Thermometric Indicators of Fluid-Rock Interaction in the Bacon-Manito Geothermal Field, Philippines

Abstract

This study focuses on the thermometric indicators of fluid-rock interaction to aid in understanding the fluid-rock interaction processes in a neutral-pH liquid-dominated Bacon-Manito Geothermal Field (BGMF) in the Philippines. Two wells: Well OP-4D, a mature production well; and Well RS-1D, an exploration well have been included in this study to compare and contrast the temperature and fluid-rock interaction indicators, and correlate across the geothermal field. Petrography and geochemical techniques have been applied to cuttings to correlate lithologic, mineralogic, and geochemical variations between wells. Well OP-4D encountered predominantly andesitic to basaltic volcanic rocks while Well RS-1D drilled through similar volcanic rocks, however, limestones and quartzose sandstones become more abundant at greater depths. Hydrothermal alteration appears to be structurally and lithologically controlled such that the volcanic rocks are more intensely altered than the limestones and quartzose sandstones. The minerals present in both wells indicate a prograde neutral-pH hydrothermal alteration based on smectite + illite + chlorite + quartz + calcite + epidote + actinolite + garnet + secondary biotite assemblage. Contact metamorphic minerals such as garnet and wollastonite are more common in Well RS-1D than in Well OP-4D. Mineral-, fluid inclusion-, and chlorite geothermometry techniques have been used to determine the temperature variations with depth. Chlorite thermometry here is correlated to variations in textures and indicates that chlorite in veins generally gives a higher temperature range (250° to 300° C) than those in vugs (~120° to 200°C); and chlorite as replacement minerals give a wide temperature range (120° to 380°C) than those in the matrix (170° to ~250°C). Moreover, chlorite as a replacement mineral occurs from as shallow as -100m whereas matrix occurs at around -1000m. Temperatures are broadly consistent between the different thermometric techniques and indicate generally higher temperatures reaching >330°C at greater depths consistent with locations in an upflow zone. Based on integration of permeability, temperature and fluid-rock interaction, Wells OP-4D and RS-1D were possibly drilled through the same upflow region, and Well RS-1D appears to have a better production potential than Well OP-4D.