POTENTIAL CONSTRAINTS TO THE DEVELOPMENT OF THE RANGAS SECTOR BASED ON PETROLOGIC EVALUATION OF THE BACMAN GEOTHERMAL FIELD, PHILIPPINES

Abstract

At production levels below -600 m RL, the reservoir in the Palayang Bayan sector of the Bacman Geothermal Production Field is hosted by andesitic lavas, tuffs and breccias of the Plio-Pleistocene Pocdol Volcanics, while the Botong-Rangas sector is dominated by calcareous sediments of the Late Miocene-Early Pliocene Gayong Sedimentary Formation. Alteration mineralogy and fluid inclusion homogenization (Th) data are generally consistent with geochemical and stable measured temperatures indicating a maximum reservoir temperature of 320?C at the upflow zone located in the vicinity of wells PAL-10D and OP-4D. The isotherms dip towards the southeast in Rangas where maximum temperatures of 300?C lie at -1800 mRL. Freezing point depression temperatures (Tm) of fluid inclusions with corresponding apparent salinities of ~7,000-9,000 ppm Clequi approximate Clres of 8,000 mg/kg. Higher Th and apparent salinities are attributed to trapped CO2 gases contributed by Gayong sediments in PAL-8D, PAL-14D and OP-6D, while progressively low salinities in PAL-14D and OP-6D suggest dilution of reservoir fluids by invading cooler waters. Fluid flow towards Rangas is channeled by the southeast-trending Makabug, Botong, and Dome faults. The low permeabilities and high NCG levels of the Botong reservoir will likewise prevail in Rangas and may impose critical constraints on the future development of this sector.