SUBSURFACE ALTERATION AND 3D MODELLING IN ASAL-RIFT GEOTHERMAL FIELD, DJIBOUTI

Abstract

The study area (Asal-rift geothermal field) is one of the most prosperous geothermal fields and tectonically active area in Djibouti. Almost all of the studied area is covered by volcanic rocks, Thermal manifestations are found in the area. This paper describes the result of X-ray diffraction mineralogical analysis in order to characterize hydrothermal processes and thermal evolution using cutting samples from new geothermal wells (three directional wells).
The formation of hydrothermal alteration minerals is usually dependent on the temperature, permeability, pressure, fluid composition and the duration of hydrothermal activity. The lithologic encountered succeed from oldest to youngest; 1) Dalha basalt Series, this unit consists of a sequence of lava flows estimated to have been formed between 8.9 to 3.8Ma, 2) Afar Stratoid composed mainly of basalt, which as formed between 4 to 1Ma, 3) Asal Series, less than 1Ma consisting of porphyritic basalt, scoriaceous basalt and hyaloclastite in the southeastern part of Asal-rift geothermal field. Mineralogical examination revealed six zones of hydrothermal alteration downward from unaltered zones, followed by smectite zones, mixed layer zones, chlorite zone, chlorite-epidote zone and chlorite-actinolite zone. The hydrothermal alteration zoning with increasing depth resulted from interaction of geothermal fluids with the basaltic host rock.
Three dimensional magnetotelluric (MT) inversion of Asal-rift using leapfrog geothermal was performed to obtain the resistivity structure on the field. The result of the interpretations shows three main resistivity structures. Thin and high resistivity layer is present at the top followed by low resistivity (conductive cap), below there is a high resistivity layer (resistive core).