Integrated organic, inorganic geochemical and palynofacies analyses to characterize the paleoenvironment of the Tortonian (Late Miocene) Wakar Formation, offshore Nile Delta, Egypt

Abstract

The subsurface Wakar Formation (Fm) has been identified in all wells of the offshore concessions in the eastern Mediterranean in Egypt. This formation is important because it represents the end member of the Late Miocene (Tortonian) siliciclastic sedimentary rock of a vertical thickness between 500 and 900 m in these fields and is a promising formation for gas production in Egypt. There are no comprehensive studies on the geochemistry and palynofacies of this Wakar Fm in literature. Therefore, sixteen collected ditch samples of the entire thickness of the Wakar Fm from a PFM-SE -1 well were investigated. The main objective is to reveal the history of depositional events that prevailed in North Africa at this time. TOC content (average = 0.81 wt%) and generative potential of the Wakar Fm are of poor and fair quantity. The organic matter (OM) belongs to types IIIC and III of coastal and delta settings as revealed by the palynofacies and geochemical data. The Tmax of the samples does not exceed 435 °C (419–432 °C) of the immature stage, which corresponds to the carbon preference index (CPI >1) (1.15–1.4). The thermal alteration index range between 2 and 2+, indicating immature to marginally mature OM. GC of the two extracted samples shows a predominance of short-chain and mid-chain n-alkanes derived mainly from algae and seagrass; the Pr/Ph ratio (3.24–3.02; suboxic) and low terrigenous/aquatic ratio (TAR; 0.23–0.47) indicate that the OM bearing sediments were accumulated in a transitional setting of the suboxic aqueous medium. The dominance of terrestrial OM and low amorphous organic matter also suggests deposition in a marginal basin under dysoxic-anoxic to suboxic-anoxic conditions. Pr/n-C17 versus Ph/n-C18 diagrams confirm that preservation occurred in weakly oxidized environments from a source of kerogen type III gathered from a swamp and transitional marine zones. The inorganic geochemical analyses (XRD and XRF) and the calculated paleoenvironmental indices (i.e; log (Fe2O3/K2O) versus log (SiO2/Al2O3)) indicate that the main composition of the Wakar Fm is a combination of iron shale and sandstone facies. The Fe-sandstone facies occupies the middle part of the formation with a thickness of 170 m, which is the pay zone in the present concession. The parent protolith of Wakar Fm is a combination of intermediate and mafic igneous rocks. The prevailing paleoclimate is arid and semi-arid conditions synchronize the base, and top of the Wakar Fm, which is responsible for weak to moderate chemical weathering as revealed from chemical index of alteration values (CIA average = 67.99). Discriminant functions exhibit a depositional setting belonging to an active continental margin and oceanic island arc between marine and terrestrial zones under oxic-dysoxic environments.