GEOTHERMAL RESOURCES OF YEMEN AND THEIR GEOTHERMOMETRIC CHARACTERISTICS

Thermal springs and gas vents are widely distributed throughout the country, but the majority is found discharging from igneous centers of Tertiary and Quaternary volcanic fields of the western Yemen province. Structurally these volcanics are connected to N-NW faults that are parallel to the main Red Sea trend, and partially associated with relatively shallow Tertiary acidic intrusions. In the Gulf of Aden region the heat flow varies from 50-100 mW/m². The present geothermal gradient in the Red Sea region ranges from 49 to 77 °C/km, and the present heat flow varies from 94 to 154 mW/m². Such high and moderate heat flow values have more or less been affected by the thermal equilibrium between the upper mantle and the crust. This had resulted in the formation of the relatively large and widely distributed epithermal alteration haloes throughout the Yemen Cenozoic volcanic areas and also, the occurrence of thermal springs. Temperature and pH values of the thermal springs range between 37 °C and 96 °C, and 6.3 and 8.7, respectively. However, TDS values range between 1000 ppm and 3500 ppm and in some cases may exceed 10000 ppm. The condensates collected from fumaroles and steaming ground have low pH values (<4.5) and TDS (<250 ppm). The Yemeni thermal waters indicate high variability in composition since they are Na (K)-Cl, Na-HCO₃, and Ca (Mg)-SO₄ types, whereas the surficial waters have the typical worldwide Ca (Mg)-HCO₃ composition. The water samples from the southern Provinces of Yemen have a Na/Cl ratio approximately equal to 1, suggesting variable degrees of mixing with seawater. The remaining samples display a higher, but relatively constant, Na/Cl ratio (4-6). This can be related to the fact that the thermal water in the southern and eastern Provinces is often associated with N₂-gases while the others present a CO₂-rich gas phase. The latter increases the water-rock interaction processes leading to a higher alteration degree and favoring ion-exchange reaction with Na-rich silicates. These waters thus evolve to Na-HCO₃ in composition. Equilibrium temperature evaluation of the thermal reservoirs has been performed by using different liquid phase geothermometers, such as SiO₂, K²/Mg and Na/K. Estimated reservoir temperatures range between 70 and 140 °C.