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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.

This paper presents the study of the heterogeneity in lithofacies, porosity, permeability, mineral grain density, pore-throats sizes and hydraulic rock types and the intra sand-body compartmentalization in cored delta-front deposit of Greater Ughelli depobelt of Niger Delta. Sedimentological study of core samples results in the identification of nine lithofacies and the interpretation of environments of deposition as mainly proximal and distal delta-front mouth bar. The sand unit indicates moderate to excellent reservoir quality with core porosity between 16.2 and 29.5% and permeability between 16.8 and 7,560 md. Dykstra-Parsons core permeability distribution coefficient of 0.97 indicates that the studied reservoir sand-body is vertically highly heterogeneous, with high potential for vertical intra sand-body compartmentalization. Graphical cluster analysis of Flow Zone Indicators (FZI) led to the identification of nine Hydraulic Flow Units (HFU) with distribution controlled by depositional facies. The prediction of permeability, porethroats (r35) and flow zone indicator values of the reservoir in an uncored-well using predictive mathematical models developed with multiple regression analysis enabled the inter-well correlation of hydraulic flow units and indicates lateral continuity of reservoir compartments. An intra sand-body compartmentalization evaluated with core permeability values, Winland r35 coefficients and flow zone indicators was corroborated with formation pressure data analysis. Results show that the studied delta front-mouth bar reservoir is vertically compartmentalized by intercalated shales. Fractures in over-pressured zones were found to reduce vertical reservoir fluid compartmentalization by intercalated shales. However, reduction in reservoir pressure due to hydrocarbon production can result in dynamic compartmentalization and consequently, reduction in hydrocarbon recovery.

Sediment deposition has increased at a meander section of Mbaa river resulting in a drastic reduction of water storage. It has necessitated calls for sand-mining or periodic dredging which could have far-reaching impacts on the aquatic ecosystem, depending on the texture and chemical compositions of the sediment. Therefore, surficial sediments samples were analyzed for textural characteristics, mineralogy and chemical compositions. Grain sizes and mineralogical analysis showed that the sediments were moderately sorted, mesokurtic, near symmetrical - slightly positive skewed, subangular – subrounded medium grained-sands, and consist of about 2.5% feldspar, 2.7% rock fragments, 5.4% hematite and 89% quartz. Mean TOC and pH were 0.39 wt% and 6.48 respectively. Atomic Absorption Spectrophotometer (AAS) analysis results showed general low concentrations of metal elements: mean values of alkali/alkali earth metals ranged from 1.33 - 3.72ppm, transition metals ranged from 0.12 - 23.07ppm; while Pb (a poor metal) was not detected. General low concentrations of metals and TOC; and non-detection of Pb suggest lack of mineralization zones and minimal anthropogenic impacts in the upstream areas.  Textural and chemical characteristics and moderate energy of river water flow suggest that human resuspension of the sediment deposit will result in minimal lateral dispersion of sediment in the water column, except during heavy rainfall. Therefore, periodic dredging or sand mining will cause minimal impact on the aquatic ecosystem, especially in the distant downstream areas, whereas within the meander section and nearby areas of the river, it is expected that there will be a significant change in water quality parameters due to the increase in the concentrations of suspended and dissolved compounds.

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.