Ajali Formation Research Articles

Provenance Interpretation and Depositional Environment of Sandstones of the Ajali Formation, South of Ifon, Western Flank of Anambra Basin, Southwestern Nigeria

Provenance Interpretation and Depositional Environment of Sandstones of the Ajali Formation, South of Ifon, Western Flank of Anambra Basin, Southwestern Nigeria

Stratigraphic palynology and micropaleontological study of the Cretaceous-Paleogene succession of the Nani-1 well of the Anambra and Niger Delta basins: Implications for hydrocarbon exploration

Integrated palynological and micropaleontological analyses were carried out on the Nani-1 well drilled into the Anambra and Niger Delta basins of Nigeria. The study is aimed at reconstructing biozones and paleoenvironments of deposition, as well as deducing the sequence stratigraphic framework of the Cretaceous-Paleogene successions of an area containing key elements for hydrocarbon exploration. Encountered lithofacies succession of sandstone, siltstone, dark-grey to black shale, and coal beds comprises six lithostratigraphic units, namely the Nkporo Group (Campanian-Maastrichtian), Mamu Formation (Middle Maastrichtian), Ajali Formation (Middle-Late Maastrichtian), Nsukka Formation (Late Maastrichtian), Imo Formation (Danian-Thanetian) and Ameki Group (Eocene). Fourteen dinoflagellate zones (A-N) and seven pollen and spore subzones (A1-G1) are recognized based on the first occurrence of two or more species. Two major stratigraphic events are documented: (1) dinoflagellate cyst (dinocyst) zone G at 880 m depth marked by first appearance of Danea aff. californica and Danea cf. mutabilis in association with Cordosphaeridium fibrospinosum, and the last occurrence of Danea aff. californica and Phelodinium magnificum place the Cretaceous-Paleogene (K/Pg) boundary in the lowermost part of the Imo Formation; and (2) a hiatus characterized by a starved section observed at 380 m directly overlain by the Early Eocene assemblages of the Ameki Group indicates a sequence boundary at Imo/Ameki Boundary. Thus, a chronostratigraphic chart is developed for parts of the Anambra Basin and the Niger Delta. Six depositional environments which include marginal to shallow marine, marshy lagoon to estuarine, shallow neritic and tidally influenced fluvial channel are identified, and palynostratigraphic evidences indicate that fresh water palynomorphs are present in the Lower Maastrichtian strata. Four sequence boundaries and four maximum flooding surfaces are also identified within the Maastrichtian–Early Eocene sediments. The paleoenvironmental reconstruction shows that possible hydrocarbon reservoirs in the basin are the marine sandstone facies of the Nkporo Group and Mamu Formation. Their shale members constitute potential source rocks, while the overlying shale units of the Nsukka and Imo formations may provide regional seals for hydrocarbon habitation.

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 geotechnical investigations of part of the Anambra Basin, Southeastern Nigeria: implication for gully erosion hazards

BackgroundGeologic and geotechnical conditions of soils where Nanka and Ajali Formations outcropped in Anambra Basin, Southeastern Nigeria were investigated and accessed. This was done using detailed mapping and mechanical soil laboratory tests to unravel the genesis and continued expansion of gully erosion in the study areas.ResultsField study revealed that gully erosions are more pronounced in the study area with poor vegetation cover and a high degree of slope steepness. Grain size analysis revealed that the soils of the Nanka Formation have an average sand content value of 90.90% (sandy) and silt content value of 3.0% (low fine portions). The plasticity index of the fine portions indicates that the soils are weak plastic, with a mean value of 5.29%. The soils have an average cohesion value of 0.30 kg/cm2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.67 × 10–3 cm/s. The compaction test further revealed that the soils are loosely compacted. The soils for the Ajali Formation have an average sand content value of 95.10% (sandy) and silt content value of 1.43% (low fine portions). The plasticity index of the fine portions indicates that the soils are weak plastic, with a mean value of 2.70%. The soils have an average cohesion value of 0.30 kg/cm2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.70 × 10–3 cm/s. The compaction test revealed that the soils are loosely compacted.ConclusionAfter field surveys and laboratory analyses, it was found that the gully erosions have been developing respectively on steep slopes and non-vegetated areas, and their genesis facilitated by the cohesionless and very permeable nature of the sandy formations. Following those key findings, it was proposed many practices (agronomic and engineering mainly) that can help mitigate the formations as well as the expansion of this very damaging hazard type. The potential implications of these gully erosion include damaging of buildings, residential houses, bridges, and roads, loss of farmland and vegetation, isolation of villages and towns, increased migration of inhabitants as well as degradation of agricultural fertile land.

English

Tropical landscapes are generally known to store much carbon in its varied ecosystems, but information required to prioritize and conserve areas with much soil carbon concentration is scanty. This study provides insight on the patterns of carbon storage and its dynamics in an extensive humid zone in SE Nigeria. Soil carbon data was downloaded from soil grids of 250 m carbon stocks for the first 30 cm of the soil profile and compared with the land use/cover, geology and digital elevation model of the region. Carbon estimates varied over the four epochs used and ranged from 29-177, 29-172, 12-172 and 4-177 tons/ha in 1992, 2001, 2010 and 2018, respectively. Even though land cover variation was quite distinct in the zone (between the northern zone with much cover and the southern zone with less), it was not seen as the major factor that influenced soil carbon in the zone. Much of the explanations on carbon concentration and variability were mainly from the geology of the zone which is categorized into six major classes. But three out of the categories were seen to dominate the carbon store: Ajali formation, Mamu formation and Nsukka formation. This exogenous factor (geology), which is not uniform, was seen as the major variable to consider in deciding areas that will be prioritized for carbon conservation in the zone. Proper land use strategies and policy frameworks that would help to maintain proper baselines and further enhance adequate carbon conservation were equally recommended. Key words: Carbon management, conservation, land use change, modified ecosystems, prioritization, tropical.

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

SEDIMENTARY ARCHITECTURAL ELEMENTS AND SANDY BRAIDED FLUVIAL SUCCESSIONS IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Lithofacies, bounding surfaces and sedimentary architectural elements exposed in two ridges at Uturu being quarried for construction sands were analysed to determine the paleoenvironment of deposition and the factors that control the deposition of sand units. Mainly outcropped is Ajali Formation overlying locally exposed Mamu Formation in Western Afikpo basin. Lithofacies identified include: Trough cross bedded medium- to coarse-grained (St), Planar cross bedded fine- to coarse-grained sandstone (Sp), Small scale planar cross bedded sandstone (SSp), Lenticular mudstone (Fm), Heterolithic sandstone/mudstone (Fsm), Horizontal stratified sandstone (Sh), Cross ripple laminated sandstone (Sr), Reddish muddy sand (Fl), Siltstone (SSm) and Shale (Fsh). The associations of lithofacies and bounding surfaces gave four fluvial and one marine architectural element. The fluvial elements which mainly characterized the Ajali Formation include: Channel-fill (CH), Macroforms Accretion (MA). Flood-Plain Fines (FF) and Channel Abandonment Fines (CAF). Offshore-shoreface fines (OSF) element defined marine Mamu Formation. The profiles of the ridges show dominance of MA followed by CH while FF is limited in occurrence and in some zones pinch-out to lenticular inter-bar mudstone. The MA is characterized by planar cross beddings, reactivation surfaces, internal grading, steep dipping ferruginized accretion surfaces and abrupt flat top which indicate mid-channel bars deposition in typical sandy braided fluvial depositional system. Generally, there is vertical aggradation/ amalgamation of channel deposits and dominance of sheet alluvial architecture. Low rate of channel avulsion, moderate rate of lateral migration and aggradation, variable discharge rate and high rate of sediment supply and subsidence were considered as factors that controlled the deposition and preservations of sand units. This study provided an understanding of mesoscopic heterogeneities and compartmentalization style inherent in hydrocarbon bearing sandy braided reservoirs which can be used as analog model for its development.

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

Reassessing Groundwater Potentials and Subsurface water Hydrochemistry in a Tropical Anambra Basin, Southeastern Nigeria

This review presented a detailed re-assessment of the hydrogeology and hydrochemistry of the Tropical Anambra Basin. It identified and discussed the major geological formations and their groundwater potentials. The geological examination showed that the Ajali Formation is confined in places forming an artesian condition; the potentials of this aquifer decline in the western basin due to a decrease in thickness. The sandstone associates of the Nsukka Formation are aquiferous and have produced high-pressure artesian boreholes along the Oji River. The Imo Shale is characterized by permeability stability all over much of the intermediate unit. The Bende-Ameki aquifer has a lesser amount of groundwater when equated to other formations; the geologic characteristics do not produce favorable hydrogeological conditions for groundwater occurrence. The stratigraphical and structural framework suggested the presence of an efficient throughflow in the basin. Based on physical and chemical parameters of water quality, the basin holds water of acceptable quality. While there are considerable investigations on the hydrogeology and hydrochemistry, studies are short of analysis of the hydrogeochemical evolution of groundwater, water quality index, heavy metals pollution index as well as total hazard quotient. Suitability of groundwater based on agricultural water quality indices (e.g. SAR) is also salient. Therefore, future studies should address these owing to increasing dependence on groundwater.

Evaluation of Physico-Chemical Parameters of Stream Water and Their Cytotoxic Effects on Allium Cepa Around Ibobo Coalmine, Kogi State, Nigeria

The Ibobo coal mine is an abandoned mine located in the middle belt of Nigeria and lies in the Ajali Formation within the Anambra Basin. The adjoining rivers and streams in this community serve as sources of water for human consumption and plant use. The study was conducted in January 2016 at the peak of the dry season. Major and trace elements were measured in the water samples collected and subjected to the following analysis; conductometry and electro-analytical techniques for physical parameters; titrimetry and spectrophotometry for anions; and flame atomic absorption spectrophotometry for the cations. Further study was carried out to evaluate the cytotoxic effects of the water samples from plants and other living organisms using the root meristems of Allium cepa (onion). This study revealed that the average concentrations of Iron (0.344mg/L), Lead (0.024 mg/L) and Cadmium (0.004 mg/L) are higher than the maximum permissible limits of Standard Organisation of Nigeria (SON) and World Health Organisation (WHO) guidelines for domestic and drinking water qualities. Water from the streams should, therefore, be discouraged for both domestic and agricultural purposes through public health awareness program. Keywords: Physico-chemical analysis, spectrophotometry, TDS, Allium cepa, Mamu Formation, Cytotoxic DOI : 10.7176/JEES/9-6-07 Publication date :June 30 th 2019

Signatures of key petroleum system elements: outcrop examples from the Anambra Basin, Southeastern Nigeria

Although several works have proved the existence of oil and gas in the Campanian to Maastrichtian strata of the Anambra Basin, it is still considered a frontier basin due to the challenges associated with interpretation of stratigraphy and structure arising from non-availability of subsurface data. This has led to poor understanding of the petroleum system and hindered exploration activity within the basin. This research paper aims to discuss the signatures of essential petroleum system elements, based on the study of high-quality outcrop examples. Detailed field studies reveal the presence of source, reservoir and seal rocks, and traps exposed within the four lithostratigraphic units outcropping in the basin, across the southeastern part of Nigeria. The shales of Enugu Formation, offer good source rocks. The shales, coals, coaly shales and sandstones with repeated occurrence of shale–silt–sand heteroliths of the Mamu Formation provide possible source and reservoir rocks. Thick extensive sandstones with clay laminae of the Ajali Formation offer potential reservoir that are partly compartmentalized. The shales with interstratified sandstone of the Nsukka Formation provide potential cap and overburden rocks. The presence of anastomosing fracture bands, extensional joints and faults, small-scale growth faults, and rollover anticlinal structures are good evidence of structural and stratigraphic configuration that could allow for possible migration, accumulation and entrapment of hydrocarbon. These recognized signatures of the key elements of a petroleum system and oil seeps encountered in the basin are an indication of the presence of an active petroleum system.

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.

Lithostratigraphic characterization of the Upper Campanian – Maastrichtian succession in the Afikpo Sub-basin, southern Anambra Basin, Nigeria

Lithostratigraphic mapping of the Afikpo Sub-basin of southeastern Nigeria was undertaken to reappraise and characterize lithostratigraphic package of the Upper Campanian – Maastrichtian in the sub-basin. The study involved descriptive lithostratigraphic and sedimentological methods supported by palynological biofacies. Five component lithostratigraphic units were identified: the Afikpo Sandstone, Nkporo, Mamu, Ajali and Nsukka formations. The Afikpo Sandstone is the oldest lithostratigraphic unit in the sub-basin. The sandstone unit overlies an angular unconformity above the Turonian Eze-Aku Group facies around Amasiri in the southern Benue Trough. It consists of fine to very coarse and occasionally pebbly sandstone interbedded with fossiliferous siltstones/sandstones, mudstones and coals. The Nkporo Formation overlies the Afikpo Sandstone and consists of thick succession of fossiliferous dark – grey to black shales, micritic limestones, oolitic ironstone and a distinct sandstone body enclosed within the dominant shale facies. The Nkporo Formation in the sub-basin comprises of three lithologically homogenous subunits: the Asaga Amangwu Shale, Owutu Sandstone and Amaiyi Edda Shale. The type locality is retained in Ndagbo Nkporo. The Owutu Sandstone is intervening between the Asaga Amangwu Shale and Amaiyi Edda Shale subunits. The Mamu Formation overlies the Nkporo Formation and is conformably overlain by the Ajali Formation. The Nsukka Formation is the topmost lithostratigraphic unit mapped in the sub-basin. The type localities for the Mamu, Ajali and Nsukka formations are retained in the Central Anambra Basin. This study has de-emphasized usage of the term “Nkporo Group” in addressing the Afikpo Sandstone and Nkporo Formation because the two lithostratigraphic units are distinct formations.

Granulometric and pebble morphometric applications to Benin Flank sediments in western Anambra Basin, Nigeria: proxies for paleoenvironmental reconstruction.

Integrated granulometric and pebble morphometric study of Late Cretaceous rocks of the Benin Flank, Nigeria, were used to delineate depositional process and environment of the rocks in the study area. Granulometric analysis reveals that the sandstones of the Lokoja Bassange Formation have an average graphic mean (Mz) of 0.59Φ (coarse grains), inclusive graphic standard deviation (δi) of 1.36Φ (poorly sorted), graphic kurtosis (Ka) of 1.12Φ (leptokurtic), and inclusive graphic skewness (Ski) of - 0.14Φ (stongly coarse skewed). The sandstones for the Mamu Formation have an average Mz of 1.42Φ (medium grains), inclusive graphic standard deviation (δi) of 0.74Φ (moderately sorted), graphic kurtosis (Ka) of 1.43Φ (leptokurtic), and inclusive graphic skewness (Ski) of 0.02Φ (nearly symetrical). The sandstones for the Ajali Formation have an average Mz of 1.23Φ (medium grains), inclusive graphic standard deviation (δi) of 1.21Φ (poorly sorted), graphic kurtosis (Ka) of 1.46Φ (leptokurtic), and inclusive graphic skewness (Ski) of - 0.17Φ (nearly symetrical). Pebble morphometric analysis of the larger grains reveals that the mean values of elongation ratio (ER) = 0.69, coefficient of flatness (FI) = 54.90%, oblate-prolate index (OPI) = 3.34, the maximum projection sphericity index (MPSI) = 0.75, form = 0.67, and roundness (ρ) = 36.28% for the Lokoja Bassange Formation; ER = 0.76, FI = 37.07%, OPI = -3.20, the MPSI = 0.53, form = 0.38, and ρ = 52.93% for the Mamu Formation; and ER = 0.72, FI = 47.44%, OPI = 1.12, the MPSI = 0.67, form = 0.54 and ρ = 42.68% for the Ajali Formation. The integration of the various granulometric and pebble morphometric data, different bivariate and ternary plots indicate fluvial environment for the Lokoja Bassange Formation, dominantly shallow marine environment for the Mamu Formation and fluvial - shallow marine environment for the Ajali Formation. This study shows that the Benin Flank, western Anambra Basin has a depositional setting radiating between fluvial and shallow marine environments.

Sequence stratigraphy and controls on sedimentation of the Upper Cretaceous in the Afikpo Sub-basin, southeastern Nigeria

Outcrop-based sequence stratigraphic analysis and palynological biofacies were used to define depositional sequences and their bounding surfaces, and build a sequence stratigraphic model for the Upper Cretaceous succession of the Afikpo Sub-basin. Four unconformity-bounded third-order depositional sequences were identified. Sequence 1 comprises the Nkporo Formation and is subdivided into lowstand system tract (LST) representing an incised valley fill and transgressive systems tract (TST) consisting of estuarine and marine shales and mudstones. The base of the sequence is an angular unconformity correlated to the 77.5 Ma sequence boundary (SB) and the maximum flooding surface (MFS) is dated at 76 Ma. Sequence 2 is diachronous and straddles the lithostratigraphic boundary of the Nkporo and Mamu formations. The upper SB is dated at 71 Ma while associated MFS is dated at 73.5 Ma. Sequence 3 consists of the upper Mamu Formation and the Ajali Formation. The upper SB of sequence 3 is at 68 Ma while the MFS is dated at 69.8 Ma. Sequence 4 is the topmost depositional sequence belonging to the Nsukka Formation. The upper SB is placed at 66.5 Ma. The MFS within this sequence is dated at 67.8 Ma. The sequences encompass from tidally influenced bay head delta and central estuarine environments to coastal and shallow marine shelf environments. Stratigraphic architecture and facies types show that sequence development was controlled to a great extent by eustatic sea level variations though differential subsidence rates encouraged differential rates of sediment supply and rates of sea level change along different segments of the shoreline.

Soft sediment deformation structures in the Maastrichtian Ajali Formation Western Flank of Anambra Basin, Southern Nigeria

Soft sediment deformation structures were recognized in the Maastrichtian shallow marine wave to tide influenced regressive sediments of Ajali Formation in the western flank of Anambra basin, southern Nigerian. The soft sediment deformation structures were in association with cross bedded sands, clay and silt and show different morphological types. Two main types recognised are plastic deformations represented by different types of recumbent folds and injection structure represented by clastic dykes. Other structures in association with the plastic deformation structures include distorted convolute lamination, subsidence lobes, pillars, cusps and sand balls. These structures are interpreted to have been formed by liquefaction and fluidization mechanisms. The driving forces inferred include gravitational instabilities and hydraulic processes. Facies analysis, detailed morphologic study of the soft sediment deformation structures and previous tectonic history of the basin indicate that the main trigger agent for deformation is earthquake shock. The soft sediment deformation structures recognised in the western part of Anambra basin provide a continuous record of the tectonic processes that acted on the regressive Ajali Formation during the Maastrichtian.

Geoelectrical sounding for estimating groundwater potential in Nsukka L.G.A. Enugu State, Nigeria

Sixty six vertical electrical soundings (VES) have been used to evaluate the ground water potential in Nsukka Local Government Area of Enugu State, Nigeria. The project domain lies within longitudes 7°13′00″ - 7°35′30″ and latitudes 6°43′30″ - 7°00′30″ and covers an area of about 480 km² over three main geological formations. The resistivity and thickness of the aquiferous layer at various observation points were determined by the electrical survey. Also zones of high yield potentials were inferred from the resistivity information. Transmissivity values were inferred using the calculated Dar Zarrouk parameters. Results show highly variable thickness of the aquiferous layer in the study area. Aggregate transverse resistance indicates greater depth of the substratum in the southeastern part of the study area, underlain mostly by the Ajali Formation. High values of transmissivity also predominate, thus suggesting thick and prolific aquiferous zone. Key words: Resistivity, transverse resistance, transmissivity.

The application of sedimentology in the characterization of borehole gravel packs and screen slot sizes for the Nanka and Ajali formation aquifers, Anambra Basin, southeastern Nigeria

The drilling of a borehole into a poorly cemented and unstable reservoir/aquifer system like the Ajali and the Nanka Sands and the withdrawal of its fluids trigger movement, because unconsolidated grains and matrix materials are dislodged and entrained in the produced fluid (water, gas or oil). The undesirability of pumping any fluid togetherwith sand and matrix material is reflected in the dangers posed to the equipment by way of ; abrasion of downhole pumps and accessories, casing pipes and surface equipment on one hand; and the loss of production, compaction of reservoir and handling/disposal problems on the other hand. Produced sand thus must be stopped, while allowing formation fluid to pass into the borehole. Two measures of sand control for the Ajali and Nanka fluid production are discussed herewith; a screen placed at the reservoir interval and a gravel pack in the screen-hole annulus. Gravel pack character and screen slots design are directly dependent on the size of formation/aquifer sand size. Relevant equations were used to compute the sizes of gravel pack material and screen slot sizes for twenty boreholes; eleven (11) that penetrated the Ajali and nine (9) that penetrated the Nanka Sands, (the two most prolific and most exploited aquifers within the Anambra basin). Results show that gravel pack sizes for the Ajali Sands range from 1.42mm, through 2.13, 3.37 to 5.06mm, indicating an average gravel diameter size range of 2.13 to 5.06mm. For the Nanka Sands, the gravel pack values range from 0.57mm, through 0.75, 3.17 to 4.73mm indicating an average range of 0.57 to 4.75mm. Uniformity coefficients (Uc) for the Ajali and Nanka Sands are 0.6 and 0.32 respectively, indicating perfect or nearly so, but it is broad for most individual boreholes. Under broad uniformity conditions, recomputed screen slot widths are 0.33 and 0.18mm for the Ajali and Nanka Sands respectively. In commercial terms, these figures generally translate into ‘24inches gage’ and ‘7inches gage’ screen slot sizes for the Ajali Sands and ’13 inches gage’ for the Nanka Sands.KEY WORDS: Ajali and Nanka Sands, Sand Control, Gravel Pack and Screen Slot Sizes

An investigation of groundwater condition in Agbede by Geelectrical resistivity method.

Vertical soundings (VES) for the purpose of drilling groundwater boreholes for the inhabitants were conducted in Agbede to determine: the depth to the aquifer (Ajali Formation which consists of porous and permeable coarse sandstones); the thickness of overlying aquiclude (Imo Shale which consists of non porous/ permeable thick clays) and to locate where the small lenticular sands within the Imo Shale called perched aquifer exists. Perched aquifers are hydraulically separated, are relatively small, and they occur above the water table when there is an impermeable layer of rock (aquiclude) above the main aquifer. The VES curves of the area were qualitatively interpreted and the result showed an ascent at the first decade (dry top soil), a decent at the second decade (Imo Shale) and with the right most segment ascending int6o the third decade which is an indication of the presence of the Ajali Formation below the Imo Shale. The geoelectric section from the from the VES revealed that the Ajali Formation could not be encountered even at a depth of 494.03m, indicating that the clay is as thick as 500m. Also perched aquifer could be encountered between the depths of 52.76-55.43m with thicknesses 9.89-10.86m but not in all locations. Journal of the Nigerian Association of Mathematical Physics Vol. 10 2006: pp. 71-76