The Sorik Marapi Geothermal Reservoir is currently being developed by Sorik Marapi Geothermal Power (SMGP) in northern Sumatra, Indonesia on the northeastern flanks of the Sorik Marapi Volcano along the Great Sumatran Fault Zone (GSFZ). A broad area of thermal surface manifestations exists with manifestations located on the summit, north and eastern flanks, and to the north of the volcano. While the flanks of the volcano support primarily steam discharge as fumaroles or acid-sulfate or acid-sulfate-chloride condensate pools, neutral sodium-chloride hot and warm springs are found to the north. Two-phase geothermal fluid samples have been collected from well discharge tests and after plant commissioning compiled into a data base with the fluid chemistry of surface manifestations. Geochemical modeling of reservoir chemistry based on two-phase samples collected at the surface indicates that the reservoir fluids of all wells is a similar, neutral sodium chloride brine. Silica geothermometer temperatures from analysis of well samples range from 240 to 270°C, matching measured temperatures well, whereas cation geothermometers are approximately 10 to 30°C higher. Slight variations in chemistry between wells appear to indicate slight temperature variations within the field and may also reflect the variations in reservoir rock and the location of geothermal upflow and outflow. Comparison of the reservoir and surface manifestation geochemistry indicates the acid-sulfate and acid-chloride-sulfate waters on the flanks of the volcano are likely geothermal steam condensate and ground-water mixtures, and the neutral chloride waters to the north may be a distant outflow of the reservoir or represent a separate, lower temperature geothermal system.