The geophysical recognition of a vapor-cored geothermal system in divergent plate tectonics: The Alalobeda (Alalobad) field, Ethiopia

Description

We show the results of a geophysical survey carried out in the Alalobeda (Alalobad) geothermal field (Afar, Ethiopia). The site is located on the western margin of the NW-SW Tendaho Graben, at the intersection with the NNE-SSW Main Ethiopian Rift. The survey included 121 Magnetotelluric (MT) and Time Domain Electromagnetics (TDEM) soundings, and 300 gravity records. We applied 3D MT and gravity inversion to target a previously inferred high-T (220 °C) near neutral pH liquid-dominated chloride reservoir. Geophysical modelling identified the graben basin structure, with normal fault stepping in the Afar Stratoid basalts (2485 kg/m3), underlying a low-density (2035 kg/m3) and conductive (1 Ohm m) sedimentary cover. Along the graben shoulder, a density anomaly (+200 kg/m3) suggests a propylitization zone corresponding to the reservoir, underlying the typical conductive clay cap updomings and downdomings of high-T geothermal systems. A major conductive plume, without an appreciable gravity signature, occurs in the graben shoulder. Integration with the geochemical results allows to interpret it as a vapor-cored system. The hot spring water CO2, H2S and Cl content indicates the contamination of magmatic gases into the reservoir that produce fluid acidification while ascending from a deep source. The local bulk resistivity (10 Ohm m) is explainable by assuming conductive permeating acid fluids (10 S/m), an enhanced porosity (20%) and a significant gas saturation (20%). The absence of an appreciable density increase is also justified by the hypothesised porosity and the gas fraction. Since the acidic fluids are neutralised by interaction with the host rock, the sampled waters are nearly pH-neutral. Our results suggest the first occurrence of a vapor-cored system in divergent plate tectonics, such as the Tendaho Graben sector of the Afar rift zone.

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