Integrated seismic stratigraphic and structural analysis of Southern main Ethiopian rift basin: implications for hydrocarbon potentials

In East Africa and the Arabian Peninsula, Mesozoic and/or Tertiary rift basins are well developed. These rift basins have proven their importance to economic development of the region through major oil discoveries including the Jobi-Rii field in the Albertine basin, Uganda, the Ngamia field in Lokichar Basin, Kenya and the oil fields of Yemen. The Main Ethiopian Rift (MER) developed during the Oligocene-Miocene time as part of the East African Rift System and is commonly divided into the Northern, Central and Southern parts. The subsurface rift architecture of the MER remains poorly understood due to lack detailed geophysical studies. This study aims to investigate the subsurface stratigraphic and structural setup of the Southern MER by using 2D high-resolution seismic reflection surveys integrated with nearby well data. Analysis of 2D-seismic reflection data in the Southern MER reveals four horizons from which three horizons show good correlations in Northern Abaya and Gelana basins. The basement structure delineated the main basins in the study area: Northern Abaya, Southern Abaya, Chamo and Gelana sub-basins. The sub-basins showed a typical rift-basin development: pre-rift, syn-rift and post-rift sedimentation and the faulted basement is dominantly oriented NE-SW parallel to the regional structural trend of the area. The deepest basement is greater than 3600 m in Northern Abaya basin followed by Gelana Basin which reaches to 3100 m. Southern Abaya and Chamo basins have shallow basement depths less than 2250 m. Interpretation of the interval velocity integrated with the geology and nearby well data helped to identify the different stratigraphic units ranging from Jurassic to Quaternary time span in the area. Possible source rocks of Late Jurassic to Upper Cretaceous marine shales and fluvial-lacustrine sediments of Lower Miocene thickness 1200 m and 1800 m are identified in Northern Abaya Basin, respectively, where the thicknesses of those units are 900 m and 1000 m in Galena Basin. We also infer from seismic interpretation that the upper Miocene sand/sandstone inferred on both sub-basins can be considered as a reservoir, whereas distinctive fault closures against the basement with horst and grabens or tilted half grabens and anticlinal structures can smoothly provide the hydrocarbon trapping mechanism. Numerous faults mapped on seismic sections play a major role for migration of hydrocarbon from source to reservoir rocks. The interpreted NE-SW major faults located on the western side of most of the seismic sections reactivated during the East African rifting at Neogene period produced an appreciable throw which can also provide migration pathways for the hydrocarbon.