Geochemistry of Thermal Waters from Al-Lisi – Isbil Geothermal Field, Dhamar Governorate, Yemen

Abstract

The Al-Lisi – Isbil geothermal and volcanic field, one of Yemens important fields, is known for its fumaroles, steaming grounds and hot wells. The area is mainly composed of basaltic lava flows, stratified basic tuffs and agglomerate pyroclastics. Less common are differentiated rock-types like rhyolite. The Al-Lisi volcano is an exception which consists entirely of acidic lava flows (rhyolite, obsidian, acidic tuffs and ash rings). Chemical analyses from 29 shallow water wells from the Al-Lisi – Isbil area were reviewed. All the data were interpreted using the WATCH program for speciation and by mineral equilibrium diagrams and other graphical presentation and classification methods. The maximum reservoir temperature for the wells, predicted by calculation of various geothermometers, exceeds 100˚C. There is evidence of mixing with cold water. The thermal fluid is of bicarbonate type and the reservoir rocks consist mainly of sandstone (Tawilah Formation) at 1000 m to 1500 m depth.