and thermal histories of such a basin. This method of reconstruction or geohistory analysis has been applied by many authors using stratigraphic and other geophysical data (Angevine and Turcotte, 1981; Middleton, 1982; Onuoha, 1985; Onuoha and Ekine, 1999; Ekine and Onuoha, 2008). However, in recent times, geohistory analy- sis has been successfully applied to seismic data (Middleton and Falvey, 1983; Middleton, 1984). Seismic geohistory analysis is a technique whereby the geologic history of a sedimentary basin is reconstructed in terms of two-way travel times from observed seismic reflection data. This method provides a means of rapidly reconstructing the geo- logic history from an interpreted seismic section. Again in recent times, the great advances in exploration tools, techniques and experiences have compelled and moti- vated many explorationists to shift their exploration empha- sis from the quest for simple structural to stratigraphic traps. The difference between structural and stratigraphic traps can either be quite clear or so subtle for differentiation where both traps play significant roles as trapping mechanisms. The use of seismic techniques for the identification and defi- nition of stratigraphically trapped hydrocarbons has been made possible by the introduction of sophisticated packages and approaches in seismic data interpretation. The differen- tial interformational velocity analysis, originated by Neidel and Beard (1984), attempts to delineate anomalous low ve- locity zones by tracking laterally the interval velocity (stack- ing velocity) along a seismic profile by using the travel times for the reflectors that have been identified. These two tech- niques for basin analysis have been applied in the Anambra basin using some interpreted seismic sections (see Fig. 2). This attempt is to aid in the identification of anomalous ve- 90 EKINE, A.S. AND ONUOHA, K. M. 7°00’6°32’E 6°40’ 7°20’E 7°00’ ALD-1 OPL 447 ATU-1 129/128-776/77-18 ANR-3 ANR-1 ANR-2 ODR-1 NZAM-1 447-84-216 OKP-1 ALO-1 44-84-259 IJI-1 AJR-1 447-84-248 447-84-217 ALO-1 O P L 44 7 AKU-1 AMA-1 6°30’ 0 20 km 6°00’N Figure 2. Location of Seismic Sections. locity zones which in normal circumstances could be attrib- uted to the occurrence of reefs, sand bars, geomorphic features and other features commonly associated with strati- graphic traps. Also the study will demonstrate the applica- bility of these non conventional techniques in the search for the subtle traps. Methodology P-waves travelling through sedimentary layers are found to obey a power law equation given by Acheson (1981) as t az bn� � (1) where t = the one-way travel time; z = depth to reflector; a, b and n = constants. Consequently, the two-way travel time T can be ex- pressed as T = 2(a zn + b) (2) According to Falvey and Middleton (1981), and Middleton, (1984), the behaviour of porosity �z with depth z can be sufficiently approximated by the expression 1 1 0� �( )z kz� � (3) where �0 = the depositional porosity and k = a constant. The above expression has been used in this study as the main thrust of the study is not the evaluation of reservoirs as in Ehrenberg et al., (2009). To decompact and reconstruct the geologic history of a sedimentary basin from interpreted seismic section, a proce- dure similar to burial history reconstruction is applied. In this case depths are replaced with arrival times. According to Sclater and Christie (1980), the height of sediment grain hsg for a unit cross-sectional area between the intervals z1 and z2 is given by the expression � �h dzsg z z z � �� 1 1 2 � ( ) (4) and z h z k n k z k n k zstg2 1 0 1 0 2 1 1 1 1 1 1’ ’ ’( ) ( )� � � � � �� � (5a) or z k n k z h z k n k zstg2 0 2 1 0 1 1 1 1 1 1 1’ ’ ’ ’( ) ( )� � � � � �� � (5b) where z1 ’ and z2 ’ are the depths to the top sedimentary units when z z1 1� ’ is at the surface. Substituting equation (2) into equation (3) gives 1 1 2 20 1 � � ( ) � � � � � � �k T b a n (6) and equation 5(b) now becomes T b a k k T Fn n2 1 0 22 2 1 1 1 2 ’ ’ ( � � � � � � � � � � � � � � � � � � �n 2b a � T T T1 1 2, , )’ (7) with Fn T T T T b a T Tn n ( , , )’ ’ ’ 1 1 2 1 1 2 1 12 2 2 � � � � � � � � � � � � � 2b a � � � � � � � � � � � � � � 2b a n 2b a 2b a 2 1 1 1 2 1 2 1 0 2 0 2 n k k T k T � � ’ ’ � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1 1 1 2 0 1 1 k k T n n 2b a � ’ � � � � (8) T1 ’ and T2 ’ are the reflector times when T T1 1� ’ is at the surface. (T T z2 1 2 ’ ’, and have been defined in the equations as re- quired) Equation (8) is the basis for the seismic geohistory anal- ysis. This equation is solved numerically for T2 ‘, with T1 ‘ = 0 at the surface for the first instance. Fig. 3 is a schematic rep- resentation of the process of seismic history analysis. For subsequent reflectors and cycles a similar procedure as em- ployed for the burial history reconstruction was used in this study (Ekine and Onuoha, 2008). Using the two-way travel time versus depth plots obtained at Anambra River-1, Oda River-1 and Nzam-1 well sites (Figs. 4a, b, and c), a time-depth relation was obtained for the basin. This relation can be expressed as t = 0.46 z0.83 + 18.99 (9) where t is in milliseconds and z is in feet. Seismic geohistory analysis was carried out along three interpreted seismic sections which were calibrated at Iji-1, Nzam-1 and Okpo-1 wells. The generalized porosity-depth relations were obtained from each well along individual seismic line and not a generalized relation for the whole ba- sin. This approach significantly reduced the possible errors associated with simple assumed basinal generalized func- tions and thus enhances the representativeness of the poros- ity-depth and the time-depth functions. The reconstructed 91 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA seismic sections as obtained from the solutions of equation (7) are shown on Figs. 5(i), (ii) and (iii). Finally, the method of differential interformational ve- locity analysis (DIVA) was applied to some interpreted seis- mic section within the basin. These velocity sections include those calibrated at Anambra River-1, Anambra River-3, Iji-1 and Nzam-1 wells. The graphics of the analysis are dis- played on Figs. 6a, b, c and d. Discussions The results of seismic geohistory analysis carried out along three interpreted seismic sections indicate that most of the major faults in the basin originated in the Post-Maestrichtian times and mainly in the Paleocene. The observed major faults are found to commence mainly within the Nkporo shales at Okpo-1 well site (Fig. 5(i) (d, e)); within the Lower Coal Measure at Iji-1 (Fig. 5(ii)(d, e)) and Nzam-1 (5(iii)(c)) well sites. These formations were deposited in the Post-Maestrichtian times. This observation correlates with the renewed high rate of subsidence and sedimentation re- vealed by the reconstructed burial analysis (Ekine and Onuoha, 2008). A very good correlation also exist between sediments depths obtained from the decompaction method and that obtained from seismic geohistory analysis by con- verting travel-times to depths using the time-depth relation obtained for the basin. Similarly, some faults arising proba- bly from compressional stresses, due to the upliftment of the Abakaliki anticlinorium (Ekine, 1989) were observed to be significant mostly in the northern and southeastern regions of the basin. These faults which originated in the Santonian are probably related to the first folding episode in the evolu- tion of the Benue Trough. The Maestrichtian / Campanian faults observed towards the north of Anambra River-3 well may also have been influenced by these stresses or are di- rectly related to the Post-Maestrichtian folding episode in the Benue Trough. The fault system KP identified on the in- terpreted seismic section [see Figs. 5(i)(b) and 5(iii)(b)], is a Pre-Eocene event which cuts across from the south of Okpo-1 and Nzam-1 to the north of Iji-1 well. It is observed that sediments to the north of Nzam-1 well site have experi- enced more faulting during the Lower-Maestrichtian times. However, fault systems that occurred after the Paleocene times are more prominent in the southern parts of Nzam-1 towards Iji-1 well sites. Generally, we observed that most of the major faults occurred in the Paleocene. Some other faultings which occurred in the Santonian were mainly sig- nificant in the northern and the southeastern parts, whereas 92 EKINE, A.S. AND ONUOHA, K. M. 0 1.0 2.0 3.0 4.0 5.0 T3 Reflector A T = 01 2 Reflector B Reflector C Reflector C Reflector B Reflector A T1 T2 0 1.0 2.0 3.0 4.0 5.0 T3 T2 Reconstructed Seismic Section (Reflector A Time) Interpreted Seismic Section (Present Day) Figure 3. Schematic Diagram of the Seismic Geohistory Analysis Method. 93 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA 0.2 0.4 0.6 0.8 1.0 1.2 1.4 One-Way Time (sec) 0 1.0 2.0 3.0 4.0 True t-z curve Computed (c) Nzam - I Well D ep th (k m ) 1 = 0.46 Z + 18.990.83 Figure 4. Time vs Depth. 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0 1.0 2.0 3.0 t = 0.46 Z + 18.990.83 (a) Anambra River - I Well True t-z curve Computed 0 1.0 2.0 3.0 True t-z curve t = 0.46 Z + 68.980.83 t = 0.46 Z + 18.990.83 One-Way Travel Time (sec) (b) Oda River - I Well D ep th (k m ) 94 EKINE, A.S. AND ONUOHA, K. M. 80 120 160 200 240 280 320 360 90 120 160 200 240 280 320 360 IJI 0.0 1.0 2.0 3.0 4.0 Base of Eocene Horizon within Imo HF FTB GL Top ICM HK FT VB Top LCM KR 0.0 1.0 2.0 (d) Top UCM at Surface (a) Present Day (b) Top Imo Shale at Surface 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 (c) Lower Imo Shale at Surface Top UCM Top LCM (e) Top LCM at Surface UCM = Upper Coat Measure LCM = Lower Coat Measure FTB = Major Faults As Indentified on Seismic Sections Figure 5(ii). Reconstructed Line 447-84-259 at IJI-1. 610 650 690 730 770 810 610 650 690 710 770 810 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 OKPO Base of Eocene KP KN Top UCM Top NKP (a) Present Day (b) Top Imo Shale at Surface (c) Top UCM of Surface (d) Top Nkporo Shale at Surface (e) End if Santonian UCM = Upper Coal Measure RP = Major Faults as identified on Seismic Sections Figure 5(i). Reconstructed Line 447-84-216 at OKPO-1. 95 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA 140 200 260 320 380 440 500 560 620 680 740 800 860 920 980 1040 1100 1160 NZAM 0.0 1.0 2.0 3.0 4.0 Base of Eocene GL HK Too UCM Top LCM VB HA GL GA KP KN MP KH LE LA LB (a) Present Time 0.0 1.0 2.0 3.0 (b) Top Imo Shale at Surface (c) Top UCM at Surface UCM = Upper Coat Measure LCM = Lower Coat Measure GA = Major Faults As Indentified on Seismic Sections KR Figure 5(iii). Reconstructed Line 447-84-217 at NZAM-1. 1.5 3.5 1.5 3.5 1.5 3.5 2.5 4.5 1.5 3.5 2.5 4.5 V el o ci ty (k m - S ) -1 CDP 1331 1033 735 437 138 H2 H1 H1 H3 H3 H1 H4 H4 H2 H2 H4 H3 H2 = Top Mamu Fm.H1 = Top Nsukka Fm. H4 = Top Awgu Shale.H3 = Top Nkporo Shale. Figure 6(a). “DIVA” Display for Line 129/128-76/77-78. the Post-Eocene faults were predominantly in the southwest- ern portions of the basin. The differential interformational velocity analysis, ‘DIVA’, carried out along four seismic sections indicated mainly minor ‘DIVA’ anomalies. However, the ‘DIVA’ dis- play for line 129/128 – 76/77 – 78, calibrated at Anambra Rivers-1 and -3 wells, show major anomaly beyond CDP 437 and to the end of the seismic line southeast of Anambra River-1 [see Fig. 6(a)]. This anomaly is within the tops of Mamu Formation and the Agwu Shales. A similar anomaly, not clearly defined, is indicated northwest of Anambra River-3 well. This anomaly is within the same levels with beds dipping southwest, or shows the trend of migrating flu- ids and hydrocarbons. The top of Agwu Shales contains the column of water and liquid hydrocarbons observed at the Anambra River-1 well. The major ‘DIVA’ anomaly observed southeast of Anambra River-1 well correlates with the zone of overpressures within which liquid hydrocarbon, water and gas have been discovered in the basin. The absence of any other well defined anomaly along this section implies that the overpressured condition and the hydrocarbon de- posit do not extend in that direction or that the section ana- lyzed is not long enough to clearly show this trend. 96 EKINE, A.S. AND ONUOHA, K. M. H2 H1 H1 H3 H3 H1 H4 H4 H2 H4 H3 2.3 4.3 2.3 3.3 2.3 3.3 2.3 3.3 2.3 4.3 2.3 4.3 V el o ci ty (k m - S ) -1 0 50 100 200 350 IJI-ISP H2 = Top Nsukka Fm. H4 = Top Mamu Fm.H3 = Horizon in Imo Shale. H1 = Top Imo Shale. Figure 6(b). “DIVA” Display for Line 447-84-259. Conclusions The method of seismic geohistory analysis has shown that the timing of faulting in a basin is very important for the un- derstanding of the problem of hydrocarbon migration and accumulation. The lack of adequate stratigraphic or struc- tural trappings may lead to vertical or lateral migration of liquid hydrocarbon and their possible dissemination. When the time of hydrocarbon maturation (Ekine, 1989), is related to the timing of the observed fault systems, the absence of accumulated liquid hydrocarbons in the north-central parts of the Anambra basin can be explained. Results of the differential interformational velocity analysis have further confirmed the existence of velocity anomalies resulting from changes in rock type and or fluid content, which substantially enhances the magnitude of the anomaly when the presence of gas is indicated. The DIVA re- sults indicate that the Maestrichtian to Paleocene sediments in the southern and mainly in the southwestern sector of the Anambra basin should be the major sedimentary strata with liquid hydrocarbon potentials, whereas the Lower Creta- ceous and particularly the Santonian sediments exhibit the highest potentials for gaseous hydrocarbons. The above geophysical tools employed in this study for basin analysis, 97 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA 2.5 3.5 0 100 200 350 400 2.5 3.5 2.5 3.5 2.5 4.5 2.5 45 2.5 4.5 SP IJI-1 H2 H1 H1 H3 H3 H1 H4 H4 H2 H4 H3 H2 H2 = Top Nsukka Fm. H4 = Top Mamu Fm.H3 = Horizon in Imo. H1 = Top Imo Shale. Figure 6(c). “DIVA” Display for Line 447-84-248. have demonstrated their effectiveness for obtaining infor- mation about the tectonism, hydrodynamics, hydrocarbon and geothermal resource potentials of basin. Consequently, we suggest that the methods of seismic geohistory analysis and DIVA be further pursued using high fidelity seismic sec- tions with greater coverage. References Acheson, C. H., 1981. Time-depth and velocity-depth rela- tions in sedimentary basins: A study based on current in- vestigations in the Arctic Islands and an interpretation of experience elsewhere. Geophysics, 46: 707 – 716. Adeleye, D. R., 1975. Nigerian late Cretaceous stratigraphy and paleogeography. A. A. P. G. Bull., 59: 2302 – 2313. Agagu, O. K., E. A. Fayose and S. W. Peters, 1985. Stratig- raphy and sedimentation in the Senonian Anambra ba- sin of Eastern Nigeria. Nig. Journ. Min. Geol., 22: 25 – 36. Angevine, C. L. and D. L. Turcotte, 1981. Thermal subsi- dence and compaction in sedimentary basins: Applica- tion to Baltimore Canyon trough. A. A. P. G. Bull., 65: 219 – 225. Ehrenberg, S. N., P. H. Nadeau and Ø. Steen, 2009. Petro- leum reservoir porosity versus depth: Influence of geo- logical age. A. A. P. G. Bull., 93(# 10): 1281 – 1296. Ekine, A. S., 1989. Empirical heat flow studies, geohistory analysis and hydrocarbon maturation modelling in the Anambra Basin, Nigeria. Unpublished Ph. D. thesis, University of Nigeria, Nsukka. Ekine, A. S. and K. M. Onuoha, 2008. Burial history analy- sis and subsidence in the Anambra basin, Nigeria. Nig. Journ. Phys., 20 (#1): 145 – 154. Falvey, D. A. and M. F. Middleton, 1981. Passive continen- tal margins: Evidence for break-up deep crustal meta- morphic subsidence mechanism. Oceanologica Acta, suppl., 103 – 110. Hoque, M. and C. S. Nwajide, 1984. Tectono-sedimentological evolution of an elongate intracratonic basin (Aulacogen): The case of the Benue Trough of Nigeria. Nig. Journ. Min. Geol., 21: 19 – 26. Merki, P., 1972. Structural geology of the Cenozoic Niger delta. In African Geology, ed. T. J. F. Dessauvigie and A. J. Whiteman. University of Ibadan Press, pp. 251 – 268. 98 EKINE, A.S. AND ONUOHA, K. M. 2.0 4.0 2.0 4.0 2.0 4.0 V el o ci ty (k m - S ) -1 0 138 338 538 738 938 1138 SP NZAM-1 H1 H2 H1 H3 H2 H3 H2 = Top Nsukka Fm. H3 = Top Mamu Fm. H1 = Top Imo Shale. Figure 6(d). “DIVA” Display for Line 447-84-2178. Middleton, M. F., 1982. The subsidence and thermal history of the Bass basin, Southeastern Australia. Tectonophysics, 87: 383 – 397. Middleton, M. F. and Falvey, D. A. 1983. Maturation mod- elling in Otway Basin, Australia. A. A. P. G. Bull., 67(#2): 271 – 274. Middleton, M. F., 1984. Seismic geohistory analysis – A case history from the Canning Basin, Western Australia. Geophysics, 49: 333 – 343. Murat, R. C, 1972. Stratigraphy and paleogeography of the Cretaceous and Lower Tertiary in Southern Nigeria. In African Geology. ed. T. J. F. Dessauvigie and A. J. Whiteman, University of Ibadan Press, pp. 635 – 648. Neidel, N. S., and J. H. Beard, 1984. Progress in strati- graphic seismic exploration and the definition of reser- voirs. Distinguished Author Series, Journ. Pet. Tech. pp. 709 – 726. Onuoha, K. M., 1985. Basin subsidence, sediment decompaction and burial history modelling techniques: Applicability to the Anambra basin. N. A. P. E. Proc., 2: 6 – 17. Onuoha, K. M. and A. S. Ekine, 1999. Subsurface tempera- ture variations and heat flow in the Anambra basin, Ni- geria. Journ. African Earth Sc., 28(#3): 641 – 652. Peters, S. W., 1978. Stratigraphic evolution of the Benue Trough and its implication for the Upper Cretaceous paleogeography of West Africa. Journ. Geol., 86; 311 – 322. Reyment, R. A., 1965. Aspects of the geology of Nigeria: The stratigraphy of the Cretaceous and Cenozoic depos- its. University of Ibadan Press. Sclater, J. G. and P. A. F. Christie, 1980. Continental stretch- ing: An explanation of the Post-Mid-Cretaceous subsi- dence of the Central North Sea basin. Journ. Geophysical Research, 85: 3711 – 3739. Whiteman, A. J., 1982. Nigeria: Its petroleum geology, re- sources and potential. Vols. I and II, Graham and Trotham, London. 99 SEISMIC GEOHISTORY AND DIFFERENTIAL INTERFORMATIONAL VELOCITY ANALYSIS IN THE ANAMBRA BASIN, NIGERIA