Deep Geothermal Resources in the Philippines: an Update of Two PNOC EDC Exploration Projects

Abstract

Results of deep drilling, flow-testing, and surface exploration are integrated to provide updated working models of the Mt. Labo and Northern Negros geothermal fields. In Mt. Labo, a diorite intrusion mapped from gravity survey at 3-4 Ian depth is believed to actively supply heat and magmatic volatiles into the overlying hydrothermal system. High-temperature (260-280°C) but acidic fluids are encountered in the region closest to this presumed intrusion. West of the acidic reservoir, neutral-pH fluids with temperature of 265°C are present at depths of over 2000 m. The exact extent and capacity of this neutral chloride resource are the focus of current investigations. In Northern Negros, an intrusive body at depths of 3.5-5 Ian has also been inferred based on gravity data. Unlike in Mt. Labo, the wells drilled closest to the inferred intrusive body show no preferential increase in temperature suggesting that such intrusion is not the heat source of the hydrothermal system. The latter is a 250°C reservoir at depths of 2200-2500 m containing neutral-pH chloride brine with no indications of magmatic input.
Delineation drilling are currently on-going in both areas to determine their commercial capacity. But their long-term development and utilization will have to contend with the thick cover of the reservoir, their relatively lower temperature and fair to moderate permeability compared with currently producing Philippine geothermal fields.