Update on the Stable Isotope Geochemistry of the Leyte Geothermal Production Field

Abstract

Isotopic data monitoring from geothermal wells, springs, ground water and rainfall aims to update the stable isotope systematic of the Leyte Geothermal Production Field, Philippines. Isotope collection was conducted during the exploration stage and in 1990-2002 but was terminated due to the breakdown of the mass spectrophotometer isotope analyzer in 2002. With the procurement of a new isotope analyzer, sampling in Leyte resumed in 2010 and now becomes a routine geochemistry activity. Isotope data, correlated with the geochemical characteristics of the fluids, elucidates the reservoir processes affecting the performance of the Leyte Geothermal Production Field including Tongonan-1 Sector which continues to produce for over 30 years. For example, production wells affected by incursion of injected brine show enrichment of stable isotope (from -1.19 to 0.22 δ18Ores‰) coinciding with reservoir chloride increase. Conversely, for some wells affected by dilute, cooler waters depict relatively depleted oxygen-18 isotope (at -1.6 to -1.70 ‰) which corresponded to lower chloride and gas levels. In areas where boiling is prevalent, there is an observed enrichment of stable isotopes, increasing CO2TD and discharge enthalpies greater than 2000 kJ/kg. Monitoring of isotopic trends and distribution in the area, together with geochemical trends, aids in mapping out the evolution of these reservoir processes. This also serves as a useful tool in evaluating the effectiveness of implemented strategies. As such, isotope geochemistry has been considered a valuable tool in managing the production field and in formulating resource strategies. Thus, isotope monitoring correlated with geochemical data must be continued to help us understand changes in field hydrology with continued mass extraction.