Geochemical Re-Assessment of Mt. Balut Geothermal Prospect, Sarangani, Davao del Sur: an Update

Abstract

Surface exploration surveys were conducted in Balut Island geothermal prospect in 2013 as part of the Department of Energy (DoE) project “Detailed Assessment of Selected Low Enthalpy Geothermal Areas in the Philippines”. The primary objective of the abovementioned project is to accelerate the development of low to medium enthalpy geothermal resource areas (900C to 1500C) in the Philippines mainly for power generation. Secondary project objectives included: (1) the assessment and realization of the economic feasibility of small scale geothermal power projects for local power needs, and (2) the preparation of a comprehensive data package that will showcase this type of geothermal resource for future private investor participation. Hot springs and hydrothermally-altered grounds are found in four barangays located on the west and northwest apron of the Balut island volcano namely, Lipol, Gomtago, Tinina and Batuganding. The hot springs have discharge temperatures ranging from 38-750C. The waters from Barangay Lipol are the hottest and the most acidic with field pH of 2. Those from Barangays Tinina and Batuganding are warm with neutral to near neutral pH. The chloride levels of the waters are low, ranging from 123-279 ppm; HCO3 range from nil to 606 ppm while sulfate levels range from 26-564 ppm. The higher temperature waters have higher chloride and sulfate levels than the warm waters which are bicarbonate rich. The large altered region of Tacol Solfatara, named after the Tacol Collapse Structure in Barangay Gomtago, is characterized by a very dry, hot, gassy, and intensely altered ground. All thermal springs have high Cl and SO4 content although the previous and present report has not found a representative hot spring of reservoir fluids. Tolentino, et. al. (2014a) further investigated to explain the source of high temperature, Cl and acidity of the thermal springs by cross-plotting the SO4 vs. pH and Cl vs. Mg. In the report, it is confirmed that an increase in temperature shows an increase in SO4 content and pH. The positive correlation indicates that the springs are actually steam heated. The result of the Cl vs Mg on the other hand shows that the spring waters are a product of mixing and dilution of seawater and groundwater, thus are all shallow in origin. To explain the source of high temperature, it is then inferred the existence of a gas cap underneath that supplies heat and H2S to the thermal springs. Reservoir temperature on the other hand is estimated to range from 175-2000C based on gas geothermometry.