Eastern Dahomey Research Articles

Evaluation of source rock potential and hydrocarbon composition of oil sand and associated clay deposits from the Eastern Dahomey Basin, Nigeria

Oil sands are classified as unconventional hydrocarbon plays and are being exploited to augment global energy needs. Nigeria has the largest conventional oil industry in Africa, but is also endowed with abundant oil sand deposits found within the Afowo Formation of the Eastern Dahomey Basin. In this study, outcrop samples of clay, oil sand, and bitumen seepage from a forestry reserve (J4) in Ogun State, South Western Nigeria were evaluated for organic matter maturity and hydrocarbon composition. Pyrolysis results for the clay indicate that the total organic carbon ranges from 5.95 to 18.46 wt % with a maturation temperature (Tmax) within the range of 417–426 °C. A complementary gas chromatography (GC) analysis of the free hydrocarbons in the clay reveals that both light and heavy hydrocarbon components are present. In contrast, oil sand and bitumen contain mostly heavy end hydrocarbon components with hydrocarbon chains C6–C12 missing, confirming oil biodegradation. This study demonstrates that these oil sands and clays represent viable exploration targets. With vast oil sand deposits in the Eastern Dahomey Basin, exploration and exploitation of this resource will open new frontiers to increase Nigeria's hydrocarbon production.

Geology, Palynomorphs Distribution, Stratigraphy and Depositional Environments of Lewumeji and Idogun Wells, Eastern Dahomey Basin Southwestern, Nigeria

Selected composited samples from Lewumeji (0-111m) and Idogun (0- 54m) wells, from Abeokuta Group, Eastern Dahomey Basin, Nigeria, were subjected to detailed lithologic and palynological studies. The studies aimed at determining the lithological sequence, relative age, palynological zone and paleoenvironments of deposition. The core samples were subjected to lithological description and palynological analysis using standard procedure in order to determine palynomorphs contents such as pollen, spore and dinoflagellates. The lithologies from both wells consist of reddish to brown clay, reddish to brown colour, rounded to sub rounded sandstone, dark grey shale denoting possibly fluvial, brackish, lagoonal and marine environments. A total of 31 well preserved low to moderate diverse palynomorphs were recovered from the studied area. The palynomorphs frequency percentage distribution shows that both wells have a higher ratio of land-derived pollen and spores to the marine dinoflagellates; (75%, 25% and 61%, 39%) for Lewumeji and Idogun wells respectively. The microfloral assemblages include abundant Cyathidites sp., Cyathidites minor, Tubistephanocolpites cylindricus, Proteacidites sp., Trilete spore, Foveotriletes margaritae, Monocolpites marginatus, Monoporites annulatus, Pteris sp, Distaverrusporites simplex and Laevigatosporites sp. The dinoflagellates recovered were characterized by the likes of Leiosphaeridia sp., Senegalinium sp., Oligosphaeridinium sp., Paleocytodinium sp., Cerodinium sp. and Subtilisphaera sp. The wells fall within Cyathidites Minor zone, characterized by the diagnostic occurrence of Cyathidites minor, Cyathidites sp. and Monocolpites marginatus dated Upper Maastrichtian to early Paleocene. Paleoenvironmental interpretations based on the abundance of freshwater swamps pollen and Spores, diagnostic dinoflagellates cyst and the Palynomorphs Marine Index (PMI) are suggestive of environmental settings that vary from continental to brackish water and shallow marine.

The assessment of potential source rocks of Maastrichtian Araromi formation in Araromi and Gbekebo wells Dahomey Basin, southwestern Nigeria

Drilled core samples of the Araromi Formation in the eastern Dahomey basin penetrated by Araromi and Gbekebo exploratory wells were investigated to establish the source rocks potentials in the onshore area of the basin. The sediments are of Maastrichtian age deposited in the shallow marine environment with varying thicknesses.Rock-Eval data of forty seven (40) shales give Total Organic Carbon (TOC) range of 0.50–4.78 wt%, Hydrogen Index (HI) value range of 1 - 327mgHC/gTOC, Tmax values from 398 °C–437 °C and Source Potential (SP) values range from 0.01 - 14.56kgHC/ton of rock. The maceral compositions of the shales are liptinite (av. 26.0%), abundance vitrinite (av. 38.1%) and inertinite (av. 35.9 %) with vitrinite reflectance (VRo) ranging from 0.51 - 0.68 %Ro. Hydrocarbons and biomarkers results reveal a bimodal n-alkane envelope between (nC16 and nC18) and (nC27 and nC29) suggesting organic matter of mixed origin of algae and higher plant generally in the two well. The Significant contribution of marine algae in the deeper part of Gbekebo well was observed by the presence of C30 24-n-propyl cholestane (%C30 sterane range from 0.45 to as high as 5.23%).Integration of the Rock-Eval, organic petrology and biomarkers data reveal that the kerogen constituents of the source rocks in Araromi well are mainly Type II/III, III and IV with a high amount of inertinite constituents suggesting they have been reworked. Type II and II/III kerogen derived from marine algae are better preserved in the deeper part of Gbekebo well located more southerly in the basin than in the Araromi well. The source rocks are generally immature to marginally mature and hydrocarbon exploration effort should be targeted towards Gbekebo well area where we have more promising potential source rocks capable of generating more hydrocarbons essentially at a deeper depth.

Biostratigraphic Analysis of the Cretaceous Abeokuta Group in the Eastern Dahomey Basin, Southwestern Nigeria

Shales of the Abeokuta Group in the Dahomey Basin yielded abundant foraminiferal tests. The study investigated sediments from three wells that were drilled for petroleum exploration: Well – X located in the offshore deep water, Orimedu-1 and Ise-2 in the coastline region to establish their Cretaceous biostratigraphic units through planktonic foraminiferal zonation. Calcareous benthonic foraminifers occur abundantly and consistently throughout the sequences in the three wells while planktonic foraminifera are more abundant in the Well X than those in the Orimedu-1 and Ise-2.The identified planktonic foraminiferal species are: Rotalipora greenhornesis, Rotalipora cushmani, Dicarinella primitiva, Praeglobotruncana helvetica, Marginotruncana sp. Heldbergella delrionensis, Helbergella planispira, Hedbergella simplex, Whitenella inonata, Whitenella baltica, Globotruncana aegytiaca, Globotruncana sp., Heterohelix reymenti, Heterohelix moremani, Heterohelix globulosa, Abathomphalus mayaroensis and occurrence of Orthokarstenina oyea, Ammobaculites corprolithiformis, Ammobaculities sp and Bolivina sp. Calcareous benthonic foraminiferal species were more abundant than the planktonic species in Orimedu-1 and Ise-2 wells. The benthonic species decrease drastically in the well-X offshore deep water where the planktonic species were most abundant. This trend may suggest an increasing water depth from near coastline to offshore regions in the Cretaceous times.Six planktonic foraminiferal zones were recognised across the three exploratory wells. These include Rotalipora greenhornesis (middle – late Cenomanian), Helvetoglobotruncana helvetica (early – late Turonian), Marginotruncana pseudolinneiana (late Turonian), Marginotruncana renzi (Coniacian – Santonian), Globotruncana calcarata (late Santonian – early Campanian) and Globotruncana aegytiaca (Maastrichtian) Zones. The Rotalipora greenhornesis and Praeglobotruncanae helvetica Zones (Cenomanian – Turonian) were correlated with the standard zones in the Tethyan regions.The Well-X indeed penetrated older Cenomanian to Turonian intervals in the investigated wells, thereby confirming the presence of older marine sequences in the Eastern Dahomey Basin contrary to previous suppositions.

Reservoir geology and effects on exploitation of natural bitumen deposits (Nigerian deposits as a case study)

Geological investigation have led to discovery of huge tar sand deposits within Afowo Formation of the Turonian–Maastrichtianage (95.9–66.0 Ma) in the Nigerian sector of the Eastern Dahomey Basin. This study aims at determining the feasibility of exploiting the major hydrocarbon resource steam assisted gravity drainage enhanced recovery technique. Samples from three core holes were dry sievied to determine the particle size distribution and their sections studied using a petrographic microscope. Clay mineral content was determined using X-ray diffraction scanning electron microscopy. The granulometric analysis shows the bituminous sediments to be generally fine grained and moderate to well sorted, and the grains are angular to subangular. Porosity ranges from 15.5 to 33.6 ɸ with average value of 26.4 ɸ, while permeability ranges from 270 to 4800 mD, with an average value of 4800 mD (very high) recorded for the sandstones. Petrographic study, scanning electron microscopy and X-ray diffractometry showed quartz as the dominant mineral component, with subordinate feldspar and other accessory minerals. The predominance of quartz is probably due to its mechanical stability. The low frequency of feldspar is attributable to its susceptibility to chemical breakdown and alteration, respectively, during transport and after deposition, with latter accounting for the observed secondary porosity. Kaolinite is the common clay mineral present in the oil sands and may not have sufficiently reduced the reservoir quality to negatively impacting enhanced recovery operation by steam assisted gravity drainage.

Delineation of Saline Water Intrusion Lateral Extent Using Hydro-chemically and Geo-electrically Derived CRS - Model Parameters - A Case Study of Eastern Dahomey Basin, Nigeria

This study was another attempt to map the subsurface lateral extent of saline water intrusions into aquifers at the eastern part of Dahomey basin, Nigeria. The study comprised geo-electric sounding method consisting of 108 vertical electrical soundings (VES) data acquired using Schlumberger array technique and hydro-chemical analysis of 61 water samples. The geoelectric survey results was presented as average longitudinal Conductance and average longitudinal Resistivity maps. The longitudinal conductance values vary from moderate to high (20 - 180 mhos) in the costal areas, while lower values (lesser than 10 mhos) were recorded in the mainlands. Conversely the average longitudinal resistivity values were low to moderate (0.26 - 41.7 ohm-m) in the coastal areas and in few places such Agbabu, Legbogbo and Sabomi in the mainland, while higher values (more than 60 ohm-m) were obtained from the mainlands. The hydro-chemical analysis results was presented as equivalent Salinity map; its values range from 25.8 mg/l (Ode-Irele) to 2808 mg/l (Asisa), high values were obtained from Ugbo, Ugbonla, Odonla, Idogun and Ayetoro area in the southeastern part of the study area and likewise at Agbabu in the north central part of the study area. The results were synthesized using CRS-model. The final CRS-model map showed that the saline water intrusion extended to 35 % of the total area which covers; most part of the coastal area and Agbabu in the mainland.

Geochemical characterisation of the

The geochemistry of the <2 μm fractions of Cretaceous-Tertiary Kaolins from Eastern Dahomey and Niger Delta Basins, Nigeria was investigated to infer their paleoenvironment, provenance, and tectonic settings using major, trace, and rare earth elements (REE). Twenty-eight (28) samples comprising of fifteen (15) Cretaceous and thirteen (13) Tertiary kaolins, respectively, were collected. Geochemical compositions of the <2 μm fractions were obtained using X-ray fluorescence (XRF) Spectrometry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analytical methods. Weathering indices such as chemical index of alteration (CIA, 96.98–99.39) and chemical index of weathering (CIW, 98.95–99.89) suggested intense chemical weathering under a wet tropical climatic condition. Ni/Co, V/Ni ratios and bivariate plot of V versus Ni indicated that they were deposited under marine oxygen-poor environment which is consistent with the horst and graben structural framework model of the Eastern Dahomey and Niger Delta Basins. The major, trace and REE data suggest that these deposits were derived from predominantly felsic source rocks with contributions from intermediate source rocks. The geochemical discrimination plots showed that the Cretaceous and Tertiary kaolins have been deposited in passive margin tectonic settings.

Mineral Constituents and Kaolinite Crystallinity of the &lt;2 μm Fraction of Cretaceous-Paleogene/Neogene Kaolins from Eastern Dahomey and Niger Delta Basins, Nigeria

Abstract This study examines the mineral composition and kaolinite crystallinity of Cretaceous and Paleogene/Neogene kaolins from Nigeria based on X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy data of the &lt;2 μm fraction. The purpose of this was to assess the minerals present, structural order, and possible industrial applications of the kaolins. A total of twenty-eight (28) samples comprising of fifteen (15) Cretaceous and thirteen (13) Paleogene/Neogene kaolins were analysed. The XRD results showed that both the Cretaceous and Paleogene/Neogene kaolins have kaolinite as the dominant phase followed by quartz with anatase, hematite, and goethite occurring in minor quantities. Based on the mineralogical composition, the Paleogene/Neogene deposits were classified as sandy kaolin and the Cretaceous deposits were classified as pure kaolin. The kaolinite structural order for the Cretaceous pure kaolin was higher than that obtained for the Paleogene/Neogene sandy kaolin with more quartz, iron, and titanium oxides. The economic consideration of the deposits showed that the sandy kaolin type can be used in pigment industrial applications whereas the pure kaolin type can be used in refractory, fiberglass, and ceramic industrial applications.

Source Rock Evaluation of Paleocene Akinbo shale, Eastern Dahomey Basin

Selected subsurface core samples of the shale of Akinbo Formation as penetrated by an exploratory well in Ibese, Eastern Dahomey basin were investigated to ascertain the quality and quantity of organic matter, the hydrocarbon potential and kerogen type.The samples were subjected to Total Organic Carbon (TOC) and Rock Eval analyses and various cross plots were generated from the data obtained.The TOC and Free oil content (S1) of all the shale samples range from 0.96wt% to 2.82wt% and 0.07mgHC/g to 0.17mgHC/g with mean values of 1.67wt% and 0.11mgHC/g respectively while the source rock potential (S2) ranges from 0.01mgHC/g to 0.17mgHC/g with an average value of 0.08mgHC/g. Also, the Hydrogen Index (HI) and the Oxygen Index (OI), ranges from 0.35mgHC/g TOC to 16.7mgHC/g TOC and 11.4mgCO/g TOC to 38.33mgCO/g TOC with an average value of 5.77mgHC/g TOC and 19.04mgCO/g TOC respectively. The Production Index (PI) and the Generative Potential (GP) range from 0.38 to 0.94 and 0.12mgHC/g to 0.34mgHC/g with mean values of 0.61 and 0.19mgHC/g respectively.The results obtained from the cross plots of HI versus OI, S2 versus TOC and TOC versus GP; It shows that the shale samples from the Akinbo Formation have good organic matter richness to generate hydrocarbon, dominantly gas prone and from a Type III kerogen.

Frequency-Dependent Electrical Characterization of Rock Types from Ewekoro, Eastern Dahomey Basin, Nigeria

Dielectric measurements (40 Hz-110 MHz) conducted on samples of limestone and its associated rocks from Ewekoro, Eastern Dahomey Basin, Nigeria has yielded vital information for characterization. Cole-Cole plots manifest a distribution of relaxation times in the rock samples common for multicomponent systems. All the rock types show dielectric dispersion in dry and partially saturated conditions, but the frequency range differs for the rock types and depends on wettability. At partial water saturation there is: (i) enhanced polarization resulting in increase in real and imaginary permittivities; (ii) shortened region of dielectric dispersion; (iii) broadened electrode polarization plateau; and (iv) steeper and shorter dispersion region. Irrespective of the state of the rocks, dielectric parameters for shale and glauconite are at least an order greater than for limestone and sandstone. Geometric or textural effects are partly responsible for the observed differences coupled with the presence of charged clay/clay-like particles in shale and glauconite. Decrease in relaxation and critical frequencies in partial saturation for shale in contrast to the increase in these frequencies for the other three rock types is due the effect of pore geometry on overall dielectric relaxation. This study shows that dielectric measurement can complement geochemical analysis in laboratory evaluation and characterization of rock raw materials.

Lithotype characterizations by Nuclear Magnetic Resonance (NMR): A case study on limestone and associated rocks from the eastern Dahomey Basin, Nigeria

Three representative rock types (limestone, sandstone, and shale) and glauconite samples collected from Ewekoro Quarry, eastern Dahomey Basin in Nigeria were characterized using low field 2 MHz and 20 MHz Nuclear Magnetic Resonance (NMR) techniques. NMR T2 relaxation time decay measurement was conducted on disc samples under partial water-saturation and full water-saturation conditions using CPMG spin-echo routine. The T2 relaxation decay was converted into T2 distribution in the time domain to assess and evaluate the pore size distribution of the samples. Good agreement exists between water content from T2 NMR distributions and water imbibition porosity (WIP) technique. Results show that the most useful characteristics to discriminate the different facies come from full saturation NMR 2 MHz pore size distribution (PSD). Shale facies depict a quasi-unimodal distribution with greater than 90% contribution from clay bound water component (T2s) coupled to capillary bound water component (T2i) centred on 2 ms. The other facies with well connected pore structure show either bimodal or trimodal T2 distribution composed of the similar clay bound water component centred on 0.3 ms and quasi-capillary bound water component centred on 10 ms. But their difference depends on the movable water T2 component (T2l) that does not exist in the glauconite facies (bimodal distribution) while it exists in both the sandstone and limestone facies. The basic difference between the limestone and sandstone facies is related to the longer T2 coupling: T2i and T2l populations are coupled in sandstone generating a single population which convolves both populations (bimodal distribution). Limestone with a trimodal distribution attests to the fact that carbonate rocks have more complex pore system than siliclastic rocks. The degree of pore connectivity is highest in sandstone, followed by limestone and least in glauconite. Therefore a basic/quick NMR log run on samples along a geological formation can provide precise lithofacies characterization with quantitative information on pore size, structure and distributions.

Sedimentological interpretation of coastal ditch cuttings: implications for subsurface geology and provenance in Dahomey Basin, Nigeria

Understanding sediment provenance and subsurface geology of an area plays an important role in geological exploration and environmental studies. This research characterized subsurface geology and provenance of coastal sediments in some parts of the eastern Dahomey Basin, southwestern Nigeria. One hundred twenty-six ditch cuttings were retrieved from four boreholes drilled along the coastal–near-coast area of the basin. Samples were subjected to sedimentological descriptions, heavy mineral analysis and geochemical analysis (atomic absorption spectrometry (AAS)), using standard laboratory procedures. The presence of stable–ultra-stable heavy minerals and the paucity of unstable minerals characterized the analyzed samples. AAS showed that SiO2 was the dominant oxide in all the 42 representative samples analyzed, with concentration that generally exceeds 90 wt%, thereby complementing ZTR index to ascertain sediment maturity. Careful comparison of the sedimentological data and heavy mineral indices shows several non-correlatable depositional cycles in the wells, while five distinct sequences were revealed from bivariate geochemical plots. Dominance of plutonic provenance, with very minor contributions from metamorphic rocks, continental depositional environment and repeated cycles of long transportation and deposition were inferred. This study demonstrated the usability of ditch cuttings to predict provenance, paleoenvironment and transport history in terrains with paucity of outcrops, core data and fossil-deficient intervals.

Geoelectric Soundings for Delineation of Saline Water Intrusion into Aquifers in Part of Eastern Dahomey Basin, Nigeria

This study was aimed at mapping the subsurface extent of saline water intrusions into aquifers at the eastern part of Dahomey basin, Nigeria. The study adopted geoelectric sounding methods. 108 vertical electrical soundings (VES) and 9 induced polarization soundings (IPS) data were acquired using Schlumberger array technique. Three aquifer units were delineated across the study area. The resistivity of the first, second and third aquifer layers varies from 0.2 to 1569 ohm-m, 0.5 to 904 ohm-m and 0.4 to 665 ohm-m respectively, while depth to the top of first, second and third aquifer varies respectively from 0.7 to 151.5 m, 1.4 to 305.5 m and 12.9 to 452.9 m. The depth to the first aquifer layer is shallow (less than 5 m) in the coastal area which makes this area to be highly vulnerable to anthropogenic pollution while their proximity to Atlantic Ocean makes them susceptible to saline water intrusion. In all the three aquifer units, the coastal area, Agbabu and other few locations in the mainland are characterized by low resistivity values (below 60 ohm-m) indicating possible presence of brackish or saline water. IP sounding results showed that all the low resistive layers in the mainland are characterized by clayey materials. The integration of VES and IPS results enabled the delineation of the saline water lateral extent across the study area. There is a strong direct correlation (r2 = 0.8564) between location distance from the saline water source and depth to saline water in the study area. This can therefore serve as a predictive model to determine depth to saline water at any location within the saline water zone in the study area.

Geochemical Evaluation of Arimogija-Okeluse Limestones, Eastern Dahomey Basin, Southwestern Nigeria

10 representative samples of limestone were collected from the Arimogija-Okeluse Axis of the Dahomey Basin and subjected to X-Ray Fluorescence analysis. Major and Trace elements were used to classify and predict the depositional environment of Arimogija-Okeluse limestone.The results for the major oxide composition revealed that CaO content ranges from 47.6-52.31% with a mean value of 50.06%. SiO2 concentration varies from 1.94-5.24% with an average of 3.36%. Fe2O3. MgO, and Al2O3 contents have mean values of 1.702%, 1.52%, and 1.09% respectively. Other oxides; Na2O, P2O5, K2O and TiO2 are low in concentration. The Loss on Ignition also varies from 38.9-41.8 with a mean value of 40.43% suggesting a high carbonate content for the limestone. Standard Ca/Mg and Mg/Ca ratios varies from 25.87-37.63 and 0.03-0.04 with a mean of 33.31and 0.031 respectively. The high strontium content reveals a high saline environmental condition of formation for the limestone.The results obtained, however showed that the Arimogija-Okeluse limestone is a Magnesian limestone type deposited in a shallow marine environment and suitable for cement production.

Geochemical Evaluation of Arimogija-Okeluse Limestones, Eastern Dahomey Basin, Southwestern Nigeria

10 representative samples of limestone were collected from the Arimogija-Okeluse Axis of the Dahomey Basin and subjected to X-Ray Fluorescence analysis. Major and Trace elements were used to classify and predict the depositional environment of Arimogija-Okeluse limestone.The results for the major oxide composition revealed that CaO content ranges from 47.6-52.31% with a mean value of 50.06%. SiO2concentration varies from 1.94-5.24% with an average of 3.36%. Fe2O3. MgO, and Al2O3contents have mean values of 1.702%, 1.52%, and 1.09% respectively. Other oxides; Na2O, P2O5, K2O and TiO2are low in concentration. The Loss on Ignition also varies from 38.9-41.8 with a mean value of 40.43% suggesting a high carbonate content for the limestone. Standard Ca/Mg and Mg/Ca ratios varies from 25.87-37.63 and 0.03-0.04 with a mean of 33.31and 0.031 respectively. The high strontium content reveals a high saline environmental condition of formation for the limestone.The results obtained, however showed that the Arimogija-Okeluse limestone is a Magnesian limestone type deposited in a shallow marine environment and suitable for cement production.

Petrographic Studies and Paleoenvironmental Reconstruction of Some Outcropping Sediments in Parts of Eastern Dahomey Basin, South Western, Nigeria

Petrographic Studies and Paleoenvironmental Reconstruction of Some Outcropping Sediments in Parts of Eastern Dahomey Basin, South Western, Nigeria

Evaluation of the natural radioactivity level of Nigerian tar sand deposits, eastern Dahomey basin, southwestern Nigeria

The specific activities of natural radionuclides in twenty seven samples collected from the eastern Dahomey basin in southwestern Nigeria were evaluated. Experimental results were obtained by using a 3" x 3" sodium iodide NaI(Tl) detector. A major trace element assessment of the samples was made by Inductively Coupled Plasma Mass Spectrometry techniques. Gamma ray radioactive standard sources, 137Cs and 60Co, were used to calibrate the measurement system and the International Atomic Energy Agency SOIL-375 radioactive standard source was also used to analyze and compute the specific activities of desired natural radionuclides. Three radioelements, viz. 238U, 232Th and 40K, were identified in the samples with the following specific activities of 238U ranging from 9.88 ? 4.70 Bq/kg to 69.15 ? 12.37 Bq/kg with an average of 26.50 ? 7.18 Bq/kg; 232Th from 12.78 ? 5.16 Bq/kg to 36.86 ? 13.35 Bq/kg with an average of 22.77 ? 4.28 Bq/kg; 40K ranging from 189.82 ? 79.51 Bq/kg to 518.77 ? ? 119.54 Bq/kg with an average of 297.69 ? 16.21 Bq/kg. The result was compared with the world mean values of 35, 30 and 400 Bq/kg, respectively, specified by the United Nations Scientific Committee on the Effects of Atomic Radiation. The mean result obtained for the radium equivalent, total absorbed dose rate, external hazard index, internal hazard index, and the annual effective dose equivalent, were 79.90 Bq/kg, 38.50 ?Sv/h, 0.22 Bq/kg, 0.29 Bq/kg, and 47.22 ?Sv, respectively. With respect to radiological risk to human health, the absorbed gamma dose rate in air was estimated to be in the range of 21.7 ? 0.4 to 155.7 ? 2.2 ?Sv/h; the outdoor annual effective dose equivalent was evaluated to vary from 26.6 ? 0.4 to 190.9 ? ? 2.7 ?Sv with the arithmetic mean value of 79.06 ? 33.23 ?Sv and compared to the world-wide effective dose of 70 ?Sv. Also, the values of the radium equivalent and the external hazard index for all samples in the study area were found to be lower than the accepted safety limit value of 370 Bq/kg and equally below the limit of unity, respectively. The results indicate that, at present, the radiation hazard from radionuclides in all samples analyzed is within permissible limits.

Provenance Analysis of Sediments in ILU-TITUN and Environs, Eastern Dahomey Basin, Ondo State, Nigeria

Twenty eight sediment samples were obtained from road cuts and stream cuts in Ilu Titun area of Ondo State, Nigeria and were subjected to Grain Size Distribution (GSD) analysis, from which statistical parameters were calculated and used to determine the environment of deposition of the sediments. Six representative samples were selected for petrographic studies to determine the heavy mineral content of the sediments with a view to establishing the sediments source of Ilu Titun and environs which mark the eastern margin of the Dahomey basin. The Graphic Mean (Mz) ranges from 0.700F to 1.667&Phi while the Inclusive Graphic Standard Deviation (&sigma i ) range from 0.960 to 1.319. The Inclusive Graphic Skewness ( SK I ) of the samples ranges from -0.665 to 0.803 while Kurtosis range from 1.652 to 3.309. The bivariate plot of mean size against the Inclusive graphic standard deviation show that the sediments were deposited by river and beach processes. The plot of skewness against the Inclusive graphic standard deviation confirmed that the sediments were deposited by river processes. The results of the petrographic studies show that Slides EK 01 and EK 08 have Kyanite and Staurolite. Slides EK 13 and EK 24 have Staurolite, Tourmaline and Garnet. Slides EK 26 and 28 have Tourmaline and Zircon present in them. Bivariate plots using the statistical parameters show that the sediments were deposited by fluvial processes. The results from the petrographic studies indicate that the sediments were sourced from metamorphic rocks and pegmatites.

Petrography, compositional characteristics and stable isotope geochemistry of the Ewekoro formation from Ibese Corehole, eastern Dahomey basin, southwestern Nigeria

Subsurface samples of the predominantly carbonate Ewekoro Formation, obtained from Ibese core hole within the Dahomey basin were used in this study. Investigations entail petrographic, elemental composition as well asstable isotopes (carbon and oxygen) geochemistry in order to deduce the different microfacies and depositional environment. Petrographic study reveals the presence of dolomite, biomicrite, sandy biomicrite, and biosparite facies. Preserved pore types such as; intercrystaline, moldic and vuggy pores were observed as predominant conduits for fluids. The major element oxides ranges are; CaO (39.79 - 53.98wt. %), SiO2 (1.29-25.37 wt. %), Al2O3 (0.43-3.77 wt. %), K2O (0.04-0.27wt. %), Fe2O3 (0.74-2.35 wt. %), and Na2O (&lt;0.01-0.05 wt. %). Cross correlation of the major oxidecontents shows that Al2O3, K2O, Na2O, TiO2, Fe2O3, and P2O5 are associated with aluminosilicates and their low concentrations indicates that the basin was free of suspended clays and other aluminosilicates materials during carbonate sedimentation. The relatively high concentration of SiO2 and its interrelationship with other major oxides, suggests SiO2 has strictly influenced the sedimentation of the carbonate hence would have been contributed as free silica, probably from beaches and not as aluminosilicates. The Mn/Sr ratio reveals an influence of diagenesis on the mineralogy of the carbonates. The Mg/Ca ratio ranges from 0.02 and 0.1(av. 0.04), with an outlier value of 0.1, indicating a strong diagenetic influence and the precipitation of dolomite in the mixing zone. The isotopic composition of δ13C (-1.9‰ to 0.79‰) shows a discrepancy from modern carbonate sediments. The average estimated formational temperature of the limestone is 32.50C. The δ13O versus δ18O bivariate diagram indicates that the limestone is predominantly average marine limestone with chalk and late cement. The values of depositional setting (Z), estimated from δ13O and δ18O composition support a diagenetic influence on the Ewekoro Formation. The U/Th ratios (~0.25 to 2.3; av. 1.2) and Ce anomaly (0.70 - 0.94, av. 0.85) indicate a predominantly oxygenated environment with little fluctuation in bottom water oxygen level.

Lithostratigraphy, Palynostratigraphy and Palynofacies Indications of the Depositional Environments of Upper Cretaceous to Paleogene Sediments, Offshore Eastern Dahomey Basin, SW Nigeria

The ditch cuttings from an offshore well, 'JX' were studied for their sedimentology, palynostratigraphy, palynofacies and depositional environment history. Lithologically, poorly sorted sandstones with variable interbeds of shales and siltstones overlain by dark grey shales and interbedded limestones with occasional pyrite occurrence (Abeokuta-Afowo Formations) constituted the basal sections. Overlying is a glauconitic sequence consisting of shale with little clay and lenses of limestones (Araromi Formation). The uppermost section consists of predominantly of shales with some glauconitic materials (Oshosun Formation). Occurrences of Canningia cf. senonica, Nelsoniella; Microdinium ornatum (dinoflagellates), Crybelosporites pannuceus, Cicatricosisporites spp. (spores), and Classopollis spp., Ephedripites (pollen) assemblage suggests a late Cenomanian age for the lowermost section while the occurrences of Lejeunecysta hyaline, Selenopemphix selenoides and Hystrichokolpoma rigudae (dinoflagellates) and Peregrinipollis nigericus, Retitricolporites irregularis and Ctenolophonidites costatus (pollen) and Foveotriletes margaritae (spore) at the uppermost section indicate an Eocene age. Palynological marine index (PMI) curve and the Peridinioid to Gonyaulacoid ratio (P/G ratio) between the different palynological assemblages showed very strong fluctuations. Six broad lithofacies units with characteristic palynofacies assemblages reflecting different paleoenvironment were distinguished, which varied from estuarine through shallow inner neritic (shelf) to middle neritic shelf settings.