Microfossil Constraints on the Age, Paleoenvironments and Source Rock Potential of the Eyasi-Wembere Basin, Tanzania

Abstract

The timing and processes governing organic matter accumulation in East African Rift basins remain poorly constrained due to limited subsurface data. We present the first integrated microfossil-based study for the Eyasi Wembere Basin (EWB). Core samples from three stratigraphic boreholes drilled in the basin have been utilized in this study. Palynological, palynofacies, ostracods and sedimentological analyses were used to determine age, paleoenvironment and source-rock potential of the basin fill. Based on the palynomorph species Laevigatosporites ovatus and Triorites minor, and the ostracod species Cytherissa lacustris and Ilyocypris bradyi, the age of the studied material ranges from the Pliocene to the Holocene. From sedimentological and micropaleontological evidence, the studied sediments were deposited within a shallow lacustrine system marked by spatial variations in water depth and hydrological conditions, with periodic fluvial input into the basin. Palynofacies analysis shows organic matter dominated by amorphous organic matter and freshwater algae (Botryococcus), indicating oil-prone kerogen types I and II. Spore-color observations from the studied material indicate a marginally mature source rock. Therefore, the basin represents a potentially promising but underexplored segment of the East African Rift System, where deeper drilling and additional subsurface investigations are required to fully evaluate its petroleum system potential