Discovery of New Geothermal Manifestations as Indicators of the Upflow Zone in the Lumut Balai Geothermal Field, Indonesia

Abstract

Recent field investigations in the Lumut Balai geothermal field, South Sumatra, Indonesia, have identified previously unrecognized geothermal surface manifestations in the southern part of the producing area, within the Tanjung Tiga prospect. These manifestations comprise fumaroles, hot springs, mud pools, and hydrothermally altered grounds, expanding the known extent of surface expressions associated with the system. This study evaluates the geochemical characteristics of the newly identified manifestations to assess their connection to the main geothermal reservoir and to delineate the geothermal upflow zone. Water and gas samples were analyzed for major ion chemistry, and gas composition, complemented by in situ measurements of temperature and pH, following established geothermal geochemical approaches (Giggenbach, 1988; Nicholson, 1993). Results indicate that fumaroles in the Tanjung Tiga prospect area contain higher non-condensable gas (NCG) concentrations than those observed in the currently developed sector, suggesting elevated subsurface temperatures and closer proximity to the upflow zone. Geothermometric calculations, including aqueous and gas geothermometers, estimate reservoir temperatures of approximately 275–300 °C beneath the Tanjung Tiga prospect, consistent with high-temperature upflow conditions (D’Amore and Truesdell, 1985). These integrated geochemical findings refined the conceptual model of the Lumut Balai geothermal system and provide a robust basis for upflow zone delineation and future exploration targeting in the southern sector of the field.