| Title | The Geophysical Structure of the Leyte Geothermal Production Field, Philippines |
|---|---|
| Authors | Nilo A. Apuada, Rhoel Enrico R. Olivar, David M. Rigor, Felixberto C. Maneja and Maribel C. Zaide-Delfin |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | LGPF, Bouguer anomaly, velocity model, resistivity anomaly, micro-earthquake |
| Abstract | Various geophysical studies have been conducted at the Leyte Geothermal Production Field (LGPF). During the exploration and the early development stages, resistivity and gravity survey results were used to identify and site drilling targets. Seismic epicentral and frequency studies were also conducted to monitor the occurrence of micro-earthquake events before and during exploitation. Re-interpretation of these geophysical data and correlation with subsurface reservoir data allowed the characterization of two high temperature water-dominated geothermal systems in the LGPF. Two different gravity signatures define the geothermal resources of LGPF. The Mahiao production field in the northwest is located inside the gravity high (2-6 mGals) while the Mahanagdong production field in the southeast is associated with the gravity low (<-2 mGals). The gravity high in the Mahiao production field is attributed to the combined effects of the Mamban Formation (MF) which is mainly composed of volcanic lava and breccias with very minor amount of sedimentary layers, and the Mahiao Sedimentary Complex (MSC) composed principally of conglomeratic microdiorite, quartz monzodiorite and volcanics. Furthermore, this field is defined by the 10 ߟ-m anomaly that coincides with the upflow zone. The subsurface resistivity structure at about 500 meters depth exhibits an increasing resistivity value with depth. The formation of the low gravity anomaly in the Mahanagdong production field is due the presence of thick layers of (. 200 m) of siltstones and claystones found intercalating with MF and MSC. Based on drilling results, the upflow zone is associated with the 20. 50 ߟ-m resistivity gradient. However, this field is further manifested by the occurrence of two conductive zones (10 and 20 ߟ-m closures) that signifies the outflow areas. Twodimensional modeling reveals a decreasing resistivity values with depth. Microearthquake monitoring before and during exploitation reveals that the earthquake activity at LGPF is dominated by the events occurring outside the geothermal field while the number of local events during the monitoring period remained fairly constant. The microearthquake activity at LGPF is associated with the Philippine Fault although induced seismicity was noted during injection and hydraulic experiment. The microearthquake low velocity (< 3.75 km/sec) model at 1.5-km depth correlates well with the 20 ߟ- m resistivity contour and the hydrological flow pattern. |