RISK ASSESSMENT OF TECTONIC STRUCTURES - RELATED TO RESERVOIR LEAKAGE AT TENDAHO RESERVOIR; AFAR DEPRESSION, NE ETHIOPIA

The proposed Tendaho reservoir is located at the center of the Afar Depression where the Main Ethiopian Rift (MER), the Red Sea Rift and the Gulf of Aden Rift join at the extensional triple junction. The Main Ethiopian Rift and the Red Sea Rift propagation structures are inter-fingered along the Tendaho Goba’ad Discontinuity (TGD) ridge. During the propagation of the MER structures towards the Afar Depression, the Gesye graben was formed. Then after the graben was bisected by magmatic intrusions and later on it was cross cut by the NW-SE trending Red Sea rift propagation structures. The Gesye graben has NE-SW orientation with a maximum and minimum width of 9 and 3 km, respectively. The entire proposed reservoir bed, which lies along the TGD ridge, is covered by lacustrine deposits except the 2.4 km² magmatic dyke intrusion at the center of the Gesye graben.

Ten parameters were considered for the risk assessment of the tectonic structures (unforeseen geological conditions during the feasibility stage) related to the reservoir leakage: Tectonic Parameters – tectonic status Ts, tectonic regime Trr, tectonic history Th, aerial extent Ex, cross cut index Cci; and External Parameters – hydrogeology G, proximity P, relative position Ps, elevation difference Ed and hydraulic pressure Hp. Semi-quantitative and qualitative approaches of simple averaging, pairwise comparison and analytic hierarchy processing were applied to analyze the risk level of the tectonic structures. Tectonic structures, which are very close to the proposed reservoir, formed by extensional tectonic activities, higher aerial extent and cross cut index, are very high risks concerning the reservoir leakage. The risk of the reservoir leakage was estimated for the worst and average scenarios as 25.4m³/s and 2.4m³/s, respectively.