PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202 1 Geochemistry of the North Western Algerian Geothermal System Mohamed BELHAI, Yasuhiro FUJIMITSU, Fatima Zohra BOUCHAREB-HAOUCHINE, Tatsuto IWANAGA and Masami NOTO Mailing address: belhai_Mohamed89@mine.kyushu-u.ac.jp E-mail address: mkhader89@gmail.com Keywords: western Algeria, geothermometer, isotopes, mixing, shifting. ABSTRACT The western Algerian low enthalpy geothermal system is geographycally divided into four important geothermal fields: the Hammam bouhdjar, Hammam Bouhnifia , Hammam Boughrara and Hammam Rabbi geothermal fields, geologically located in the Occidental Tell of the Alpine-Magrebide belt. The upper Jurassic limestones constitute the main reservoir rocks of the geothermal area. The thermal water surface temperatures ranged between 42.9 to 66.1°C, with a near-neutral pH (6.2 to 7.56). The thermal water samples studied show a high total dissolved solids (TDS) of up to 4003 mg/kg. The water chemistry revealed four types of waters: Na-HCO 3 -Cl, Na-Ca-HCO 3 , Na-Ca-SO 4 -HCO 3 and a few Na-Mg-HCO 3 facies. The isotopic results indicate a meteoric origin of the thermal water of the western Algerian geothermal systems. The δ O 18 and δD shifting is due to the residence time of the thermal water in the reservoir, the faraway of the recharge zone area, and mixing with shallow groundwater. The estimated temperature, given by silica and cationic geothermometers, ranges between 125 to 250°C. The deep circulation of water in the study area is supplied by the high geothermal gradient around 4.4 °C per 100 m, which is supported by the NE reverse faults of Miocene age. 1. INTRODUCTION The Hot springs area is located in the western part of Algeria (Fig1), also called the Occidental Tell, it extends for over 300 km, from the Ouarsenis Mountain and the Chelif basin in the East up to the Rif and the pre-Rif at the western borders of the country, for more than 100 km. The samples were collected from the four most important geothermal areas of western Algeria, Hammam Bouhdjar, Hammam Bouhnifia, Hammam Boughrara and Hammam Rabbi. The objective of this paper is to make a convincing conceptual model for the western Algerian hydrothermal systems relying on water chemistry and to estimate the reservoir temperature for each geothermal area using several geotheromometrical techniques and mixing models.In order to find any relationship with the Plio-Quaternary alkaline volcanism and the geothermal fluid of the study area, the isotopic results were also applied in this study. Figure 1: Detailed map showing the location of the Western Algerian geothermal system in Algeria, black circles show the studied geothermal fields.