Potential Constraints to the Development of the Rangas Sector Based on Petrologic Evaluation of the Bacman Geothermal Field, Philippines

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 OP6D, 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. INTRODUCTION The Bacon-Manito (Bacman) Geothermal Production Field, (Fig. 1) has a total installed capacity of 150 Mwe from Palayang Bayan (Bacman-I, 110 MWe), Cawayan (Bacman-II, 20 MWe), and Botong (Bacman-II, 20 MWe). Bacman-I has been in commercial operation since September, 1993 while Cawayan and Botong were commissioned in 1994 and 1998, respectively. An earlier evaluation on the Rangas-Tanawon sector (PNOC-EDC, 1992) shows an upflow zone in the vicinity of OP-4D with a major outflow towards the northwest and minor outflow paths to the west, southwest, and east-northeast. Although a well has yet to be drilled in Rangas, this sector was considered part of the upflow zone as there were no indications of pressure and temperature decline south of OP-3D. On the other hand, geochemical baseline studies (Ruaya et al., 1994) and magnetotelluric interpretations (Los Baños et al., 2000) both indicate that Rangas lies within a possible outflow path. The present study focuses on the extent of hot geothermal resource from the Palayang BayanBotong production field towards the southeast in Rangas through a detailed petrologic evaluation of production wells in Palayang Bayan (PAL-8D, PAL10D, PAL-13D, PAL-14D, PAL-15D) and Botong (OP-3D, OP-4D, OP-5DA, OP-6D, OP-7D). These petrologic data are integrated with available magnetolelluric, geochemical and other reservoir parameters to a model showing the location of Rangas relative to the existing production field and to assess the viability of this frontier sector for future expansion. Pa ng as D u m . C Pa lay an Ba yan Ce ntr al