Constrained 3D gravity interface inversion for layer structures: implications for assessment of hydrocarbon sources in the Ziway-Shala Lakes basin, Central Main Ethiopian rift

Abstract

Multi layer 3D gravity inversion for layered structures and density interfaces are performed in the Central Main Ethiopian rift bounded between 38000′–39030′ E and 7000′–8030′ N. The inversion is carried out in wave number domain using Parker-Oldenburg algorithm and is constrained with initial model information. The previous studies in the region focused on mapping crustal structures and Moho depths and least is known about the shallow earth. This study thus targets on mapping layers relief of shallow earth origin. Stacked horizons with depth to tops of density contrast are obtained from well log data and previous geophysical studies. These stacked grids represent major geological boundaries where density contrast exists. The model utilizes observed residual gravity anomaly and generates the structural relief maps of the respective layers with their corresponding gravity anomaly responses and the associated errors. Successive structural inversions are performed on three layers with their corresponding acceptable mean misfits’ errors. The iteration process converges successively for each layer in each structural inversion and the result is validated against a priori information. In addition to the topography/thickness of each layers, this study for the first time identified a new Mesozoic horizon laying between a Tertiary ignimbrite layer and the crystalline basement at depths between –2499 m and –3060 m and having estimated maximum thickness of 561 m. The identified Mesozoic sediment formation underlies a thick volcanic cover of 2.5 km which might be a suitable geologic setting for the growth of hydrocarbon reserves in the area and could probably be the source of CO2 degassing.