Geochemical interpretation of thermal water and borehole water from the Asal-Ghoubbet region, Djibouti

Abstract

The Asal area, Djibouti, is characterized by volcanic activity with extensive faulting, hot springs and fumarole manifestations on the surface. Geologically, this area is dominated by volcanic rocks like basalts. Twenty-eight samples were collected from springs and Asal wells in 1990 from the Asal-Ghoubbet area. The purpose of this project is to interpret the chemical data obtained from these samples using the ClSO4-HCO3 and Na-K-Mg ternary diagrams for classification of the water and to estimate the temperature and origin of the reservoir fluids from the results of chemical analysis. Apparently, chalcedony controls the silica solubility and the results of original cation geothermometer calculations suggest a much higher temperature for the Na-K-Ca geothermometer than the Na/K geothermometer, for the spring waters. The high Na-K-Ca temperatures are due to the influence of cold groundwater and after the application of a magnesium correction, values much closer to those of the other geothermometers are obtained. Quartz controls the silica solubility in the fluids of the Asal wells, and the quartz geothermometer yields temperatures close to measured temperatures as do the cation geothermometers (NaK and Na-K-Ca). The Cl/B concentration ratios suggest that the spring water composition is influenced by water-rock interaction, but that of the borehole water suggests evaporated seawater with a relatively low boron concentration. Thus, the results of this study suggest that the spring waters are mostly relatively low temperature waters whose origin is in seawater and whose composition has been modified by evaporation and water-rock interaction. The Asal well water is predominantly evaporated seawater from a reservoir with temperatures of 220- 260°C.