Eastern Dahomey Research Articles

REE signatures in quaternary deposits: implication for provenance and economic potentials

The Rare Earth Element (REE) geochemistry of siliciclastic sediment in the Ilaje area, eastern Dahomey basin, southwestern Nigeria was conducted to determine the provenance, geotectonic setting as well the REE economic potential in the sediments. Geochemical analysis results (ppm) revealed the concentration of La varies from 1.87–21.47 while Ce ranges from 1.16–45.98. Others include; Pr (1.07–39.56), Nd (0.71–26.02), Sm (0.42–12.51), Eu (0.20–4.66), Gd (0.27–8.79), Tb (0.29–3.97), Dy (0.28–3.62), Ho (0.22–2.76), Er (0.29–2.64), Tm (0.33–3.83), Yb (0.37–3.75) and Lu (0.29–3.03) respectively while the ∑REE ranges from 2.25 to 6686.75 ppm. The REE normalization plot is characterized by an inclined shape depletion indicating depletion trend. The patterns show an enriched Light Rare Earth Elements (LREE) and Heavy Rare Earth Element (HREE) pattern indicating upper continental crust sourced materials. The plot also revealed a negative Eu anomaly which may be associated with plagioclase fractionation in the acidic source rocks. The high ∑REE in the sediments has uncovered the economic potential of the sediments as a useful raw material resource for technological advancement.

Evaluation of heavy hydrocarbon liquid in the Eastern Dahomey Basin, Southwestern Nigeria (case study of Agbabu Bitumen in Ondo State)

Solvent-aided dilution processes are often employed to recover bitumen by reducing its viscosity. In this study, methanol, toluene, and xylene were investigated as potential hydrocarbon solvents for solvent-aided hydrocarbon recovery of Agbabu bitumen. To achieve this, the solubility of the sample in the solvents was determined with a solubility test, its viscosity was measured with the Saybolt Furol Viscometer using the ASTM D88 method, and its SARA analysis was conducted using high-performance liquid chromatography. Agbabu bitumen was found to have a high content of saturates and aromatics. Viscosity decreases as pressure increases, while solubility reduces as temperature increases. The sample was most soluble in toluene, and its viscosity was reduced most in it, followed by xylene. Methanol reduced the saturates content and significantly raised the asphaltene content, keeping the mixture viscosity high. In terms of asphaltene stability of the mixtures, the bitumen-methanol system with the lowest colloidal instability index of 0.874 will give a mild asphaltene deposit issues compared with others that are unstable. This approach of combining multiple tests with the CII can accurately predict the behavior of Agbabu bitumen in solvents and enhance decision on the choice of bitumen recovery technology.

Physiochemical Parameters and Groundwater Resource in the Eastern Dahomey Basin, Southwestern Nigeria

Physiochemical Parameters and Groundwater Resource in the Eastern Dahomey Basin, Southwestern Nigeria

Lithofacies, bio-sequence stratigraphy and paleoenvironment of the cretaceous-neogene at the BG-1 well, offshore Eastern Dahomey Basin, Nigeria: implications for future exploration and development efforts

Lithofacies, bio-sequence stratigraphy and paleoenvironment of the cretaceous-neogene at the BG-1 well, offshore Eastern Dahomey Basin, Nigeria: implications for future exploration and development efforts

Geophysical and Sedimentological Characterization of Oil Sand Deposit in Part of Eastern Dahomey Basin Southwestern Nigeria

Unconventional oil is the group name given to unconventional petroleum deposits such as oil sands and heavy oils. Oil sand, commonly referred to as tar sand or bituminous sand is a sandstone formation impregnated with viscous bitumen. This study utilized the two-dimensional electrical resistivity imaging technique to characterize oil sand and validate its true resistivity value and sedimentological analysis to evaluate the physical properties of the oil sand deposit. The two-dimension electrical resistivity imaging data acquisition was done along seven profiles systematically in a grid format using dipole-dipole electrode configuration and processed using the RES2DINV software. Fresh samples were collected from three points within the study area for grain sizes and textural properties analysis. The result of the 2-D ERI in the study area showed two to three geoelectric layers and the oil sand being the second layer as evident in the 3-D model obtained from the systemic data acquisition procedure. The resistivity obtained for oil sand in the study area range between 2,500 Ωm to 105 Ωm which validates the report of high resistivity value of oil sand in the study area. The sedimentological analysis showed that the sediments in the study area are of medium sand moderately sorted sand grain and leptokurtic to mesokurtic sand suggesting unimodal and bimodal parent source.

Calcareous nannofossils biostratigraphy of well Uche 1, Offshore Lagos, Eastern Dahomey Basin, South Western Nigeria

Well Uche 1 drilled in the offshore of Dahomey Basin was biostratigraphically analyzed, based on its calcareous nannofossils content. Thirty analyzed samples were collected from the Well at an interval range of 20m at a depth of 1284m- 1860m. The microscopic examination of the processed samples was carried out (using simple smear method) and this was found useful in the identification of the different biostratigraphic zones present. Moreover, it is equally found useful in age determination. In all, fifteen (15) calcareous nannofossil species (Braarudosphaera bigelowii, Calcidiscus leptoporus, Coccolithus nitescens, Coccolithus pelagicus, Coronocyclus nitescens, Discoaster deflandrei, Discoaster druggii, Discoaster sp (6rays), Helicosphaera cateri,Pontosphaera multipora, Reticulofenestra haqii (3-5 microns), Reticulofenestra pseudoumbilicus (5-7 microns), Reticulofenestra pseudoumbilicus (>7 µ), Sphenolithus moriformis, Triquetrorhabdulus carinatus) were identified and used for biozonation and identification of significant time surfaces. The distribution of the calcareous nanofossil species in the well enabled the establishment of zones belonging to the Early Miocene; NN2 zone. The most abundant calcareous nannofossils identified are Triquetrorhabdulus carinatus, Cyclicargolithus abisectus and Discoaster druggi. Two biozones of calcareous nannofossils were identified from the well as well as three significant surfaces.

Sedimentary: Carbonate microfacies and mineralogy of the southern Benue Trough and eastern Dahomey basin, Nigeria

This study presents a detailed investigation into the microfacies, geochemistry, and depositional environments of carbonate rocks from the southern Benue Trough and eastern Dahomey Basin. This analysis involved a combination of techniques, including X-ray diffraction, X-ray fluorescence, scanning electron microscopy with energy-dispersive X-ray spectroscopy and thin-section petrographic examination. Samples from the Igumale Formation of the southern Benue Trough and samples from Ewekoro Formation of the eastern Dahomey Basin were analyzed, offering insights into their mineral compositions, elemental distributions, and depositional environments. In each basin, two carbonate microfacies were identified. The two microfacies found in the Igumale Formation were recognized as micritic bioclastic packstone and sandy bioclastic wackestone. In the Ewekoro Formation, the two microfacies recognized were bioclastic packstone and sparitic peloidal wackestone. The X-ray diffraction analysis of samples from both basins revealed a dominance of calcite. X-ray fluorescence analysis showcased calcium oxide and calcium as the predominant oxide and elements in both basins with varying concentrations. Scanning electron microscopy-energy-dispersive X-ray spectroscopy results unveiled the elemental composition with calcium and silicon emerging as major constituents. The photomicrographs suggest distinct depositional conditions for both samples with those from the Igumale Formation showing evidence of shallow marine and that of the Ewekoro Formation suggesting calm marine environments comprised of fined grained carbonate rocks containing fossils. This study has provided detailed understanding on the nature, microfacies types, mineralogical composition, depositional environment and diagenetic processes of Cretaceous-Paleocene carbonate rocks within the studied basins.

Estimating The Hydraulic Conductivity and Depositional Environment of Quaternary Deposits in Parts of Eastern Dahomey Basin

Estimating The Hydraulic Conductivity and Depositional Environment of Quaternary Deposits in Parts of Eastern Dahomey Basin

Hydrogeophysical Investigation in Parts of the Eastern Dahomey Basin, Southwestern Nigeria: Implications for Sustainable Groundwater Resources Development and Management

Geoelectrical resistivity measurements were conducted in five locations within the eastern portion of the Dahomey basin for the purpose of subsurface evaluation and detecting saturated zones. The locations are Covenant University (L1), Bells University (L2), Oju-Ore-Ilogbo Road (L3), Obasanjo-Ijagba Road (L4), and Iyana Iyesi (L5). The study was carried out to avert the common challenges of drilling low-yield groundwater boreholes in the area. A total of 30 Vertical Electrical Soundings (VES) and five two-dimensional Electrical Resistivity Tomography (ERT) data sets have been acquired along the study areas. The geoelectrical resistivity results were integrated with the borehole logs to generate the spatial distribution of the subsurface lithologies in the area. The delineated subsurface lithologies include the topsoil (lateritic clay), clayey sand, sandy clay, fine silty sand, coarse sand, and shale/clay units. The fine silty sand and coarse sand units were identified as the two main aquifer units within the area. The depths to the upper aquifer unit in the area include 31.7–96.7 m, 38.5–94.0 m, 30.7–57.5 m, 39.1–63.4 m, and 46.9–57.5 m for locations L1, L2, L3, L4, and L5, respectively. At the same time, the depths to the lower aquifer unit in the area include 43.4–112.7 m, 52.2–108.0 m, 44.2–72.5 m, 53.7–78.5 m, and 63.5–72.9 m for locations L1, L2, L3, L4, and L5, respectively. The estimated hydraulic parameters for both aquifers show they are highly productive with mean porosity, mean hydraulic conductivity, and mean transmissivity of 20–22%, 12.4–17.0 × 10−2 m/s, 1.56–2.18 m2/s for the upper aquifer, and 48–50%, 371–478 × 10−2 m/s, 50.00–62.14 m2/s for the lower aquifer. By focusing on these aquifer systems during exploration, sustainable groundwater resources can be secured, providing relief to homeowners within the study area who might otherwise face the frustration of drilling unproductive and low-yield boreholes. However, it is crucial to consider the presence of sub-vertical faults in the study area, as these faults can significantly impact groundwater development and management. These sub-vertical structural faults may lead to changes in the permeability, hydraulic conductivity, and transmissivity of the delineated aquifers, affecting their productivity across the divide and ultimately influencing the overall water availability in the area. Careful consideration of these geological factors is essential for effective aquifer management and sustainable groundwater utilisation.

N-Alkane bound biomarker fingerprints from asphaltenes in the bitumens of Eastern Dahomey Basin, Nigeria: Source and genetic implications

The asphaltene fractions of the bitumens of Eastern Dahomey Basin in Nigeria, were analyzed by flash pyrolysis-gas chromatography (Py-GC) method in order to unravel its geochemical history and properties. The distributions of the initial biomarkers of the original oils from the pyrolysates are related to the assessment of organic matter source, paleo-redox conditions and source environment during deposition. Also, it effectively establishes the genetic relationship of the bitumens. The n-alkane distributions in the pyrolysates reveal nC9-nC32n-alkanes, maximizing at nC14, isoprenoids-pristane (Pr) and phytane (Ph), and some n-alkene peaks. High peaks of low to medium-weight nC9-nC20n-alkanes and low peaks of nC21+n-alkanes characterize the distributions. These reveal that abundant algal organic matter with some terrigenous inputs contributed to the source rock of the bitumens. The high concentration of marine organic matter inputs to the source rock is further confirmed by the nC17/nC27 ratios which range from 5.39 to 19.82 and shows the predominance of nC17 alkanes. The general unimodal n-alkane distributions in the bitumens indicate derivation from similar organic matter types showing that they are genetically related. The anoxic to suboxic environmental conditions that prevailed during the deposition of the sediments is revealed by the isoprenoids, Pr/Ph ratios (0.72–1.28). Pristane/nC17 and Phytane/nC18 range from 0.16 to 0.33 and 0.22 to 0.56, revealing that the bitumens were from predominantly marine organic matter (type II kerogen) preserved in a reducing environment with no evidence of biodegradation. However, the Ph/nC18 ratio and the cross plot of Pr+Ph/nC17+nC18 allow the classification of the bitumens into two subfamilies/groups (A and B). The bitumen samples have low wax content as indicated by the degree of waxiness ranging from 0.21 to 0.38 which confirms low terrigenous input. Based on the carbon preference index (CPI: 0.92 to 1.55) and odd-even predominance (OEP: 0.70 to 1.36), it is concluded that the bitumens are immature to marginally mature.

Diagenetic evolution of continental to shallow marine Mesozoic–Cenozoic deposits, Eastern Dahomey Basin, Nigeria: Implications for reservoir quality

Diagenetic evolution of continental to shallow marine Mesozoic–Cenozoic deposits, Eastern Dahomey Basin, Nigeria: Implications for reservoir quality

Provenance and Paleoenvironments reconstruction of Sandstones from Onshore Dahomey Basin, Southwestern, Nigeria: Implications for Reservoir Potential

The aquiferous sandstone units in Daniel-1, Olambe and Yakoyo boreholes located in the onshore part of the eastern Dahomey Basin, Southwestern Nigeria, were investigated through grain size, petrography and geochemical studies to evaluate their provenance, paleoenvironments and the preliminary aquifer or reservoir quality of the sandstones. Average mean, standard deviation (sorting), skewness and kurtosis across the three boreholes ranges from 0.37 mm – 1.37 mm, 0.54 mm – 0.82 mm, 0.18 mm – 0.84 mm and 0.89 mm – 2.07 mm respectively suggesting dominance of medium to coarse grain, and moderate to moderately well sorted sandstones. The mineral grain sizes, sorting and sub-angular to sub-rounded shapes indicate a sub-mature texture. Mineralogy reveals average 78 % quartz, 9 % feldspar and 13 % rock fragments typical of sub-arkose to quartz arenites sandstones. Major oxides of SiO2 ranges from 90.70 – 99.62 %, Al2O3 0.49 – 3.34 %, Fe2O3 0.07 – 1.65 %, while other oxides are less than 1.0 % further suggesting dominance of quartz mineral grains. Bivariate plots of Th, La, Sc, Co trace elements further revealed dominance of quartz minerals sourced from felsic granitic igneous rock of a passive margin continental basement. The sub-mature to mature textures and mineralogy coupled with their shallow depths of the sandstones at 130 ft – 160 ft are suitable factors that enhances good porosity and permeability in an aquifer or reservoir rock. These are probably the positive attributes favouring the storage and efficient water production of the aquiferous sandstones. They have similar textural and mineralogy features with the Turonian – Coniacian Afowo sandstones reservoir producing oil and gas in the deep subsurface offshore area of the basin and the onshore Tar sand in Nigeria and part of Benin Republic, West Africa

Geoelectrical resistivity mapping and characterization of bituminous sands at some parts of Eastern Dahomey Basin, SW Nigeria

Predicting the variability and subsurface distribution of bituminous sand deposits rely significantly on integrating geophysical techniques such as electrical resistivity mapping with borehole information. While several studies have been conducted to characterize the bitumen-rich sequence of the sedimentary terrain of the study area, there are no or few studies documenting the subsurface architecture of bituminous sands in the transition zones in the study area. This study utilized the integration of 2D electrical resistivity mapping and borehole lithology logs to delineate and characterize bituminous sands in the study area to provide insights into the electrical resistivity signature of bituminous sand deposits within the transition zone in Ondo state, SW Nigeria. Two boreholes (BHI and BH2) and 2D electrical resistivity mapping using dipole–dipole array along five traverses were used for the investigation. A systematic and data-based approach was employed for the mapping of the deposits. The 2D electrical resistivity survey acquisition was carried out on traverses oriented north–south in the direction perpendicular to the sedimentary strike to effectively map the bituminous deposits. The results obtained from the approach employed indicated a maximum of three to four geoelectric layers delineated within the study area. This includes the topsoil, the upper X-horizon, mudstone, and lower Y-horizon. The upper/shallow bituminous sands in the X-horizon, as well as the deeper bituminous sands in the Y-horizon sands, are characterized by a very high resistivity range of about 1000 to about 100,000 Ωm. Findings from this study reveal that the resistivity of bituminous sands in the area is generally high in contrast to the low resistivity signature observed in the sedimentary parts of the area. The results obtained from this study would serve as a means to guide future exploration studies within transition zones in Eastern Dahomey Basin, SW Nigeria.

Morphotype analysis and paleoecological implications of Maastrichtian - Paleogene benthic foraminifera from RCCG BH4 Borehole, Eastern Dahomey (Benin) Basin

The Maastrichtian-Eocene paleoecological conditions of eastern Dahomey (Benin) Basin were investigated using benthic foraminiferal morphotypes from the RCCG BH-4 borehole located on kilometer 46, Lagos-Ibadan Expressway, southwest Nigeria. Many previous studies on the basin and other Nigerian sedimentary basins centered on paleobathymetric studies using depth indicator benthic foraminiferal assemblages and planktic/benthic ratio. This present study attempts to categorize benthic species into morphological categories (morphotypes) based on general outline and shape, chamber arrangement, and mode of coiling, and use the distribution of the morphotypes to determine certain paleoecological conditions such as organic matter content, sediment pore water oxygen availability and sediment depositional setting. 25 ditch cutting samples subjected to the kerosene method of micropaleontological sample processing yielded few Maastrichtian – Eocene planktic species (Planoheterohelix globulosa, Morozovella acuta, Parasubbotina pseudobulloides, and Subbotina velascoensis) and fairly abundant and diverse benthic foraminiferal assemblages which we categorized into nine (9) morphotypes designated B2, B3, C1, F, H, I, J, N and Q. Six (6) of these morphotypes (B2, B3, I, J, N, and Q) are adapted to the epifaunal mode of life, while morphotypes C1, F, and H belong to the infaunal types. The dominance of the epifaunal morphotypes within the limestone Ewekoro Formation, and the underlying shaly sand unit of the Araromi Formation revealed a shallow marine (inner – middle neritic), low organic matter content, and oxygen-rich environment of deposition for these units. The infaunal morphotypes show sparse occurrence within the borehole except within the organic intercalation within the limestone unit in which 8 counts of Afrobolivina afra were recorded. The disappearance of species (faunal turnover) within the overlying shale unit (Akinbo Formation) is believed to be a response to heightened organic matter flux, reduced oxygen availability, and other climatic perturbations characteristic of the Paleocene-Eocene boundary. Eponides pseudoelevatus (morphotype N) is believed to be an opportunistic species based on the species’ continued occurrence and increased abundance within the shale unit (Akinbo Formation). The species may have recolonized the environment following improved environmental conditions that ensued after the cessation of the end Paleocene climatic perturbation.

Metal–Metal Correlation of Biodegraded Crude Oil and Associated Economic Crops from the Eastern Dahomey Basin, Nigeria

The presence of heavy metals in plants from oil sand deposits may reflect mineralization resulting from petroleum biodegradation. Petroleum composition and heavy metal analyses were performed using thermal desorption gas chromatography and atomic absorption spectrophotometry on oil sand and plant root samples from the same localities in the Dahomey Basin. The results from the oil sand showed mainly heavy-end hydrocarbon components, humps of unresolved complex mixtures (UCM), absences of C6-C12 hydrocarbon chains, pristane, and phytane, indicating severe biodegradation. In addition, they showed varying concentrations of vanadium (2.699–7.708 ppm), nickel (4.005–11.716 ppm), chromium (1.686–5.733 ppm), cobalt (0.953–3.223 ppm), lead (0.649–0.978 ppm), and cadmium (0.188–0.461 ppm). Furthermore, these heavy metals were present in Citrus, Theobroma Cacao, Elaeis guineensis, and Cola. The chromium, nickel, vanadium, lead, cobalt, and cadmium concentrations in the Citrus were 7.475, 4.981, 0.551, 0.001, 0.806, and 0.177 ppm, respectively. For the Theobroma Cacao, the concentrations of chromium, nickel, vanadium, lead, cobalt, and cadmium were 7.095, 16.697, 2.151, 0.023, 3.942, and 0.254 ppm. Elaeis guineensis also showed the presence of chromium (32.685 ppm), nickel (32.423 ppm), vanadium (11.983 ppm), lead (0.190 ppm), cobalt (4.425 ppm), and cadmium (0.262 ppm). The amounts of chromium, nickel, vanadium, lead, cobalt, and cadmium in the Cola were 9.687, 9.157, 0.779, 0.037, 0.695, and 0.023 ppm. The World Health Organization’s (WHO) safe and permissible limits for Cd (0.003 ppm), Cr (0.1 ppm), Ni (0.05 ppm), and Pb (0.1 ppm) in agricultural soils were all exceeded in the oil sand. The presence of these metals in the oil sands and their uptake by the plants could potentially be toxic, resulting in high mortality. The metal–metal correlation of the plant’s rootsto the oil sand indicates the nonanthropogenic origin of the heavy metals, which leads to the conclusion that their source is related to the hydrocarbon accumulation in the Afowo sand.

Integrated geophysical characterization of saltwater intrusion in Lagos State, Eastern Dahomey Basin, Southwestern Nigeria

Integrated geophysical methods of Electrical resistivity tomography (ERT), Induced Polarization (IP), and Well Logging were employed in this study to delineate the lateral and depth extent of saline water intrusion in Oniru area of Lagos State, South-western Nigeria due to reported saltwater contamination of some wells at relatively shallow depths. Ergo, improving the scientific understanding of fresh and saline waters occurrence in the coastal areas of Lagos State.Five traverses, each of variant lengths (100 to 200 m) were established for both ERT and IP as space permitted. For each profile level, Wenner electrode configuration was adopted with a minimum electrode spacing of 3 m and maximum electrode spacing of 50 m. Ten levels were achieved for profiles 1, 2, 4, and 5 while eight levels were accomplished for profile 3. The ERT and IP data were processed using RES2DINV 2-D resistivity data processing software. The well log data utilized were acquired from two existing lithological logs within the study area to validate the results of the ERT and IP.The unconsolidated sandy formation was characterized by high resistivity with corresponding low to medium chargeability while the clay units were characterized by low resistivity with equivalent high chargeability. The sandy formation saturated by saltwater was characterized by very low resistivity with corresponding low chargeability while that of freshwater was characterized by medium to high resistivity with corresponding low chargeability. The integrated geophysical methods of ERT, IP and borehole logging in the study area identified three lithological sequences. The first layer corresponded to high resistivity values of unconsolidated sandy topsoil, mostly less than 5 m. However, in profiles 3 and 4, it occurred as saturated marshy topsoil. The top layer was underlain by sand/clayey sand formation, which was observed to have pockets of saline water intrusion as evinced from the results of the ERT and IP data. The saline water-bearing zones were observed to intermingle with the freshwater-bearing sand units. The freshwater-saline water transition tends to be conspicuous especially in profiles 3 and 5. The base of profiles 1 and 2 was occupied by clay units which were evident from the results of the borehole log, the ERT, and the IP sections.The integrated geophysical method was able to successfully delineate the fresh and saline water interface and extent of saltwater intrusion within the study area and thus would provide a reliable database for future groundwater development and management within the study area.

Hydrocarbon Potential and Biomarker Studies of EE-1 Well, Offshore Eastern Dahomey Basin, SW Nigeria

Abstract The hydrocarbon potential, organic source input, and paleodepositional environment of subsurface sediments from EE-1 well, offshore Eastern Dahomey Basin, were assessed using Rock-Eval pyrolysis and biomarker geochemistry. The total organic carbon (TOC) and soluble organic matter (SOM) in the sediments ranged from 0.96wt% to 8.92wt% and 676.12 ppm to 2883.85 ppm, respectively, indicating adequate to excellent organic richness. The pseudo-Van Krevelen plot classified the sediments as types II and III kerogen, which have the potential to generate both oil and gas. The Tmax and production index (PI) ranged from 422°C to 431°C (average, 426°C) and 0.03 to 0.24, respectively, suggesting low thermal maturity. The presence of C27–C29 steranes, oleanane, and hopane/sterane ratio (1.53/16.11), indicated organic matter from mixed sources with more terrigenous input. Cross plots of Pr/nC17 against Ph/nC18, C35/C31 – C35 homohopane index (0.05 – 0.17) and other related biomarker ratios such as C21/C23 trycyclic terpane ratio (0.33 – 0.82) signified that the sediments were deposited in mixed marine/terrigenous environment under oxic – suboxic conditions. This study has demonstrated that sediments had adequate organic matter with the potential to generate both oil and gas at appropriate thermal maturity.

Mapping the possible origin of anomalous saline water occurrence in Agbabu, Eastern Dahomey Basin, Nigeria: insights from geophysical and hydrochemical methods

Many researchers have suspected saltwater occurrence in Agbabu in the eastern Dahomey Basin. However, there is a need to get an insight to the possible origin of saltwater in the area. The aim of this study is to determine the source of saltwater in the inland aquifers of Agbabu, southwestern Nigeria for proper management of scarcely available freshwater resource in the area. 23 vertical electrical sounding, 2D electrical resistivity tomography (ERT), aeromagnetic and hydrochemical data (11 subsurface water samples) analyses were utilized in this study. The results from the magnetic analysis revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending fracture zone cuts across the entire eastern Dahomey Basin, including Agbabu and its environs. Four (4) geoelectric layers were delineated across Agbabu namely, the topsoil (4 - 2806 Ωm), clayey sand (3 - 1280 Ωm), clayey (3 - 161 Ωm) and bituminous sand (1 - 10390 Ωm) layers. Anomalously low resistivity (1 - 20 Ωm) zones suspected within the bituminous sand correspond to the saltwater intrusion zones. The bituminous sand unit harbouring freshwater exhibits characteristic resistivity values of 33 – 160 Ωm. The interface between the freshwater and saltwater intruded zones in the area was suspected at 10 m depth with resistivity values ranging between 21 Ωm and 33 Ωm. The results from the hydrochemical water analysis also confirmed the presence of saltwater intrusion in the area. The salinity extent chart which was generated from the VES results indicated the mobility of the saltwater into the groundwater system. The fracture connectivity between the complex aquifers of Agbabu and the Atlantic Ocean (CFZ), that was mapped from the aeromagnetic and electrical resistivity results, is suspected to be primarily controlling the localization of saltwater in the study area.

Frequency domain electromagnetic and electrical resistivity geophysical investigation of tar sands deposits in the Ijebu Waterside area, Eastern Dahomey Basin, southwestern Nigeria

The use of electromagnetic conductivity and electrical resistivity geophysical techniques has been employed in this study to investigate the occurrence, thickness, and spatial distribution of bitumen deposit around Makun-Omi in the Ijebu Waterside area of southwestern Nigeria. Subsurface apparent conductivity distribution data obtained using Geonic 34-3 equipment along fifteen traverses which sounded from different depths of 7.5, 15, 30 and 60 m at inter-coil separation of 10, 20 and 40 m in vertical and horizontal coupling modes and ground resistivity distribution measurements, obtained using Geopulse Tiger Allied resistivity meter at fourteen Vertical Electrical Sounding (VES) stations, were processed and employed to characterize the subsurface in terms of tar sands distribution. The conductivity and resistivity distribution in the form of a 3D earth model, iso-depth maps, 2D sections generated from the processed conductivity and resistivity data indicate relatively low conductivity <20 mS/m and high resistivity >1300 Ωm values around tar/bitumen impregnated sands which mostly occurred in the southern part of the study area. Clearly defined conductivity and resistivity anomalies which delineate the lateral and vertical occurrence of tar impregnated sands underscore the efficacy of integrating electromagnetic and electrical resistivity geophysical techniques to identify occurrence of economic deposits of tar sands in parts of southwestern Nigeria.

Petrography and reservoir quality controls in shallow transitional marine Cretaceous-Paleogene deposits in the Dahomey Basin, Nigeria

Upper Mesozoic and Early Cenozoic sedimentary rocks are of great significance to the petroleum geology of the Eastern Dahomey Basin as they are valuable prospects for hydrocarbon reservoirs. Diagenesis and reservoir quality (porosity and permeability) controls are deduced from core samples recovered from six shallow exploration wells in the onshore area of the Eastern Dahomey Basin using petrographic, petrophysical and mineralogical tools. The studied lithostratigraphic intervals consist of limestone, calcareous siliciclastic sandstone (Ewekoro and Araromi Formations), quartz arenite and clay-matrix supported sandstone (Afowo Formation) which were deposited within a shallow marine depositional environment.Siliciclastic samples of the Ewekoro Formation consist predominantly of calcareous sandstone and limestone, which are classified as packstone and wackestone. The porosity values in the sandstones range from 5 to 36% whereas permeability ranges from 0.006 to 109 mD. Ewekoro Limestone porosity ranges from 7 to 32% and permeability ranges from 0.003 to 13 mD. Samples of the Araromi Formation exhibit a porosity of 4 to 45% with permeability between <0.001 and 1764 mD in sandstones whereas a single limestone sample has 8% porosity and permeability of 0.03 mD. Porosity in quartz arenitic sandstones of the Afowo Formation ranges from 15 to 47% with permeability of 0.05 to >10,000 mD, while in the clay-matrix supported sandstones, the values range from 18 to 42% and 0.05 to 5 mD for porosity and permeability, respectively. The main pore-occluding authigenic minerals are carbonate cements (Fe-dolomite, Fe-calcite, siderite and dolomite), iron oxide, kaolinite, and illite-smectite mixed layer clays. The best reservoir quality is observed in the quartz arenitic sandstones of the Araromi and Afowo Formations, except for the low permeability, clay-matrix supported sandstone facies.The most significant controlling factors in sandstones are clay mineral matrix contents and the presence of carbonate cements. Fe-calcite, Fe-dolomite and kaolinite formation occurs during early diagenesis in the studied lithologies and has helped to reduce the effect of compaction. The results of this study indicate fair to good reservoir quality in the shallow marine sedimentary rocks of the Eastern Dahomey Basin, which could be targeted in further exploration campaigns in the region.