Stable isotope systematics of fluids and epidotes in Bacon-Manito Geothermal Field, Philippines: indicators of fluid origin and evolution

Abstract

The Bacon-Manito Geothermal Field (Bacman) in the Philippines is a liquid-dominated, volcanic field-type geothermal system in a convergent setting. Expansion initiatives in the geothermal field prompted additional studies to better understand the past and present processes that have affected the nature and configuration of the geothermal reservoir. This study contributes to the knowledge of stable isotope systematics and the existing hydrogeological model of Bacman. Geochemical data from geothermal well discharges were supplemented with hydrogen and oxygen isotopic data and elemental composition data from hydrothermal epidotes and reservoir rocks. Focus was given on hydrothermal epidote because of the mineral’s sensitivity to fluid composition, which is critical in distinguishing reservoir changes and processes that are not observed when only analyzing fluid samples. Also, it has an extensive presence in geothermal systems and is very useful in estimating reservoir temperatures based on morphology. Chemical and isotopic analyses of well discharges revealed short-term processes that are affecting the Bacman reservoir from the onset of fluid extraction due to field operations. Boiling in the Botong sector and the incursion of peripheral fluids and injection fluids into the main reservoir of Palayang Bayan and Cawayan sectors were observed from the changes in isotopic composition and chemical parameters in well discharges. Meanwhile, petrologic, isotopic, and elemental composition analysis of epidotes revealed variations in epidote chemistry which are closely linked to reservoir rock composition, fluid chemistry, and temperature. These factors dictate the extent to which water-rock interaction and fluid mixing, the two most prominent processes in the isotopic evolution of epidotes and fluids in Bacman, affect each particular sector in the reservoir. This study concludes that fluids in the Rangas sector originated from the main reservoir upflow in Botong and were isotopically enriched through the interaction with reservoir rocks. Fluids from Palayang Bayan and Cawayan sectors evolved through the continuous mixing of isotopically enriched parent waters with isotopically depleted peripheral waters. Further, data from this study has constrained the reservoir in Kayabon sector, which was thought to have produced ancient fluids that were isotopically distinct through the interaction with preexisting hydrous silicates in a relatively hotter past reservoir conditions.