Ajali Sandstone Research Articles

GEOLOGY, SEDIMENTOLOGICAL AND HYDRAULIC CONDUCTIVITY OF AJALI POTENTIAL RESERVOIR SANDSTONE EXPOSED AROUND GRA IN IDAH, NORTHERN ANAMBRA BASIN

The Ajali sandstone is a possible reservoir rock in the Lower Benue Trough (LBT)/Anambra Basin (AB), and understanding its geological, sedimentological distribution is essential for accurate prediction of fluid flow dynamic. Geological, granulometric analysis and laboratory hydraulic conductivity test are integrated for this investigation. Results from geological studies reveals three sandstone lithofacies; namely; AJALI L2A a sandstone intermixed with siltstone, lamination, bioturbated, and crossed bedded, AJALI L2B sandstone mixed with silty clay, crossed bedded and bioturbated and AJALI L2C an excessive weathered sandstone with visible ripple and ridges with all sedimentary features supporting the earlier proposed fluvio-deltaic depositional environments. The granulometric analysis reveals medium to coarse grain, poorly sorted sandstone lithofacies. Hydraulic conductivity test for AJALI L2A crossed bedded and lamination sandstone contributes smaller pore spaces between grains, which reduce overall interconnectivity of the pore network primarily decreasing the fluid hydraulic flow conductivity and thus lowers the permeability value to 75mD. While in the AJALI L2B intermixed with siltstone and clay sandstone, lowers porosity; but cross-bedding may have contributed positively to increase in permeability value to 1850 mD in this lithofacies. The weathered AJALI L2C sandstone with visible ripples and ridges significantly affect the physical properties and pore spaces obliteration by iron oxide, by contributing to reduction in the permeability value to 60 mD. In overall it is concluded that the Ajali sandstones generally will exhibits an average moderate permeability of 662 mD.

Late maastrichtian- early eocene dinoflagellate cyst biostratigraphy and aleoenvironmental study of sediments in Akukwa-1 well in the Anambra Basin

Dinoflagellate cyst biostratigraphy and palaeoenvironment of deposition of sediments penetrated by Akukwa-1 Well was carried out with a total of 253ditch cuttings using the conventional maceration technique for recovering of acid insoluble organic-walled microfossils from sediments. Sedimentological description delineated five (5) lithological units within the well, which included those of Nkporo Shale, Mamu Formation, Ajali Sandstone, Nsukka Formation and Imo Shale. A total of 14 genera and 35 species of dinoflagellate cyst were identified. The recovered dinocysts were used to establish palaeoenvironment which ranged from marginal to shallow marine, the forms were also used to propose four (4) informal dinoflagellate cyst zonations labelled (A-D) based on the first and last occurrences of two or more species. They included Zone A (Apectodinium paniculatum zone), occurring at 7990ft(Maastrichtian), zone B(Paleocystodinium australinium zone) occurring between 7990ft -4000ft (Upper Maastrichtian), zone C(Apectodinium homomorphum zone) occurring between 4000ft -2550ft (Upper Maastrichtian), and zone D(Homotryblum palladium zone) occurring between 5550ft-1520ft(Paleocene- Early Eocene). The erected dinocyst zones were correlated with the existing biozones in tropical areas of Africa and northern South America.

Textural and Depositional Environment Analyses of Maastrichtian Ajali Sandstone Outcrops at Igbere-Abiriba Area Afikpo Basin South Eastern Nigeria

The study presents the Textural characteristics of the Ajali sandstone at Igbere-Abiriba area in Afikpo basin, southeastern Nigeria. The intent is to highlight possible constraints of the environment of deposition of the source material on one hand and to infer the provenance on the other hand. The investigation approach includes field studies involving grain size analysis. Field studies shows that the Ajali Sandstone is friable at all location and range in colour from white in freshly cut stone, to reddish brown on weathering. In addition, the Ajali Sandstone units are cross-bedded and show graded bedding, exemplified by fining upward sequence. Textural examination indicates that the sandstone ranges from fine to medium grain and few coarse grain sizes constituting about 76-99% sand fraction, with graphic mean grain size of 0.93-2.60. Standard deviation (sorting) ranges from 0.71-1.48 and implies poor – moderately sorted sediments, also symmetrical, mesokurtic to leptokurtic were observed in skewness and kurtosis. Bivariate plot from the grain size parameter combination gives a more satisfactory approach toward predicting the sedimentary environment. General studies inferred from the Textural index, the depo-environmental description of the Ajali Sandstone revealed a Fluvial-river system-dominated sedimentary process.

Sedimentology and Diagenetic Evolution of Maastrichtian Fugar Sandstone, Western Flanks of Anambra Basin, Nigeria

Diagenetic studies unravel the different diagenetic processes a rock has been subjected to and this plays an important role in the formation of sedimentary rocks, therefore the detailed study of the sedimentology and diagenetic evolution of the Fugar Sandstone depicts the diagenetic history and the paragenetic sequence of the cements present. The Fugar sandstone is an extension of the Upper Cretaceous Ajali Sandstone unit of the Anambra Basin in South western Nigeria and this study was aimed at understanding the sedimentological and petrographical characteristics of the sandstone with a view to model the sandstone diagenesis. The field study and granulometric analysis results reveals that the sandstones are unconsolidated, friable, fine to medium-coarse grained, poorly sorted, planar and herringbone cross-bedded and exhibits a repeated succession of fining upward cycles with the inferred depositional environment of fluvial to marginal-marine. The samples were analyzed using a range of techniques: X-ray diffraction, X-ray fluorescence and Optical microscope which shows that the sandstone is quartz arenite with an average of 95% quartz consisting of subangular to subrounded grains and this implies that the sandstone is mineralogically matured but texturally immature. Detailed diagenetic study using thin section petrography reveals that the rocks have undergone different diagenetic processes; compaction, cementation, dissolution, alteration and replacement, and the paragenetic order of the cementing minerals were from authigenic quartz which occurs as quartz overgrowth, clay and then Iron oxides which serves as the predominant mineral cements in the rocks.

Sedimentologic and Petrographic Study of Outcrops of Ajali Sandstone in Okigwe, Uturu and Isiukwuato, Anambra Basin Southeastern Nigeria

The Ajali Formation outcrops at Okigwe, Ihube, Onyekaba mine, Uturu and Isiukwuato in the Anambra Basin southeastern Nigeria were evaluated for an integrated field, sedimentologic and petrograpic methods. The study was aimed at presenting a better understanding of the depositional processes and provenance. Five lithofacies were altogether described namely: planar Cross bedded sandstone facies A, Massive sandstone facies B, Ferruginized sandstone facies C, herringbone cross bedded sandstone facies D, and Bioturbated sandstone facies E. The facies were deposited in a fluvially influenced shallow marine environment. The overall grain size interpretation shows that sediments from the study area are fine to coarse grained with values ranging from 0.2 to 2.0Φ, poorly sorted to very well sorted with range value of 0.28 to 1.2Φ, nearly symmetrical to strongly fine skewed with values of -0.04 to 0.59Φ and platykurtic to very leptokurtic with values of 0.79 to 2.46Φ. An inspection of the cumulative frequency curves of the studied samples shows that the sediment population is dominated by saltation population. Texturally, the sandstone sediments are sub-mature containing grains that are moderately to poorly sorted and not well rounded. The Mineral Maturity Index of the sandstone in the study area ranges from 24-49 which falls within the mature to super mature class. The high quartz content and insignificant amount of feldspar and rock fragments further support this assertion. The study suggest the sandstones must have been subjected to substantial high degree of reworking that resulted in the removal of the ferromagnesian minerals and feldspar typical of recycled sediments.

Geology and Geochemical Assessment of Maastrichtian Coal Quality in Abocho area, Northern Anambra Basin, Nigeria

Detailed geologic mapping and geochemical analysis of coal samples around Abocho area, northern Anambra Basin, Nigeria was conducted in order to assess the quality of the coals in the area. Proximate and ultimate analyses were carried out on coal samples from the Mamu Formation to determine its chemical characteristics. Physical analysis was also carried out on the coal samples to determine the specific gravity, density and hardness. Geochemical analysis was also carried out on the associated rocks in the study area (Abocho), particularly Shales and Clays to determine their major oxides composition. The area is composed of the Maastrichtian Mamu Formation overlain by the Ajali Sandstone of the same age both dipping between 16°E and 19°E. The geologic mapping of Abocho area revealed two mappable lithologic units: The Mamu Formation and the Ajali Sandstone. The Proximate analyses indicates that the coal contains an average 7.15%, 35.53%, 36.24% of moisture content, volatile matter and fixed carbon respectively. These burns to generate 4,339 kcal/kg calorific value with 20.80% ash yield. The result of the ultimate analysis shows 57.81% organic carbon, 4.15% hydrogen, 8.41% oxygen, 1.39% nitrogen and 0.3% Sulphur. The physical analysis revealed that, the coal has an average specific gravity of 1.5g/cm, average density of 1.4g/cm3 and average hardness of 1.2. These characteristics qualify the coal to be ranked as high volatile sub-bituminous to marginal lignite. The coal is thus, suitable for combustion, gasification, electric power generation and industrial uses. Geochemical results show that the Shale contains 60% Silica (SiO2) and 26%Alumina (Al2O3) constituting 86% of bulk chemical composition. The Clay contains 70% Silica (SiO2) and 25% Alumina (Al2O3), constituting 95% of bulk chemical composition. The occurrences of CaO, NaO and K2O which are the major component of feldspar in clay suggests the clay to be of granitic origin possibly from Oban massif, east of the Anambra Basin. It also suggests low feldspar content.

Hydrogeophysical Investigation and Aquifer Evaluation of the Formations in Nsukka Area, Southeastern Nigeria

The study targeted the aquiferous Ajali Sandstone and Nsukka Formations that underlies the Nsukka metropolis and its environs (within Latitudes 6 o 80 1 N and 6 o 89 1 N; Longitudes 7 o 21 1 E and 7 o 30 1 E), covering an area of about 210 square kilometres. Thirty four (34) cross array Vertical Electrical Sounding (using the Schlumberger electrode configuration) and records from four (4) selected boreholes have been used to investigate the subsurface distribution of electrical resistivity and to delineate hydraulic properties of aquifers in Nsukka area, Southeastern, Nigeria.The electrical surveys of the area show high values of permeability, ranging from 0.0075 to 0.015cm 2 , hydraulic conductivity, ranging from 8.29 to 16.64m/s, transmissivity, between 455.87 and 1002.48m 2 s -1 , and specific yield, ranging from 2.35 to 4.76m 3 . A tie with the results of borehole logs analysis reveals the presence of at least four layers of contrasting hydraulic conductivity. Analyses, however, suggests that the Nsukka Formation (which overlies the Ajali Sandstone) is thick and poorly conductive while the Ajali Sandstone unit is homogenous and anisotropic and has higher conductivity (especially beyond the depth of 100 m), suggesting a prolific water-bearing formation at such depth and beyond. The Ajali Sandstone aquifer in the area is thick and prolific. The distribution of major geological units, well log data, static water level data were shown to have influence on the groundwater occurrence and flow pattern in the area. Keywords: Ajali Sandstone Aquifer, Nsukka Metropolis, Resistivity measurements, Well data DOI: 10.7176/JEES/11-7-04 Publication date: July 31 st 2021

Socio-economic influence of hydrogeology in regions adjoining coal bearing formation: Water policy in Anambra Basin

Abstract The Coal Province lies within the central portion of Anambra Basin and along the axis of regional water resources catchment of Southeastern Nigeria. Although this area includes a prolific aquifer, the high cost of development and treatment of the groundwater degraded by AMD has created water scarcity problems against water policy initiatives. Social and economic impacts were investigated, even as policy measures for rehabilitation purposes through water resource management were highlighted. Several geoscientific methods were integrated for the investigations. Mean resistivity up to 30 Ω-m relatively at shallow depths indicated aquitards of Enugu/Mamu Formations, whereas the aquifer reflected resistivity ≤1,000 Ω-m at depths of ≥180 m across Ajali Sandstone. Alkaline-rich seepage flushed Fe-rich AMD at aquifer-aquitards interfaces adjoining the river catchment area. The AMD-induced oxidation process produced noxious Ca-Mg-Na-HCO3-SO4-Cl facie. These results were correlated with policy related questionnaires. Plans for water security were proposed, mainly to channel groundwater directly from the regional aquifer, or the chemically degraded seeped water from the fluvial system, into constructor water reservoir (treatment) columns prior to distribution to the town-water supply. Such water resource development is cost effective; and with management policies regulated by relevant decision-making agencies, sustainable supply is assured.

Low temperature grain-scale retrograde alteration of detrital minerals in Ajali formation from Benin Flank Of Anambra Basin, Nigeria

Petrographical study of detrital and authigenic minerals from the Ajali Formation in the Benin flank of Anambra Basin was carried out with the aim of interpreting the textural characteristics and alteration patterns. Samples were collected from Ayowgiri sand quarry site and along Fugar-Agenebode road where good exposures of the Ajali Sandstone occurred. Heavy minerals were concentrated using bromoform according to standard procedures. The heavy mineral concentrates were mounted on glass slides with araldite glue. In addition, thin sections of sandstone were prepared. The samples were studied with the aid of polarizing microscopes. The textural features and relationships among the heavy minerals were used in the interpretations. The results indicate that the detrital heavy minerals (Fe-Ti oxides and garnet) have undergone a very low-temperatureretrograde changes rather than ordinary in-situ dissolution by weathering and burial diagenesis. The alteration of Fe-Ti oxides resulted into the formation of leucoxene and chlorite while that of garnet resulted only into the formation of chlorite. Detrital magnetite was partially dissolved with skeletal remains while the authigenic grains showed well preserved structures. The re-equilibration of these minerals and partial dissolution were probably due to rise in temperature and change in the chemistry of the formation water. Reducing and acidic conditions at elevated temperature probably favoured dissolution and alteration of the detrital Fe-oxides while the formation of authigenic hematite was aided by oxidizing condition. Temperature increase was probably induced by either underneath thin lithospheric plate or igneous activities at the center of the basin. The conversion of the detrital minerals to authigenic chlorite and partial replacement structures represent a typical case of retrograde reaction which could be described as evidence of a very low-temperature anchi-metamorphism at mineral grain level.
 Key words: Authigenic, chlorite, Fe-oxides, garnet, retrograde, anchi-metamorphism.

Rare Earth Elements of Ajali Sandstone, SW, Anambra Basin in Nigeria: Implication for Soil Genesis

The Ajali Sandstone, South-western, Anambra Basin, was studied for Rare Earth Elements (REEs), using a Phillips 45 channel Inductively Coupled Plasma (ICP) Emission Spectrometer. The standard deviation of the REEs (Dy, Er, Eu, Y, Yb, La and Nd) averages 3.5 mg/L, 0.34 mg/L, 0.32 mg/L, 3.93 mg/L, 1.96 mg/L, 4.02 mg/L and 2.24 mg/L respectively for each of the sampled locations. The REEs concentrations showed both positive and negative concentration anomalies. The enrichment or depletion of the REEs is generally attributed to their tendency to be incorporated into plagioclase preferentially over other minerals. The Ajali Formation is predominantly sand/sandstone, with clay occurring only as intercalations/thin beds of clay drapes with the increasing particle size of the sand grains, makes the rate of adsorption and distribution of REEs in the soil low, and explains the low values of REEs concentration. Soil organic matter (SOC) plays an important role in the adsorption and migration of REEs in the Ecosystems. The SOC is linearly associated with REEs Concentration as it increases with increasing content of SOC. The Ajali Sandstone studied falls in the vicinity of abandoned cropland to cropland in a lower/gentle slope region, correspondingly low in soil organic matter content and as such slow rate of distribution and migration of REEs concentrations in the Ajali Sandstone. The sesquioxides such as Fe oxides increases as REEs contents increases. The 0.46 mean value of Fe2O3 in the Sandstone of the area studied implies a low REEs adsorption rate and subsequently the very low REEs concentrations of the soils. The istribution of REEs within the Ajali Formation soils were grossly affected by the factors that influenced the adsorption and migration rate of the minerals during magmatic fractionation process, hence the low REEs concentration of the Sediments.
 Keywords`: Sansdstone, Anambra Basin, Rare Earth Elements, Magmatic fractionation

SOFT-SEDIMENT DEFORMATION STRUCTURES AND GRANULOMETRIC PROPERTIES DISTRIBUTION IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Three Ajali sandstone ridges (L1, L2 and L3) at Uturu being quarried for construction sands were studied for soft-sediment deformation structures (SSDS) and granulometric properties distributions. SSDS that includes recumbents foresets, sands dykes, flame structures and fluid escape tubes were identified only in ridge L3. The geometry of the SSDS indicates sediment loading/density contrast, fluidization and liquefaction as the mechanisms for their formation but with liquefaction as the most dominant mechanism. Gran size analysis and granulometric curves properties calculations show that: Mean grain size ranged from fine (1.18 ɸ) to medium (2.57 ɸ); Sorting ranged from rarely poorly sorted (1.13ɸ) to well sorted (0.37ɸ) but with mean values in each ridge as moderate sorted; Skewness ranged from strongly fine skewed (1.0) to strongly coarse skewed (-1.57); and Kurtosis ranged from very platykurtic (0.27) to very leptokurtic (2.0) but with sands of ridge L3 mainly very platykurtic. Granulometric curves and bivariate plot of properties indicate fluvial deposition with rare marine influence. Results show that there is no significant variation in sediment properties and depositional environments across the three ridges. The localization of SSDS and non-proximity to any fault suggest that liquefaction, as the dominant mechanism for soft-sediment deformation, was not triggered by an earthquake. Possible mechanisms include rapid sediment loading, localised sudden subsidence induced by loading of localised oxidized compressible peats and coal; and increased in sediments’ water saturation via localised groundwater seepage. Fine grains, well sorting, fine to strongly skewed very platykurtic characteristics of sediments made it more susceptible to liquefaction.

Grain Size and Heavy Minerals Analysis of Maastrichtian Sandstone, Anambra Basin, Nigeria: Implication for Aquifer Properties

The Maastrichtian sediments of Anambra Basin, south western Nigeria was studied for grain characteristics and maturity of the sediments, so that by combining sedimentological characteristics, maturity and its paleoenvironment of deposition of the Ajali sandstone, in order to ascertain the aquifer properties of the groundwater within the study area and by extension the availability of drinking water for both the urban and rural settlers. Grain size analysis (51 samples) and heavy mineral analysis (15 samples each) were analyzed. The textural parameters show that the Sandstone are medium sand, poorly to moderately sorted, coarse to strongly coarse skewed with mesokurtic to leptokurtic grains. The heavy minerals present in lithologic sand and sandstone units of Ajali Formation are mainly zircon, rutile, tourmaline, apatite, staurolite, and opaque minerals (Goethite, hematite, ilmenite), these heavy minerals found in the sandstone are associated with igneous and metamorphic source which indicates that these maastritchian sediments are from basement complex rock The ZTR index of 67.96% and the values of the kurtosis indicates a sub-mature sandstone to mature sandstone, which gives an excellent aquifer properties. Also, the paleoenvironment of deposition of the Maastrichtian sediments is Fluvial. The sand and sandstone bodies deposited in a fluvial system have sheet-like geometries and due to the differences in the extent of aquifer compartmentalization, Sandstone deposited in fluvial environment normally has better hydraulic conductivity which again reveals a good aquifer property. The sedimentological characteristics for the Sandstone exposed at Fugar and environs, is a coarsening upward sequence (increasing grain sizes) with a progressive increase in sorting. since hydraulic conductivity increases with increasing grain sizes and sorting, hence, excellent aquifer characteristics for the groundwater within the study area. The foregoing revealed that the aquifer geometry and properties of the study areas is favourable for the availability of drinking water for both the Urban and rural dwellers within these communities.
 Keywords: Aquifer geometry, groundwater, hydraulic conductivity, Anambra Basin

Sedimentology and maturity of Ajali formation, Benin flank Anambra basin, Nigeria

The Ajali Sandstone, western flank, Anambra Basin, was studied for textural characteristics and maturity of the sediments. Grain size analysis (51 samples), thin section and heavy mineral analysis (15 samples each) and XRF fusion for metallic oxides (15 samples) were analyzed. The textural parameters show that the Ajali Sandstone are medium sand, poorly to moderately sorted, coarse to strongly coarse skewed with mesokurtic to leptokurtic grains. The thin section analysis reveals grains that are sub-angular to sub-rounded (this typifies grains that have travelled fairly long distance to the deposition site), of moderate to well sorted grains, with both monocrystalline and polycrystalline quarts-grains type, with a modal composition of Q F and L . A mineralogical maturity 90.4, 2.3 2.9 index (MMI) of 17.04, SiO /Al O ratio of 180.24, and a ZTR index of 67.96% were obtained. The values for the 2 2 3 MMI and SiO /Al O indicates mineralogical matured sediments, the ZTR index shows a chemically immature 2 2 3 to sub-mature sandstone, and the modal composition values are consistent with a texturally and compositionally matured sands. The mineralogical maturity is indicative of high degree of chemical weathering of source area. Furthermore, the high quartz and silica content make the sandstone prospective for glass and glassware production.
 Key words: Anambra Basin, Maturity, Mineralogical maturity index, Textural characteristics, Glass

Hydrogeochemistry Characteristics Assessment of Water Quality in Amansiodo and Environs, Ezeagu Local Government Area, Enugu State, Nigeria

This study focuses on hydrogeochemistry characteristics assessment, aimed at determining the level of water quality in Amansiodo and environs. Geologically, the area is underlain bycross bedded, friable, whitish, poorly sorted Ajali Sandstone which is overlain by heterolithic units of shale, siltstone intercalation, mudstone, lateratic sandstone and ironstone of Nsukka Formation. A total of ten (10) water samples; hand-dug wells (3), springs (3) and boreholes (4) were collected from different locations within the study area and analyzed for their physicochemical parameters (pH, EC and TDS), cations (Ca, Na, Mg and (K), anions (Cl, HCO 3 , NO 3, PO 4 and SO 4</su

GIS-Based Study of Prioritized Gully Erosion and Flood Locations in Ebonyi State

Gully erosion and flood is a major environmental disaster in south eastern part of Nigeria. This study adopts Geographic Information System (GIS) as a tool in the identification of flood and gully erosion locations in Ebonyi State. 31 flood and erosion locations were identified using GPS receiver and field ground investigation. Gully erosion and flood severity map was produced using the SRTM (Shuttle Radar Topographic Mission), which was imported into the ArcGIS platform. The result shows that gully erosion sites were well dispersed around Ajali Sandstone geological Formation. The slope characteristic was ascertained to be greater than 15 o which encourages flood and gully erosion activities. The flood and erosion severity map produced showed risk and vulnerability of settlements and infrastructure while the observation on site showed loss of properties, economic trees, biodiversity and obstructed access; thus endangered human existence. This study therefore recommended public enlightenment on their role in flood and erosion control, enlistment of geologists, environmental managers and other experts in flood and erosion control planning for better understanding of the terrain before adopting any measure of control. Also the production of gully and flood severity map for other areas within the south-eastern Nigeria is necessary to enable better control of these environmental hazards. Keywords : Gully Erosion, Flood Locations, GIS DOI: 10.7176/JEES/10-2-11 Publication date: February 29th 2020

Geotechnical properties of Ajali Sandstone in Enugu, Nigeria for engineering use

Twelve soil samples collected at different intervals from 0.1 meter deep in Onyeama Mine, Enugu State from the Ajali sandstone were subjected to geotechnical analysis (moisture content, sieve analysis, Atterberg’s limits) to determine their suitability for the siting of some engineering structures. The results show that the soils are cohesive at shallow depths (1.8m – 1.0m). It is cohesionless with greater sandy fraction at greater depths. The moisture content values are low and averages 10.65% (from depth 0.00m– 1.8m). For the Atterberg limit; liquid limit ranges from 31% - 36% while the plasticity limit was 25%. On the AASHTO classification and plasticity chart, part of these results show that the top soils at shallow depths (1.8m – 1.0m) are inorganic clays. Consequently, the top inorganic layer needs to be stabilized or excavated and back-filled with more stable materials for engineering structures purposes. Keywords: Engineering, Geotechnical Analysis, Geotechnical Properties, Moisture Content, Sandstone, Structures

Assessment of natural radioactivity levels in the Ajali Formation, Enugu, South Eastern Nigeria

Activity concentrations of the radionuclides (238U, 232Th and 40K) were analysed using high resolution co-axial HPGe gamma ray spectrometer system to know the implication where groundwater is sourced from Enugu, South-East Nigeria. The activity concentration of 238U ranges from 37 ± 4 to 74 ± 6 Bq kg−1 with the highest value of 74 ± 6 Bq kg−1 noted in the thin coal sample. The 232Th activity level in the rock samples ranges from 58 ±5 to 85 ± 7 Bq kg−1 with the higher value of 89 ± 7 Bq kg−1 reported in the coal sample. For 40K, the activity concentration varies from 140 ± 19 to 293 ± 25 Bq kg−1 with the highest value of 293 ± 25 Bq kg−1 reported in coal samples whereas lowest value of 140 ± 19 Bq kg−1 was noted in Ajali sandstone in the study area (Enugu State). The higher activity levels of 238U, 232Th and 40K reported in coal samples may be due to the closer clusters of weathered surface of Ajali sandstone that overlays the thin coal layer with an escarpment being spread out, implying higher permeability towards deeper direction. All the values are within the recommended level when compared with United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000). Further research on groundwater activity concentration and rock geochemistry is required within the study area. This will allow for a comprehensive conclusion to be drawn on the level of exposures to the inhabitants relying on groundwater for consumption.

Density and reservoir properties of Cretaceous rocks in southern Benue Trough, Nigeria: implications for hydrocarbon exploration

Densities and reservoir properties of rocks were determined using 167 representative fresh rock samples of Cretaceous sedimentary rocks outcropping in the southern portion of the Benue Trough, Southeastern Nigeria, through the application of the Archimedes principle and grain size analysis. Result of the analysis indicates that the mean bulk densities for the rocks in the study area are as follows: Asu River Group (1.65 g/cm3), volcanic rocks (1.60 g/cm3), Eze-Aku Group (2.44 g/cm3), Awgu Formation (1.83 g/cm3), Nkporo Group (1.77 g/cm3), Mamu Formation (1.79 g/cm3), Ajali Sandstones (2.20 g/cm3), Nsukka Formation (1.74 g/cm3). Porosity and permeability values of the sandstones are as follows: Asu River (6.81%; 98.36 mD), Amasiri Sandstone (11.12%; 120.30 mD), Afikpo Sandstone (8.90%; 133.11 mD), Owelli Sandstone (11.15%; 659.37 mD), Mamu Sandstone (14.53%; 1596.45 mD), Ajali Sandstone (18.52%; 759.61 mD), Nsukka Sandstone (14.64%; 3838.61 mD). Porosity of the pre-Santonian shales is controlled by age and burial history, whereas those of the post-Santonian shales are more controlled by their physical properties than age and depth of burial. Porosity of the pre-Santonian sandstones is controlled by texture, depth of burial, and rock matrix, whereas that of the post-Santonian sandstones are controlled by depth of burial and rock matrix. While compaction and diagenetic alterations have influenced the permeability of the pre-Santonian sandstones, degree of sorting cum depth of burial has been a major factor influencing the permeability of the post-Santonian sandstones. The reservoir properties of the Asu River Group sandstones indicate that they cannot make good reservoirs. The reservoir properties of the Amasiri, Afikpo, Owelli, Mamu, Ajali, and Nsukka sandstones indicate that they can make good reservoirs. However, aside from the Amasiri, Afikpo, and Owelli sandstones, the sandstones of Mamu, Ajali, and Nsukka formations are not stratigraphically positioned to serve as hydrocarbon reservoirs in the study area.

Application of electrical resistivity method in estimating geohydraulic properties of a sandy hydrolithofacies: a case study of Ajali Sandstone in Ninth Mile, Enugu State, Nigeria

Geoelectric investigation using vertical electrical sounding (VES) (Schlumberger electrode configuration) was carried out in 14 locations at Ninth Mile area, southeastern Nigeria to determine the variations and interrelationship of some geoelectric and geohydraulic parameters of a sandstone hydrolithofacies. The measured resistivity data were interpreted using manual and computer software packages, which gave the resistivity, depth, and thickness for each layer within the maximum current electrodes separation. The aquifer resistivity values range from 86.56 to 4753.0 Ωm with 1669.40 Ωm average value. The values of water resistivity from borehole locations close to the sounding points range from 79.49 to 454 .55 Ωm and averaging about 264.7 Ωm. Porosity values of the sandy aquifer range from 30.19 to 34.20%. Fractional porosity values range from 0.3019 to 0.3292, while the tortuosity values vary between 2.91 and 22.85. The geohydraulic parameters estimated vary across the study area. Formation factor ranges from 0.28 to 15.29, hydraulic conductivity ranges from 1.21 to 66.54 m/day which, however, influences the natural flow of water in the aquifer while tortuosity values range from 2.91 to 23.27. The contour maps clearly show the variation of these parameters in the subsurface and the plots show their relationship and high correlation coefficients with one another. The results of this study have revealed the geological characteristics of the subsurface aquifer, established the influence on the amount of groundwater, and proposed a strategy for the management and exploitation of groundwater resources in the area and other aquiferous formations.

Sequence stratigraphic interpretation of parts of Anambra Basin, Nigeria using geophysical well logs and biostratigraphic data

The Anambra basin constitutes the southeastern lower portion of the Benue Trough, which is a large structural depression that is divided into lower, middle and upper parts; and is one of the least studied inland sedimentary basins in Nigeria. Sequence stratigraphic interpretation had been carried out in parts of the Anambra Basin using data from three wells (Alo-1 Igbariam-1 and Ajire-1). Geophysical well logs and biostratigraphic data were integrated in order to identify key bounding surfaces, subdivide the sediment packages, correlate sand continuity and interpret the environment of deposition in the fields. Biostratigraphic interpretation, using foraminifera and plankton population and diversity, reveals five maximum flooding surfaces (MFS) in the fields. Five sequence boundaries (SB) were also identified using the well log analysis. Four 3rd order genetic sequences bounded by maximum flooding surfaces (MFS-1 to MFS-6) were identified in the areas; four complete sequences and one incomplete sequence were identified in both Alo-1 and Igbariam-1 wells while Ajire-1 has an no complete sequence. The identified system tracts delineated comprises Lowstand Systems Tracts (progradational to aggradational to retrogradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational to progradational packages) in each well. The sand continuity across the fields reveal sands S1 to S5 where S1 is present in Ajire-1 well and Igbariam-1 well but not in Alo-1 well. The sands S4 to S5 run across the three fields at different depths. The formations penetrated by the wells starting from the base are; Nkporo Formation (Campanian), Mamu Formation (Late Campanian to Early Maastrichtian), Ajali Sandstone (Maastrichtian), Nsukka Formation (Late Maastrichtian to Early Palaeocene), Imo Formation (Palaeocene) and Nanka Sand (Eocene). The environments of deposition revealed are from coastal to bathyal. The sands of lowstand system tract and highstand system tract found in Ajali, Nsukka, Nkporo and Imo (Ebenebe Sandstone) Formations show good continuity and as such good reservoir qualities while the shales of the transgressive system tracts which includes the Imo Formation, Mamu, and Nkporo Formations where most of the maximum flooding surfaces were delineated, can serve as seals to the numerous reservoir units. Combinations of the reservoir sands of the lowstand system tract and highstand system tract and the shale units of the transgressive system tract can form good stratigraphic traps for hydrocarbon and hence should be hydrocarbon exploration targets.