ALBERT

Description: Redox-sensitive elements (Mo, V, U, Re, Cd, Co, As, Sb, Tl, Ni, Cr, Mn, Fe, Pb, Cu, Zn, Se) and proxies for detrital clastic sedimentation (Al, Ti, Sc, Th) along with total organic carbon (TOC) and total sulfur (TS) were analysed to constrain the paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile (Abay) Basin, Ethiopia. These samples were collected from the deep marine Antalo Limestone (Oxfordian-Kimmeridgian), Gohatsion Formation (Bathonian) and glauconitic shale-mudstone unit (Aalenian-Toarcian). Significant variations in redox-sensitive elements concentration within and between these shales were evident for variable redox conditions. In general, the concentration of proxies for detrital clastic sedimentation increases from the Antalo Limestone to Gohatsion Formation and further to glauconitic shale-mudstone unit shales, respectively. The Antalo Limestone shales show slightly higher enrichment in redox-sensitive elements than the Gohatsion Formation and glauconitic shale-mudstone unit shales. The Antalo Limestones shales have higher TOC content than the glauconitic shale-mudstone unit shales and become very low in the Gohatsion Formation shales. The TS content in contrast, is relatively lower in the Antalo Limestone shales, intermediate in the Gohatsion Formation shales and becomes higher in the glauconitic shale-mudstone unit shales. The Antalo Limestone shales were deposited under anoxic-suboxic conditions whereas the Gohatsion Formation and glauconitic shale-mudstone unit shales were deposited under suboxic-oxic conditions. The TOC content and redox conditions of the Antalo Limestone shales implies favourable organic matter preservation and future source rock explorations in the basin should be targeted there.

Description: This study aims to evaluate the hydrogeochemical and isotopic signatures of water (surface and groundwater) of the Andasa watershed in the upper Blue Nile Basin, northwest Ethiopia using integrated hydrogeochemical and isotopic methods with the help of Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) techniques. To achieve this, major cation and anion (n = 61) and stable isotope (δ2H and δ18O; n = 36) analyses have been done. The result revealed that geogenic (rock-water interaction) and anthropogenic (waste disposal landfill site, agricultural practices, diverted river via open canal) effects are the major water quality controlling factors. Rock-water interaction is more dominant factor that produced alkaline earths bicarbonate hydrogeochemical facies. Moreover, the Cl and δ18O relationship shows a shift towards the right from vertical axis (δ18O) which signifies anthropogenic effects. Using HCA and PCA techniques, EC, TDS, Ca, Na, Mg, HCO3, SO4, Cl and NO3 were identified as the leading hydrogeochemical parameters in determining the area. Accordingly, Mg-Ca-HCO3, Ca-Mg-HCO3 and Ca-Na-HCO3 found to be the main water types. Silicate minerals are responsible for controlling the hydrogeochemical property of the waters in the area through hydrolysis and cation exchange processes. Most of the samples signify evaporation prior to recharge in the wet seasons and irrigation return flow in the dry seasons. Most river samples show depleted δ18O and δ2H reflecting that their source is highland area of the watershed. From hydrogeochemical and isotopic signatures, the north and northeast parts of the area are the discharging zones and southern and central parts are the recharging zones.

Description: This work aims to unravel the provenance, depositional tectonic setting and source area weathering of the cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia from its petrographic and geochemical signatures. Both the average chemical index of alteration (CIA) and chemical index of weathering (CIW) of sandstones (CIA = 91.08, CIW = 96.75), shales (CIA = 81.89, CIW = 98.17) and siltstones (CIA = 98.10, CIW = 99.35) of the Debre Libanos Sandstone indicate very strong chemical weathering. The low Th/Sc ratio of sandstones (1.46), shales (0.85) and siltstones (0.81), and Th/Sc versus Zr/Sc relationship indicate sediment recycling that leads to a progressive enrichment of heavy minerals such as zircon which is the major host for Zr and Th. The increase of quartz content in sandstones and dense minerals in siltstones and shales can be related to hydraulic sorting, grain-size controlled fractionation and source composition. Trace element concentration and their ratios (La/Th, Cr/Th, Th/Sc, Cr/V, Y/Ni) with rare earth element proxies (Eu/Eu*, LaN/YbN) of the studied samples revealed felsic to mixed source rocks of upper continental crust with some input from recycled old sediments. The source area of the Debre Libanos Sandstone is from the northern and northwestern Ethiopia during the time of marine regression, retreat of the Indian Ocean from NW Ethiopia back to SE Somalia. Tectonic discrimination diagrams revealed continental rift setting for the sandstones and collision setting for shales and most of the siltstones. Furthermore, the studied sandstones were partly deposited in a passive margin setting which had undergone post-depositional K-metasomatic alteration. The shales and siltstones were deposited in active margin setting.

Description: This study investigates the hydrocarbon generation potential, kerogen quality, thermal maturity and depositional environment of Middle – Upper Jurassic sedimentary rocks in the Blue Nile Basin, Ethiopia, using organic petrography, Rock‐Eval pyrolysis and molecular organic geochemistry. Thirty‐seven outcrop samples were analysed for their total organic carbon (TOC) and inorganic carbon (TIC) contents. The samples came from a Toarcian – Bathonian transitional glauconitic shale‐mudstone unit, the overlying Upper Bathonian Gohatsion Formation, and the Lower Callovian – Upper Tithonian Antalo Limestone Formation. Thirteen samples with sufficient TOC contents for further analysis of the organic matter, eight from the Antalo Limestone Formation and five from the glauconitic shale‐mudstone unit, were selected and analysed using Rock‐Eval pyrolysis. Vitrinite reflectance (VRr) was measured on random particles, and qualitative maceral analysis was performed under normal incident and UV light. Nine samples were selected for molecular organic‐geochemical analyses. All the samples originating from the Gohatsion Formation showed TOC values which were too low for further analyses of the organic matter. The TOC contents of shales and limestones from the Antalo Limestone Formation and and of shales from the glauconitic shale‐mudstone unit were 3.43‐6.43% (average 4.85%) and 0.76‐3.15% (average 1.72%), respectively, and two coaly shale samples from the latter unit have average TOC values of 18.48%. HI values are very high for shales in the Antalo Limestone Formation (average 575 mg HC/g TOC) but lower for the shales in the glauconitic shale‐mudstone unit. The vitrinite reflectance of shales from the Antalo Limestone Formation ranged between 0.21% and 0.47%; coaly shales from the glauconitic shale‐mudstone unit have VRr% of between 0.29% and 0.35%. Pr/Ph ratios for samples of the Antalo Limestone Formation shales ranged from 0.8 to 1.1, indicating anoxic to suboxic depositional conditions; while shales in the glauconitic shale‐mudstone unit show higher values of up to 4.9. In terms of organic petrography, the Antalo Limestone Formation samples are dominated by finely dispersed liptinite particles and alginite; the organic material in the glauconitic shale‐mudstone unit is of higher land plant origin, with abundant vitrinite and inertinite. Sterane and hopane biomarker ratios suggest an anoxic/suboxic depositional environment for the Antalo Limestone Formation shales and limestones. These values together with Rock‐Eval Tmax (average 414 °C), the high ratio of pristane and phytane over the n‐alkanes C17 and C18, and hopane biomarker ratios indicate that the Middle – Upper Jurassic succession is of low thermal maturity in the central parts of the Blue Nile Basin. The Antalo Limestone Formation shales have a high petroleum generation potential, making them a viable target for future exploration activities.