Diagenetic evolution of the Ansian–Pliensbachian Adigrat Sandstone, Blue Nile Basin, Ethiopia

Abstract

The Blue Nile Basin is located in the Central Plateau of Ethiopia. The basin consists of Precambrian basement, Palaeozoic and Mesozoic sedimentary rocks and Tertiary volcanic rocks. The sedimentary successions and Adigrat Sandstone reach a maximum thickness of 3000 and 800 m, respectively. The formation is composed of mudstone, finely laminated siltstone, very fine-grained cross-bedded sandstone, coarse to medium-grained sandstone, massive to crudely cross-bedded gravely sandstone and massive to crudely-bedded conglomerate. In the Blue Nile Basin, the Adigrat Sandstone was deposited in alluvial fan, fluviatile and lacustrine depositional environments. The formation has a complex diagenetic history and cemented by silica, carbonate, kaolinite and hematite with minor amounts of dolomite, illite, chlorite and feldspar overgrowths. Depositional environment, burial history and diagenetic processes are the major factors, which control the porosity and permeability of the Adigrat Sandstone. Primary porosity is preserved due to framework grain stability. Dissolution of carbonate cements created a certain amounts of secondary porosity in medium to coarse-grained sandstones, siltstones and mudstones facies. The porosity and permeability reach up to 20.4% and 710 mD, respectively. The medium-coarse-grained sandstones are porous and potential for oil and gas reservoir, whilst low-permeability siltstones and mudstones are possible gas reservoir.